United States 

Department of 

Agriculture 

Forest Service 

The California Spotted Owl: 

A Technical Assessment Paciflc Southwest 

Research Station 

General Technical 

Repott PSW-GTR-133 of Its Current Status 
Verner, Jared; McKelvey, KevinS.; Noon, Barry R.; Gutikrrez, R. J.; Gould. Gordon I.. Jr.; Beck. Thomas W.,Technical 

Coordinators. 1992. The California spotted owl: a technical assessment of its current slatus. Gen. Tech. Rep. 

PSW-GTR-133. Albany, CA: Pacific Southwest Research Station, Forest Service, U.S. Department of Agriculture; 285 p. 

This renort is based an the Final Renart submitted on Mav 8. 1992 bv the Technical Assessment Team to the , . ~ ~~ 

interagency Steering Committee for the California Spotted Owl Assessment. The 13 chapters cover the assessment of 

thecurrent status of theCaliforniaspotted owl, its biology and habitat use, and forests wherethe subsoecies occurs in the 

Rerrievol Terns: Strix occidenlalis occidomlis, habitat attributes, old-growth, resource management, Sierra Nevada, 

southern California 

Technical Coordinators: 

Jared Verner, a research wildlife biologist, is Project Leader-Wildlife Monitoring and Range Research, Pacific 

Southwest Research Station, USDA Forest Service, Fresno. California. and the Technical Assessmenl Team Leader 

I<e\in S. \lcKrlvr.y 1s :% ~M#rc bwIo~#st ~NII thc St,x.on. 11111 st.~l~~:d in AKIL Cdli Harq K. Uwn. 3 ir.e,r:l~ 

u,tld.lfi. hmlogtrt wtl~ the Stat~ou 1% Prole;! l.c.dcr-hnoer Wildlife Inrc.~ict~~~. Rc,carcll. Arcdld. R. J. (iatiCrrr~ i,

professor ofwildlife, Humboldt State University, Arcata. Gordon I. Gould, Jr., is a nongame wildlife biologist with the 

California Depanment of Fish and Game, Sacmmenta. Thomas W. Beck is a forcst biologist with the Stanislaus 

National Forest, USDA Forest S"&ce, Sonora, Calif. 

Cover-: Photographs by John S. Senser, Stanislaus National Forest, Mi-Wuk Village, Calif. 

Publisher: 

Pacific Southwest Research Station 

Albany, California 

(Mailing address: P.O. Box 245, Berkeley, California 94701-0245 

Telephone: 510-559-6300) 

July 1992 
A Technical Assessment of Its Current Status 

Technical Coordinators: 

Jared Verner Kevin S. McKelvey Barry R. Noon R. J. Gutierrez Gordon I. Gould, Jr. 

Thomas W. Beck 

Contents 

Part I: Where We Are Now, and Where We Should Go Next 

Chapter 1. 

Assessment of lhe current status of the California spotted owl, with recommendations 

for management ...................................................................................................................... 3 

Jared Verner, Kevin S. McKelvey, Barry R. Noon, R. J. Cutidrl-ez, Gordon I. Gould, Jr.. 

and Thomas W. Beck 

Chapter 2. 

Future directions for the California spotted owl effort .................................................................. 27 

Jared Ver-ner and Roberl J. Taylor 

Part IT: About the Owl 

Chapter 3. 

Background and the current management situation for the California spotted owl ........................ 37 

Thomas W. Beck and Gordon I. Gould, Jr. 

Chapter 4. 

The California spotted owl: general biology and ecological relations ........................ ............. 55 

Jared Verrzer, R. J. Girtidrrez, and Gordon I. Could, Jr. 

Chapter 5. 

Habitat relations of the California spotted owl ............................................................................... 79 

R. J. Gutidrr-ez, Jared Verner, Kevin S. McKelvey, Barry R. Noon,George N. Steger, Douglas R. 

Call, William S. LaHaye, Bruce B. Bingham, and John S. Senser 

Color Photograph Section ........................ ................................................................................ 99 

USDA Forest Serv~ce Gen Tech Rep PSW-GTR-I33 1992 
Contents 

Chapter 6. 

Home-range size and habitat-use patterns of California spotled owls in the Sierra Nevada ......... 149 

Cynthia J. Zabel, George N. Steger, Kevin S. McKelvey, Gary P. Eberlein, 

Barry R. Noon, andJared Verner 

Chapter 7. 

Patterns of habitat use by California spotted owls in logged forests of the northern 

Sierra Nevada ............................. .......................................................................... 165 

Cynthia J. Zabel, Kevin S. McKelvey, and James D. Johnston 

Chapter 8. 

Estimates of demographic parameters and rates of population change ........................................ 175 

Barry R. Noon, Kevin S. McKelvey, Daryl W. Lutz, William S. LaHaye, 

R. J. Gutiirrez, and Christine A. Moen 

Chapter 9. 

Stability properties of the spotted owl metapopulation in southern California ............................. I87 

Barry R. Noon and Kevin S. McKelvey 

Chapter 10. 

General biology of major prey species of the California spotted owl 207 

Daniel F. Williams, Jared Verner, Howard F. Sakal, and Jeffrey R Waters 

Part HI: About Forests and Forest Management 

Chapter 1 1. 

Historical perspectives on forests of the Sierra Nevada and the Transverse Ranges of southern 

California: forest conditions at the turn of the century .. ................ 225 

Kevin S. McKelvey and James D. Johnston 

Chapter 12. 

Fire and fuels management in relation to owl habitat in forests of the Sierra Nevada and . . southern Cal~forn~a ..................... .................................................................................. 247 

C. Phillip Weatherspoon, Susan J. Husari, and Jan W. van Wagtendonk 

Chapter 13. 

Projected trends in owl habitat .... ................................................. 261 

Kevin S. McKelvey and C. Phillip Weather-spoon 

Appendices 

Marlene B. Verner, Compile!- 

Appendix A-Technical Assessment Team and Chronology of Activities .................................. 277 

Appendix B--Glossary ............................................................................................................... 279 

Appendix CSpecies List 283 

Appendix D-Authors and Their Chapters 285 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 


Acronyms 

AK--------- Adaptive Kernel estimates of home-range 

 size 

BLM------- Bureau of Land Management (USDI) 

CDF-------- California Department of Forestry and Fire 

 Protection 

CDFG------ California Department of Fish and Game 

CVs-------- Coefficients of Variation 

d.b.h.------- Diameter at breast height 

EIS -------- Environmental Impact Statement 

ESA ------- Endangered Species Act 

FS---------- Forest Service (USDA) 

FWS ------ Fish and Wildlife Service (USDI) 

GIS ------- Geographic Information System 

HCAs ----- Habitat Conservation Areas 

IPs --------- Industrial Private (Timber Companies) 

ISC -------- Interagency Scientific Committee 

LMPs------ Land Management Plans 

LTBMU -- Lake Tahoe Basin Management Unit 

MBF------- Thousand Board Feet 

MCP------- Minimum Convex Polygon estimates of 

home-range size 

MIS ------- Management Indicator Species 

MMBF ---- Million Board Feet 

MPs ------- Miscellaneous Private Parties 

NFs ------- National Forests 

NFMA ---- National Forest Management Act 

NPs ------- National Parks 

NPS ------- National Park Service (USDI) 

PNW ------ Pacific Northwest Research Station (FS) 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

PSW --------- Pacific Southwest Research Station (FS) 

RD&A ------ Research, Development, and Application 

 Program 

R5 ----------- Region 5, Pacific Southwest Region (FS) 

R6 ----------- Region 6, Pacific Northwest Region (FS) 

RSAs -------- Random Sample Areas 

SEIS ---------Supplemental Environmental Impact 

 Statement 

SFs ---------- State Forests 

SHPs -------- State Historical Parks 

SOHAs------ Spotted Owl Habitat Areas 

SPs ---------- State Parks 

UC----------- University of California 

USDA------- United States Department of Agriculture 

USDI -------- United States Department of Interior 

USGS ------- United States Geologic Survey (USDI) 

Metric Equivalents: 

1 inch = ------------- 2.54 centimeters 

1 foot = ------------- 0.305 meters 

1 square foot = ----- 0.09 square meters 

1 acre = -------------- 0.405 hectares 

1 mile = -------------- 1.609 kilometers 

1 square mile = ----- 2.59 square kilometers 

1 ounce = ------------ 28.35 grams 

1 pound =------------ 454 grams 

iii 
Preface 

Maintaining viability of northern spotted owl populations in 

Washington, Oregon, and California, in the face of the many 

activities that take place in their habitats, especially logging, has 

challenged public land managers for nearly two decades. Policies and guidelines established by the USDA Forest Service 

during the late 1970s and early 1980s were challenged repeatedly, and from opposing camps. The timber industry is concerned that management of the owl's preferred habitat unnecessarily restricts commodity production, while environmental interests contend that Forest Service plans for the owl fall short of 

assuring long-term viability of the bird. Although the controversy has been more hotly debated over the northern spotted 

owl, the California spotted owl has recently taken part of the spotlight. 

The Report and Its Origin 

During most of the 1980s, the Forest Service implemented a 

conservation strategy based on retention of Spotted Owl Habitat 

Areas (SOHAs) capable of supporting one to three pairs of owls, 

separated from each other by distances ranging from 6 to 12 

miles, edge-to-edge. A recent, comprehensive assessment of 

management for the northern spotted owl identified serious 

weaknesses in the SOHA strategy, however, and gave rise to a 

markedly different approach for conserving the owl.1

 Because 

the SOHA strategy is currently used to manage for the California 

subspecies, in June 1991 the Forest Service and various agencies 

of the State of California initiated a technical assessment of the 

current status of the California spotted owl, under auspices of an 

Interagency Steering Committee for the California Spotted Owl 

Assessment (the "Steering Committee"). 

The Steering Committee formed two investigative, advisory teams-the Technical Assessment Team and the 

Policy-Implementation Team. This report contains findings of 

the Technical Assessment Team, as transmitted to the Steering 

Committee on 8 May 1992. The Team's findings will be further 

evaluated by the Policy-Implementation Team, which will suggest policies and guidelines for implementing recommendations 

of the Technical Assessment Team. Final recommendations will 

then be made by the Steering Committee. 

In developing this technical assessment, we have drawn 

information from many sources-the owl's biology, including 

its geographic distribution, habitat associations, life history, 

demography, and ecological relations; historical aspects of land 

management and disturbance in the Sierra Nevada and southern 

California; fuels management; principles of conservation biology; and risk analysis. This was consistent with our assignment, 

1

Thomas, Jack Ward; Forsman, Eric E.; Lint, Joseph B.; Meslow, E. 

Charles; Noon, Barry R.; Verner, Jared. 1990. A conservation strategy for the 

northern spotted owl. Washington, DC: U.S. Government Printing Office; 

427 p. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

as set forth in the charter that established the Technical Assessment Team. The charter also directed the Team to "follow 

accepted scientific standards and practices." Accordingly, to the 

extent possible, we have used the hypothetico-deductive process 

to guide our assessment of available information. We sought to 

falsify specific null hypotheses by using empirical data, computer models, widely accepted concepts and theories, and professional judgment (common sense!), rejecting hypotheses that 

failed these tests. This does not mean that surviving hypotheses 

are necessarily correct, only that they were not falsified by 

present information. This is the way of science. It inevitably 

leaves us with areas of uncertainty. How we deal with that 

uncertainty in the end depends, of course, on society's objectives. 

The next step in following accepted scientific standards and 

practices was to submit a nearly final draft of our report for peer 

review by five other scientists not selected by the Team. This 

was accomplished by requesting that the Presidents of five 

learned societies-The American Ornithologists' Union, The 

Ecological Society of America, The Society of American Foresters, The Society for Conservation Biology, and The Wildlife 

Society-personally select one reviewer each, at their discretion. We then sent the draft document to each of these reviewers, 

whose critical comments and suggestions were taken into account in the revisions leading to this final report. 

Although the report focuses on the California spotted owl, 

much more is at issue here. The owl is designated as an "indicator species" for old-growth ecosystems even though, as detailed 

in some of the chapters beyond, it is not restricted to just those 

habitats. But owl pairs use inordinately large areas in the course 

of their daily, seasonal, and annual activities, making the species 

an effective "umbrella" for a host of other species that satisfy 

their life requisites in the same habitats. Certainly commodity 

production must figure prominently when we weigh the pros and 

cons of maintaining viable populations of spotted owls. But 

prudence demands that the assessment be exhaustive and extend 

beyond the bottom line for the next year, or even the next 

decade. For the forest ecosystems we consider, our appraisal 

must look many decades, even centuries, into the future to be 

certain that decisions we make today are sustainable into the 

future-that they do not compromise future benefits from these 

ecosystems. A truly balanced assessment, therefore, "...must 

consider water quality, fisheries, recreation, soils, stream flows, 

scenic values, biological diversity, and other species of wildlife. 

All aspects of such a decision should be weighed in the balance. 

The issues are not limited to questions of owls and timber 

supply, as important as those are. The matter is not that simple-- 

it never has been."' 

 v 
Some Guidelines for Using the Report 

The report consists of 13 chapters covering various aspects 

of the information needed for the assessment. Chapter 1 is an 

extended summary of Chapters 3-13, providing the bare essentials about key findings to establish the basis for our final 

conclusions, and presenting our recommendations. It is the essence of our assessment. Chapter 2 presents the consensus of the 

Technical Assessment Team's recommendations about what we 

think future efforts on the California spotted owl should be, and 

why. Readers can explore any or all of the remaining chapters, as 

needed or desired, for details on statements and conclusions 

summarized in Chapter 1. Chapter 3 presents the general background leading up to the present issue and summarizes what we 

know of the owl's distribution throughout California, and how 

the subspecies is being managed. The next seven chapters describe the owl's general biology and ecological relations, results 

of studies on its habitat associations, its demography and projections of population stability, and its major prey species. Finally, 

Chapters 11-13 consider how conifer forests in the owl's range 

came to be what they are today, review the fire history and 

current fuels problems in owl habitat, and identify projected 

trends in the forested habitats used by the owl. Four appendices 

conclude the report: Appendix A identifies the Technical Assessment Team and provides a brief chronology of Team activities leading to the report; Appendix B is a glossary of terms used 

in the text; Appendix C lists the common and scientific names of 

plant and animal species mentioned in the text; and Appendix D 

identifies authors of the various parts of the document. 

We adopted some policies in the preparation of this report 

that we believe will ease the reader's task. Figures and tables are 

labeled by number for the chapter in which they appear, and by 

letter for their order in that chapter (for example, figure 4C is the 

third figure in Chapter 4). This procedure allows us to cross 

reference figures and tables from one chapter to another without 

confusion. In addition, a set of color photos of owl habitat taken 

during the Team's field trips are aggregated at the end of Chapter 5; these are identified as "colored photo 5-1," colored photo 

5-2," and so on. We have substituted acronyms for several 

entities that are mentioned frequently in the text. Some are 

already familiar to most readers (for example, USDA = the 

United States Department of Agriculture), but in each chapter 

we define anew each acronym when it is first used. A list of 

many of the common acronyms appears after the Contents pages 

in this volume, for quick reference in case readers momentarily 

forget what one represents. All measurements are expressed in 

English units (for example, feet, acres, ounces) instead of metric 

units (meters, hectares, grams). This decision was made because 

English units are still used by Federal land-management agencies and by the forest industry, and because most of the general 

public still thinks in terms of English units, not metric units. 

vi 

Acknowledgments 

To complete this assessment, we have relied upon assistance, input, information, and general cooperation from literally 

hundreds of persons. To acknowledge each one is clearly not 

possible. We could not even rank them in order of the importance of their contributions. Certainly our biggest debt is to the 

many biologists from all the State and Federal agencies and 

private entities-both in management and research-who have 

spent countless hundreds of hours in the field, mostly at night, 

hooting for owls, searching for their nests, measuring nest trees 

and stands, locating owlets, recording data, and generally accumulating the wealth of information now available about the owl 

and its habitats. In particular, we thank the Forest and District 

Biologists from the National Forests within the range of the 

California spotted owl who have responded, again and again, to 

specific requests for information that we needed. They have met 

all of our requests unfailingly, pleasantly, and quickly. 

Another major group of folks that worked largely behind 

the scenes, and essentially anonymously as far as we on the 

Team were concerned, were persons on the National Forests 

who compiled and mapped habitat information for us in electronic form. We have all learned much from this exercise, which 

should result in significant improvements in the way habitat 

information is collected, classified, and stored in the future. 

Many others operated behind the scenes to implement data 

summaries, analyses, and syntheses, particularly at the Pacific 

Southwest Research Station's Redwood Sciences Laboratory in 

Arcata and Forestry Sciences Laboratory in Fresno. And a special thanks to folks in the California Department of Forestry's 

Forest and Range Resources Assessment Program, whose willing applications of their geographic information system made 

possible the mapping of all known California spotted owl sites 

and an assessment of high, medium, and low likelihoods of owls 

in riparian/hardwood forests in the Sierran foothills and Coast 

Ranges. 

The administrative side of the Team's operation was made 

possible largely through the Regional Office of the Forest Service in San Francisco, which covered nearly all of the costs, and 

the Pacific Southwest Research Station in Albany, California. 

We were blessed by the wisdom of a couple folks in the Regional Office who saw and met a need to provide us with 

"scissors" to cut through the red tape that occasionally threatened to bring our progress to a temporary halt. 

Technical Coordinators: 

Jared Verner, Kevin S. McKelvey, Barry R. Noon, 

R. J. Gutiérrez, Gordon I. Gould, Jr., Thomas W. Beck 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Chapter 1 

Assessment of the Current Status of the California 

Spotted Owl, with Recommendations for Management 

Jared Verner, Kevin S. McKelvey, Barry R. Noon, R. J. Gutierrez, Gordon I. Gould, Jr., and Thomas W. Beck 

Release of a proposed conservation strategy for the northern 

spotted owl in April of 1990 (Thomas et al. 1990) raised concern 

in Region 5 (R5) of the U.S. Department of Agriculture, Forest 

Service (FS) about the adequacy of their regional guides for 

managing the California spotted owl (Barker and Jay pers. comm.). 

This concern was amplified by a decision formally announced 

on 26 June 1990 by the U. S. Department of Interior, Fish and 

Wildlife Service (1990) to confer "threatened" status on the 

northern spotted owl throughout its range. Negotiations began 

shortly thereafter to undertake an assessment of the current 

status of the California subspecies. This process led to the formation of the California Spotted Owl Assessment Team Steering Committee, with members representing several State of 

California (Resources Agency, Board of Forestry, Department 

of Fish and Game, and Department of Forestry and Fire Protection) and Federal entities (U.S. Department of Agriculture, Forest Service; and U.S. Department of Interior-Bureau of Land 

Management, Fish and Wildlife Service, and National Park 

Service). Observers represented the California Farm Bureau, 

California State Association of Counties, California Forestry 

Association, National Audubon Society, Nature Conservancy, 

private timber companies, Sacramento Chamber of Commerce, 

and the Wilderness Society. The Steering Committee held its 

first meeting on 11 May 1991, in Sacramento, CA, and established two teams to implement the assessment--a Technical 

Assessment Team to be designated by the FS and a 

Policy-Implementation Team to be designated by State entities. 

The charter for the Technical Assessment Team specified 

submission to the Steering Committee of a report on the current 

status of the California spotted owl (the "CASPO Report"), 

following "accepted scientific standards and practices." The 

report would: 

1. Present, analyze, and interpret relevant information currently available on the biology of the owl-its distribution, 

abundance, density, movements, breeding biology, diet, 

demography, habitat associations, and so on. 

2. To the extent possible, characterize the attributes of various 

habitats used for foraging, roosting, and nesting by the owl 

throughout its range in California. 

3. Evaluate current land-management practices throughout the 

range of the owl, recognizing that more detailed informa 

tion may be available for some ownerships than for others. 

4. Evaluate a range of options to achieve an amount and 

configuration of suitable habitat to provide for the long-term 

maintenance of the owl throughout its range. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

5. Identify research, monitoring, and inventory programs needed 

to answer existing critical questions and to provide for 

adaptive management of the owl in the future. 

The Technical Assessment Team consisted of a Core Group 

of six members (see Appendix A), consultants from the State 

and Federal entities represented on the Steering Committee, 

observers from the timber industry and the environmental community, and staff. This volume is the CASPO Report to the 

Steering Committee; this chapter synthesizes the Team's findings and presents its recommendations. 

Producing the Technical 

Assessment 

We established an agenda, schedule, objectives, and operating procedures (see chronology of Team activities in Appendix 

A). We spent 19 days on field trips throughout the range of the 

owl in the Sierra Nevada and southern California, including 5 

days on private industrial timberlands. Arrangements were made 

for a professional photographer to accompany the Team on all 

field trips and to search archives for historical photos of locations that might be rephotographed now. Workshops were held 

to exchange information and explore concepts with agency biologists from throughout the State of California, with leading 

authorities in silviculture, and with some of the Nation's leading 

conservation biologists. Numerous other informational meetings were held with smaller groups and with more focused 

objectives. An extensive reference library, including most published literature and unpublished reports (from many very recent 

field studies of California spotted owls), was assembled and 

made available to the Team in its offices in Sacramento. We 

acquired all other relevant information currently available on the 

owl, its habitats, and its biology; reviewed the current-management situation; and identified the major factors leading to concern for the well-being of the California spotted owl throughout 

its range. The Team and staff analyzed and synthesized all 

information obtained from the variety of sources mentioned 

above. Various Team members and other specialists prepared 

the supporting chapters contained in this report. 

Chapter 1 3 
Background and the Current 

Management Situation 

As done for the northern spotted owl (Thomas et al. 1990, p. 

12), we have subdivided the range of the California spotted owl 

into two major physiographic provinces, based on a variety of 

factors. These are the Sierra Nevada Province and the Southern 

California Province, with Tehachapi Pass as the dividing line 

between provinces. These regions are clearly distinct geographically; owl populations in the two provinces probably seldom 

exchange individuals; most owls in the Sierra Nevada Province 

prey mainly on northern flying squirrels, but all owls in the 

Southern California Province prey almost exclusively on 

dusky-footed woodrats (table 4A); the predominant threats to 

owl populations differ markedly between provinces; and feasible options for dealing with those threats also differ markedly 

between provinces. 

The administrative history of the California spotted owl is 

closely tied to that of the northern spotted owl. Detailed research 

began in 1969, with studies on the northern subspecies, and early 

surveys for both subspecies of spotted owls in California were 

done in 1973 and 1974. Those surveys located owls at 159 sites 

(Chapter 3), primarily by visiting selected late-successional forests and areas with known historical sightings. Region 5 (R5) of 

the FS designated the spotted owl as a "Sensitive Species" on 

National Forests (NFs) throughout California in the late 1970s. 

In spite of this and increasing concern over the status of the 

northern spotted owl, FS surveys in the range of the California 

spotted owl did not begin in earnest until 1981. 

Sierra Nevada Province 

Primary concern for the status of the California spotted owl 

has been in regard to timber management on the seven westside 

Sierran NFs and on private industrial timberlands. The Sierra 

Nevada owl population is contiguous and fairly evenly distributed throughout its 400-mile length (figs. 4A and 4B) but is 

probably poorly linked to the southern California population. 

Connectivity may exist through the Tehachapi Mountains and 

the Liebre/Sawmill area east of Interstate Highway 5 (fig. 9A). 

The California spotted owl connects to the northern spotted owl 

in Shasta County (fig. 4A). 

Standard survey and monitoring protocols have been established by the FS to locate owl sites; these protocols have been 

used by most other entities involved in owl inventories. An owl 

site is an area of unspecified dimensions where a single owl or a 

pair of owls has been located, usually repeatedly. In demographic and radio-tracking study areas, where efforts to locate all 

owls are more intense than elsewhere, most owl sites with single 

owls have eventually been found to have a pair. All owl sites 

have been mapped and given unique spatial references, so they 

can be tallied. Designation of an owl site makes no assumption 

about home-range or territory boundaries of the owls, although 

usually a center of activity can be identified by the location of a 

nest or a primary roosting area. The terms "owl site" and "site" 

are general and often used generically to refer to home ranges, 

territories, or to sites designated by agencies for special owl 

management (see glossary in Appendix B). 

Surveys, inventories, and other field efforts produced an 

estimated 1,250 spotted owl sites in the Sierra Nevada during the 

period 1987-1991, 92 percent on public lands (table 1A). Pairs 

were confirmed at 697 of those sites (table 3B). Only 162 of the 

sites were in reserved lands [National Parks (NPs) and Wilderness Areas in NFs]. Biologists have estimated an additional 305 

1

 Acreages are based on forested land currently known to be suitable habitat (dominant tree size -14 inches in diameter at breast height, with  SHUFHQW FDQRS\

cover) or land that is currently not suitable but has high timber-producing capability, providing for a relatively rapid return to suitable habitat. 2

 Includes some local governmental ownerships. 

3

 Figure is only for known forested lands in private ownership in the Sierra Nevada; an unknown amount of that is unsuitable. 

4

 The quantity of suitable habitat on private ownerships in southern California and on Native American Nations' lands has not been determined. 

Table 1A-Known California spotted owl sites (1987-1991 surveys) and estimated acreages of suitable habitat, by ownership and physiographic province (see tables 

3B and 3J). 

4 Chapter 1 USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992.

 California spotted owl sites 

 Estimated acres 

Sierra Nevada Southern California of potential 

 Province Province Statewide suitable habitat

1

 Additional Additional Additional Sierra Southern 

Ownership Known estimated Known estimated Known estimated Nevada California 

Federal ownerships 

 Forest Service 1,028 250 294 190 1,322 440 3,500,000 540,000 

 National Park Service 120 55 0 0 120 55 461,000 0 

 Bureau of Land Management 1 ? 1 ? 2 ? 68,500 7,600 

 Subtotal of Federal 1,149 305+ 295 190+ 1,444 495+ 4,029,500 547,600 

State of California 3 ? 7 2 142 10 14+ 26,000 25,0002

Private ownerships 98 ? 37 ? 135 ? 2,408,000 3 ? 4

Native American Nations 0 ? 4 ? 4 ? ? 4 ? 4 

Grand total 1,250 305+ 343 204+ 1,593 509+ 6,463,500 572,600 
sites, based on unsurveyed areas in habitats considered to be 

suitable (tables 1A and 3B). We have assigned owl sites in the

Sierra Nevada Province to one of five general habitat types, 

based primarily on tree-species composition: 

1. Foothill riparian/hardwood forest-This type generally occurs at low elevations in the Sierran foothills. It includes 

denser stands of hardwoods immediately adjacent to streams, 

as well as denser stands of hardwood forests on the adjoin 

ing slopes. Tree species along streams include cottonwood, 

California sycamore, interior live oak, California buckeye, 

Oregon ash, and occasionally white alder. Tree species on 

the adjoining slopes include blue oak, interior live oak, and 

digger pine. 

2. Ponderosa pine/hardwood forest (montane hardwood)-- 

This habitat blends with the upper elevations of riparian/ 

hardwood forests. In the southern Sierra Nevada, ponderosa 

pine at its lowest elevation generally occurs with interior 

live oak, canyon live oak, and black oak, with incense-cedar 

and white fir coming into stands at slightly higher elevations. In the northern Sierra Nevada, tanoak and Pacific 

madrone commonly contribute to the hardwood component 

of this type. 

3. Mixed-conifer forest-This type is the predominant 

timber-producing forest of the Sierra Nevada, consisting of 

various mixtures of white fir, ponderosa pine (at lower 

elevations), incense-cedar, sugar pine, black oak, and red fir 

(at higher elevations). Douglas-fir is an important component from Yosemite NP northward, and giant sequoia occurs in widely scattered localities. 

4. Red fir forest-This type blends with the higher zones of 

mixed-conifer forest. It is dominated by red fir, with increasing amounts of white fir at lower elevations until it 

becomes mixed-conifer forest. At upper elevations it often 

includes some lodgepole pine and occasionally quaking 

aspen. 

5. Eastside pine forest-This type occurs generally east of the 

Sierran crest and is dominated by ponderosa and/or Jeffrey 

pine. 

Most known spotted owl sites (82 percent) on Federal lands in 

the Sierra Nevada are in mixed-conifer forests. Indeed, about 62 

percent of all California spotted owl sites on Federal lands are in 

Sierran mixed-conifer forests, making this by far the most significant habitat for the subspecies (table 1B). 

Approximately 8.6 million acres of forested or potentially 

forested lands occur in the Sierra Nevada; 71 percent are on 

public lands. Of these lands, 6.5 million acres are either suitable 

or potentially suitable owl habitat, and about 4 million acres are 

owned by the public. Because we lack a full understanding of all 

attributes that comprise suitable owl habitat, however, we cannot determine the exact amount of suitable habitat for the owls 

on any ownership. 

Forest Service 

NFs of the western Sierra Nevada with major owl populations have a total land base of 6,978,900 acres; about 5,260,611 

acres are forested and about half of that is current or potential 

habitat for spotted owls (Chapter 3). An estimated 1,028 spotted

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

owl sites, probably most capable of supporting a pair of owls. 

have been located on NFs in the Sierra Nevada (table 1A). About

80 percent of those are in the zone of mixed-conifer forests. 

about 10 percent in red fir forests, and about 7 percent in 

ponderosa pine/hardwood habitats. The remaining 3 percent are 

in eastside pine forests and foothill riparian/hardwood habitats in 

the western Sierran foothills (table 1B). 

In July 1981, the Regional Office of R5 notified Forests 

with the California spotted owl to provide in their Land Management Plans (LMPs) a strategy for maintaining viability of the 

owls. This led to the designation of Spotted Owl Habitat Areas 

(SOHAs) in a "network" on each of the westside Sierra Nevada 

NFs with owl populations and major timber-management programs. The network concept was patterned after a similar approach adopted by the FS in Washington and Oregon to manage 

for northern spotted owls. SOHAs are designated stands of 

habitat to be managed to maintain suitable owl habitat. They 

may occur singly, in pairs, or in triplets. If single, they may be no 

more than 6 miles from at least two other SOHAs, edge-to-edge; 

if pairs or triplets, they may be up to 12 miles from other 

SOHAs. Management direction for SOHAs is to maintain at 

least 1,000 acres of suitable owl habitat within a 1.5-mile radius 

of the known or potential nest site. Suitable habitat is described 

as mature timber stands having (1) multi-storied canopies with 

70 percent or greater total cover, (2) 40 percent or more of the 

total canopy in trees at least 21 inches in diameter at breast 

height (d.b.h.), and (3) extensive decadence-cavities, broken 

tops, snags, and so on (Chapter 3). 

Directions for Sierran Forests resulted in the designation of 

264 SOHAs in approved or draft Forest LMPs (Lassen 40; 

Plumas 54, Tahoe 33, Eldorado 32, Stanislaus 36, Sierra 29, and 

Sequoia 40). Of this number, 249 are on lands suitable and 

Table 1B- Distribution by major habitat types of known California spotted owl 

sites, based on 1987-1991 surveys in National Forests and National Parks (see 

tables 3A and 3I)1 

 Known Percent of Percent of 

Forest type sites province total 

Sierra Nevada Province 

Mixed-conifer 959 81.5 62.4 

 Red fir 114 9.7 7.4 

 Ponderosa pine/hardwood 79 6.7 5.1 

 Foothill riparian/hardwood 19 1.6 1.2 

 Eastside pine 6 0.5 0.4 

 Subtotal 1,177 100.0 76.6 

Southern California Province 

Live oak/bigcone Douglas-fir 147 40.8 9.6 

 Riparian/hardwood 116 32.2 7.6 

 Mixed-conifer 95 26.4 6.2 

 Redwood/California-laurel 2 0.6 0.1 

 Subtotal 360 100.0 23.4 

Grand total 1,537 100.0 

1 Subtotals and totals do not match those in table IA because values given 

here were based only on numbers supplied by National Forests and National 

Parks. The figures occasionally include habitat descriptions from sites on private 

lands within NF boundaries and from single owl locations. We believe the 

percentages shown here correctly display the relative proportions of owl sites in 

these habitat types. 

Chapter 1 5 
otherwise available for timber management. The total allocation 

for the 264 SOHAs is about 454,000 acres, of which about 

110,000 acres would be managed under low-yield, even- or 

uneven-aged management (Chapter 3). 

Other Public Ownerships 

Lassen, Yosemite, Kings Canyon, and Sequoia NPs occur 

within the range of the California spotted owl in the Sierra 

Nevada. These have a total land base of 1,719,039 acres, but 

only about 28 percent of that is judged to be suitable owl habitat, 

and only 120 owl sites are known to occur in the four NPs 

combined (table 1A). Even though the NPs experience high 

recreation impacts in local areas, NP management has not been 

an issue, because the emphasis in the Parks generally is believed 

to be compatible with habitat needs of the owls. 

The Bureau of Land Management (BLM) administers scattered public lands along the foothills and lower slopes of the 

Sierra Nevada and southern Cascades, some with forests, woodlands, and riparian habitats that are potentially suitable for California spotted owls. About 68,500 acres of BLM lands are 

potential for spotted owls (table 1A). Although owls have been 

observed at a few sites on BLM lands, a general lack of inventory precludes an estimate of the total number. 

Seven State Parks (SPs) total an estimated 16,580 acres in 

the Sierra Nevada, with perhaps as many as six sites suitable for 

owls. Because all wildlife and plants are protected in SPs, habitat 

for any owls there is probably secure. Two State Forests (SFs) in 

the Sierra Nevada, totaling 13,830 acres, are managed for demonstration of forestry practices and to support cooperative research with other agencies. Uneven-aged silviculture is featured 

on these lands, which may have three or four pairs of owls. In 

addition, the University of California administers the 3,000-acre 

Blodgett Forest in El Dorado County and the 320-acre Whittaker 

Forest in Tulare County. About 2,000 acres at Blodgett are 

suitable owl habitat and typically one or two pairs nest in the 

area (Chapter 3). 

Private Lands 

Industrial timberlands total 1,451,000 acres and miscellaneous private timberlands total 957,000 acres in the Sierra Nevada (table 3D). The latter group includes both large landowners, such as utilities and water districts, and small landowners. 

About 58 percent of the combined total of these private lands are 

surrounded by NF lands, much in the form of alternating sections of private and public lands in "checkerboard" pattern, 

especially in the Tahoe and Eldorado NFs. Significant additional 

parcels of private timberlands, not in checkerboard arrangement, 

are included within boundaries of the Lassen, Plumas, and 

Stanislaus NFs. Most of the best forest-growing lands in the 

Sierra Nevada are owned by commercial timber companies in 

the mixed-conifer zone. The majority of the private land has not 

been inventoried for owls yet, but it is apparent that some 

industry lands with a long history of logging have spotted owls 

comparable in density to adjacent NF lands. Other private lands 

lack nesting owls, however, even though nest sites occur in 

adjoining NF lands (Chapter 5). Decisions are still pending on 

6 Chapter 1 

possible enactment of forestry reform legislation to provide new 

rules for logging on private lands in California. Although we 

cannot now assess the effects of any new forestry regulations on 

the capability of private lands to support spotted owls, we believe that any new regulations would be more beneficial for owls 

than past policies. 

At the north end of the Sierra Nevada, private timberlands 

along the east side of Shasta County provide an essential habitat 

linkage for movement back and forth by both northern and 

California spotted owls, between the Lassen and Shasta-Trinity 

NFs. Maintenance of this connection is believed to be essential 

to the long-term conservation of both subspecies of spotted owls 

(Dawson et al. 1987, Thomas et al. 1990). 

Spotted owls breed in dense stands of hardwoods along 

stream channels at low elevations on the western edge of the 

Sierra and Sequoia NFs. These habitats, generally not classified 

as commercial timberlands, are similar in most respects to habitats commonly used by spotted owls in southern California. 

Similar habitats occur along riparian zones west of the boundaries of all NFs in the Sierra Nevada. Livestock grazing, type 

conversions, firewood cutting, and logging in and adjacent to 

riparian zones have affected these habitats. Many are now being 

affected by an increasing trend of residential developments in 

the foothills. These potential habitats have not been adequately 

surveyed for spotted owls, although they may support many 

pairs (see fig. 4D). 

Areas of Concern 

Our over-riding concerns for spotted owls in the Sierra 

Nevada conifer zone involve potential impacts of logging practices on their habitat (details in Chapter 13) and the extent to 

which selective logging and aggressive fire suppression in this 

century have created incendiary conditions in a majority of the 

low- to mid-elevation conifer forests (details in Chapters 11 

and 12). In addition, we have identified several conditions that 

will bear further study and evaluation (table 3G, fig. 3A). 

These involve bottlenecks in the distribution of habitat or owl 

populations, gaps in the known distribution of owls, locally 

isolated populations, fragmented habitats, and areas with low 

densities of owls. 

Concerns for spotted owls in foothill riparian/hardwood 

forests in the western Sierra Nevada primarily involve increaseing development of residential properties. This is the case for 

dispersed, rural homesites and growth of existing communities 

in the foothills. Both potentially impact spotted owls directly by 

reducing the amount of owl habitat and by bringing dogs and 

cats into potential contact with fledgling owls that may spend 

some period of time on the ground before they are capable fliers. 

These developments would also affect the owls indirectly by 

reducing the area suitable for woodrats. Grazing in the foothills 

may also impact owls by influencing shrub cover needed by 

woodrats. We cannot evaluate possible effects on owls that may 

result from the increasing need for surface and ground water to 

provide for residential developments in the foothills. None of 

these potential impacts has been studied. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Southern California Province 

Spotted owls occur in all major mountainous areas of southern California, but they are not continuously distributed like 

those in the Sierra Nevada. Instead, we believe that each major 

mountain range has a relatively isolated subpopulation of birds 

that is separated from its nearest neighboring subpopulation by 

distances ranging from 6 to 45 miles (fig. 9A, table 9A). Inventories in these mountains since 1987 have produced a total count of 

343 known owl sites; 295 of these are on Federal lands (table 

1A). Estimates by biologists of additional sites in southern California range from 155 to 254 (table 3J); taking the midpoint of 

these gives an estimate of 204 additional sites or an estimated 

total of 547 spotted owl sites in southern California. This is not 

out of line with an independent estimate of 578 (known + 

potential) owl sites in southern California by Stephenson (1991). 

We have assigned owl sites in southern California to one of 

four general habitat types, based primarily on tree-species composition: 

1. Riparian/hardwood forest-This type varies considerably 

in different parts of southern California. In deep canyons in 

the Los Padres NF, for example, it occurs in narrow strips 

adjacent to permanent or near-permanent streams. Common 

tree species include coast live oak (near coast), canyon live 

oak (interior locations), California sycamore, white alder, 

California-laurel, and cottonwood. In shallower canyons in 

the Cleveland NF, these forests may consist almost exclusively of coast live oak. 

2. Live oak/bigcone Douglas-fir forest-This habitat occurs in 

a narrow band mostly at mid-elevations in mountains of all 

four NFs in southern California. Dominant tree species are 

canyon live oak, coast live oak, and bigcone Douglas-fir. 

3. Mixed-conifer forest-This type is best developed at relatively high elevations in the San Gabriel and San Bernardino Mountains, and on Mount San Jacinto. Species composition is similar to that of Sierran mixed-conifer, although 

Coulter pine occurs and bigcone Douglas-fir occasionally 

occurs at lower elevations. Red fir, Douglas-fir, and giant 

sequoia are missing. 

4. Redwood/California-laurel forest-These forests are restricted to the coast range, where coast redwood, 

California-laurel, tanoak, Pacific madrone, red and white 

alder, coast live oak, Santa Lucia fir, and bigleaf maple 

form various mixtures. 

About 41 percent of the owl sites in southern California are in 

live oak/bigcone Douglas-fir forests, 32 percent are in riparian/ 

hardwood forests, and 26 percent are in mixed-conifer forests, 

mainly in the San Bernardino Mountains (table 1B). Southern 

California has an estimated potential of about 573,000 acres of 

suitable owl habitat (table 1A), but we still cannot characterize

the full range of conditions that comprise suitable habitats there. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

Federal and State Lands 

Regional direction for the four NFs in southern California is 

to protect all known spotted owl sites and to manage the habitat 

based on local information about suitability and availability. 

One owl site is known on BLM lands, where only about 7,600 

acres of potential owl habitat occur; habitats are managed for 

wildlife, riparian habitat quality, water quality, and dispersed 

recreation. Camp Pendleton probably had spotted owls in the 

past, but long-term effects of military training activities have 

degraded habitat to an extent that little exists today. State Parks 

have several thousand acres of potential owl habitat, perhaps 

enough for 13-15 pairs. 

Other Lands 

Over the past 5 years, 41 owl sites have been found on 

private and Native American Nation lands in southern California 

(table 1A). Most private lands are at lower elevations than 

"traditional" spotted owl habitat. 

Areas of Concern 

Several significant factors threaten the long-term maintenance of spotted owl populations in these relatively isolated 

mountain ranges in southern California. Probably of most concern is the likelihood of a decline in the capability of landscapes 

between the mountains to support owls that would otherwise 

disperse from one subpopulation to another. Only in this way 

can the decline in one subpopulation be offset naturally by 

immigration from other subpopulations (so-called demographic 

"rescue effects"). As urban and residential areas expand in the 

valleys between mountains, the suitability of dispersal habitat 

may decline to the point that successful dispersal is too restricted 

for demographic rescue. Subpopulations must then survive demographically on their own or decline to extinction (Chapters 8 

and 9). We are similarly concerned about what appears to be a 

tenuous linkage between owls in the southern Sierra Nevada and 

owls in the Transverse Ranges (see fig. 9A). 

In addition to maintaining connectivity, the integrity of each 

habitat "island" must be maintained. An additional concern, 

therefore, is a decline in the amount, or an increase in fragmentation, of currently suitable habitat within any of the many isolated 

mountain ranges. 

Direct surface-water diversions and "mining" of ground 

water for human needs deplete water in permanent or 

near-permanent streams, threatening the associated riparian woodlands. Loss of the woodlands would mean the loss of spotted 

owls and numerous other riparian species found in these habitats. 

Stand-destroying fires, and increasing concentration of recreational activities in prime owl habitat are additional threats to 

spotted owls in southern California. Maintenance of a viable 

population of spotted owls in southern California may be impossible without changes in land-use policies on private lands, 

especially those that adjoin public lands. 

Chapter 1 7 
Assessing the Owl's Status: 

The Sierra Nevada 

Determining the owl's status depends on answers to three 

fundamental questions: (1) Is the California spotted owl's population declining in all or part of its range? (2) Is the California 

spotted owl a habitat specialist? (3) If the answer to question 2 is 

yes, then is the habitat upon which the California spotted owl 

specializes declining? We have endeavored to answer these 

questions by attempting to falsify the implied null hypotheses: 

Ho: California spotted owl populations in demographic study 

areas are not declining. 

Ho: California spotted owls use all habitats in proportion to 

their availability. 

Ho: Habitats used in excess of availability by California 

spotted owls are not declining in abundance. 

All sources of information available to us have been important in 

this effort, including common sense, professional judgment, 

empirical data, widely accepted concepts and theories, and mathematical modeling. 

Failure to falsify a null hypothesis does not necessarily 

mean that it is true. When data are insufficient to provide a 

powerful test of the hypothesis, we are likely not to falsify the 

hypothesis even when it is false. This is known to statisticians as 

a type-II error. In all cases where it was possible, we have 

estimated the power of tests that failed to reject a null hypothesis. This information is critical to the conclusions and recommendations we have made in this report. 

Is the Owl's Population Declining in 

All or Part of Its Range? 

Demographic studies of California spotted owls are currently underway in five locations-Lassen NF (2 years), Eldorado 

NF (6 years), Sierra NF (2 years), Sequoia/Kings Canyon NPs (4 

years), and San Bernardino NF (5 years). Owls are captured and 

color-banded with unique band combinations that can be identified in the field without recapturing the birds. In this way, a 

history of each color-banded bird can be accumulated for as long 

as it remains in the study area. Sex is determined by voice, and 

age (up to 2 years) can be determined by plumage characteristics. Critical parameters needed to determine whether a population is stable, increasing, or decreasing are stage-specific birth 

rates and death rates. The parameter we need to estimate is 

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DQG    LQGLFDWHV D GHFOLQLQJ SRSXODWLRQ /DPEGD LV FRPputed from estimates of three classes of parameters: age at first 

reproduction, age-specific survival rates, and age-specific fecundity (for simplicity in modeling population trends, we use a 

females-only model, so fecundity is defined here as the expected 

8 Chapter 1 

number of female fledglings produced per female of age x per 

year). In the Lefkovitch stage-projection matrix model 

(Lefkovitch 1965) used for this assessment, the value of lambda 

indicates the annual rate of change in the size of a population. 

Results 

Owl banding has been underway long enough to compute 

estimates of lambda for only three study areas-Eldorado NF, 

Sequoia/Kings Canyon NPs, and San Bernardino NF. We could 

estimate juvenile survival rate only for the San Bernardino study 

area, because data were insufficient for the Eldorado and Sequoia areas. Consequently, the San Bernardino value was used 

as a reasonable approximation for the other two locations. It was 

in line with estimates of juvenile survival rates from studies of 

northern spotted owls (Chapter 8), and lambda was not especially sensitive to variations in juvenile survival rate (figs. 8C 

and 8D). Results from the Eldorado and Sequoia/Kings Canyon 

studies follow; results from the San Bernardino are deferred to 

the section dealing with southern California. 

Eldorado Study Area-The estimate of lambda for the 

Eldorado population was 0.947, suggesting about a 5 percent 

annual rate of population decline during the period of study 

(1986-91). This value was not significantly <1.0 (a = 0.05, P = 

0.1271), however, so we cannot reject the null hypothesis that 

the population is not declining. The test, however, had a power 

of only 0.30. Even if the population were truly declining at 5 

percent per year, we would fail to detect that decline 70 times in 

every 100 studies of equivalent size. The low power resulted 

from a relatively small number of marked birds, and the large 

standard errors of parameter estimates (table 8E). The correct 

inference to draw from this result is that we are uncertain about 

the true trend of this population. The power of the test is much 

too low to infer that the population is stable. 

Sequoia/Kings Canyon Study Area-The estimate of lambda 

for this population was 0.969 (table 8F), suggesting about a 3 

percent annual rate of population decline during the period of the 

study (1988-91). As in the Eldorado study, the statistical test (a 

= 0.05, P = 0.2709) failed to reject the null hypothesis of no 

decline in the populaton. The power of this test-0.30-was 

identical to that for the Eldorado study, so we must infer again 

that we are uncertain about the trend of this population. 

Is the California Spotted Owl a 

Habitat Specialist? 

Results from Landscape Studies 

This question was explored in several ways and at three 

scales, with details given in Chapters 5 and 6. We know, for 

example, that California spotted owls use forested habitats almost exclusively, although they occur and breed in a greater 

variety of habitats than does the northern spotted owl. Within 

forested landscapes, we found that 45 percent of all nests of 

California spotted owls in the conifer zone of the Sierra Nevada 

were in M4N and M4G stands, significantly more than expected 

based on availability (table 5A) [table 1C explains codes designating timber strata, or see "timber strata" in the glossary (ApUSDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
pendix B)]. All other habitat types that we evaluated were used 

less than or equal to their availability (table 5A). These results 

indicated that, for nesting, the owls selected stands with relatively large trees and closed canopies. 

Densities of owl sites in 1:24,000 U.S. Geological Survey 

quadrangles were significantly related to the percentage of forests having medium-sized and larger trees and high canopy 

closure. These results generally corroborated those of the previous analysis. 

Results from Home-Range Studies 

At a home-range scale, attributes in nest stands were compared with those in randomly selected stands in the general 

forest matrix. Significant differences were found for several 

attributes, most or all of which were consistent with the previous 

conclusion that the owls tended to select stands of large, old trees 

with closed canopies for nesting. Results of identical analyses in 

roost stands produced parallel results. Nest and roost stands 

showed consistent, often significant differences from random 

locations in the forest in having higher canopy cover, greater 

snag basal area, greater total basal area of live trees, and greater 

softwood basal area (tables 5B and 5D). Mean values for canopy 

cover ranged from about 75 to 96 percent in the different studies; 

similarly these studies suggested a range for total basal area of 

live trees from 185 to 350 square feet per acre, and basal area of 

large snags (>15 inches in d.b.h. and >20 feet tall) from 19 to 

31 square feet per acre in nest and roost stands ("Recommendations" in Chapter 5). Many of these parameters varied considerably, and not all measures of habitat used by spotted owls and at 

random locations differed significantly within a given study. 

The data were, however, consistent and mutually supportive 

among all studies. California spotted owls in these various 

studies chose to nest and roost in stands that were denser than 

average and that contained a large-tree component. Most nest 

sites were selected in dense mixed-conifer stands with average 

quadratic mean diameters of canopy trees >24 inches in d.b.h. 

We know of no studies that consistently contradicted these 

findings. 

Table 1C-- Explanation of codes used to designate timber strata in the Sierra 

Nevada. 

1

 Mean diameter at breast height of dominant trees. 

2 Code 5 has been used to designate larger size-classes, and code 6 has been 

used to designate multi-layered stands; most National Forests in the Sierra

Nevada no longer make these distinctions in their timber inventories. 

Attribute Code used Identification 

Timber type M Mixed-conifer 

 P Ponderosa pine 

 R Red fir 

Tree size-class1 2 <12 inches 

 3 12-23.9 inches 

4 >24 inches2

Canopy closure P Poor 0-39 percent 

 N Normal 40-69 percent 

G Good >70 percent 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

Results of similar analyses at foraging locations indicated 

that the owls foraged in stands characteristic of nest and roost 

sites, as well as in a wide variety of other habitats having lower 

canopy cover and a greater range of tree sizes and ages. Nonetheless, in comparison with random locations within the forest, 

owls tended to forage in sites with higher canopy closure; greater 

basal areas of live trees combined and of softwoods; greater 

basal areas of snags; and more dead-and-downed wood (tables 

5G and 5H). In general, they foraged in forests of intermediate to 

old age, typically with >40 percent canopy closure. 

Results from Studies at Nest Locations 

Data from 276 nests located throughout the range of the 

California spotted owl provided the most conclusive evidence of 

selection by the owls of very large, old trees. In Sierran conifer 

forests, for example, nest trees averaged about 96 feet in height 

and 45 inches in d.b.h., with a surrounding canopy cover of 

about 75 percent (table 5K). A prevalence in these forests of 

cavity nests (66 percent) and nests on broken-topped trees (10 

percent) showed that most nest trees were not only large but also 

old and decadent. For example, many of the natural cavities used 

for nests were created when decay invaded a wound on the side 

of the tree where a branch tore out of the trunk. These cavities 

must have room to accommodate an owl's nest, the female, and 

her (usually) two nestlings, so only very large trees have branches 

and trunks of sufficient size to produce such cavities. Not only 

were the diameters of nest trees significantly larger than the 

average tree in today's conifer forest (fig. 5K), but also they 

exceeded the mean diameter of trees in plots sampled in the 

Sierra Nevada at the turn of this century. The owls are apparently 

nesting today in a legacy of very large, old trees that were 

present in 1900 and before. 

Results from Radio-Tracking Studies 

Radio-tracking studies of California spotted owls in the 

Lassen NF and the Sierra NF provide some insights into habitat 

selection in conifer forests of the Sierra Nevada. Studies of this 

nature have shortcomings that can lower our ability to draw 

inferences from them, however. First is the need for a large 

number of owl locations during a brief period of the year (for 

example, the breeding period or the winter period). But to meet 

assumptions of independence in the locations, required by statistical tests, locations should be recorded only about every 2-3 

days. In the 6-month period that approximates the breeding 

cycle of the spotted owl, only about 72 locations could be 

obtained without violating assumptions of independence. Second is the fact that a small sample size results in low power of the 

tests to detect habitat selection. A mean of 57 locations was 

available for the radio-tagged spotted owls reported in the 

home-range studies (Chapter 6) upon which the following summary is based. The power of statistical tests ranged from about 

15 to 80 percent, so failure to detect significant overuse or 

underuse of habitats, based on availability, probably resulted 

from low power in many cases. This means that any consistent 

pattern in habitat selection among birds with samples large 

enough for tests with ample power should be given additional 

Chapter 1 9 
weight when evaluating the suitability of habitats for spotted 

owls from studies of radio-tagged birds. 

Habitat selection was more consistent and more pronounced 

for canopy closure than for tree size-class among radio-tagged 

California spotted owls in the Sierra Nevada. Chi-square values 

were consistently higher for canopy closure, and more owls had 

significant tests of habitat use for canopy closure than for tree 

size-class in 18 site-by-season comparisons. Differences between total and dominant canopy closure were minor (Chapter 

6). Because more owls showed selection for cover of the dominant 

trees in a stand, however, it appears to be a better measure of 

habitat quality for California spotted owls than total canopy cover. 

The amount of medium and large sawtimber in individual 

home ranges did not appear to be a good indicator of the amount 

of habitat needed to sustain the owls, unlike the case for the 

northern spotted owl (Chapter 6). Most owls did not have significant tests of habitat use for tree size-class. Nearly all owls in 

the Sierra NF used size-classes in proportion to their availability; 

patterns were stronger in the Lassen NF during the breeding 

season, where about half of the birds used medium and large 

sawtimber greater than expected. 

Based on overall use by radio-tagged owls of habitats with 

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cover, stands with  SHUFHQW FDQRS\ FRYHU VKRXOG JHQHUDOO\

be considered suitable owl habitat. Stands with  SHUFHQW

canopy cover should generally be considered unsuitable (Chapter 6). The data show that owls exhibited lower selectivity for 

habitats when foraging than they did when roosting (Chapters 5 

and 6). 

Results from Studies in Foothill 

Riparian/Hardwood Forest 

Results of habitat studies and home-range use by owls in 

lower-elevation, riparian/hardwood forests and adjacent stands 

of oak-pine woodlands in foothills of the Sierra Nevada generally agree with those in conifer forests. The birds nest and roost 

in stands with mean canopy cover of about 89 percent and in 

trees generally large for those habitats (mean d.b.h. = 29.5 

inches, table 5K). 

Is the Habitat Used Selectively by the 

California Spotted Owl Declining? 

Having concluded that California spotted owls are not habitat generalists, particularly for nest stands, we next must determine whether any evidence indicates a decline in the amount of 

habitat used more than expected by the owl. Forests of the Sierra 

Nevada have been markedly impacted in a variety of ways by 

human intervention, especially during the past 150 years (Chapter 11). The first major perturbation was grazing by millions of 

sheep from about 1860 to the first decade or so of this century; 

peak numbers occurred in the early 1870s. Coincident with 

sheep grazing was extensive early logging, mainly at low elevations near towns, mines, and along transportation routes. Timber 

production-in billions of board feet-reached a peak about 

1950, dropping some from that level but remaining relatively 

10 Chapter 1 

high in most years since. Fire suppression began in the early part 

of this century and has become increasingly effective and aggressive since. 

With removal of sheep and some measure of fire control in 

place, forest stands became subject to ingrowth of shade-tolerant 

conifers such as white fir and incense-cedar (Chapter 11). A 

combination of logging and natural attrition of the old forest led 

to a decline in the number of large, old trees. Past logging 

activities that concentrated on removal of the largest, most valuable trees broke up the patchy mosaic of the natural forest, 

further encouraging the development of dense conifer regeneration. These developments, especially in the ponderosa pine and 

mixed-conifer forests of the Sierra Nevada, reduced large-diameter 

trees in many areas to small remnant populations. Concurrently, 

surface fuels have been accumulating in forest stands and the 

extensive ingrowth of shade tolerant trees has resulted in vertical 

fuel ladders that essentially connect the surface fuels to the 

dominant tree crowns over much of the Sierra Nevada. These 

changes have not occurred to the same degree in the red fir type, 

where fires were less frequent historically, and logging was 

generally uncommon until recent decades. 

Because of current stand structures and excessive fuel loadings in much of the Sierran mixed-conifer type, fires that escape 

initial suppression efforts-usually those occurring during extreme weather conditions-tend to become large and severe. 

Fire trends in the Sierra Nevada can be expected to continue 

along their current trajectories. As the human population increases in Sierran forests and woodlands, the presence of so 

many houses within the forest will shift further the emphasis of 

suppression from one of saving forests to one of saving property. 

The fuels will also continue to accumulate, with the recent 

drought-induced bark beetle infestations contributing a major 

pulse of new fuels over the next few decades. We expect the net 

result to be a much higher incidence of stand-destroying fires in 

the future than was characteristic of the Sierran fire regime prior 

to this century. And with those fires we will continue to lose 

remnant, individual old trees, stands of old trees, and other 

old-growth attributes. 

Timber cutting trends also point to a continuing decline in 

the number of old trees and remnant old-growth stands. Sixty-five 

percent of the forested acres on all Sierran NFs are classified as 

suitable for timber production (Chapter 13). If we discount 

forested acres that cannot produce timber commercially because 

they are too poor in quality, they cannot be successfully regenerated, or they have unstable soils, 74 percent of the lands that can 

potentially produce timber will be harvested in some manner 

(table 13A). Seventy-two percent of the timber volume removed 

from these lands will be taken through even-aged systems-- 

mostly clearcuts. Of the 528,474 acres of suitable timberlands on 

the Tahoe NF, for example, 68 percent will be managed for 

even-aged silviculture (24 percent long rotation, 44 percent short 

rotation) (Chapter 13). On the Plumas NF, 52,000 acres are 

scheduled for even-aged cutting per decade, with 8,000 acres in 

selection cutting methods. 

Clearcut, seed-tree, and shelterwood cutting techniques all 

have the same goal: produce even-aged stands. In this regard, 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
seed-tree and shelterwood systems can generally be thought of 

as two-stage (sometimes three-stage) clearcuts. In all of these 

cutting systems, the original stand will be totally removed before 

the new stand is scheduled to be cut. In terms of owl biology, the 

primary impact of traditional, even-aged harvesting practices 

lies in the creation of simple stand structures and, probably more 

importantly, the removal of all large trees from vast areas of the 

forest. Even if prescriptions are modified so that snags and live 

culls are left at the first cutting, no provision is made for a 

predictable recruitment of replacement trees for these relics 

when they fall. This, in turn, will lead to a loss of large-diameter 

downed woody material important for production of the fungi 

that are a primary food source for flying squirrels-the main 

prey of spotted owls in the Sierra Nevada (Chapter 4). Log slash 

can create much small-diameter woody debris, but it cannot 

replace the large logs. In an even-aged system, these old-growth 

features can be created only by an extreme extension of the 

rotation interval. Even if the rotation is extended to 150 years, 

for instance, no trees will match the average age of the forest at 

the beginning of this century (Chapter 11). Decadent features in 

stands are functions of age, not just d.b.h. (fig. 13G); and any 

animals that depend on decadent features (cavities, broken-tops, 

snags), or the large woody debris that they create, will simply 

drop out of these forests (see Chapters 4, 5, and 10 for examples 

specific to the spotted owl and its prey species). The rate of 

conversion to even-aged systems in the western Sierra Nevada is 

estimated by the LMPs to be 229,000 acres per decade. 

Even on lands planned for selection harvest (about 80,000 

acres/decade), we have no guarantee that harvest prescriptions 

will leave any of the large, old trees. Ideally, stands managed for 

individual selection are harvested in a manner that brings the 

diameter distribution in the stand into conformity with an idealized distribution, which is characterized by a declining exponential function (in forestry referred to as an inverse "J"). The 

number of large trees in the stand is dictated by the slope of this 

curve and the designated diameter of the largest tree. In 

selection-logging systems, timber is removed from all diameter 

classes as required to maintain this diameter distribution. Little 

evidence exists, however, that historical patterns of partial cutting have followed the classic single-tree theory. "Selective" 

harvest in the Sierra Nevada has, in the past, primarily targeted 

the large trees. This system, sometimes called "pick and pluck," 

will not produce the simple, even-aged structures that characterize clearcutting techniques, but its effect on the presence of 

large, old trees is similar. If the large trees are removed and no 

stocking control is done on the smaller stems, replacement trees 

in these diameter classes will be produced very slowly, if at all, 

and they will consist primarily of the more shade-tolerant species. Even with classical single-tree selection, a gradual loss of 

shade-intolerant species would be likely. 

The future forest of the Sierra Nevada, as projected by the 

LMPs, will very likely be split between areas of even-aged 

plantations and areas of dense and increasingly small-diameter 

stands. Given these projections, it seems most likely that the 

forest to be generated by adherence to current LMPs will be 

susceptible to fire disturbance, nearly devoid of large, old trees, 

and depauperate in terms both of plant and animal species that 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

depend on attributes of the older forests that were common last 

century. We conclude that the key elements of spotted owl nest 

and roost stands, under current LMPs, will decline sharply over 

most of the Sierra Nevada in the next few decades. If they 

disappear, a hiatus of well over 100 years will pass before more 

can be grown to take their place. In the process, the spotted owl 

would probably be markedly reduced in numbers over most of 

the Sierra Nevada, but probably with viable subpopulations 

surviving in Yosemite and Sequoia/Kings Canyon NPs. 

Conclusions 

Is the Sierran population declining? We cannot be certain. 

Failure to detect significant declines in the two Sierran study 

areas must be interpreted cautiously, because the power of both 

tests was very low. We know nothing about the normal, long-term 

fluctuations of spotted owl populations in the Sierra Nevada. If 

the California spotted owl has experienced gradual declines in 

habitat quality in these mountains, the effects may be subtle and 

difficult to detect. Because we lack adequate, historical inventories of spotted owls in the Sierra Nevada, we have no basis for 

comparison with our current knowledge. Their current distribution and abundance, however, do not suggest that they have 

declined either in their overall distribution in the Sierra Nevada 

or that they have declined markedly in abundance within any 

forest type. 

Selective logging of the largest trees from the most productive sites in the Sierra Nevada has resulted in significant changes 

in diameter distributions of trees, leaving relatively few very old, 

large trees that are clearly selected by the owls for nesting 

(Chapter 5). Consequently, we are far from comforted by results 

from the demographic studies. Before reaching a final conclusion on this matter, we need to continue these studies until the 

power of their tests on lambda is greatly increased. 

Do Sierran owls exhibit selective use of habitats? Yes. The 

overwhelming weight of evidence is that California spotted owls 

in the Sierra Nevada select habitats differentially from among all 

habitats available to them. Selectivity is strongest for nesting and 

roosting habitats, weaker for foraging habitats. Even for foraging, however, we conclude that a target for suitable owl habitat 

should include at least 40 percent canopy cover in stands with 

trees averaging at least 11 inches in d.b.h. Data from direct 

measures of foraging stands further suggest that suitable foraging stands have snags, dead-and-downed woody debris, and 

some large trees (Chapter 5). 

Are key habitat elements declining in the Sierra Nevada? 

Yes. Of greatest concern to us at this time is the rapid disappearance of the large, old, and generally decadent trees that are the 

focus of nesting by spotted owls. Given projections from approved and draft LMPs for NFs in the western Sierra Nevada, 

where the vast majority of Sierran owls occur, these important 

stand components will disappear at a rapid rate over the next few 

decades. They cannot be replaced quickly. 

Considering the present state of our knowledge about spotted owls in the Sierra Nevada, we can identify eight major 

factors of concern about owl habitats there (table 1D). These 

have resulted from a combination of selective logging removing 

Chapter 1 11 
Table 1D-Summary of major factors of concern in habitats of California spotted owls in the Sierra Nevada, reasons for those factors, and their impacts on the owls. 

Factor Reason(s) for the factor Impact on spotted owls 

Decline in abundance of very large, old trees Selective logging of the largest trees from stands Loss of the owl's preferred nest sites 

Long recovery period for spotted owl Selective logging of the largest trees from stands Less of total landscape in suitable owl

habitat after logging habitat at any given time 

Ingrowth of shade-tolerant tree species, Selection harvest; aggressive fire suppression; sheep grazing, Increased threat of stand-destroying fires 

creating unnaturally dense stands with which created ideal seedbeds for conifer germination late last 

ground-to-crown fuel ladders century 

Excessive build-up of surface fuels Aggressive fire suppression over the last 90 years, leading to Increased threat of stand-destroying fires 

 higher densities of trees, more competition for space and 

 water, so a higher death rate of trees 

Loss of large-diameter logs from the decaying Intentional fires by sheepherders; selective logging of Potential decline in flying squirrel densities 

wood source on the ground largest trees; piling and burning logs after logging; domestic via loss of fungi that are a dietary staple for 

 fuel-wood removal the squirrels 

Decline in snag density Selective logging of the largest trees from stands; salvage Loss of potential nest sites for owls; loss of den 

 logging; fuel-wood removal sites for flying squirrels; loss of a source of 

 large logs for decay needs on the ground 

Disturbance and/or removal of duff and Sheep grazing; mechanical disturbance from logging Potential decline in flying squirrel densities 

topsoil layers equipment, skid trails, and so on; increased surface fuels that via loss of fungi that are a dietary staple for 

burn hot enough to destroy duff layer the squirrels 

Change in composition of tree species Selective logging of the largest trees, particularly pine species, Some loss of nest sites; other effects unknown 

(fewer pines and black oaks, more firs and from stands; aggressive fire suppression 

incense-cedar) 

mainly the largest trees from stands, aggressive fire suppression 

beginning shortly after the turn of this century, and the combination of human-ignited fires and extensive sheep grazing in the 

Sierra Nevada during most of the last half of last century. 

Assessing the Owl's Status: 

Southern California 

Here we summarize available evidence for the spotted owl 

in southern California as it relates to the three fundamental 

questions, and null hypotheses, posited for Sierran owls. Less is 

known about the habitat relations of spotted owls in southern 

California, but we can say much about the likely stability of the 

owl population there in relation to its pattern of distribution-in 

relatively isolated blocks with potentially hostile habitat between them, through which the owls must disperse. 

12 Chapter 1 

Does Evidence Indicate a Decline in 

the Southern California Population? 

Yes. The estimate of lambda for the San Bernardino demographic study area was 0.827 (table 8G), suggesting about a 17 

percent annual rate of decline in the resident, territorial population during the study period (1987-91). The statistical test (alpha 

= 0.05, P < 0.0001) strongly rejected the null hypothesis of a 

nondeclining population. The correct inference for this population is that it has been in a steep decline for at least the past 5 

years. 

We do not know the reason(s) for this decline. Much logging occurred there in the 1960s, but we doubt whether that 

disturbance can explain the current decline. Chronic air pollution in southern California may be directly or indirectly linked to 

the declining population of owls, for example by way of the 

plant foods important to woodrats. A more plausible hypothesis 

involves either direct or indirect effects of the drought in southern California, where precipitation from 1984 through 1990 

averaged about 60 percent of normal at one weather station and 

about 67 percent of normal at another near Big Bear Lake, near 

the center of this demographic study area. Precipitation was 

below normal in all 7 years at one station and above normal in 

only 1 of the 7 years at the other (figs. 8A and 8B). In 1991, 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
when precipitation was above normal, it exceeded the long-term 

average by about 10 percent. Most of this came in a series of 

strong storms that coincided with the laying period for most owl 

pairs that attempted to breed that year. 

One working hypothesis is that numbers of dusky-footed 

woodrats, the primary prey of the owls in the San Bernardino 

Mountains (Chapter 4), have declined as a result of the drought. 

If the decline in owl numbers is related in some way to the 

drought, it suggests that the owl population there is subject to 

high levels of environmentally induced variation in its demographic parameters. As the population declines, individuals may 

be lost from marginal habitats, where survival and reproduction 

are possible only during "good" times (Chapter 8). Individuals 

that survive, and even reproduce, during the decline may be 

those occupying better, more stable, habitats, as where more 

mesic conditions prevail (for example, riparian areas). Such 

refuges would be critically important to the species' long-term 

persistence, and any destabilization of them-by logging, water 

diversion, depression in ground-water levels, excessive development of recreational activities, or further development of communities and dispersed housing-could pose a significant threat 

to the owl's survival. 

Do Spotted Owls in Southern 

California Exhibit Selective 

Use of Habitats? 

Yes. The same basic patterns found at the home-range scale 

for Sierran owls have been observed in studies of habitat use by 

spotted owls in southern California. The most detailed study was 

done in the San Bernardino Mountains (table 5C). Compared to 

randomly located sites, nesting and roosting stands had significantly higher canopy cover, total live basal area, hardwood basal 

area, softwood basal area, and snag basal area. Nest trees were 

very large for the area, averaging 37 inches in d.b.h. and 88 feet 

in height. The mean age of nest trees in the San Bernardino 

Mountains was 230 years based just on the core length that could 

be extracted from the trees (table 5M). 

Are Key Habitat Elements Declining 

in Southern California? 

We do not know. We were not able to bring quantitative 

information to bear on this question. Little commercial logging 

occurs in southern California, but "timber-sale improvements" 

and firewood cutting have negative impacts on owl habitat there. 

In addition, wildfires occasionally burn through suitable owl 

habitat, rendering it less suitable, or even useless. The extent to 

which these events result in a net loss of suitable owl habitat is 

unknown, however. We also know that urban and dispersed 

residential expansion is occurring in suitable owl habitat in 

southern California, especially at lower elevations between relatively isolated subpopulations of the owl. Those are the places 

where dispersing owls must move from one subpopulation to 

another, and such dispersal is the only method whereby a decline 

in one subpopulation can be compensated by "rescue" via immiUSDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

grants from another. We also know that many owl pairs in 

southern California, especially in the southern portion of the Los 

Padres NF, occupy narrow strips of riparian/hardwood forest. 

These forests will survive there only as long as the stream system 

from which they get their water survives. In some of 

these areas, water mining in the forms both of diverting surface 

water and drilling into underground aquifers threatens to dry up 

streams to the point that they will lose their riparian forests. 

Stability Properties of the Spotted 

Owl Metapopulation in Southern 

California 

Spotted owls in the Southern California Province have an 

insular population structure, ranging in size from about 1-4 pair 

sites to about 125 pair sites, distributed among discrete mountain 

ranges (fig. 9A, table 9A). This distribution of habitat islands is 

discontinuous across the landscape, reflecting natural 

discontinuities in vegetation structure and composition, in topographic conditions, and in the effects of extensive human-induced 

habitat disturbance and fragmentation. The largest population is 

in the San Bernardino Mountains, with considerably lower population sizes in the other areas. This "archipelago" is estimated to 

have 376 pair sites (table 9A), with an approximate population 

of 300-350 pairs at any point in time. Based on theory and 

limited empirical data, we believe the ultimate stability of this 

metapopulation will depend on several factors, including the 

persistence of one or more populations of sufficient size to avoid 

negative effects of demographic stochasticity, and with demographic characteristics that result in production of excess individuals to serve as potential colonists for other local populations 

(Chapter 9). 

The sensitivity of the southern California metapopulation to 

a variety of perturbations was tested by performing multiple 

simulations, using a spatially explicit model developed to examine effects of spatial aspects of the distribution of the northern 

spotted owl (Chapter 9). Interpreting model results in a visual 

and spatially explicit way allows insights into areas of the landscape that are especially vulnerable to local extinction events, as 

well as those areas that represent sources for immigrants to other 

local populations. We did not, however, project extinction likelyhoods from the model runs. 

The arrangement of owls and owl habitat across the landscape shows that most of the population is concentrated in the 

San Gabriel/San Bernardino Mountain complex. Smaller populations in the archipelago will continue to function as a part of 

the larger metapopulation only if they remain connected through 

dispersal. If these smaller populations become increasingly isolated, via reduction in size of their habitat islands or creation of 

barriers to dispersal, the likelihood of their extinction increases. 

Although these small, isolated populations will be the first to go, 

even the largest, most continuous ones will experience increased 

risks as smaller populations drop out of the metapopulation. 

The many factors discussed earlier-for example, wildfires, 

urban and dispersed residential expansion, water mining, and 

increased recreational use of riparian areas that are prime owl 

habitat-can all add their seemingly insignificant, individual bits 

Chapter 1 13 
of erosion into the existing population of spotted owls in southern 

California. To the extent that this leads eventually to fewer pairs 

overall, fewer pairs in individual "islands," greater distances between pairs, and reduction in the rate of successful dispersal between populations (to maintain smaller ones), the spotted owl 

population in southern California appears to be fragile. 

On the other hand, we know that inventories of spotted owls 

in southern California have not been completed. If more pairs 

were known to occur in some of the habitat islands, it could 

markedly increase estimates of the stability properties of those 

subpopulations. Particularly important in this regard are the 

possibilities of more pairs in and near the Cleveland NF and in 

and near the San Rafael Wilderness in the Los Padres NF. 

Increasing cluster size (number of pairs with essentially shared 

home-range boundaries) to 45 or 50 pairs in each of these areas 

would have a strong stabilizing effect on the metapopulation in 

southern California. 

Conclusions 

Several uncertainties are associated with the status of spotted owls in southern California. The only population studied 

demographically has been declining at a high rate for at least 5 

years, but this has all taken place during the recent drought. We 

cannot separate the possible effects of the drought from other 

possibilities. Indeed, no other explanations are immediately evident. It is possible that some subtle, even unsuspected phenomenon is the real cause of the decline. Although the owls in 

southern California use only a subset of all available habitats, we 

do not know if those selected types are undergoing a net decline. 

Our modeling suggests that the metapopulation structure of the 

owls, there is especially sensitive to diminishing sizes of smaller, 

local populations. And it is also especially sensitive to any 

reduction in the effectiveness of dispersal by owls among the 

various "island" populations. We have identified several factors 

that could be, and probably are, affecting the sizes of the "island" 

populations and the effectiveness of dispersal among them. For 

this reason, we believe that more inventories and research are 

needed on the spotted owl metapopulation in southern California. 

Considering the present state of our knowledge about spotted owls in southern California, we have identified seven major 

areas of concern about owl habitats there (table 1E). These have 

resulted from a combination of two major factors: (1) The 

overall population is naturally fragmented into small, relatively 

isolated subpopulations by the topography, precipitation patterns, and fire regime. (2) Extensive growth in the human population in the Los Angeles basin, and in other valley and foothill 

areas within commuting distance of Los Angeles, is encroaching 

on owl habitat. 

Table 1E--Summary of major factors of concern in habitats of California spotted owls in southern California, reasons for those factors, and their impacts on the owls. 

Factor Reason(s) for the factor Impact on spotted owls 

Fragmented distribution of suitable owl Mainly a natural result of topography, precipitation patterns, Creation of a metapopulation structure 

habitat into small, relatively isolated "islands" and fire regime in southern California overall population is fragmented into 

 numerous relatively small populations 

Small population units are relatively Demographic stochasticity (random events in breeding, such Increased likelihood of local extinction of small 

unstable as most or all young in a given year being males) population units 

Extent of demographic rescue of Distances between isolated populations, and the nature of the Increased likelihood of local extinction of small 

small populations by immigration of owls habitat between them, directly affect the likelihood of population units 

from other populations is relatively impeded successful dispersal among populations by owls 

Wildfires Natural fire regimes in southern California; additional Loss of suitable habitat will exacerbate 

 human-caused fires; difficulty of fire suppression in rugged, problems of small owl populations and 

 remote terrain restricted dispersal among populations 

Expansion of communities and dispersed Human population growth in southern California Further decline in effective dispersal among 

housing developments in suitable owl isolated owl populations; possible loss 

habitat, especially in dispersal areas of suitable breeding habitat 

between isolated owl populations 

Increasing recreational impacts in owl Human population growth in southern California Possible loss of additional owl habitat; 

habitats possible disturbance effects inducing owls to 

 leave otherwise suitable habitat 

Surface and subterranean mining of riparian Human population growth in southern California Loss of suitable owl habitats in riparian/hard 

water sources wood forests 

14 Chapter 1 USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
An Assessment of Current 

Management 

Forest Service Lands 

Regional Policy-The SOHA Network 

Based on available information, we estimate that 83 percent 

of all California spotted owls occur on NFs, overall; 65 percent 

of the total are on NFs in the Sierra Nevada (table 1A), and only 

4 percent of those are on reserved lands. All known spotted owl 

sites on NFs in southern California are to be protected. We were 

not able to assess the extent to which the implementation of this 

policy is adequate for those owls, because we have incomplete 

knowledge of the range of habitats in which the birds can 

maintain self-sustaining populations. In general, we agree with 

the policy. We are concerned, however, about the current level 

of information on owl sites in southern California, as well as 

with the ability of the FS to manage habitat to provide adequate 

protection from fire and other factors. We recommend that 

current policy be reviewed periodically to determine (1) that it is 

being implemented adequately, and (2) that measures taken to 

implement it reflect the latest information available on the owls 

in each locality. 

The FS's Regional policy for maintaining a viable population of California spotted owls in the Sierra Nevada is the 

network of SOHAs described in a previous section of this chapter. Because SOHAs provide habitat for only one to three owl 

pairs in a unit, separated by 6-12 miles from other units, the 

Interagency Spotted Owl Scientific Committee (ISC) that proposed an alternate strategy for the northern spotted owl (Thomas 

et al. 1990) concluded that the SOHA strategy had an unacceptably low likelihood of maintaining the owl population over the 

next 50 to 100 years. We agree that a SOHA strategy, culminating in a network of small, relatively isolated "islands" of older 

forest suitable for breeding by spotted owls and separated by a 

"sea" of younger, less suitable or unsuitable habitat, is not a 

workable strategy to assure long-term maintenance of spotted 

owls. The underlying principles are the same whether for the 

northern or the California spotted owl: 

1. Every empirical study available on the persistence of bird 

populations in relation to the number of pairs in the population shows that the likelihood of extinction increases dramatically with decreasing numbers of pairs in a block of 

habitat. Isolated pairs exhibit excessively high extinction 

rates. Modeling studies show the same thing. Consequently, 

we expect that owl pairs in SOHAs would disappear at a 

relatively high rate, leaving the SOHAs unoccupied and at 

least temporarily nonfunctional. This loss would considerably exacerbate dispersal problems. Replacement of members lost from pairs would occur very slowly because recruits would have to search extensive areas of unsuitable 

landscape before locating a vacancy in an isolated SOHA.

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

2. Social interactions among pairs of owls almost certainly 

increase calling frequency where several pairs of birds arc 

clustered. If this were not true, observers should not be able 

to elicit calling from silent owls by imitating their calls. The 

increased calling rate in clusters of several pairs should 

provide a sort of "vocal guidance" that would help dispersing birds locate other owls of the opposite sex in good, 

occupied habitat. This effect would be minimal at SOHAs 

because calling neighbors could be too far away to hear, and 

thus to stimulate counter-calling (see Thomas et al. 1990. 

appendix O). 

3. Although SOHAs must provide at least 1,000 acres of 

suitable owl habitat, and some specified amount of replacement habitat, this can be (and usually is) accumulated by 

summing acreages of several smaller patches. A result is 

that SOHAs have a high ratio of edge to area. Some studies 

indicate that fundamental changes occur in the microclimate of a forest interior, at about 525 feet from an edge 

(Harris 1984, Franklin and Forman 1987). A 20-acre circular patch, therefore, is essentially all "edge." A 100-acre 

circular patch has a core of only 32 acres that would be 

sheltered from edge effects. In addition, allowing a SOHA 

to consist of several small patches of habitat, instead of a 

single large one, results in each patch being more susceptible to blowdown of trees around its edges. 

4. Being relatively small, SOHAs are vulnerable to small-scale 

catastrophes. Destruction of a SOHA removes it from the 

network for perhaps 80 to 150 years and increases the mean 

dispersal distance between remaining SOHAs, further reducing the chance of nonterritorial owls finding unoccupied 

but suitable sites. 

5. Floaters (nonterritorial birds) behave toward populations of 

breeding birds in ways that seem unlikely toward isolated 

pairs of breeders. We believe SOHAs would fail to provide 

sufficient conditions for recruitment of floaters into a breeding population, because the extent of suitable habitat in a 

SOHA is too limited to accommodate much more than a 

nesting pair. 

All of the above problems are markedly reduced if owl 

populations are maintained in relatively large clusters in extensive landscapes where most or all owl pairs have one or more 

adjoining neighbors (Thomas et al. 1990). 

Cumulative Effects Analysis 

In 1991, the FS implemented a cumulative effects analysis 

(CEA) to evaluate green timber sales and other projects within 

the range of the California spotted owl in the Sierra Nevada. Its 

objective is to maintain a full range of options for managing 

spotted owls in the future, while still allowing logging. First, all 

known and probable spotted owl sites for pairs or resident 

singles are identified that could be directly or indirectly affected 

by a project that might remove or affect owl habitat. Directly 

affected sites are those in which project activities will occur; 

indirectly affected sites are those in which owl use areas during 

the breeding period adjoin directly affected sites. The analysis 

area generally corresponds to the combined use areas of the 

known and probable owls that are determined to be directly and 

Chapter 1 15 
indirectly affected by a given project. The outer boundary of this 

combined area may be constrained on one or more sides by 

topographic barriers or unsuitable vegetation types. 

The process next calls for mapping all nesting and foraging 

habitat available within the analysis area before the project is 

done, using the most current information. This can include 

recent aerial photography, Landsat imagery, LMP database, 

timber stand inventory, and especially ground verification. All 

components of suitable habitat-total canopy closure, dominant 

overstory trees, multi-storied canopy structure, decadence, 

dead-and-downed wood, and hardwoods are considered. The 

amounts of suitable nesting and foraging habitat that will remain 

in each owl use area after completion of the proposed project are 

then mapped. Effects of other actions that are reasonably forseeable 

are also considered (for example, other sales under contract, 

other projects with signed decision notices, timber harvest plans, 

or predictable actions on private lands that will remove suitable 

habitat). The amounts of suitable foraging and nesting habitat 

that will remain after project completion are next evaluated 

against that determined to be needed by the owls in that locality, 

using the best available information from research and other 

sources. If the proposed action would reduce the total suitable 

owl habitat blow levels needed to support the current estimated 

number of owls in the analysis area, adjustments are made in the 

project. These may include deleting portions of the sale, modifying prescriptions so that suitable habitat remains after logging 

entry, or moving sale units into unsuitable habitat. If the project 

would leave the needed amount of suitable foraging and nesting 

habitat per owl use area, it may proceed subject to any other 

Forest standards and guidelines that apply. 

We have reviewed this process and believe that it will 

accomplish its objectives in many cases. We are concerned, 

however, that it lacks specific guidance for retaining the very 

large trees that are selected for nesting by the owls. Although the 

procedure calls for at least half of the canopy cover retained in 

the project area to be in the dominant overstory, which would 

undoubtedly spare many larger trees in a project area, it would 

not necessarily spare the largest or the oldest trees. The process 

also lacks specific guidance for retaining snags and maintaining 

some quantity of dead-and-downed woody material in specific 

size-classes. Finally, the CEA process has no provision to retain 

important habitat attributes in areas not now classified as suitable nesting or foraging habitat, even though these may have the 

potential to become suitable at varying times in the future-- 

some sooner and some later. Results presented in Chapter 5 of 

this report could be used to craft specific recommendations for 

these attributes. 

Other Public Ownerships 

Only 130 owl sites located from 1987 to 1991 were on SPs 

and NPs, where management appears to be consistent with 

maintaining their habitat. The single known owl site on BLM 

lands certainly underestimates the true number of owl pairs, 

present, although the final count is not likely to be large. Logging occurs on much of the forested land managed by the BLM. 

Even though BLM's stated management emphasis will shift 

16 Chapter 1 

toward managing for old-growth, wood products, stand maintenance, and to meet wildlife and vegetation objectives, we cannot 

be certain that this will suffice to maintain the number of owl 

pairs that probably occur now on BLM lands. The matter needs 

to be addressed in detail along lines recommended in later 

sections of this chapter. A similar approach should be taken for 

State-owned forests where logging occurs. 

Private Timberlands 

Timberlands in the Sierra Nevada that are owned by commercial timber companies and miscellaneous private parties 

exceed 2 million acres. Presumably much of this acreage has 

habitat suitable for spotted owls. Inventories have not been 

completed on most of these lands, and we have not been provided with full information about results of some inventories 

that have been done. It is clear, however, that much commercial 

timberland still supports breeding pairs of spotted owls, even 

though that has not been an objective. Breeding pairs are missing 

from other private timberlands, however (Chapter 3). Management across all private timberlands is consistent to the extent that 

policies and practices mandated by the State Forest Practices Act 

are followed on all private lands. Even with these constraints, 

however, we cannot easily characterize timber management on 

private ownerships because practices differ markedly among 

them. The fact that some private timberlands have breeding pairs 

of owls, while others do not, suggests to us that existing State 

regulations do not assure maintenance of owl sites on private 

lands. The difference lies in the different policies and practices 

of individual land owners. Whether or not new forest management practices will be enacted by the State of California remains 

to be seen, as does their contribution to the maintenance of 

breeding pairs of spotted owls on private lands. 

Management Recommendations for Southern California 

We regard the status of the spotted owl in southern California as serious and meriting annual attention into the foreseeable 

future. We are deeply concerned that the largest subpopulation 

in southern California, in the San Bernardino Mountains, has 

been declining at an average annual rate of about 17 percent, at 

least since 1987. Of equal concern is the fact that the overall 

population is fragmented into many smaller populations. This 

metapopulation structure is mainly a natural result of vegetation 

patterns created by topography, precipitation, and fire regimes. 

Consequently, we are unaware of significant management opportunities to create additional, large areas of suitable dispersal 

habitat between the isolated populations, or to add markedly to 

the amount of suitable breeding habitat within those population 

areas. Our modeling studies strongly suggest that the stability of 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992.
the entire southern California metapopulation depends on the 

populations in the San Bernardino and San Gabriel Mountains 

(Chapter 9). If they collapse, the entire metapopulation will 

collapse with them. Although the observed steep decline in the 

San Bernardino population may be related to current drought 

conditions, and so be transitory, this is not a certainty (Chapter 8). 

The large number of factors leading to concern for the owls 

in southern California (table lE) only add to our concern for 

what appears to be a very fragile balance for the spotted owl 

metapopulation. Accordingly, we recommend the following: 

1. Immediately implement a program to complete inventories 

of spotted owls in the remainder of their range in southern 

California (Chapter 2). If inventories and assessments of 

total populations, based on our understanding of habitat, 

have led us to markedly underestimate the number of owls 

occurring in various parts of their range, it could significantly change conclusions from our modeling. 

2. Continue to monitor the demographics of the San Bernardino population annually, and implement at least two additional demographic studies-one centered on Palomar Mountain and the other in an area with reasonable road access in 

the Los Padres NF. These additional demographic studies 

would help (1) to determine whether our conclusion from 

modeling is correct that owl subpopulations in these locations depend for their maintenance on immigrants from the 

San Bernardino and San Gabriel subpopulations, and (2) if 

so, to establish the rate of immigration from other subpopulations needed to maintain them. 

3. Continue existing management direction on FS lands, and 

extend that policy to other Federal lands and to State lands, 

to maintain all known pairs of spotted owls in southern 

California. To the extent possible, implement the same 

policy on private lands. 

4. Finally, we recommend that a team of specialists be assembled immediately to formulate guidelines that they believe would best assure maintenance of owl pairs in various 

parts of their distribution in southern California. This team 

should include biologists with the most knowledge of spotted owl biology and habitats in southern California, silviculturists, specialists in fuels management and wildfire suppression, county planners, and probably others. 

Management Recommendations for Private Timberlands 

in the Sierra Nevada 

Management of private timberlands in California are regulated by the State, which appears to be in the process of promulgating new policies in this arena. We hope that some of the 

information provided in this full report may influence the final 

form of those new policies. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

Recently, some of the larger private timber companies have 

begun to develop guidelines for their lands that they contend will 

maintain populations of breeding spotted owls. We laud those 

efforts and believe they should be encouraged, but under the 

provision that results are carefully studied and documented using standard, scientific methods, including scientific peer review, and that they are shared openly with the public at least 

annually. Specifically, for private timberlands, we recommend 

the following: 

1. Private timber companies that have developed management 

practices that they contend will maintain nesting or foraging 

habitat, or both, for spotted owls, should be permitted to test 

those practices, contingent upon submission of detailed 

plans to, and subsequent approval from, the State Board of 

Forestry. It would be the Board's responsibility to determine whether a particular plan has reasonable merit, vis-á-vis 

spotted owl biology. 

A. These plans should clearly identify how resulting forest 

structures and configurations are likely to provide owl 

habitat, as it is presently understood, or additional information presently known only to a given timber company 

should be made public, in detail, for evaluation. 

B. Approval of a plan by the State Board of Forestry 

would be contingent upon the concurrent implementation by the timber company of a long-term demographic study (see Chapter 8) over a large enough 

sample of its ownership to determine whether or not 

its management leads to predicted results. Such a 

demographic study would follow the same standards 

and protocols already established for spotted owls, 

and results would be open for scrutiny, at any time, 

by the public. 

2. Operations on other private timberlands should continue to 

be regulated by existing State policies. 

A. All information about spotted owls on these lands 

should be shared openly with all adjoining ownerships. Indeed, this needs to be a two-way street so 

that all parties can maximize the efficiency of their 

planning and the evaluation of their land 

managagement, vis-á-vis the owls. 

B. Further, overall plans for management of spotted owls 

need to result from coordinated efforts with adjoining 

landowners, including all public ownerships. This recommendation is not leveled as a criticism of private 

landowners. On the contrary, we believe that all parties-public and private-share equally in the general 

failure to work cooperatively to develop solutions to 

common problems. 

Chapter 1 17 
Management Recommendations for Public Timberlands 

in the Sierra Nevada 

A successful strategy for the California spotted owl in the 

Sierra Nevada must be designed to ameliorate the negative 

effects on owls of several important trends that have been underway for at least the past 100 years (table 1D). Furthermore, if the 

FS moves ahead with its current generation of LMPs, the dramatic shift toward clearcutting would add considerably to the 

fragmentation of Sierran forests. This would lessen the ability of 

spotted owls to find mates and increase the distances that the 

birds would need to fly to find sufficient food. In addition to 

ameliorating the several negative trends itemized in table 1D, a 

successful long-term strategy for spotted owls in the Sierra 

Nevada must result in the clustering of pairs such that many 

occur as neighbors with overlapping home ranges in the same 

general area.-This is the same reasoning advanced by the ISC in 

the case of the northern spotted owl (Thomas et al. 1990), which 

recommended multiple Habitat Conservation Areas (HCAs) large 

enough to provide habitat for at least 20 pairs of owls. 

Evaluation of an HCA Strategy for 

the California Spotted Owl 

Both the northern and California spotted owls select forest conditions commonly associated with very old forests. 

Consequently, logging is responsible for much of the concern 

about long-term maintenance of the populations of both subspecies. As two of our five peer reviewers pointed out, the ISC 

strategy proposed for the northern spotted owl (Thomas et al. 

1990) set new precedents in conservation biology, so it should 

not be lightly dismissed as an option for the California spotted 

owl. We agree. In five important ways, however, the current 

situation for the California subspecies differs from that of the 

northern subspecies. 

First, by some estimates the numbers and distribution of the 

northern spotted owl have been reduced by about 60 percent as a 

direct result of logging, land clearing for agriculture, urbanization, and other human developments (Thomas et al. 1990, p. 20). 

We have no evidence of similar declines in the number or 

distribution of California spotted owls, however, either in the 

Sierra Nevada or in southern California. In spite of the fact that 

logging has occurred over nearly all of the conifer forests of the 

Sierra Nevada in the past 100 years, and especially in the past 50 

years, spotted owls continue to be widely distributed throughout 

most of the conifer zone. Indeed, spotted owls may be more 

abundant in some areas of the Sierra Nevada today than they 

were 100 years ago. Late last century, sheep and sheepherders so 

depleted the understory vegetation and the supply of 

dead-and-downed wood at some locations in the Sierra Nevada 

18 Chapter 1 

that flying squirrel populations may have been depressed. We 

would expect owl numbers to decline proportional to the decline 

in numbers of flying squirrels (see table 4A), unless the owls 

preyed mainly on other species in the latter part of last century. 

With the burst of regeneration that followed removal of the 

sheep and introduction of reasonably effective fire suppression 

(fig; 11I), stand densities increased markedly and this led to an

increase in the amount of decaying wood on the ground. The 

absence of periodic fires also permitted greater accumulations of 

duff and decaying wood. 

Second, clearcutting is still held by many foresters and 

silviculturists to be the prescription of choice for most of western 

Washington and Oregon, west of the Cascade crest. Partial 

cutting there leads to extensive blowdown of remaining trees, 

and regeneration of preferred timber species is poor after partial 

cutting compared with clearcutting. The result is a scant record 

of experience with partial cutting in most of the Pacific Northwest, and certainly no experience with how to maintain spotted 

owls in logged forests by applying a variety of partial-cutting 

prescriptions. As a result, the ISC opted for a strategy that 

separated HCAs from areas where logging could occur, and they 

prudently held that experience with silvicultural procedures that 

could both generate timber volume and maintain owls should be 

acquired outside of HCAs. On the other hand, partial cutting has 

been the predominant method over most of the Sierra Nevada for 

decades. We know that stands there do not "fall apart" when 

partially cut. We also know that most of what has been done 

there has not yet excluded spotted owls from Sierran forests. 

Third, because clearcutting practices have dominated silviculture in the Pacific Northwest, most forests there today are 

either relatively undisturbed or they are in various stages of 

regeneration from clearcuts done mostly within the past 50 

years. Consequently, distinguishing between suitable and unsuitable owl habitat in the Pacific Northwest was relatively easy, 

compared with the same task in most of the range of the California spotted owl. Tallying total acres of suitable owl habitat, 

although not easy, was nonetheless feasible over most of the 

range of the northern spotted owl. This has not been the case 

throughout the range of the California spotted owl for three 

primary reasons: (1) Logging practices in the Sierra Nevada and 

southern California have not typically involved the creation of 

nonforests where once forests stood. Instead, logging's impacts 

have been incremental. (2) We have no studies to show what 

sorts of forest stands can support self-sustaining populations of 

California spotted owls. (3) Nearly all of the quantitative research done on the California spotted owl began in 1987 or 

later-the same time the present drought began (fig. 4H). Therefore, all results must be interpreted against that background. 

Fourth, fire is not a major threat to most existing stands west 

of the Cascade crest in Oregon and Washington (Agee and 

Edmonds 1992). Setting aside large blocks of forested land to be 

left nearly intact, with little or no logging or other stand-altering 

activities, does not entail a big risk that fires will destroy major 

portions of. those blocks at an unacceptable rate. We have little 

confidence that the same is true in the Sierra Nevada. Sierran 

mixed-conifer forests, where most California spotted owls ocUSDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
cur, are drier and, given the effects of fire exclusion, much more 

prone to stand-destroying fires than are most forests in western 

Washington and Oregon. This creates a challenge when trying to 

establish procedures for maintaining spotted owls in Sierran 

conifer forests. An HCA strategy there could deal with the 

uncertainties associated with logging, but HCAs would sometimes be reduced in extent by stand-destroying fires. Prescribed 

fires and other methods of fuel treatment can be used to reduce 

the excessive fuel loads that are now so common in Sierran 

forests (Chapter 12). These procedures are costly, however, and 

we believe it is folly to imagine that sufficient funds would be 

forthcoming to implement an effective fuels management program in HCAs excluded from logging. Furthermore, regulations 

on air quality standards are making it increasingly difficult for 

agencies to obtain the burning permits needed to implement 

effective prescribed burning programs. 

Fifth, the northern spotted owl is considerably more numerous than the California spotted owl. This contrast is even greater, 

of course, for the Sierra Nevada-the only area where an HCA 

strategy might be considered for the California spotted owl (see 

prior discussion of the southern California case). Thomas et al. 

(1990, p. 20) stated that "...results indicate about 2000 pairs 

located during the last 5 years, representing some unknown 

fraction of the true number of pairs. Because a census of the total 

population is not available, we have no statistically reliable 

population estimate. Recent claims of actual counts of some 

6000 birds in 1989 are not out of line with other information 

from monitoring and inventory efforts." The HCAs recommended by the ISC were estimated to set aside habitat for 1,743 

pairs of northern spotted owls (Thomas et al. 1990, p. 33). "In a 

worst-case scenario, we estimate that the strategy could result in 

a 50 to 60% reduction in current owl numbers," stated Thomas et 

al. (1990, p. 34). Given the relatively large number of northern 

spotted owls, and the extensive distribution of HCAs throughout 

the range of the subspecies, the ISC believed that such a reduction in total population would not preclude attaining a stable, 

equilibrium population within 100 years. 

We expect that an HCA strategy in the Sierra Nevada could 

be implemented only on Federal lands, where we have estimated 

1,454 known and possible owl sites (table 1A). If 75 percent of 

the owl sites have pairs at any given time, and assuming that an 

HCA strategy in the Sierra Nevada might result in only a 40 

percent decline in the number of owl pairs, we would expect 

only about 650 pairs of owls to be protected by HCAs (which 

would be structured to include reserved lands-NPs and Wilderness Areas). This number may be sufficient to maintain a viable 

population of owls over the short- to mid-term in the Sierra 

Nevada, depending on the sizes and positioning of the HCAs. 

But the number is small enough to introduce additional risks 

associated with catastrophic events, such as stand-destroying 

fires in HCAs. Because fire events and subsequent impacts on 

owl numbers are inevitable, we must maintain a balance between the rate of habitat loss to fires and the rate of habitat 

recovery from fires. 

From the above considerations, we believe an HCA strategy 

for the Sierra Nevada has as many faults as it has benefits. It 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

should not be undertaken lightly, and evidence of the need for 

such a strategy must be compelling. Here we briefly summarize 

that evidence. 

Although both study populations suggest the possibility of 

population declines, evidence of declining owl populations in 

the Sierra Nevada was inconclusive. Because the power of the 

test of the null hypothesis of no decline was very low in both 

cases, the correct inference to draw from results is that we are 

uncertain about the status of these populations. Condition this 

inference, however, with the additional facts that (1) one of the 

two demographic studies was done in Sequoia/Kings Canyon 

NPs, where logging and other habitat disturbances have not 

occurred, and (2) the second demographic study was done in an 

area of checkerboard ownership where owl density was considerably less than in areas of continuous public ownership in the 

Sierra Nevada, and where no nests and few roosts of owls were 

found on the private timberlands that were part of the checkerboard. Even if this population actually declined during the period of study, it may not have been representative of owl populations occupying areas of more contiguous, suitable habitat in 

Sierran conifer forests. 

As for northern spotted owls, strong evidence from several 

sources indicates that California spotted owls select nest and 

roost sites in stands with very large, old trees, high canopy 

closure, and snags. Clear evidence from past logging practices 

and from the LMPs for Sierran NFs indicates that most of these 

stands will soon be gone if the direction of forest management in 

Sierran conifer forests is not changed. At the present time, however, the owls are widely and evenly distributed throughout nearly 

all of the westside conifer forests on NF lands. We know less 

about their occurrence on private lands, but we do know that owls 

occur on many of them. Apparently, even though the total amount 

of old-growth forest has been markedly reduced in the Sierra 

Nevada during the past century, enough very old trees remain 

today, widely distributed, that the owls do not exhibit major gaps 

in their distribution that can be clearly attributed to logging. 

Given these circumstances, we do not find a case sufficiently compelling at this time to recommend setting aside large 

blocks of Sierran forests as HCAs for the California spotted owl. 

Instead, we believe the situation calls for several steps needed 

during an interim period to preserve for the future significant 

management options for owls in the Sierra Nevada. These are 

aimed primarily at saving the older forest elements that the owls 

appear to need for nesting and roosting, and at reducing the 

excessive build-up of surface and ladder fuels. 

A Recommended Interim Approach 

We believe the current status of the California spotted owl 

in the Sierra Nevada is more amenable to improved management 

practices throughout public lands (Federal and State) than it is to 

any of the variety of reserved block designs we have examined. 

Because spotted owls are still widely and fairly evenly distributed throughout the conifer forests of the western Sierra Nevada, 

we favor an alternative strategy that maintains that number and 

distribution at least for an interim period. Management of the 

Chapter 1 19 
forests during this interim should not foreclose options for whatever long-term management scenario may be adopted for the 

owl at the end of the interim period. The desired objective, of 

course, would be to determine how to maintain spotted owls 

throughout Sierran conifer forests in a manner compatible with 

some sustainable level of timber production. The advantages of 

such a strategy are many. No decisions must be made about the 

number of owl pairs needed in blocks of habitat or how far apart 

to space blocks, because most of the Sierran conifer forest would 

be suitable for foraging by owls, and nesting and roosting habitat 

would be widely available. Commodity production associated 

with maintenance of suitable owl habitat would result in funds 

for fuels management. And much of the fuels management 

problem could be approached physically as part of the strategy to 

maintain suitable owl habitat by removing the dense surface and 

ladder fuels that now facilitate stand-destroying fires. Finally, 

we contend that such a strategy is more likely to sustain viable 

populations of most or all other plant and animal species in the 

Sierra Nevada than is any block strategy. 

Whatever interim strategy may be adopted, it should accomplish three primary objectives: (1) protect known owl nest 

stands (or main roost stands if nest stands are not known) from 

any significant degradation; (2) protect very large, old trees 

throughout Sierran conifer forests; and (3) begin to cope with the 

excessive fuels problem. The duration of the interim period will 

depend on how quickly we can determine, with certainty, the 

status of the owl population in the Sierra Nevada and attain a 

relatively full understanding of the range of habitats in which the 

owls can maintain self-sustaining populations. We recommend 

an initial period of 5 years, although whatever period is chosen 

must extend well past the present drought into the next "normal" 

or "wet" climatic period. 

General Recommendations 

1. Maintain all existing SOHAs, as presently specified in LMPs, 

until a long-term strategy is implemented. Although the 

SOHAs do not, by themselves, constitute a viable strategy 

for the owls, we cannot anticipate what role they may play, 

if any, in a long-term strategy. 

2. Continue to monitor the demographies of spotted owls in 

the Lassen NF, Eldorado NF, Sierra NF, and Sequoia/ 

Kings Canyon NPs. Enlarge these studies enough that the 

power of their tests of lambda can provide a reasonable 

likelihood of detecting real population declines when they 

occur (Chapter 8). 

3. Implement ecological studies of the primary prey species of 

spotted owls in the Sierra Nevada Province, especially the 

northern flying squirrel in the conifer zone and the dusky-

.footed woodrat at lower elevations (Chapter 2). The objective should be to develop a full understanding of the key 

ecological linkages among trees, soil, water, prey, and owls 

(Chapter 4). 

4. Undertake an extensive inventory of potential spotted owl 

habitat in riparian/hardwood forests and adjoining woodlands in the foothills of the western Sierra Nevada and in the 

20 Chapter 1 

inner coast ranges to estimate the number of nesting pairs 

there (Chapter 2). 

5. Through coordination between managers and researchers, 

initiate a program of experimental forestry in Sierran NFs 

within the interim period, as recommended in Chapter 2 and 

elucidated in Chapter 13, so that we may observe these 

modified forest systems and determine their effects on spotted owls. 

6. Develop a time schedule that identifies specific, annual 

accomplishments that can be monitored to assure satisfactory progress toward attaining information needed to craft a 

longer-term strategy for the California spotted owl. 

Specific Recommendations 

The following guidelines (table 1F) should not preclude

options for the future. The protections afforded to owl sites and 

preferred nesting habitat are intended to stabilize owl habitat 

acreage in the short-term. Taking a longer look, the basal area 

retention in larger tree size-classes will ensure that old-growth 

elements will not be lost from these systems. Existing Experimental Forests and Demonstration Forests are expressly exempted from all of the following recommendations. 

Spotted Owl Sites on Public Lands 

1. Establish a Protected Activity Center at all known California spotted owl sites in the Sierra Nevada. Locate owl sites 

using the California Department of Fish and Game's database, and identify the activity center in each, defined either 

by a known nest site or by what is judged by a Forest 

Biologist knowledgeable about owl biology to be the best 

roost location in the site. Delineate an area of 300 acres 

around this activity center (see "Size of Activity Centers" in 

Chapter 5) following boundaries of known habitat polygons 

and topographic features such as ridgelines, as appropriate. 

The intent here is to include in the 300 acres the best 

possible owl habitat available, blocked up into as compact a 

unit as possible. Ideally, each unit would consist of 300 

acres of P4G, M4G, or better stands (M5G, M5N, M6) (see 

table 1C for code definitions), but this will likely not be 

possible in all instances. To assure that the unit includes the 

best owl habitat available, augment the acreage of P4G, 

M4G, or better with the following timber strata, arranged 

here in descending order of priority: M3G, P3G, M4N, 

M3N, P3N, R4G, R4N, P4P, and M2G (see table 5A and fig. 

5B, and the discussion of ponderosa pine strata under the 

heading "Selective Use of Forest Types" in Chapter 5). 

2. Undertake no stand-altering activities within Protected Activity Centers, other than light underburning. 

3. Light underburning in these stands would be permissible, 

given careful review by biologists and fuels management

specialists, on a case-by-case basis. Any underburning should 

be done in a manner that minimizes removal of duff and 

large woody debris. 

4. Remove no snags or large culls from Protected Activity 

Centers. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992.
Table 1F-Summary of primary recommendations for stand retention and special stand treatments to maintain options for spotted owls on public timberlands in the 

Sierra Nevada during an interim period. 

 Protected1 Selected2 Other3

 Activity Timber Timber 

Attributes Centers Strata Strata 

Large, old trees 

 Basal area No logging Retain 40 percent basal area from Retain 30 percent basal area from 

 the largest healthy trees and culls the largest healthy trees and culls 

 Retain at least 50 square feet 

 basal area per acre 

D.b.h.4 No logging Retain all live trees  LQFKHV Retain all live trees  LQFKHV

in d.b.h. in d.b.h 

Percent canopy cover No reduction  SHUFHQW No restriction 

Snags No reduction Save the largest snags  LQFKHV LQ GEK WR D PD[LPXP RI HLJKW VQDJV

per acre; if this is < 20 square feet basal area per acre, save snags < 30 inches

in d.b.h., from the largest down, to a total of eight snags per acre or 20 

square feet basal area per acre, whichever comes first; need not retain snags 

<15 inches in d.b.h. or < 20 feet tall 

Downed woody material Reduction only in relation to Beginning with the largest downed logs (by volume), sequentially retain pieces 

light underbuming of downed wood until an average of at least 10-15 tons/acre are retained over a cut 

unit. Do not include pieces <11 inches in diameter to meet the tonnage limit. The 

intent here is to retain as many as possible of the existing large pieces of decaying 

wood present on a site before any treatment (for example, a timber sale or prescribed 

burn). Cull logs created by a sale should be left at or near where they fall and be 

included when totalling the downed wood to be retained 

Fire threats Light underburning Positive fuels Positive fuels 

 management management 

1 Block of 300 acres of suitable nesting/roosting habitat delineated around nest site or primary roost site in all known spotted owl sites in the Sierra Nevada, as 

identified in the California Department of Fish and Game database. 2 Timber strata selected for nesting by spotted owls in the Sierra Nevada (P4G, M4N, M4G, and better-strata codes explained in table 1C). 3 Other timber strata used for nesting by spotted owls in the Sierra Nevada (MG, P3N, P4P, M2G, M3P, M3G, M3N, M4P, R3P, R3G, R4G, R4N-strata codes 

explained in table 1C). 4 Diameter at breast height, in inches. 

Other Forested Public Lands 

1. Selected Timber Strata: Stands shown to be selected for 

nesting by the owls (P4G, M4G, M4N, M5G, M5N, M6-- 

see table 5A and section on "Selective Use of Forest Types" 

in Chapter 5) may be entered only once for commercial 

logging prior to implementing a long-term strategy for managing the California spotted owl on public lands. Remove 

no live tree 30 inches in d.b.h. Retain 40 percent of the 

basal area, consisting of the largest of the healthy trees and 

culls in each cut unit, using the following steps: 

A. Do not rely on current timber inventories to determine 

stand strata. Base this determination on field verification of each cut unit during stand inventories in preparation for sales. When a cut unit is borderline between 

two timber strata (for example, between M4N and 

M3N, between M4G and M3G, or between M4N and 

M4P), assign it to the stratum with higher canopy cover 

and/or larger stem diameter. Develop diameter distributions of live trees (including culls) from the inventory data for each cut unit separately, by 2- to 4-inch 

d.b.h. groupings. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

Figure 1A-Diameter distribution (stems per acre), basal area distribution (square feet per acre), and cumulative basal area distribution of a 

hypothetical, uneven-aged stand of trees. 

Chapter 1 21 

0 5 10 15 20 25 30 35 40 45 50 

 Diameter at Breast Height (inches) 
          

Diameter at Breast Height (inches) 

Figure 1 B-A cumulative basal-area function from a hypothetical stand, 

showing how to relate the proportion of the total basal area subject to 

logging to the diameter limit of the trees to be retained. 

22 Chapter 1 

B. For each cut unit separately, construct basal-area distributions for live trees. 

C. Next form cumulative basal-area distributions for live 

trees in each cut unit (see fig. lA), and draw a horizontal line from the 0.6 proportion on the y axis to the 

cumulative basal-area distribution curve. From that point, 

draw a vertical line down to the x axis (see fig. 1B). 

D. Cut no live trees with diameters equal to or larger than 

the diameter intersected by the line perpendicular to the 

x axis. 

Maintain an average crown closure 40 percent in the 

remaining overstory. If the largest live trees retained in the cut 

unit do not yield a canopy closure 40 percent, make up the 

difference with stems 12-24 inches in d.b.h. Treat surface and 

ladder fuels as necessary to create a mosaic of fuel profiles that 

will minimize the probability of extensive, stand-destroying 

wildfires. Fuel profiles should consider other objectives of land 

management, including the needs for site productivity and for 

habitat of species other than spotted owls. 

2. Other Timber Strata: Stand types used for nesting by the 

owls, but not significantly selected based on availability 

(P3G, P3N, P4P, M2G, M3P, M3G, M3N, M4P, R3P, R3G, 

R3N, R4G, R4N-table 5A), may be entered only once for 

commercial logging prior to implementing a long-term strategy on public lands. Remove no live tree 30 inches in 

d.b.h. Retain 30 percent of the basal area, consisting of the 

largest of the healthy trees and culls in each cut unit, following the same steps described for Selected Timber Strata. In 

this case, begin the horizontal line to the cumulative basal 

area distribution curve from 0.7 on they axis (fig. 1B). Live 

trees remaining in these stands must have a cumulative 

basal area of at least 50 square feet per acre. Remove 

surface and ladder fuels that would threaten to carry fire into 

the crowns of remaining trees, and undertake on the sale 

unit other fuels treatments that are considered necessary. 

3. Snag Retention: Retain all snags in Protected Activity Centers. In all other habitat that is currently or potentially 

suitable for foraging, roosting, and/or nesting by spotted 

owls, including salvage sales and instances of catastrophic 

stand loss, use the following guidelines: Save the largest 

snags  LQFKHV LQ GEK VWDUWLQJ ZLWK WKH ODUJHVW VQDJ

and working down) to a maximum of eight snags per acre, 

averaged over the cut unit. If this guideline does not result in 

at least 20 square feet basal area of snags per acre, continue 

adding snags, from the largest ones remaining, down to a 

total of eight snags per acre or 20 square feet basal area, 

whichever comes first. Snags <15 inches in d.b.h. or <20 

feet tall need not be retained. 

4. Downed Wood Retention: In all habitat that is currently or 

potentially suitable for foraging, roosting, and/or nesting by 

spotted owls, use the following guidelines: Beginning with 

the largest downed logs (by volume), sequentially retain 

pieces of downed wood until an average of at least 10-15 

tons/acre are retained over a cut unit. Do not include pieces 

smaller than 11 inches in diameter to meet the tonnage limit. 

The intent here is to retain as many as possible of the 

existing large pieces of decaying wood present on a site 

before any treatment (for example, a timber sale or pre-

=scribed burn). Cull logs created by a sale should be left at or 

near where they fall and included when totalling the downed 

wood to be retained. For the mass calculation, assume a 

specific gravity of 0.4. 

5. Exceptions to Guidelines 1 and 2: Guidelines 1 and 2, 

above, require that large, live trees be left where they are 

found. In certain cases, based on concurrence between wildlife biologists and silviculturists, compelling reasons may 

exist to reduce the areas of tree crowns and roots in portions 

of a cut unit. These include the need to break up a uniform 

distribution of leave trees to allow regeneration of shade 

intolerant species, to reduce the spread of dwarf mistletoe 

from the overstory to regenerating conifers in the understory, or to protect dense leave patches from bark beetle 

attack by killing high-risk trees. In such cases, we recommend the following guidelines: 

If the total basal area of snags 30 inches in d.b.h. is less 

than 20 square feet per acre, live trees greater than the diameter 

limit for Selected Timber Strata or Other Timber Strata, as 

appropriate, may be girdled to create snags. Add the basal area 

of the girdled trees to the snag basal area, but leave an equivalent 

live basal area in the dominant and codominant tree classes to 

compensate for the loss in basal area of large live trees that were 

girdled. A maximum of 10 square feet basal area per acre, or one 

stem per acre if that stem contains 10 square feet basal area, 

may be girdled. As with all other retention figures, the evaluation is averaged over the cut unit, which means that a minimum 

of 40 trees may be treated in this manner on a 40-acre cut unit. 

The following steps would serve to implement this strategy: 

A. Develop a diameter distribution and mark leave trees in 

a given cut unit. 

B. Determine whether the stand is deficient in large snags. 

C. If more large snags are desirable, mark desired snags as 

"wildlife" trees and measure their diameters at breast 

height. 

D. After all "wildlife" trees have been selected, mark to 

leave enough additional live trees from the dominant 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992.
remain for basal-area retentions between 20 and 50 percent 

(table 1G). Evaluation criteria were based on the need to leave 

both the large, old trees and to leave sufficient "dominants" as 

replacement trees so that these structures would be retained into 

the future. Because we wished to establish a limit, not a target, 

we aimed to set minimum retention values. 

An additional objective for Selected Timber Strata was to 

leave such stands in or near a structural condition corresponding 

to suitable foraging habitat for spotted owls. For these stands, it 

is clear from study of values for M4G strata in table 1G that

basal-area retentions of <30 percent would not accomplish our 

main objective of retaining large, old trees AND providing a 

succession of replacement trees for them. The smallest of the 

large trees under 20-percent retention would be 34 inches in 

d.b.h.-too large to be considered candidates for replacement of 

the large, old trees when they die and fall. Only six large trees 

per acre would remain with 20-percent retention, but the 30-inch 

Chapter 1 23 

and codominant classes to equal the basal area of trees 

to be girdled. 

E. Girdle the marked "wildlife" trees after the sale. If trees 

marked to leave were inadvertently removed during 

harvest, make up for the lost basal area by NOT girdling some of the "wildlife" trees. 

A Rationale for the Recommendations 

Having decided that a major part of an interim strategy for 

spotted owls in the Sierra Nevada needed to save the largest, 

oldest trees, we then had to determine what level of retention 

would be sufficient. For this, we obtained plot-level data from 

timber-strata inventories in the Tahoe NF. From these data we 

defined diameter-distribution and basal-area functions for each 

stratum in the mixed-conifer group. We then generated cumulative distributions for each stratum, determined the diameter 

limits, and computed the number of trees per acre that would 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

1

 Strata codes defined in table 1C. 

2

 Diameter at breast height, in inches. 

3

 Does not satisfy the 30-inch d.b.h. rule, which then takes effect. 

4

 Does not satisfy the rule of retaining a basal area of at least 50 square feet. 

Table 1G-Estimates of the effects of leaving the largest trees in stands, accumulating to various percentages of total stand basal area, for four timber strata, using 

actual mean stand-diameter distributions from the Tahoe NF. When the value in column seven ("Percent of basal area retained by 30-inch limit") exceeds the 

corresponding value in column one ("Percent basal-area retention"), the 30-inch limit is more constraining. Similarly, when the value in column five ("D.b.h. of smallest 

tree left") is larger than 30, the 30-inch limit is more constraining than "Percent basal-area retention." 

 Number of 

 Percent of trees left 

 Percent Basal area D.b.h. of Mean d.b.h. basal area per acre 

basal-area Initial left after, smallest of trees retained by with basal-

 retention Strata1 basal area logging tree left2 remaining 30-inch limit area rule 

Stands affected by the 40-percent retention rule 

20 M4G3

 254.61 50.92 34 37 27 6 

25 M4G3

 254.61 63.65 31 37 27 7

30 M4G 254.61 76.38 29 36 27 10 

35 M4G 254.61 89.11 27 34 27 14 

40 M4G 254.61 101.84 25 32 27 18 

45 M4G 254.61 114.57 23 30 27 23

50 M4G 254.61 127.30 22 30 27 26 

Stands affected by the 30-percent retention rule 

20 M4P3,4 126.45 25.29 32 38 24 5 

 M3G3,4 213.43 42.69 34 38 26 4 

 M3P4

 138.06 27.62 30 38 19 3 

25 M4P4

 126.45 31.61 30 38 24 4 

 M3G3

 213.43 53.36 31 38 26 6 

 M3P4

 138.06 34.52 28 35 19 5 

30 M4P4

 126.45 37.94 26 35 24 6

 M3G 213.43 64.03 29 37 26 8 

 M3P4

 138.06 41.39 26 33 19 7 

35 M4P4

 126.45 44.26 24 33 24 7 

 M3G 213.43 74.70 27 35 26 11 

M3P4 138.06 48.32 25 32 19 9 

40 M4P 126.45 50.58 22 30 24 10 

 M3G 213.43 85.37 25 32 26 15 

 M3P 138.06 55.33 24 31 19 10 

45 M4P 126.45 56.90 21 28 . 24 13 

 M3G 213.43 96.04 24 31 26 17 

 M3P 138.06 62.13 22 29 19 13 

50 M4P 126.45 63.23 20 27 24 14 

 M3G 213.43 106.72 22 30 26 21 

M3P 138.06 69.03 20 28 19 16 
d.b.h. rule takes over in this case and leads to retention of 10 

trees per acre. A 40-percent retention nearly doubles the number 

of trees retained per acre, and the smallest of the large trees 

would be 25 inches in d.b.h. Trees of this size can easily be 

produced in less than 100 years on most commercial timberlands 

in the Sierran mixed-conifer zone, providing ample replacements 

for the large, old trees that range upward in age from 200 years. 

A challenge arises with stands having a very high proportion of very large trees. Even with a 40-percent retention rule, the 

smallest of the large trees retained could be too large to qualify 

as a replacement tree. For this reason, we added a recommendation to retain all trees  LQFKHV LQ GEK :KHQ WKH EDVDO-area 

retention rule fails to include such trees, the 30-inch rule is more 

constraining and determines the smallest stem diameter to be 

retained. 

The criteria outlined above for retaining large trees will 

result in unique residual stand structures. In contrast to the 

uniform spacing of trees common to seed-tree and shelterwood 

methods, we expect large trees to be irregularly distributed in a 

stand and to exhibit varying degrees of clumping. This pattern of 

distribution would result in some large openings in the canopies 

of units logged following the retention guidelines, and thus 

promote the regeneration of shade-intolerant species like ponderosa pine and black oak. Our next concern was with the final 

canopy closure in these stands, which should be  SHUFHQW WR

be within the range of suitable owl foraging habitat. Given 18 

trees per acre with a 40-percent retention, and assuming an 

average crown diameter of 30 feet, final canopy closure would 

be <30 percent. This led to our final recommendation of retaining sufficient trees 12-24 inches in d.b.h. to bring total canopy 

closure up to  SHUFHQW 1RW RQO\ ZRXOG WKLV SURYLGH DGHTXDWH

canopy closure, but also it would provide an intermediate range of 

tree sizes as later candidates for replacement of large, old trees. 

The thought process was similar for Other Timber Strata, 

represented by M4P, M3G, and M3P stands (table 1G), although 

it was not our intention that these should qualify structurally as 

suitable foraging habitat for owls after logging. Retaining 30 

percent of the total basal area in the largest tree sizes would 

maintain some trees <30 inches in d.b.h., which would provide 

replacements for the largest trees. As added protection for very 

sparse stands, we have recommended a minimum basal area of 

50 square feet per acre, accumulated from the largest trees in the 

stand. For the M3P and M4P stands (table 1G), approximately 

40 percent of the basal area must be retained to leave 50 square 

feet per acre, and 10 of the largest trees per acre would remain. 

As described in Chapter 5, under the section entitled "Selective Use of Forest Types," we lacked data to analyze whether or 

not spotted owls select nest sites in various ponderosa pine strata 

in excess of expectation. Because we have strong reason to 

believe that at least P4G strata would be selected, however, we 

recommend a cautious approach in treating ponderosa pine types 

during the proposed interim period, especially because NFs are 

likely to classify them as unsuitable owl habitat for lack of 

sufficient crown closure, even though they may have plenty of 

hardwood cover in the understory. Accordingly, we recommend 

that P4G stands, so classified after hardwood and understory 

conifer components have been included in an assessment of

24 Chapter 1 

stem diameter and canopy closure, should be treated as "Selected Timber Strata" during the proposed interim period. P3N, 

MG, and P4P, again as classified after inclusion of their hardwood components, should be treated as "Other Timber Strata." 

Evaluation of the Recommendations in Relation to 

the Problem 

Our recommendations address all factors in Sierran conifer 

forests that we believe have negative effects on California spotted owls (table 1H). Of the eight factors identified, six would be 

alleviated by a strategy that saves the largest trees in stands and 

removes some significant proportion of the smaller trees. In 

effect, the approach recommended here tends to invert silvicultural practices of the last 100 years. What has been characterized 

as "top down" logging (concentrating on the largest trees) would 

become primarily a "bottom up" approach (leaving the largest 

trees and concentrating on the smaller trees). Although not

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

Table 1H Summary of major factors of concern in habitats of California spotted 

owls in the Sierra Nevada, and which of the recommended actions are intended 

to ameliorate effects of those factors. 

Factor Recommended actions 

Decline in abundance of very Saving live trees  LQFKHV LQ

large, old trees diameter at breast height; saving the 

largest trees in stands, by percentage 

of basal area 

Long recovery period for spotted Emphasizing retention of largest, trees 

owl habitat after logging shortens the recovery period after 

 logging 

Ingrowth of shade-tolerant tree Thinning; emphasizing removal of 

species, creating unnaturally small-diameter trees; clearing fuel 

dense stands with ground-to-crown ladders from around largest trees 

fuel ladders 

Excessive build-up of surface Implementing aggressive fuels 

fuels management program, especially to 

reduce amount of downed wood 

<10 inches in diameter 

Loss of large-diameter logs from Retaining the largest trees and snags 

the decaying wood source on the in stands, which will eventually be 

ground recruited as logs; meeting guidelines 

for large-diameter logs on the ground 

Decline in snag density Retaining at least 20 square feet basal 

area per acre of the largest snags in 

stands; in addition, sparing the 

largest trees will assure a continuing 

source of large snags and downed logs 

Disturbance and/or removal of Reducing surface fuels so ground fires 

duff and topsoil layers burn cooler; reducing mechanical 

impact of logging and other projects on 

top soil and duff layers 

Change in composition of tree Leaving largest trees and removing 

species-fewer pines and black smaller trees should result in many 

oaks, more firs and incense-cedar openings large enough for germination 

by shade-intolerant species 
excluding certain kinds of silvicultural prescriptions that are 

sometimes associated with clearcut logging, the steps recommended here preclude "clearcutting" in the sense that much of 

the public perceives it-as creating unsightly patches of land 

from which all trees have been removed. It also prevents the 

high-grading of scattered remnant trees. 

We do not contend that the approach recommended is 

without risks. No approach is risk-free. The guidelines suggested for Other Timber Strata are not so restrictive that they 

would guarantee stand conditions suitable for owls immediately 

after logging. We have focused on setting strong rules to retain 

stand components that are most at risk and hardest to replace. 

For instance, to replace large logs in late decay-classes after 

clearcutting, we first must grow large-diameter trees, allow them 

to become snags, fall over, and subsequently rot. This is a 

process measured in centuries. On the other hand, a clearcut site 

can return to a dense stand of small to medium sawlogs in a few 

decades (see figs. 11P-11S). We are therefore more concerned 

about the former than the latter. The spotted owl population in 

the Sierra Nevada persists despite 100 years of logging injurious 

to its habitat, and it is still widely and relatively evenly distributed. We thus believe the recommended changes in traditional 

silvicultural practices in Sierran forests are unlikely to significantly degrade spotted owl habitat over the short-term, and they 

may even improve habitat over the long-term. 

In contrast to the case for Other Timber Strata, recommendations for Protected Activity Centers should maintain existing 

nest/roost habitats in a condition suitable for continued use by the 

owls for those purposes. And guidelines for Selected Timber 

Strata should at least maintain suitable foraging habitat, as 

recommendations would retain all structural attributes associated with foraging owls (Chapters 5 and 6). We know, for 

example, that spotted owls regularly used some stands, but not 

others, that had been recently logged in the Lassen NF (Chapter 

7). We would not be surprised to find that a brief period (probably less than 5 years) elapses after logging operations before the 

owls resume foraging in Selected Timber Strata. This is a primary question to be studied through radio-tagged owls (see 

recommendation in Chapter 2). If the approaches recommended 

here can be implemented faithfully and studied carefully, we are 

hopeful that they might lead us to a feasible, long-term solution 

to the owl problem and to many other problems that follow from 

the loss of attributes associated primarily with forests in late 

seral stages. 

The recommendations DO NOT REPRESENT TARGETS. 

Instead, they should be viewed as limits that allow a wide range 

of silvicultural options. Their main purposes are to arrest the 

decline of very large, old trees; to save younger, "dominant" 

trees as replacements for older trees as they die and fall; to 

reduce risks of catastrophic fire; to promote recruitment of 

shade-intolerant tree species; and to retain large-diameter deadand-downed woody materials. These goals can be achieved, in 

most cases, by leaving more trees than the guidelines suggest. 

For instance, guidelines proposed for Selected Timber Strata 

would be perfectly acceptable for Other Timber Strata. Examples of some silvicultural options compatible with the recommendations are described in Chapter 13. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

Most or all biomass sales (see glossary) are probably also 

consistent with the guidelines. In particular, these sales can deal 

effectively with the serious problem of accumulating surface 

and ladder fuels. This situation strikes us as being akin to the 

general deterioration in the nation's infrastructure-bridges, highways, railroads, and so on. We have enjoyed relative luxury 

while postponing the inevitable costs of maintaining these structures. Someday, someone must pay for this negligence, and it 

will certainly cost more in the long run than it would if we 

simply dealt with maintenance needs as they arise. In the case of 

the current, serious fire threats to conifer forests of the Sierra 

Nevada, the money spent to suppress just one very large, standdestroying fire would go a long way toward lessening the threat 

of such fires if it were spent in an aggressive fuels management 

program. Instead, we continue to behave like a person who fails to 

see the wisdom of owning fire insurance on an expensive home. 

Conclusions 

We suspect that some of the interim guidelines proposed 

here-those intended to retain various stand attributes found 

most often in older forests-would also be a necessary part of 

any strategy to maintain California spotted owls over the longterm. The best long-term solution for the owl in the Sierra 

Nevada would be to maintain the population in its current, 

relatively even distribution throughout the forests of the western 

slopes. If that distribution can be maintained, no need may arise 

to block up numerous areas large enough to contain many pairs 

of owls that can share home-range boundaries, as proposed for 

the northern spotted owl (Thomas et al. 1990). Such a policy in 

the Sierra Nevada would bring with it a high risk of standdestroying fires (Chapter 12) that was not a major concern over 

most of the range of the northern spotted owl. Further, if an 

acceptable way can be found to maintain attributes of older 

forests generally throughout the Sierra Nevada, such a plan 

might go a long way toward meeting the needs of most or all 

other plant and animal species that thrive in older forests. The 

spotted owl issue is only the first in a potentially long list of such 

species awaiting our attention. 

Chapter 1 25 
References 

Agee, James K.; Edmonds, Robert L. 1992. Forest protection guidelines for the 

northern spotted owl. Seattle: College of Forest Resources, University of 

Washington. Unpublished draft supplied by authors; 56 p. 

Barker, Paul A., Regional Forester, USDA Forest Service, Region 5, San 

Francisco, CA. [Personal communication]. May 1990. 

Dawson, William R.; Ligon, J. David; Murphy, Joseph R.; Myers, J. P.; 

Simberloff, Daniel; Verner, Jared. 1987. Report of the scientific advisory 

panel on the spotted owl. Condor 89:205-229. 

Franklin, Jerry F.; Forman, R. T. T. 1987. Creating landscape patterns by forest 

cutting: ecological consequences and principles. Landscape Ecology 1:5-18. 

Harris, Larry D. 1984. The fragmented forest: island biogeographic theory and 

the preservation of biotic diversity. Chicago, IL: Univ. of Chicago Press; 

211 p. 

Jay, David, Deputy Regional Forester, USDA Forest Service, Region 5, San 

Francisco, CA. [Personal communication]. May 1990. 

Lefkovitch, L. P. 1965. The study of population growth in organisms grouped 

by stages. Biometrics 21:1-18. 

Stephenson, John. 1991. Spotted owl surveys on the National Forests of 

southern California: a status report and recommendations for the future. 

San Bernardino, CA: Forest Service, U.S. Department of Agriculture; 13 

p. 

Thomas, Jack Ward; Forsman, Eric E.; Lint, Joseph B.; Meslow, E. Charles; 

Noon, Barry R.; Verner, Jared. 1990. A conservation strategy for the 

northern spotted owl. Washington, DC: U.S. Government Printing Office; 

427 p. 

U.S. Department of Interior, Fish and Wildlife Service. 1990. Endangered and 

threatened wildlife and plants; determination of threatened status for the 

northern spotted owl; final rule. Washington DC: Federal Register 

55(123):26114-26194. 

26 Chapter 1 USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Chapter 2 

Future Directions for the California 

Spotted Owl Effort 

Jared Verner and Robert J. Taylor 

The chapters that follow summarize most of what we presently know about California spotted owls-their biology and 

general ecology, their habitat requirements, their demography, 

and so on. Most chapters contain references to information still 

needed and suggest further studies and inventories of California 

spotted owls-their habitat associations, their prey, and their 

demography. Here we offer tentative directions for proceeding 

with these tasks, which we believe will speed our progress 

compared with the past. This chapter is intended to satisfy item 

#5 in the Charter that established the Technical Assessment 

Team for the California Spotted Owl: "Identify research, monitoring, and inventory programs needed to answer existing critical questions and to provide for adaptive management of the owl 

in the future." The need to continue any phase of the effort to 

fully understand the current status of the owl must be conditioned on the extent to which remaining areas of uncertainty will 

lead to incorrect decisions. The objectives of this chapter are to 

identify remaining areas of uncertainty that can be reduced by 

further inventory, monitoring, and research work, and to recommend at least a broad approach for moving forward with these 

efforts in a more systematic and coordinated way than has been 

the case in the past. 

Major Uncertainties About 

California Spotted Owls 

Inventory 

Significant areas of uncertainty still preclude conclusive 

recommendations about how best to manage for California spotted owls. For example, inventories have not yet been completed 

over much of the range of the owl in southern California. Our 

present information indicates that the largest area of fairly clustered owl sites in the Cleveland National Forest (NF) and adjacent areas has only 37 sites. Similarly, the largest group anywhere west of the San Gabriel Mountains in southern California 

contains only 25-30 sites. Sites in both localities are not tightly 

clustered, so they would not function efficiently to foster internal 

recruitment of dispersing juveniles. The numbers of sites in both 

are also near the lower limit of stability for isolated populations 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

(see fig. 9G). Consequently even small increases in numbers of 

pairs there can be expected to have large, positive effects on 

estimates of stability properties of the populations in those two 

areas and of the entire metapopulation in southern California. 

Further, we know little about the full extent of winter use or 

the presence of breeding owls in riparian/hardwood habitats in 

foothill woodlands of the western Sierra Nevada or other parts of 

the owl's distribution in the Coast Ranges (Chapter 4). This 

information is critical for two reasons. First, significant numbers 

of breeding pairs in these habitats, combined with owls in the 

adjacent conifer forests, would markedly increase the overall 

size, and thus the stability, of these owl populations. Second, 

their presence and importance there would merit serious consideration in planning for residential expansion into those localities. 

We know little about the possible impacts of current land uses in 

relevant hardwood habitats on spotted owls or their prey species. 

These are major uncertainties that need resolution. Inventories in 

these hardwood types could be planned in a way that tests, on a 

sample basis, habitat suitability models developed by Steger and 

Greenwood (pens.' comm.) to estimate the possible distribution 

and abundance of spotted owls in foothill woodlands (fig. 4D). 

Finally, the presence of California spotted owls along the 

central coast of California is well-established as far north as 

Monterey County, where they occur in mixed conifer-hardwood 

habitats and coast redwood/Douglas-fir forests. Similar habitats 

occur to the north, especially in the coastal mountains from the 

Santa Cruz area northward at least into San Mateo County. 

Inventories are needed there to determine whether the coastal 

arm of the California spotted owl's distribution extends northward beyond Monterey County. 

Monitoring 

A detailed monitoring procedure was designed to detect 

declining trends in the occupancy and breeding status of territorial spotted owls in the SOHA network (see Chapter 1) on Forest 

Service lands in California. In the range of the California subspecies, this procedure was implemented in 1987, 1988, and 

1989. Analyses done in the course of developing a proposed 

conservation strategy for the northern spotted owl, however, 

showed that this monitoring procedure was relatively insensitive 

to declines in the overall population (Thomas et al. 1990, appendix M). This was the case because of two significant lag effects 

that accompany habitat-induced declines in the number of territorial owls. 

Chapter 2 27 
The first lag effect arises from the fact that spotted owls can 

live for 10-20 years. Even in a SOHA that has declined in 

suitability to the point that a pair of owls can no longer breed 

successfully there, the pair may survive for a long time, still be 

counted as a viable pair, but never replace themselves. Spotted 

owls often do not breed annually; they may even fail to do so for 

several successive years. For this reason, little concern is raised 

when a pair does not nest in a given year. Furthermore, the 

monitoring protocol does not always detect nesting when it does 

occur. Consequently, a gradual decline in habitat quality could 

be overlooked and, over the short term, the overall population 

would be judged to be stable. 

The second lag effect results from the fact that only the 

resident territorial population is monitored. Floaters (nonterritorial 

birds) are an integral part of the total owl population (Chapter 4), 

but they are seldom or never detected by any standard sampling 

protocols. When territorial adults die in the vicinity of floaters, 

their places are quickly filled, leaving no indication that the total 

population has actually declined. This continued occupancy of 

pair sites would last as long as required to exhaust the floater 

population. Only then would we expect to see a decline in the 

territorial component of the total population. This may be the 

reason why 1991 was the first year in the several years of 

studying the declining population of spotted owls in the San 

Bernardino Mountains that the number of occupied territories 

showed a marked drop (LaHaye pers. comm.). 

Modeling work done for the northern spotted owl (Thomas 

et al. 1990, appendix M) indicated that the combination of these 

lag effects might delay by as much as 15-20 years the period 

before a significant decline could be detected in the territorial 

component of a population of spotted owls. By that time, conditions might already have worsened to a point that some critical 

threshold in the amount or quality of suitable habitat could have 

been crossed, resulting in a precipitous decline in the owl population with little chance of recovery. For these reasons, Thomas 

et al. (1990, appendix R) recommended that the emphasis of the 

monitoring program be shifted from tracking trends in the occupancy rate and breeding status of owls in SOHAs to detailed 

monitoring of birth and death rates, turnover rates (replacements 

of territorial birds lost from the population), and finite rates of 

population change (lambda-see Chapter 8) of spotted owls in 

several demographic study areas. Only in this way could we 

know whether a population is reproducing with sufficient success to maintain itself. 

We are still uncertain about whether the Sierran population 

has been declining during the years since demographic studies 

began (Chapter 8). Lambda was <1.0 in two populations in 

which it could be estimated (Eldorado NF and Sequoia/Kings 

Canyon National Parks), but not significantly less. The power of 

the tests on lambda, however, were so low that we were not 

likely to detect a significant decline even had it been occurring. 

Furthermore, even if the test results had suggested that a significant decline had taken place, it would be impossible to infer 

whether the population decline resulted from changes in the 

amount or quality of available habitat or because of ecological 

relations mediated by the present drought (fig. 4H). 

28 Chapter 2 

Research 

Habitat Capability for Spotted Owls 

Explicit characterizations of superior, suitable, marginal, or 

unsuitable habitats for California. spotted owls still elude us. This 

contrasts sharply with the case for the northern spotted owl, for 

which most land capable of growing forests in the Pacific Northwest could easily be identified as either suitable or unsuitable for 

the owls-most was either regenerating from clearcuts less than 

50 years old, or it had been altered little or not at all from its 

pristine condition. Either it was or it was not suitable habitat. 

The past 150 years or so of forest management in the Sierra 

Nevada, where most California spotted owls occur, have involved relatively little clearcutting, especially on lands presently 

within NF boundaries (Chapter 11). The present character of 

these forests resulted from combined effects of (1) the natural 

fire regime before European settlement (Chapter 12), (2) massive overgrazing by sheep over a 50-year period beginning in the 

early 1860s, (3) increasingly effective fire suppression since 

about 1900, and (4) extensive selection logging in the past 5-6 

decades. Currently, most of these forests are unnaturally dense, 

have large components of young and intermediate-aged trees but 

few surviving older trees, have extensive vertical fuel ladders 

linking ground fuels with crowns, and are highly susceptible to 

stand-destroying fires (Chapters 11, 12, and 13). But these same 

forests have a relatively uniform density of spotted owls, 

dispersed in a mostly even pattern over the forested landscape. 

At the landscape scale, we see little in the overall distribution pattern of California spotted owls to suggest how we 

might distinguish between suitable and unsuitable habitat 

(but see Chapter 5). 

We have learned much about particular stand attributes that 

are used selectively by California spotted owls (Chapters 5 and 

6), but we have been unable to connect them with studies of the 

owl's reproductive success-or failure. We still are uncertain 

about what levels of canopy cover, tree densities and sizes, snag 

densities and sizes, quantities and sizes of downed woody debris, and so on, are found where owls reproduce consistently and 

well. Only by linking demographic rates with habitat attributes 

can we eventually distinguish among superior, suitable, marginal, and unsuitable habitats. 

Owl/Prey Relations 

Studies of many owl species from around the world suggest 

that variations in available prey affect their breeding behavior in 

many ways (table 4B). We contend that the spotted owl is typical 

in this regard and that an essential component of suitable owl 

habitat is a consistent abundance of available prey. We know 

much about the diets of California spotted owls, but no studies 

have been done on relations between the availability of prey 

species and the presence, density, or reproductive success of 

California spotted owls. Only one definitive study of this sort 

has been completed on northern spotted owls (Carey et al. 1992). 

Carey et al. found evidence that the area used for foraging by 

northern spotted owls increased as the densities of their primary 

prey species declined. Our ultimate understanding of why some 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
habitats tend to be superior for the owls, while others are only 

marginal, may depend on knowledge of this sort. It remains as a 

major uncertainty. 

In addition, we know little about the ecologies of the prey 

species that dominate the owl's diet in different parts of its 

range. For example, it is generally unknown if the primary prey 

species show cyclic variation in response to either endogenous 

or exogenous factors, or to what extent their populations track 

environmental variation. Furthermore, understanding how prey 

availability differs with changes in habitat structure is essential 

if we are to indirectly manage spotted owl populations by increasing suitable habitat for their prey. 

Addressing Uncertainties 

Additional Inventories 

Deficiencies in our knowledge about the presence and reproductive status of California spotted owls in localities like the 

hardwood zone of the Sierran foothills, or in large parts of 

southern California, can be met with relative ease and speed, and 

at a modest cost compared with other major uncertainties. The 

needed inventory work can follow established protocols and 

draw upon a wealth of prior experience in such endeavors. 

Planning for it should begin immediately to assure that field 

work is completed no later than September 1993. If inventories 

show that a substantial number of owl pairs breed in these 

hardwood habitats, studies will be needed to investigate the 

range and scale of the impacts on owls and their prey that result 

from the variety of current uses of these lands by humans. Such 

studies are almost certainly needed in relation to where migrant 

owls from the conifer zone spend their winters. 

Results of complete inventories in potential habitats throughout the range of the spotted owl in southern California will 

probably require additional modeling efforts to ascertain the 

stability properties of that metapopulation. 

Monitoring 

Population Trends 

We recommend that the direction of the monitoring program for California spotted owls be shifted to demographic 

studies. Using this approach, uncertainties about population trends 

can be resolved more quickly and with a better chance to 

identify cause-and-effect relations than would be the case with 

simple measures of occurrence or breeding by owls at selected 

sample points. 

Existing demographic studies in the Sierra Nevada must 

continue and be enlarged (see recommendations in Chapter 1). It 

is critical that the continuity of these be maintained, because just 

a single year's interruption in the series of years needed to 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

estimate vital demographic rates means that the study essentially 

must begin anew. Generally, 5-6 consecutive years of banding 

all territorial owls in a demographic study area are required to 

attain a reasonable estimate of the vital rates, assuming that the 

study area is large enough that a sufficient number of birds is 

banded and reobserved each year (see Chapter 8). Enlarging the 

studies to include more owls in each year's sample can shorten 

the time needed to attain accurate and precise estimates of 

lambda. For example, if current vital rates remain stable and the 

study is not enlarged, we would need 9 more years of banding in 

the Eldorado Study Area to attain a statistically significant estimate of the population's trend. On the other hand, by enlarging 

the study to double the number of banded owls, we might 

confidently estimate the population trend in only 4-5 more years. 

In any case, demographic studies in these populations need to 

continue well beyond the current drought cycle before we will 

have a chance to fully understand the significance of observed 

birth and death rates, turnover rates, and estimates of lambda. 

The demographic study in the San Bernardino Mountains 

must also be continued, especially considering the modeling 

results indicating that the collapse of this population would 

probably result in the extinction of the entire population of 

spotted owls in southern California (Chapter 9). Banding in the 

San Bernardino study area has been done for 5 years, so the 

study is just getting to a stage that payoffs from its continuation 

become especially significant. In addition, we need to enlarge 

the effort in the San Bernardino to include studies of those owl 

sites where breeding is most consistent and successful. If the 

present drought is causing the significant decline in the population there, sites that still allow successful breeding are certainly 

the ones most in need of protection. They are the "sources" of 

birds that can maintain this population through the "crunch" of 

the present drought (Chapter 9). 

Additional demographic studies are needed to broaden our 

monitoring sample in southern California-one centered on 

Palomar Mountain and one in the southern block of the Los 

Padres NF (Chapter 9). These would also serve to test our 

current hypothesis that these subpopulations depend for their 

maintenance on immigrants from the relatively large population center in the San Gabriel and San Bernardino Mountains. 

If that proves to be the case, the studies would also provide 

quantitative measures of the rates of immigration needed to 

maintain each subpopulation. 

Effects of Logging 

Monitoring the effects of various logging prescriptions on 

the foraging, roosting, and nesting activities of spotted owls is an 

especially high priority. Analyses reported in Chapter 7 provide 

one model of how such studies can be done. If recommendations 

proposed in Chapter 1 are implemented to maintain options for 

spotted owls in the Sierra Nevada during some interim period, 

we consider it essential to monitor owl activities and movements 

before and after logging operations that follow those recommendations. This can be done on a sample basis, following a study 

plan developed jointly by managers and researchers. It will 

require studies of radio-tagged owls, although some concern 

Chapter 2 29 
exists about possible effects of radio tags on breeding and survival of the owls. Using the latest tail-mounted radio transmitters, however, should reduce impacts on the owls compared to 

back-pack models used in earlier studies. 

Research 

Uncertainties About Habitat Capability 

Because an accurate determination of the suitability of owl 

habitat must be linked to the birds' reproductive success (specifically, demographic rates), we believe that advances in dealing with these uncertainties will be most rapid when habitat 

studies are coupled with demographic studies. The specific details of what habitat parameters should be measured, how they 

should be measured, and at what scale, need to be given a 

thorough review by a team of experienced owl biologists, foresters, silviculturists, and others. Implementing these efforts should 

be contingent upon developing a fully interactive Geographic 

Information System (GIS) for each demographic study area, 

with layers for all relevant attributes (for example, vegetation 

types, locations of owl nests and roosts, hydrographic features, 

topography, and so on). 

Uncertainties About How Silvicultural Systems 

Affect Owl Habitat 

Current Land Management Plans (LMPs) of the various 

NFs propose to emphasize silvicultural methods and to attain 

levels of timber removal that will result in significant changes in 

forest structure. Conversion of most of the lands suitable for 

timber production to a regulated state will largely result in 

even-aged plantations and areas with dense, and increasingly 

smaller-diameter stands (Chapter 13). The LMPs make no reference to the natural conditions of forest ecosystems when proposing the timber programs leading to regulated forests, but the 

short- and long-term effects of these changes are unknown. No 

certainty exists that ecosystem processes, whether the population dynamics of spotted owls or the successional trends of 

multi-species plant communities, will be maintained in regulated forests. We propose that the FS move away from its goal of 

regulating forests and develop, instead, the knowledge to manage for more natural stand structures, compositions, and functions. Given incomplete knowledge of natural forest conditions 

and, once known, the uncertainties of how to produce such 

stands, this is clearly an area for high-priority research. 

To the extent that spotted owls nest in stands that retain 

remnants of historical structure (Chapter 5), data available from 

habitat-capability studies can provide a useful starting point. In 

this regard, extensive studies within NPs of stands occupied by 

owls may prove particularly valuable. We propose that quantitative characterizations of the structure and composition of stands 

used by owls be expanded and that these data be made available 

to research silviculturists. 

Uncertainties About the Role of Prey 

First in priority, we need to know whether prey availability 

is a primary causative factor in whether or not the owls breed in 

30 Chapter 2 

a given year. If that proves to be the case, as we believe is likely, 

we will then need to understand better how we might manage 

habitats to assure sufficiently dense (and available) prey populations. As with studies of habitat suitability, studies of prey 

ecology and owl-prey relations would be best done within the 

boundaries of demographic study areas. For this reason, some 

decisions about where to initiate additional demographic studies could be determined according to perceived needs to study 

prey relations. 

Several approaches might be suggested to falsify the null 

hypothesis that the availability of prey is unrelated to the likelihood that a pair of owls will or will not breed in a given year. The 

hypothesis might be tested by measuring densities of key prey 

species in many owl home ranges over some period of time 

(probably a few years) and comparing prey densities available to 

pairs that did and did not breed. We believe this approach is 

unfeasible. We know from experience, for example, that it can 

cost as much as $50,000 to $100,000 per year to obtain reasonably precise estimates of prey densities in just a few owl home 

ranges. A second option might be to supplement artificially the 

prey available to various pairs of owls in a set of pair-wise 

comparisons among owl pairs randomly selected from the population. The latter experiment might be more direct and less costly 

than the former, but it may also prove to be unfeasible for a 

variety of reasons. Other alternatives need to be considered 

before we move forward with research on this question. This 

example points up the need to establish a coordinated effort and 

a rigorous process for deciding what the major uncertainties are 

that we need to study, and what are the most efficient ways to 

reduce those uncertainties. 

Establishing a Process 

Coordinating studies of owl demographics, habitat attributes, 

and prey relations promises to significantly streamline research 

and to speed answers to critical questions needed for a more 

definitive assessment of the owl's status. Simultaneously, we 

should learn more about the spatial scales at which these components of the overall system should be considered and how differences in functional scales can best be dealt with in management 

plans. For example, frequent disturbance events in pristine forests, resulting from fire, insect outbreaks, and wind storms, 

probably affected relatively small areas ranging from 0.5-2 acres 

(Chapters 12 and 13). The result was a relatively fine-grained 

pattern of variability in the vegetation, modified by topography, 

at a landscape scale. Logging and fire suppression have changed 

the scale of frequent disturbance to one of about 5-30 acres. 

Spotted owls, on the other hand, operate in home ranges of 

hundreds of acres (hardwood habitats where woodrats dominate 

the diet) to thousands of acres (conifer habitats where flying 

squirrels dominate the diet) (Chapters 4 and 6). As a result, their 

home ranges generally incorporate a mosaic of stand structures, 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
the distribution of which may determine habitat quality. Finally, 

owl prey populations probably respond to variation in vegetation 

at scales more like those of pristine forests. 

Research Approaches 

The style of research best suited to issues at hand is a fusion 

of two emerging trends in wildlife science. First is an emphasis 

on hypothesis testing (Romesburg 1981, 1989, 1991; Bailey 

1982; MacNab 1983; Eberhardt 1988; Keppie 1990; Murphy 

and Noon 1991; Nudds and Morrison 1991; Sinclair 1991). 

Second is adaptive management (Holling 1978, Walters 1986, 

Walters and Holling 1990). 

Hypothesis Testing 

Experimentation is the essence of the hypothetico-deductive 

method. It may be done in many ways, not all requiring direct 

manipulation of a system under study (Sinclair 1991). It requires 

formulation of null hypotheses, based on application of common 

sense to observed natural phenomena and stated in terms that 

can be falsified. For example, the null hypothesis that the population of spotted owls in the San Bernardino Mountains did not 

decline from 1987 to 1991 was falsified by estimates of demographic rates from the area. Problems with routine experiments 

in wildlife biology are many: the scale of the landscape is often 

immense; variability in data may be too large to reveal treatment 

effects; replication may be difficult or impossible. These challenges are not insurmountable, but they require that careful 

attention be given to experimental design. 

No style of experimental research has been more effective 

than a protocol called "strong inference" (Platt 1964). Strong 

inference is simple in principle. The phenomenon to be explained is described and defined thoroughly. An exhaustive set 

of explanations (hypotheses) is then devised, which constitutes 

the core of the process. All hypotheses must be initially plausible, and they should be as diverse and complete as possible to 

avoid the common pitfall of premature commitment to a single 

explanation (Chamberlin 1965). They must lead to precise, 

falsifiable predictions. This process is facilitated if the hypotheses can be stated as quantitative predicitons. The predictions are 

examined and compared until natural circumstances are found 

that can, if studied, discriminate among hypotheses. 

These circumstances define the conditions of a traditional 

"crucial experiment" (Romesburg 1991). The experiment may 

involve only the taking of a set of measurements in the field 

without causing disturbance; the important feature of the experiment is not that it is manipulative but that it allows alternative 

hypotheses to be compared in a definitive way. In principle, only 

one hypothesis will survive the tests and comparisons. In practice, however, crucial experiments are seldom perfectly unambiguous (Hempel 1965). Their design and conduct come under 

attack. More often, the "exhaustive" set of alternative hypotheses is found to be missing a reasonable new idea. In spite of its 

logical flaws, this style of research has proven to be remarkably 

productive in those sciences, such as chemistry and physics, 

where it is routinely practiced. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

Platt's (1964) paper on strong inference is widely cited and 

equally widely misunderstood. In particular, strong inference is 

not equivalent, as Sinclair (1991) claimed, to the 

hypothetico-deductive method-it is a subset of that method. To 

illustrate the difference, the rejection of the null hypothesis that 

California spotted owls use lakes in their home ranges in proportion to their availability is a reasonable application of the 

hypothetico-deductive method. The fact that it is a trivial null 

hypothesis, the rejection of which does not really advance our 

understanding of the owl's habitat needs, renders this research 

far from strong inference. 

A strong-inference approach to habitat selection would consist first of compiling a list of alternative hypotheses that span 

the range of possible cause-and-effect relations. The only constraint on these hypotheses is that they must be consistent both 

with what is known of owl biology and with previous research in 

animal ecology. This list might contain, for example, a physiological hypothesis (owls choose habitats that minimize thermal 

stress), a foraging hypothesis (owls require a mosaic of habitats 

that support their preferred prey), and a cover hypothesis (owls 

choose habitats that minimize risks of attack by great horned 

owls and goshawks). The hypotheses are not necessarily mutually exclusive, and thus may allow the possibility of multiple 

causation. Some set of predictions from the hypotheses, however, must differ so that all predictions cannot be jointly true 

(Romesburg 1991). 

Implementing tests of these hypotheses would require some 

quantitative assessment of specific attributes of both spotted 

owls and their territories, resulting in sets of measurements 

collected across a range of occupied sites. Further, each hypothesis would need to be formulated in terms of specific predictions 

that discriminate among them. Data used to test the truth of these 

predictions could be collected from either observational 

(nonmanipulative) or manipulative studies, in which the researcher experimentally alters one or more components of the 

system. The design of such studies is difficult and requires much 

creativity, but those sciences in which it is taught and routinely 

practiced profit enormously from the exercise. 

Adaptive Management 

This is an emerging style of management planning that 

provides an intelligent response to uncertainty (Walters and 

Holling 1990). Its essence is the use of management decisions to 

gain knowledge, which may then be used to improve subsequent 

management decisions. In effect, it integrates management with 

research and uses feedback loops to progress from acquired 

knowledge to iteratively improve management paradigms. One 

might respond that this is not new, that the history of environmental management has always been "adaptive." In its precise 

meaning, however, the point of adaptive management is to 

progress from mere trial-and-error learning to a systematic and 

efficient process of acquiring new knowledge. 

Adaptive management offers a major additional benefit 

deriving from the fact that most conflicts over environmental 

issues result as much from uncertainty as from fundamental 

differences in the social valuation of resources. If these uncerChapter 2 31 
tainties are acknowledged and a systematic effort is begun to 

reduce them, most management conflicts tend to moderate. 

Walters (1986, p. 9) suggested that adaptive management 

requires attention to four basic issues: "(1) bounding of management problems in terms of explicit and hidden objectives, practical constraints on action, and the breadth of factors considered in 

policy analysis; (2) representation of existing understanding of 

managed systems in terms of more explicit models of dynamic 

behavior, that spell out assumptions and predictions clearly 

enough so that errors can be detected and used as a basis for 

further learning; (3) representation of uncertainty and its propagation through time in relation to management actions, using 

statistical measures and imaginative identification of alternative 

hypotheses (models) that are consistent with experience but 

might point toward opportunities for improved productivity; (4) 

design of balanced policies that provide for continuing resource 

production while simultaneously probing for better understanding and untested opportunity." 

The construction of models is central to the 

adaptive-management approach. Models are not new to wildlife 

biology, but Walters intended for them to be used in a novel way. 

If one can say that a model-building tradition exists in wildlife 

biology, then it is to build a single model that captures the 

essence of existing data and leads to the formulation of rational 

projections of a population, habitat, or whatever the variable of 

interest. This activity, usually the capstone on a series of empirical studies, is not what Walters intended. He meant for the 

scientific manager to identify areas of uncertainty and then build 

a set of models that both incorporate what is known and allow 

rational evaluation of the consequences of what is not known. 

We contend that adaptive management, in its most elegant 

form, is the application of strong inference to environmental 

problems, with the additional constraint that experimental treatments are limited to management protocols that are at least 

minimally acceptable. That is, most treatments should involve 

some level of logging with positive economic value. In this light, 

adaptive management can be the best hypothetico-deductive 

science applied directly to a management problem. 

The Next Phase 

The question of how to implement an adaptive management 

approach is open to discussion. Holling (1978) and Walters 

(1986) advocated a workshop approach, which has both advantages and disadvantages. Its primary advantage is its contribution toward conflict resolution-if the owl is determined to be in 

trouble, conflicts will probably be intense. When all interested 

parties participate both in formulating the set of models considered as management alternatives and in identifying hypotheses 

to be tested, it strips the mystery from those activities and turns 

resulting products into common property. Most or all participants feel "ownership." 

A problem with relying entirely on a workshop approach is 

that modeling is a difficult process under the best of circumstances, requiring much effort, time, and creativity. A workshop 

environment is far from the best arena-time is short, tensions 

may be high, and participants are often tired and irritable. We 

32 Chapter 2 

contend, however, that a relatively structured "workshop" format is appropriate to formulate a program to proceed with 

management and research on the California spotted owl. 

The process requires a team and a leader; the leader should 

not be affiliated with any of the entities involved. The team 

should include at least a computer scientist with GIS experience, 

a population biologist, a community ecologist, a forest ecologist, 

and a forest products economist. These theoreticians could come 

from the organizations with a stake in the outcome of the plan, 

with the stipulation that no single organization can be allowed to 

dominate the team. In addition, the team should have access to 

other experts, as needed, to deal with various phases of the 

planning. A central office/work facility should be provided, with 

a staff to implement routine duties. We envision seven steps in 

the overall process: 

Step 1: Define Issues and Develop an Agenda 

The initial stage of the effort should bring together 

representatives of all affected parties. Each must attempt to gain a 

clear understanding of the components of the problem and understand both objectives of and constraints on solutions. Existing datasets should be examined to assess their usefulness, and 

unavailable but needed datasets should be identified. 

Step 2: Develop Alternative Models 

This is a brainstorming exercise designed to identify as 

large a set of component processes, and alternative representations of each, as possible. These should be cast in symbolic form 

suitable for computer models. The modeling team must include 

the best available theoreticians in all important aspects of the 

problem. The resulting models should be compared with available data to see if the models are at least consistent with what is 

known. The necessary precision of outputs will almost certainly 

require that alternative models be cast in spatially explicit forms 

and be based upon GIS-derived, landscape descriptions. 

Parameters to which the models appear to be sensitive should 

be identified. 

Step 3: Assess Uncertainty 

This is an intensive workshop exercise to identify uncertainty, both socio-economic and physical-biological. Again, participants should represent all parties with a stake in the issue, and 

they need to be at ease with quantitative reasoning. They should 

be teamed with the model builders from Step 2 to forge a set of 

alternative models that bracket our range of ignorance. 

Step 4: Design Experiments 

Models coming from Step 3 should be tightened and analyzed by the model builders until they yield predictions that lead 

to crucial experiments. To the extent possible, the modeling 

team should focus on predictions that differ qualitatively and 

that do not require precise parameter estimates. Assuming that 

multiple experiments will emerge from this exercise, the participants have an added responsibility to prioritize them in rank 

order, listing first those likely to remove the most uncertainty 

from critical aspects of our ignorance about the owls in relation 

to their environment. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Step 5: Forge a Management Plan 

Representatives of all groups involved in Steps 3 and 4 

should meet to examine the models and the proposed crucial 

experiments. If the experiments seem plausible, the group should 

consider the practical problems of their implementation, determining how they can be done and ensuring that sufficient data 

are collected to discriminate among alternative models. 

Step 6: Conduct Experiments 

This task would fall to various scientists and study teams 

from among the entities cooperating in the effort. Success will 

depend to a major extent on the degree to which the several 

participating entities can cooperate: (1) to assure that funds are 

available on a sustained basis to see each experiment through to 

a logical conclusion; (2) to plan the total set of studies to assure 

that sensible priorities are followed; (3) to coordinate all studies 

to avoid needless duplication; (4) to standardize the data to be 

collected, the methods for collecting them, the forms used to 

record them, the computer filing and operating systems for 

manipulating them, and the methods used to analyze them; (5) to 

assure that political boundaries become as invisible to researchers and their respective affiliations as they are to the owls; and 

(6) to assure that all results obtained are freely and openly 

shared, discussed, and interpreted. The nature of the experiments, and their likely duration and cost, are impossible to 

predict. Ideally they would involve existing manipulations of 

habitats, but this must not be assumed a priori. 

Step 7: Feedback Loops 

Knowledge gained from the experiments and tests of hypotheses (models) feeds back to managment, and the process 

goes through another iteration. 

Completing the Tasks 

It is no more likely that a single pass through this process 

will completely reduce uncertainty than it is that a few laboratory experiments will solve a major scientific question. These 

steps may need to be retraced a second or third time. The 

likelihood is high, however, that the process will identify a 

management solution much faster than current process-oriented 

research. In particular, the likelihood is high that a blend of 

strong inference and adaptive management will distil] the extent 

of our uncertainty about the biology of California spotted owls 

and highlight areas of ignorance that matter the most. 

Conclusion 

The primary barrier to finishing what is needed to remove 

the remaining uncertainty about the status of the California 

spotted owl is administrative, not scientific. Before it can begin, 

much discussion is needed at the highest levels of the organizations involved to discover whether they are prepared, or even 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

legally able, to accept and implement the changes that will be 

needed. For example, if a crucial experiment requires designating as "experimental land" a substantial fraction of a forest-- 

public or private, will support be there for retraining local 

managers and for temporary reorganization of management 

authority? Will permitting processes be re-examined? Will 

citizen and environmental groups refrain from pressuring local 

managers as experiments proceed? If the answers to these 

questions are "no," then the political will is missing for any 

sort of systematic hypothesis testing and adaptive management 

in forest/wildlife issues. 

References 

Bailey, James A. 1982. Implications of "muddling through" for wildlife management. Wildlife Society Bulletin 10:363-369. 

Carey, Andrew B.; Horton, Scott P.; Biswell, Brian L. 1992. Northern spotted 

owls: influence of prey base and landscape character. Ecological Monographs 65:223-250. 

Chamberlin, T. C. 1965. The method of multiple working hypotheses. Science 

148:754-759. 

Eberhardt, L. L. 1988. Testing hypotheses about populations. Journal of Wildlife Management 52:50-56. 

Greenwood, Greg, Range Ecology Specialist, California Department of Forestry and Fire Protection, Sacramento, CA. [Personal communication]. 

February 1992. 

Hempel, Carl G. 1965. Philosophy of natural science. Englewood Cliffs, NJ: 

Prentice Hall; 116 p. 

Holling, C. S. 1978. Adaptive environmental assessment and management. 

London, England: John Wiley and Sons; 377 p. 

Keppie, Daniel M. 1990. To improve graduate student research in wildlife 

education. Wildlife Society Bulletin 18:453-458. 

LaHaye, William S., Research Associate, Wildlife Department, Humboldt 

State Univ., Arcata, CA: [Personal communication]. August 1991. 

Macnab, John. 1983. Wildlife management as scientific experimentation. 

Wildlife Society Bulletin 11:397-401. 

Murphy, Dennis D.; Noon, Barry R. 1991. Coping with uncertainty in wildlife 

biology. Journal of Wildlife Management 55:773-782. 

Nudds, Thomas D.; Morrison, Michael L. 1991. Ten years after "reliable 

knowledge": are we gaining? Journal of Wildlife Management 55:757-760. 

Platt, John R. 1964. Strong inference. Science 146 (3642):347-353. 

Romesburg, H. Charles. 1981. Wildlife science: gaining reliable knowledge. 

Journal of Wildlife Management 45:293-313. 

Romesburg, H. Charles. 1989. More on gaining reliable knowledge: a reply. 

Journal of Wildlife Management 53:1177-1180. 

Romesburg, H. Charles. 1991. On improving the natural resources and environmental sciences. Journal of Wildlife Management 55:744-756. 

Sinclair, A. R. E. 1991. Science and the practice of wildlife management. 

Journal of Wildlife Management 55:767-773. 

Steger, George N., Biological Technician, Wildlife, USDA Forest Service, 

Pacific Southwest Research Station, Fresno, CA. [Personal communication]. February 1992. 

Thomas, Jack Ward; Forsman, Eric D.; Lint, Joseph B.; Meslow, E. Charles; 

Noon, Barry R.; Verner, Jared. 1990. A conservation strategy for the 

northern spotted owl. Washington, DC: U.S. Government Printing Office; 

427 p. 

Walters, Carl J. 1986. Adaptive management of renewable resources. New 

York, NY: Macmillan; 374 p. 

Walters, Carl J.; Holling, C. S. 1990. Large-scale management experiments 

and learning by doing. Ecology 71:2060-2068. 

Chapter 2 33 
Chapter 3 

Background and the Current Management Situation 

for the California Spotted Owl 

Thomas W. Beck and Gordon I. Gould, Jr. 

The administrative history of the California spotted owl is 

closely tied to that of the northern spotted owl. Detailed research 

first began in 1969, with studies on the northern subspecies 

(Forsman 1976). Forest Supervisors in California were first 

alerted in 1972 by the Pacific Southwest Regional (R5) Office of 

the U.S. Department of Agriculture, Forest Service (FS) about 

concern for the owl in Oregon, based on logging-related habitat 

changes. Wildlife biologists were advised to seek information on 

the presence of spotted owls in their respective forests. 

Administrative Events 

1972-1980 

Shortly before passage of the Endangered Species Act (ESA) 

of 1973, the spotted owl was designated by the U.S. Department 

of Interior, Fish and Wildlife Service (FWS) as a "threatened 

species," including the California subspecies. This designation 

was intended to stimulate interest, impart knowledge, and solicit 

information about wildlife that appeared to be endangered. With 

passage of the ESA, however, an official list of Threatened and 

Endangered Wildlife and Plants did not include the spotted owl 

(USDI, FWS 1973). 

In 1973 and 1974, the PSW Region, the California Department of Fish and Game (CDFG), and the U.S. Department of 

Interior, National Park Service (NPS) jointly financed the first 

survey for spotted owls in California. Locations of historical 

observations and selected late-successional forests were chosen 

for surveys. Spotted owls were located on most National Forests 

(NFs) and were found to be much more abundant than previously known, with 159 owl locations documented within the 

range of the California spotted owl (Gould 1974). 

The Oregon Endangered Species Task Force was formed in 

1973, at a time when spotted owls had been located at only about 

100 sites in that state. The Task Force recommended that 300 

acres of old-growth be retained around each northern spotted 

owl location, as interim protection until permanent guidelines 

could be adopted. FS Region 6, Pacific Northwest Region (R6) 

and the Oregon State Office of the U.S. Department of Interior, 

Bureau of Land Management (BLM) rejected the recommendation (Thomas et al. 1990). 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

Concern over the status of the California spotted owl was 

slow in developing compared to that for the northern spotted owl 

in the Pacific Northwest. During the mid-1970s, additional surveys were done on individual Forests and Ranger Districts in the 

Sierra Nevada but not in response to Regional direction or policy. 

Passage of the National Forest Management Act (NFMA) 

in 1976, and regulations pursuant to that Act, established the 

basis for maintaining well-distributed, viable populations of all 

native vertebrate species on NF lands. NFMA also required the 

use of Management Indicator Species (MIS) to guide management for certain wildlife groups or critical habitats. The spotted 

owl was designated as an MIS for wildlife requiring large 

areas of late-seral-stage conifer forests by all NFs in the western 

Sierra Nevada. 

Early in 1977, the Oregon State Director of BLM and the 

Regional Forester for R6 agreed to protect northern spotted owl 

habitat in accordance with interim recommendations of the Oregon Endangered Species Task Force. This included protection 

of habitat where spotted owls and their nest sites had been found. 

Late in 1977, agency administrators agreed to the Oregon Spotted Owl Management Plan, which recommended clusters of 3-6 

pairs, 300-acre core areas for each pair, pairs in a given cluster 

no more than 1 mile apart, and clusters no more than 8-12 miles 

apart, edge-to-edge. The plan was to be implemented through 

the land-management process and individual land-management 

plans for NFs and BLM Districts (Thomas et al. 1990). 

In 1978, the Oregon-Washington Interagency Wildlife Committee replaced the Oregon Endangered Species Task Force with 

the Spotted Owl Subcommittee, to represent both Oregon and 

Washington. In 1979, a Washington Spotted Owl Working Group 

was established, and in 1980 the FS afforded the same protection to spotted owls in Washington that had already been 

provided in Oregon. 

Because of the rapidly growing issue of logging vs. habitat 

management for the northern spotted owl, surveys to locate owls 

increased sharply in 1978 on NFs in Oregon, Washington, and 

northern California, but FS surveys in the range of the California 

spotted owl did not begin in earnest for another 3 years. The 

initiation of systematic surveys in the Sierra Nevada responded 

to the need to verify designated spotted owl territories for land 

management plans, as well as to provide wildlife input to planned 

timber sales. Designated spotted owl territories were later called 

Spotted Owl Habitat Areas, or SOHAs. 

The spotted owl was designated a "Sensitive Species" on 

NFs throughout FS R5 in the late 1970s (Carrier pers. comm.). 

Chapter 3 37 
CDFG, with jurisdictional boundaries almost identical to R5, 

classified both subspecies as "Species of Special Concern" in 

1978 (Remsen 1978). 

1980-1991 

Based on study results by Forsman (1980, 1981), the Spotted Owl Subcommittee revised the Oregon Spotted Owl Management Plan in 1981, recommending that 1000 acres of 

old-growth forest within a 1.5-mile radius of the nest area be 

maintained for each pair. R6 agreed to the new recommendations only to the extent that they would "maintain the option" to 

manage for 1000 acres if further research proved it to be necessary. The BLM in Oregon continued to protect only 300 acres 

for managed pairs (Thomas et al. 1990). 

In early 1981, R5 initiated a meeting on spotted owls that 

brought interested State and Federal agencies together for the 

first time. This included BLM, NPS, California Department of 

Forestry and Fire Protection (CDF), CDFG, and both the administrative and research branches of the FS. The purpose of this 

meeting was both to share information and to discuss development of management policies and research needs. 

FS Regional planning direction for both subspecies of spotted owl in California was issued in June 1981 and closely 

followed the Oregon Spotted Owl Management Plan. The final 

revised direction issued in 1984 included the concept of replacement habitat, explained below. In July 1981, the Regional Office, R5, notified Forests with the California spotted owl to 

provide a strategy for maintaining viability in their Land Management Plan (LMP). This led to the development of designated 

SOHAs in a grid configuration or "network" on each of the 

seven westside Sierra Nevada NFs, and prompted those NFs to 

verify occupancy of the designated sites. 

The Portland Regional Office of the FWS conducted a 

status review of the northern spotted owl in 1981 because of 

concerns about the effects of the decline of old-growth forest 

(USDI, FWS 1982). Although the species was described as 

"vulnerable," the FWS review concluded that the species did not 

meet the listing requirements of the ESA. 

In 1982, the Washington Office of the FS analyzed and 

clarified existing national policy for management of Sensitive 

Species on NF lands. This policy was seen as a visible, preventive strategy with the following key objectives: (1) identify plant 

and animal populations that currently or potentially qualify for 

Federal or State listing as a result of NF management activities, 

and (2) develop a consistent, systematic, biologically sound 

strategy to manage these species and their habitats to provide for 

viable populations so Federal listing as Threatened or Endangered would not be required. 

In cooperation with the BLM, R6 initiated the Old-Growth 

Wildlife Research and Development Program in 1982, which 

entailed field studies of old-growth wildlife species and their 

forest habitats in western Washington and Oregon. 

In 1982, R5 recognized differences in vegetation patterns 

between the Sierra Nevada and Pacific Northwest forests. Considering these differences and the observed use of habitats by 

spotted owls, R5 speculated that home ranges of the California 

38 Chapter 3 

subspecies might be smaller and owls might be using younger 

and more diverse timber stands than in areas where research had 

been done to date. Based on this concern, the FS and CDFG 

planned the first research studies for the California spotted owl. 

The first of these studies began on the Eldorado NF in 1982. 

Throughout 1984, the R5 Regional Office approved the 

final spotted owl habitat networks for proposed LMPs on most 

NFs in northern California and the western Sierra Nevada. The 

policy was to implement Regional standards and guidelines for 

the networks in advance of LMP approvals to avoid losing 

management options. No Regional policy was set, however, for 

management of spotted owls outside of approved networks. 

Some NFs provided protection for 1000-acre "non-network" 

SOHAs to maintain options for full spotted owl management, 

but most provided minimal or no protection. 

In 1985, the Deputy Assistant Secretary of Agriculture 

upheld an appeal against the R6 planning guide. The successful 

appeal contended that the standards and guidelines for northern 

spotted owls were inadequate and that an environmental impact 

statement (EIS) was required. Work on a supplemental EIS (SEIS) 

began in 1985 and was completed in 1988 (USDA, FS 1988). 

The National Audubon Society established a "Blue Ribbon" Advisory Panel in 1985 to review the status of the spotted 

owl in Washington, Oregon, and northern California. The Panel's 

report was issued in 1986, with findings and recommendations 

for the northern spotted owl throughout its range, and for the 

California spotted owl in the Sierra Nevada portion of its range 

(Dawson et al. 1987). Recommendations on numbers, home 

ranges, and distribution relevant to the California spotted owl 

were: (1) Maintain no fewer than 1,500 pairs in the combined 

range of the northern subspecies and the Sierra Nevada portion 

of the California subspecies and discard the earlier view that 500 

individuals could represent an effective breeding population; (2) 

maintain the current distribution with a network system similar 

to that being used by the agencies, but with maximum emphasis 

on having networks comprised of SOHAs with verified reproduction; and (3) provide at least 1,400 acres of late-seral forest 

habitat for each pair home range in the Sierra Nevada. Maintaining the population linkage through Shasta County, where the 

California and northern subspecies come together, was emphasized to be critically important. The Audubon Panel also recommended a well-designed program of intensive and extensive 

research on a sample basis to obtain reasonable estimates of the 

proportion of network home ranges supporting breeding pairs of 

spotted owls and to document reproductive rates in relation to 

population maintenance. 

In 1987, the Old-growth Wildlife Research and Development Program of 1982 was supplemented by the Spotted Owl 

Research, Development and Application Program (the RD&A 

Program) for a period of 5 years. This was a cooperative effort of 

R6, R5, and the Pacific Northwest (PNW) and Pacific Southwest 

(PSW) Research Stations. The new spotted owl RD&A program, funded as a line item by Congress, had a mission to 

develop, apply, and integrate new and improved information 

into guidelines and procedures needed to manage suitable spotted owl habitat to assure maintenance of viable populations in 

California, Oregon, and Washington. Under the auspices of this 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
program, numerous studies of spotted owls, associated habitats, 

and old-growth wildlife species were done in Oregon, Washington, and California (Thomas et al. 1990), but most research 

funds went to studies of the northern spotted owl. The monitoring phase of the RD&A Program included the Sierra Nevada 

portion of the California spotted owl's range. 

In early 1987, the FWS received a petition to list the northern spotted owl as an endangered species under the ESA. A new 

status review was done and the FWS determined that listing was 

not warranted. That decision was litigated by conservation groups 

in 1988 in the Seattle Federal Court. The court ruled that the 

FWS's decision was not biologically based and ordered the 

FWS to reconsider listing (Thomas et al. 1990). 

In December of 1988, the Chief of the FS approved the 

SETS for spotted owls in R6. The selected alternative maintained 

the existing concept of network owl reserves but increased the 

sizes of the SOHAs in Washington and portions of Oregon to 

range from 1,000 acres in southern Oregon to 3,000 acres on the 

Olympic Peninsula (USDA, FS 1988). 

In 1989 the California State Legislature passed AB 1580 to 

improve the analysis of the cumulative impacts of logging. The 

measure directed the CDF to develop a system to better track 

how harvest planning decisions are made, and to develop a 

scientific database on timberland habitats and wildlife species 

(Thomas et al. 1990). 

In response to Federal court action, the FWS initiated an 

other status review of the northern spotted owl in January 1989 

to supplement the 1987 review. That review team concluded that 

the northern spotted owl warranted protection as a Threatened 

Species under the ESA (USDI, FWS 1989). As required under 

Section 7 of the Act, FS and BLM immediately began to confer 

with FWS regarding evaluation of timber sales. A FWS listing 

team began a review in 1989 of the proposal to list the owl. Its 

final recommendation, released in June 1990, was to designate 

the northern spotted owl a Threatened Species throughout its range. 

The listing became official on 23 July 1990 (Thomas et al. 1990). 

An Interagency Agreement among Agency heads of the 

BLM, FS, FWS, and NPS was signed in August 1988; the intent 

of this agreement was to enable these four Federal Agencies to 

work together toward a common goal of population viability for 

the spotted owl throughout its range. Under authority of this 

agreement, the Agency heads acted upon a 1989 recommendation from the FS to establish an Interagency Scientific Commit 

tee (ISC) to address the issue of the northern spotted owl. The 

committee was officially chartered in October 1989 and given 6 

months to "develop a scientifically credible conservation strat 

egy for the northern spotted owl." The ISC's final report was 

released to Agency heads and Congress on 1 April 1990 (Tho 

mas et al. 1990). It concluded that existing management for 

northern spotted owls on NF and BLM lands would lead to 

increasing loss and fragmentation of suitable owl habitat and 

imperil the continued existence of the owl. The ISC recom 

mended an alternative conservation strategy that would desig 

nate large reserves, called Habitat Conservation Areas (HCAs), 

to be well-distributed over the range of the owl. Ideally, each 

HCA should provide sufficient suitable habitat for at least 20 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

pair-sites, with the lands between managed to facilitate -movement (dispersal) of owls among HCAs. 

The implications of the ISC report for management of the 

California spotted owl became apparent immediately. In May of 

1990, the R5 Regional Forester requested assistance from PSW 

Research Station in additional data collection and analysis for 

the California subspecies. Emphasis on Sierra Nevada NFs was 

changed from conducting a third year of RD&A monitoring to 

inventory of presumed owl habitats not previously surveyed. 

These habitats were primarily low-elevation oak woodlands, 

pine-oak forests, and high-elevation red fir forests. 

In April of 1991, R5 and PSW Research Station began a 

cooperative venture with CDF and other State and Federal agencies to assess current information on the California spotted owl. 

A Steering Committee with representatives from various State 

and Federal agencies and other interested parties was formed to 

oversee the entire effort (Chapter 1). The Steering Committee 

established two teams. A Technical Assessment Team was designated to synthesize and interpret all current information available on the California spotted owl, and to evaluate various 

management alternatives for the subspecies. A Policy-Implementation Team was designated to evaluate impacts of the Technical Assessment Team's recommendations, to determine whether 

they fall within existing policies and regulations, and to propose 

least-cost/low-impact ways to implement them. The Steering 

Committee will make final recommendations to the responsible 

agency heads based on reports from the two teams. This report is 

the product of the Technical Assessment Team. 

Current Management 

The range of the California spotted owl is considered to 

include the southern Cascades south of the Pit River in Shasta 

County, the entire Sierra Nevada province of California, all 

mountainous regions of the southern California province, and 

the central Coast Ranges at least as far north as Monterey 

County (Grinnell and Miller 1944, Gould 1977). Within this 

range, the owl occurs on 13 NFs, the Lake Tahoe Basin Management Unit (LTBMU) administered by the FS, four National 

Parks (NPs), several State Parks and Forests, private timber 

lands, scattered BLM lands, and other minor ownerships (fig. 4A, 

4B, and 4C). This includes a portion of the Toiyabe NF, which is 

administratively a part of the Intermountain Region of the FS. 

One of two major subdivisions of the total California spot 

ted owl population occupies the southern Cascades and the 

Sierra Nevada, an area roughly 400 miles long by 50 miles wide. 

The other occupies mountains of southern California, primarily 

the San Bernardino Mountains and isolated mountains south 

ward nearly to the Mexican border, the San Gabriel Mountains, 

the Tehachapi and nearby mountains westward to the coast, and 

the central coastal mountains (fig. 9A). 

Chapter 3 39 
Sierra Nevada Province 

The Sierra Nevada Province, as used here, includes the 

conifer forests and foothill riparian/hardwood forests of the 

Sierra Nevada and the southern end of the Cascade Mountains. 

Most known California spotted owls and suitable habitat occur 

on public lands on the west slope of the Sierra Nevada, primarily 

in seven NFs and in Yosemite and Sequoia/Kings Canyon NPs. 

Spotted owls also occur on scattered BLM lands along the 

Sierran foothills, but few surveys have been done on those lands. 

Large blocks and scattered parcels of private timberland occur 

within most NFs within the range of the owl in the Sierra 

Nevada. Recent surveys by timber companies indicate considerable spotted owl use on some of those lands. 

Since 1973, agency and public concern for the status of the 

California spotted owl has been in regard to timber management 

on the seven westside Sierra NFs and on private industrial 

timberlands. Even though the NPs experience high recreation 

impacts in local areas, NP management has not been an issue 

because the emphasis of the NPS generally is believed to be 

compatible with maintaining habitats needed by the owls. 

The Sierra Nevada owl population is contiguous throughout 

the mountain range's 400-mile length but is probably poorly linked 

to the southern California population through the Tehachapi 

Mountains and the Liebre/Sawmill area east of Interstate Highway 5. The California spotted owl connects to the northern 

spotted owl in eastern Shasta County. 

Forest Service, Pacific Southwest Region (R5) 

From north to south, the Lassen, Plumas, Tahoe, Eldorado, 

Stanislaus, Sierra, and Sequoia NFs in the Sierra Nevada all have 

major populations of California spotted owls. These NFs have a 

total land base of 6,978,900 acres, about 73 percent of which 

(5,093,600 acres) is forested. About 50 percent of this land base, 

or 3.5 million acres, are forest types considered current or potential spotted owl habitat (USDA, FS 1986b, 1988a,1988c,1988e, 

1990, 1991a, 1991b). The major forest types comprising known 

and potential habitat are Sierran mixed-conifer, red fir, ponderosa pine/hardwood (montane hardwood), eastside pine, and 

foothill riparian/hardwood (Chapters 5 and 6) (descriptions in 

Chapter 1). Mixed-conifer is the most abundant type with the 

most owl sites; the others are less abundant and have far fewer 

owl occurrences (table 3A). 

The LMP directions for Sierran Forests resulted in the 

designation of 264 SOHAs in approved or draft Forest LMPs 

(Lassen 40, Plumas 54, Tahoe 33, Eldorado 32, Stanislaus 36, 

Sierra 29, and Sequoia 40) (table 3B). Of the 264, 249 are on 

lands suitable and otherwise available for timber production. 

The other 15 are on lands unavailable for timber production, 

either because of Congressional classification (Wilderness or 

Wild Rivers) or because of other Forest LMP allocations. The 

total land allocation for these 264 SOHAs is about 454,000 

acres, of which about 110,000 would be managed under low-yield, 

even- or uneven-aged management (USDA, FS 1986b, 1988a, 

1988c, 1988e, 1990, 1991a, 1991b). 

The management direction for SOHAs is to maintain at 

least 1,000 acres of suitable spotted owl habitat within a 1.5-mile 

radius of the known or potential spotted owl nest site. Suitable 

habitat is described as mature timber stands having (1) multi-storied 

canopies with 70 percent or greater total cover, (2) 40 percent or 

more of the total canopy in trees 21 inches or more in d.b.h., and 

(3) well-developed decadence (USDA, FS 1984). 

Further, the direction calls for replacement of the suitable 

habitat over time, based on the certainty that forest stands unTable 3A-Known California spotted owl sites, by habitat type on National Forests and National Parks in the Sierra Nevada, 1987-1991. 

 Ponderosa pine/ Number Breeding 

 Eastside Red Mixed- montane Riparian/ sites pairs 

Location pine fir conifer hardwood hardwood sampled1

 found2

National Forests 

Lassen 8 97 105 36 

 Plumas 3 9 195 15 222 56 

 Tahoe 11 135 8 154 38 

 Eldorado 10 149 1 160 53 

 Stanislaus 11 97 10 118 26 

 Sierra 41 86 23 10 160 25 

 Sequoia 3 9 95 12 9 128 22 

 Inyo 1 1 1 

 Lake Tahoe Basin 

 Management Unit 2 1 3 

National Parks 

Yosemite 9 46 1 56 6 

 Sequoia/Kings Canyon 3 58 9 60 12 

Grand total 6 114 959 79 19 1,177 275 

Percent <1 10 81 7 2 100

1

 Data based on NF and NP information for all known owl sites where habitat information was available. (This included some single owl sightings not 

considered to be "owl sites.") Table 3B should be used for official, total count of spotted owl sites, as it is based on interpretation of NF data along with 

information from private lands to avoid duplication. 2

 Data on sites where young were produced provided by NFs and NPs. 

40 Chapter 3 USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
dergo natural events such as fire, insect and disease attack, and 

windthrow that may render them unsuitable for spotted owls. 

The Forests were given a choice of three strategies for replacement: (1) no scheduled harvest, (2) uneven-aged management 

and (3) even-aged management. An option of no scheduled 

harvest requires that 650 additional acres of suitable or 

near-suitable habitat be designated to supplement the 1,000 

acres, for a total of 1,650 acres for each SOHA. SOHAs selected 

for uneven-aged management must have replacement habitat of 

1,000 acres and all 2,000 acres can be managed using selection 

harvests and an implied rotation period of 300 years. SOHAs 

selected for even-aged management must have 1,650 acres of 

replacement habitat, and the entire 2,650-acre SOHA is to be 

managed using clearcut and shelterwood prescriptions and an 

implied rotation of 240 years (USDA, FS 1984). 

Most NFs have already designated management of their 

SOHAs, but a few will designate the management system as 

timber sales are planned. Essentially all SOHAs on the Lassen, 

Eldorado, Stanislaus, Sierra, and Sequoia NFs will be managed 

as "no scheduled harvest." SOHAs on the other NFs will be a 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

combination of management strategies. The Inyo NF and LTBMU 

have the same direction as westside NFs, but they were not 

required to establish SOHA networks because the LTBMU has 

no regulated timber program and the Inyo timber program is 

small. Both units have 1-3 spotted owl sites, and essentially all 

suitable habitat at these sites is protected. The Toiyabe NF, 

which is adjacent to the Inyo NF and LTBMU on the east slope 

of the Sierra Nevada is part of Region 4 of the FS. It has only 1-3 

spotted owl sites and essentially all suitable habitat is protected 

(Coburn pers. comm.) 

From 1981 to 1987, most spotted owl surveys done on NFs 

in the Sierra Nevada focused on verification of pairs in network 

SOHAs. Prior to 1987, this was generally done using a three-visit 

method without rigid survey requirements. In 1987, however, 

the Spotted Owl RD&A Program established strict protocols for 

monitoring SOHAs in R5 and R6. These protocols require up to 

six visits, methodical coverage of the SOHA and special 

nest-detection techniques that markedly increased the informa 

tion acquired. In 1988 and 1989, NFs in the western Sierra 

Nevada participated in the RD&A Program by monitoring ranChapter 3 41 

Table 3B-Estimated numbers of California spotted owl sites in the Sierra Nevada, by ownership. 

 Total known sites Sites in 

 (known pairs) SOHAs reserved areas 

 Estimated Total per Number Total 

 additional National with number of 1970- 1987- 

Land 1970-91 1987-91 sites Forest pairs1 pairs2 1991 1991 

Federal Lands 

National Forests 

Lassen 111 (73) 111 (73) 26-31 40 36 42 23 1 

 Plumas 228 (113) 228 (112) 38-50 54 39 46 5 3 4 

 Tahoe 150 (83) 118 (75) 46-57 33 28 33 2 2 

 Eldorado 180 (125) 177 (123) 10-21 32 28 41 0 0 

 Stanisalus 131 (47) 118 (49) 15-25 36 23 23 3 3 

 Sierra 175 (86) 157 (81) 50-52 29 16 20 153 12 

 Sequoia 127 (77) 114 (71) 32-41 40 27 31 22 3 17 

 Lake Tahoe Basin 

 Management Unit 3 (1) 2 (1) ? 0 0 0 0 0 

 Toiyabe 2 (0) 0 (0) ? 0 0 0 0 0 

 Inyo 5 (3) 3 (1) 0 0 0 0 2 1 

 Subtotal 1112 (608) 1028 (586) 217-277 264 197 236 51 40 

National Parks 

Lassen 5 (2) 4 (2) ? 5 4 

 Yosemite 56 (17) 56 (16) 26-41 56 56 

 Sequoia/Kings Canyon 60 (42) 60 (41) 20-22 60 60 

 Subtotal 121 (61) 120 (59) 46-63 121 120 

Bureau of Land Management 

All Lands 6 (3) 1 (1) ? 0 0 

 Total Federal 1239 (672) 1149 (646) 263-340 172 160 

State of California 

State Parks 1 (1) 1 (1) ? 1 1 

Other State 2 (1) 2 (0) ? 1 1 

 Private Lands 

Industrial forests 58 (32) 49 (29) ? 

Other private 67 (26) 49 (21) ? 

Total Sierra Nevada 1367 (732) 1250 (697) 174 162 

1

 Number of SOHAs with at least one pair. 

2

 Total number of pairs in all SOHAs (counting all pairs in each SOHA). 

3

 Indicates that FS data were augmented by information from CDFG's database. 
dourly selected SOHAs and Random Sample Areas (RSAs). 

Seventy-two SOHAs and 86 RSAs were selected as part of the 

sample in the Sierra Nevada. Verification of other SOHAs with 

little or no occupancy history was also made a high priority 

during this period. Many of those other SOHAs, especially at 

remote sites, had not been surveyed previously. 

In 1988, RD&A monitoring of RSAs began to add information to a meager database of owl locations outside network 

SOHAs. In 1989, RS direction specified that field surveys be 

done for the preparation of biological evaluations for spotted 

owls in proposed timber sales. Surveys in 1989 and 1990 included improved coverage of project areas, similar to the RD&A 

monitoring method, but they required only three visits. Because 

of an extended drought affecting the Sierra Nevada, most NFs 

have had extensive insect mortality among conifers in the last 

few years, resulting in a much-expanded salvage sale program 

that involved large portions of the NFs. Spotted owl surveys for 

proposed green and salvage sales since 1989 have covered much 

more land outside SOHAs, and to a higher standard, than all 

previous surveys combined. In 1991 the protocol for project 

surveys for spotted owls was changed to match that used for the 

northern spotted owl. This entailed six visits with longer time 

periods between visits, ensuring a higher likelihood of detecting 

owls that might be present. 

Protecting owl habitat in network SOHAs has been the 

policy since 1982 except where natural events have impacted the 

habitat to the extent that management actions to benefit the owl 

are justified. Direction for management of, spotted owl habitat 

outside network SOHAs, however, has not been as static. Sites 

outside network SOHAs received little or no protection before 

1989. FS direction in 1989 on field surveys and biological 

evaluations for owl sites outside network SOHAs created concern and controversy in the NFs. Most Forests were hurrying to 

complete LMPs based on Regional Guidelines for networks 

intended to ensure viability. Emerging research on the northern 

and California subspecies cast doubt on the adequacy of that 

approach, however, and many biologists preparing biological 

evaluations concluded that many non-SOHA habitat areas might 

be needed to maintain viability of the California spotted owl. 

Release of the ISC report (Thomas et al. 1990) further eroded 

confidence in the efficiency of the SOHA network. 

Protection was given to non-network spotted owls on a 

case-by-case basis in 1989 and 1990. Generally, more protection 

was given to pairs and nest sites than to single owls, but policies 

varied among Forests, Districts, and cases. Where extra measures were afforded, they usually involved habitat protection 

ranging from 40 to 300 acres and exclusion of sale-related 

activities between 1 March and 30 August for pairs known to be 

nesting. For insect salvage sales, protection entailed snag retention in the protected zone comparable to network SOHA standards. Occasionally, similar protection was given to some single 

birds where pairs were suspected. 

In 1991, commensurate with higher standards for field surveys, RS established interim policy for evaluating and mitigating 

effects of timber sales on spotted owls not otherwise protected in 

the Sierra Nevada. The direction to NFs was to see that projects 

did not directly or indirectly affect spotted owls by removing 

42 Chapter 3 

nesting or foraging habitat. This involved the use of home-range 

study data on the California spotted owl to estimate the territorial 

needs of known and potential owl pairs. The objective of the 

policy was to assure a full range of options for managing spotted 

owls by maintaining the population at current levels until the 

agencies could consider new directions based on recommendations of the Steering Committee. 

Since 1970 biologists have identified California spotted 

owls at 1,112 sites (an area with either a pair or a single where a 

pair could be supported-see glossary) in NFs in the Sierra 

Nevada province. In the last 5 years (1987-1991) 586 pairs have 

been verified at 1,028 of these sites (table 3B). The NFs estimate 

a potential for approximately 250 more sites. Of the 264 network 

SOHAs identified in the LMPs that are managed exclusively for 

California spotted owls, pairs have been verified at 197 sites, 

including 107 sites where reproduction has been documented. 

National Park System, Western Region 

Four NPs are located in the southern Cascades and Sierra 

Nevada, within the range of the California spotted owl. Sequoia/ 

Kings Canyon NPs, Yosemite NP, and Lassen Volcanic NP 

have land bases of 863,710 acres, 749,031 acres, and 106,298 

acres, respectively, for a total of 1,719,039 acres. Thirty-six 

percent (315,933 acres) of Sequoia/Kings Canyon NPs is forested and within the elevational range of the owl, and 15 percent 

(133,561 acres) is considered suitable for foraging or nesting. In 

Yosemite NP, 75 percent (564,500 acres) is forested, and 35 

percent (263,126 acres) is considered suitable for foraging or 

nesting by owls (Steger pers. comm.). Lassen NP is approximately 94 percent (100,100 acres) forested, but few surveys 

have been done there; about 60 percent (64,600 acres) of that is 

in yellow pine and fir forests (Husari 1990), and may be suitable 

spotted owl habitat. 

The NPS manages the Parks to emphasize preservation and 

maintenance of natural communities and ecological processes. 

Their policy states that "Natural processes will be managed with 

a concern for fundamental ecological processes as well as for 

individual species and features. Managers and resource specialists will not attempt solely to preserve individual species (except 

threatened or endangered species) or individual natural processes; rather, they will try to maintain all the components and 

processes of naturally evolving park ecosystems, including the 

natural abundance, diversity, and ecological integrity of the 

plants and animals" (USDI, NPS 1988). 

In addition, the NPS "will identify all state and locally listed 

threatened, endangered, rare, declining, sensitive, or candidate 

species that are native to and present in the parks, and their 

critical habitats. These species and their habitats are considered 

in NPS planning and management. Based on an analysis of the 

status of state and locally listed species throughout their native 

ranges and throughout the national park system, the National 

Park Service may choose to control access to critical habitats or 

to conduct active management programs similar to activities 

conducted to perpetuate the natural distribution and abundance 

of federally listed species." 

The spotted owl is recognized as a special status species by 

the NPS, but specific management direction has not been develUSDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
oped. Resource management plans for the NPs will be revised as 

each unit determines what special management, if any, is needed 

(van Wagtendonk pers. comm.). 

Known owl sites since 1986 total four in Lassen Volcanic 

NP, 56 in Yosemite NP (Roberts et al. 1988), and 60 in Sequoia/ 

Kings Canyon NPs (Roberts 1989, Steger pers. comm.) (table 

3B). We estimate that the NPs may contain owls at another 55 

sites. Most of the information for Yosemite and Sequoia/Kings 

Canyon NPs was obtained by the CDFG and PSW Research 

Station in special surveys and studies done since 1988. Yosemite 

and Sequoia/Kings Canyon NPs are considered critical by the 

CDFG and FS as reserves for maintaining population viability and 

for movement by spotted owls between NFs in the Sierra Nevada. 

Bureau of Land Management, California Office 

The BLM administers scattered public lands along the foothills and lower slopes of the southern Cascades, Sierra Nevada, 

Transverse Ranges, central Coast Ranges, and southern California mountain ranges. Some of these support forest, woodland, 

and foothill riparian/hardwood habitats that are potentially suitable for California spotted owls. Commonly, potential owl habitat on BLM land occurs in small tracts (less than a section) and 

may be located adjacent to NFs or intermingled with private 

timberlands. Along the Merced, Tuolumne, American, Yuba, 

Stanislaus, and Mokelumne Rivers, however, their lands are 

concentrated in blocks of several thousand acres (Saslaw 1991). 

In the northern Resource Areas, forest management includes green timber sales, thinning for timber stand improvement, fire and insect salvage, and forest sanitation. Forested 

lands of the BLM in California are managed primarily under 

uneven-aged prescriptions that remove 40 to 60 percent of the 

volume and up to 30 percent of the stems. Between 1981 and 

1990, 25,691 acres were logged under a partial-cut prescription, 

and 238 acres (1 percent) were logged as clearcuts, generally 

between 2 and 4 acres in size. Many unharvested locations were 

within riparian buffer areas, usually 100 feet on each side of the 

stream, and may be within special recreation management areas 

along major streams. Future management in the Folsom Resource Area is expected to be custodial, with forest management 

prescriptions designed to meet wildlife and vegetation objectives. Emphasis will be on managing for old-growth, special 

wood products, and stand maintenance. Logging will be directed 

to fire and insect salvage and forest sanitation. Forest management on the Redding, Alturas, and Eagle Lake Resource Areas 

will continue to use uneven-aged methods. 

Potential suitable habitat has been mapped based on BLM 

forest operation inventories, Timber Production Capability Classification maps, CDF oak woodland maps, BLM vegetation 

surveys, and records of owls from the CDFG and nearby NF 

lands. About 68,500 acres of BLM land are considered to be 

potential habitat for the California spotted owl, based on habitats 

they are known to use elsewhere in the Sierra Nevada (Saslaw 

1991) (table 3C). 

Inventories for California spotted owls on BLM lands were 

initiated in 1989 for forest management or land exchange proposals that could potentially affect spotted owl habitat. Because 

the BLM has only recently begun systematic inventories for 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

spotted owls, information on their distribution and numbers is 

unavailable. This precludes an estimate of the number of California spotted owls inhabiting BLM lands. They have been 

observed at six sites on BLM lands in the Sierra Nevada, however, and we believe there is potential for more. 

California State Parks 

Seven units of the California Department of Parks and 

Recreation with known or potential spotted owls occur in the 

Sierra Nevada. Management on State Park (SP) lands is similar 

to that of the NPs. All wildlife and plants are protected and 

sensitive species receive special care. Most units are designated 

as State Parks (SPs) and State Historical Parks (SHPs), with a 

total area of 16,580 acres. The SPs with the most potential for 

spotted owl are Plumas-Eureka SP with about 3,000 acres of 

conifer forest, Malakov Diggins SHP with 1,500 acres of conifer 

forest, South Yuba River Project with 1,500 acres of conifer and 

oak habitats, and Calaveras Big Trees SP with 5,750 acres, 

mostly in old-growth conifer forest (Bakken pers. comm.). Spotted owls have been found on adjacent NFs and Calaveras Big 

Trees SP (2 pairs), Malakov Diggins SHP (single owl) and 

Growers Hot Springs SP (single owl). Surveys, none of which 

are recent, have been conducted only at Calaveras Big Trees and 

Growers Hot Springs SPs and the potential exists for owls at the 

other larger units. 

California State Forests 

Mountain Home State Forest (SF) and Latour SF are 

State-owned forests managed by the CDF for demonstration of 

forestry practices and to support cooperative research with other 

agencies. Uneven-aged silviculture is featured in both SFs, 

including both single-tree and group-selection methods on a 

sustained yield basis. Other multiple uses are also emphasized, 

primarily recreation. Known owl activity centers would be 

protected from planned projects through review and advice 

from the CDFG. 

Located in Tulare County, Mountain Home SF includes 

4,807 acres, with Sierran mixed-conifer and giant sequoia stands 

that provide potential owl habitat on the entire property. About 

960 acres of old-growth forest remain. Surveys have been done 

only around the edges of this SF by FS biologists, but they 

provide evidence of use by spotted owls. Probably two or three 

pairs occur there (Dulitz pers. comm.). 

Table 3C--Potential suitable habitat for California 

spotted owls on Bureau of Land Management lands. 

BLM District 

Resource Area Acres 

Susanville 

 Alturas 13,337 

 Eagle Lake 18,936 

Ukiah 

 Redding 12,250 

Bakersfield 

 Folsom 24,000 

Total 68,523 

Chapter 3 43 
Latour SF is located in Shasta County and encompasses 

9,033 acres. It was surveyed for spotted owls in 1990 and 1991, 

with one pair discovered in 1990 (McNamara pers. comm.). This 

SF has 7,000 acres of mixed-conifer and fir forest that are 

potential owl habitat, but only 2,000 acres have significant large-tree 

components (6+ trees per acre at least 32 inches in d.b.h.). 

University of California 

Blodgett Forest Research Station, located in Eldorado County 

within the Eldorado NF, is a 3,000-acre facility owned and 

operated by the University of California, Berkeley. It provides a 

setting for research on all aspects of forest management in Sierra 

Nevada mixed-conifer forests, especially research done as part 

of the forestry program at the University. Blodgett Forest has 

approximately 2,000 acres of suitable habitat for California 

spotted owls and typically one or two pairs nest in the area 

(Heald pers. comm.). 

Whitaker Forest is a 320-acre unit located between the 

Sequoia NF and the General Grant area of Kings Canyon NP. 

Most of it is suitable owl habitat including giant sequoia stands. 

The property is used for research, with emphasis on sequoia 

natural history (Gasser pers. comm.). This unit has a long history 

of spotted owl observations and has supported at least a single 

owl over the last 5 years. 

Private Lands 

Private lands in the Sierra Nevada that are classified as 

timberland and have the potential to grow forest habitats used by 

California spotted owls encompass about 2.4 million acres, much 

of which is currently in suitable condition. Of this, 1.45 million 

acres are owned by industrial private timber companies (IPs) and 

0.96 million acres are owned by miscellaneous private parties 

(MPs) not primarily engaged in commercial timber production 

(Colclasure et al. 1986, Hiserote et al. 1986, Lloyd et al. 1986) 

(table 3D). The latter group includes both large landowners, 

such as utilities and water districts, and many small landowners. 

NFs in the Sierra Nevada include approximately 1.4 million 

acres of private land within their administrative boundaries 

(USDA, FS 1986b, 1988a, 1988c, 1988e, 1990, 1991a, 1991b) 

(table 3E). The other 1 million acres of IP lands are outside of 

NF boundaries. NF inholdings are much greater in extent in 

northern NFs (especially the Lassen, Plumas, and Tahoe) than in 

the southern Sierra Nevada. The Tahoe NF, with about 375,000 

acres, includes the most private inholdings, and the Sequoia NF 

(with about 54,000 acres) has the least. 

The spatial arrangement of private lands within NFs varies. 

Much of the private lands within the boundaries of the Lassen, 

Plumas and Stanislaus NFs tend to be in contiguous blocks, 

leaving NF lands also fairly contiguous. Most private lands on 

the Tahoe NF are in "checkerboard" ownership-alternating 

sections of private and NF ownership-and the Eldorado NF has 

a combination of checkerboard and contiguous block patterns. 

The checkerboard pattern is a vestige of Federal land policies 

used to stimulate development of the Nation's railroad system 

prior to establishment of the NFs. The Sierra and Sequoia NFs 

have relatively little interior private lands, and the four NPs have 

negligible amounts. 

Except for historical railroad holdings, most lands belonging to IP companies were acquired with consideration for growth 

and yield potential. Therefore, they occupy most of the best 

tree-growing lands in the Sierra Nevada. Most IP timberlands 

are at middle elevations, in the mixed-conifer zone, but some are 

higher and farther east, in red fir and eastside pine zones. Almost 

all are within the elevation and geographic ranges of the California spotted owl. About 12 major companies have relatively large 

holdings in timberlands of the Sierra Nevada with eight owning 

at least 75,000 acres (Swing pers. comm.) Table 3D shows the 

total amount of IP and MP timberlands by county. 

Systematic surveys for owls have not been done on most 

private lands, although several timber companies conducted 

surveys, using established FS protocols, starting in 1989, 1990, 

or 1991. Locations of owls found are not always available from 

timber companies, but it appears that some IP lands with a long 

Table 3E---Approximate acres of private lands within 

National Forest boundaries (data from Forest LMPs as 

listed in references) 

. 

National 

 Forest Private land 

 Lassen 230,000 

 Plumas 226,000 

 Tahoe 375,000 

 Eldorado 190,000 

 Stanislaus 195,000 

 Sierra 109,000 

 Sequoia 54,000 

Total 1,379,000 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

Table 3D-Acres of forest lands owned by commercial timber industries and 

miscellaneous private parties in the Sierra Nevada, by county, within the range 

of the California spotted owl.1 

 Forest Miscellaneous 

County industry private 

Shasta 194,000 54,000 

Lassen 267,000 42,000 

Tehama 120,000 19,000 

Plumas 215,000 82,000 

Butte 135,000 92,000 

Yuba 26,000 23,000 

Sierra 49,000 44,000 

Nevada 60,000 137,000 

Placer 95,000 79,000 

El Dorado 121,000 117,000 

Amador 25,000 31,000 

Calaveras 68,000 71,000 

Tuolumne 60,000 50,000 

Mariposa 1,000 40,000 

Madera 5,000 12,000 

Fresno 10,000 22,000 

Tulare Trace 16,000 

Kern 0 26,000 

Total 1,451,000 957,000 

1

 From Colctasure et a1.1986, Hiserote et al. 1986, and Lloyd et al. 1986. 

44 Chapter 3 
history of logging have a substantial number of owls, comparable in density to adjacent NF lands. We also know from one 

study that some IP lands with a history of intensive logging lack 

nesting owls, even though nest sites occur in adjoining NF lands 

(Bias and Gutiérrez 1988). 

The public has recently attempted to pass forestry-reform 

initiatives for California, and the Legislature and Board of Forestry are also moving to provide new rules for logging on private 

lands. Emergency Regulations were recently in effect for IP 

timberlands in California, with the expectation they would be 

replaced with permanent policies. These policies will probably 

include requirements for sustained yield, less clearcutting, riparian protection, and special requirements for older forest stands. 

Although we cannot now assess the effects of new forestry 

regulations on the capability of private lands to support spotted 

owls, we believe those regulations will be more beneficial than 

past policies. 

At the north end of the Sierra Nevada, a relatively continuous block of private timberlands in eastern Shasta County provides the habitat linkage for movement between populations by 

both northern and California spotted owls. Maintaining this 

connection is considered to be critical to the long-term conservation of both subspecies of spotted owls (Dawson et al. 1987, 

Thomas et al. 1990). 

Owls are known to inhabit dense stands of hardwoods along 

stream channels at low elevations on the western edge of the 

Sierra and Sequoia NFs. These habitats resemble, in most respects, habitats used by many spotted owls in southern California. Similar habitats, with sparse stands of digger or ponderosa 

pine over denser stands of mixed hardwoods, occur along riparian zones west of NF boundaries from the Stanislaus NF north. 

Generally, these areas are not classified as commercial timberland. Livestock grazing has been the primary activity in these 

low-elevation habitats in the past, and type conversions, firewood cutting, and logging in and adjacent to riparian zones have 

also occurred. An increase in residential developments in the 

Sierra Nevada foothills is now impacting many of these potential 

owl habitats. The combination of residential development and 

indirectly related increased fire potential represent growing threats 

to habitat for both year-round and wintering spotted owls (Chapter 13). These areas have not been adequately surveyed for owls, 

although we suspect that they support many pairs. 

Over the last 20 years, spotted owls have been located at 58 

sites on IP lands and at 67 sites on MP lands in the Sierra 

Nevada. Twenty-nine and 21 pairs have been verified on IP and 

MP lands, respectively, in the last five years (tables 3B and 3F). 

There is no estimate of the potential number of spotted owl sites 

on private lands because of the small percent of those lands 

which have been surveyed and the lack of specific habitat information which could be used to estimate the distribution and 

numbers of spotted owls in lower elevation habitat types. We 

expect that these areas contain many owl sites but we estimate, 

based on the gross mapping of hardwood types, that number to be 

no more than one-third the total number of sites in conifer types. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

Areas of Concern: Sierra Nevada 

Spotted owl distribution in the Sierra Nevada Province is 

characterized by its continuity and relatively uniform density 

(fig. 4B). The ability of existing habitat to support owls does not 

appear to be a major problem except at the peripheries of the 

subspecies' range. This condition is normal for animal populations, however, and is not a concern unless the range begins to 

shrink. Five conditions give rise to some concern for the integrity of the California spotted owl's range in the Sierra Nevada: 

(1) bottlenecks in the distribution of habitat or owl populations; 

(2) gaps in the known distribution of owls; (3) locally isolated 

populations; (4) highly fragmented habitat; and (5) areas of low 

crude density of spotted owls. Areas occur throughout the length 

of the Sierra Nevada where one or more of these conditions 

currently limit the owl population; it might be of critical concern 

if conditions were exacerbated, or if they limit our knowledge of 

the owls there. This might lead to erroneous or more risky 

management decisions (fig. 3A, table 3G). These conditions 

may be caused solely or jointly by fire, land-ownership patterns, 

natural or human-caused fragmentation of suitable habitat, and 

by natural geographic features that control vegetation patterns. 

Rather than reflecting current negative effects on spotted owls, 

areas of concern identified in figure 3A simply indicate potential 

areas where future problems may be greatest if the owl's status 

in the Sierra Nevada were to deteriorate. 

Areas A and B (fig. 3A) are bottlenecks in distribution 

where even relatively small losses of habitat could sever the 

interchange between adjacent populations of owls. In Area A, 

this would decrease the likelihood of interchange between the 

California and the northern spotted owl populations. This area of 

concern was identified for its importance by the Audubon Society's 

Spotted Owl Advisory Panel (Dawson et al. 1987) and by the 

Interagency Scientific Committee for the northern spotted owl 

(Thomas et al. 1990). 

Areas A, B, 1, 4, 5, 7, and 8 are characterized by habitat 

fragmentation that decreases the density of. owl pairs, makes 

successful dispersal more difficult, and reduces the likelihood of 

quick replacement of owls in vacated habitat. 

Area 2 is a gap in the known distribution of spotted owls 

from the western to the eastern edge of the owl's range in the 

northern Sierra Nevada. If few birds and little habitat exist in this 

area, north-south dispersal could be impeded. This area and 

others (for example, areas 4 and 5) where few surveys have been 

done are, however, capable of producing owl habitat and may 

not be breaks in distribution. 

Areas 5 and 6 also have low population densities. As in 

areas with fragmented habitat, low densities make dispersal more 

difficult and less likely to result in reoccupation of vacated sites. 

Area 8 is characterized, in part, by small isolated populations that are more vulnerable to extinction by local stochastic or 

catastrophic events. 

Chapter 3 45 
Table 3F-Estimated numbers of owl sites (confirmed pairs) on private lands in the Sierra Nevada, by county. 

Industrial lands Other private 

County 1970-1991 1987-1991 1970-1991 1987-1991 

Shasta 5 (0) 5 (0) 4 (0) 3 (0) 

Lassen 0 0 3 (1) 3 (1) 

Tehama 6 (4) 6 (4) 0 0 

Plumas 8 (5) 7 (4) 10 (4) 10 (5) 

Butte 9 (0) 5 (0) 7 (4) 5 (4) 

Sierra 3 (2) 3 (2) 2 (0) 2 (0) 

Yuba 0 0 3 (1) 1 (0) 

Nevada 7 (6) 7 (6) 3 (2) 3 (1) 

Placer 3 (3) 3 (3) 5 (1) 2 (1) 

El Dorado 13 (8) 9 (6) 8 (3) 7 (2) 

Amador 0 0 1 (0) 1 (0) 

Calaveras 0 0 5 (0) 1 (0) 

Tuolumne 4 (4) 4 (4) 7 (3) 3 (0) 

Mariposa 0 0 4 (2) 3 (2) 

Madera 0 0 0 0 

Fresno 0 0 4 (4) 4 (4) 

Tulare 0 0 0 0 

Kern 0 0 1 (1) 1 (1) 

Totals 58 (32) 49 (29) 67 (26) 49 (21) 

Southern California Province 

Spotted owls in southern California occur in 11 major mountain ranges and mountain complexes, predominantly on NF 

lands. These ranges and areas are the Santa Ana, San Diego 

(including the Laguna, Cuyamaca, Pine Hills/Vulcan, and Palomar 

mountain areas), San Jacinto, San Bernardino, San Gabriel, 

Liebre/Sawmill, Tehachapi, Tecuya Mountain area, Los Padres, 

southern Santa Lucia Mountains, and northern Santa Lucia Mountains (fig. 9A). Movement between these major ranges and 

complexes may be uncommon to nonexistent (Stephenson 1991). 

Within a given range, however, separate blocks of suitable 

habitat are relatively close together. For instance, the San Diego 

ranges include five of the mountain ranges mentioned above 

with populations 4.5 to 8 miles apart, but distances from the San 

Diego to the San Jacinto ranges and Santa Ana Mountains are 18 

and 33 miles, respectively (Stephenson 1991). Historic records 

from the turn of the century documented spotted owls in riparian 

areas on the coastal lowlands, but their occurrence there today is 

unlikely because of the extensive loss of habitat to urbanization 

(Bloom pers. comm.). 

Besides our concern that successful movement by owls 

among some of these relatively isolated islands of habitat may be 

declining or no longer possible, the southern California populations of owls may be isolated from the southern Sierra Nevada 

because of the nature of available connecting habitat. The 

Tehachapi Mountains, Liebre/Sawmill and Tecuya Mountain 

areas are critical for maintaining this linkage, if it still exists, and 

they are relatively distant from the nearest other known spotted owls in the San Gabriel Mountains and the southern Los 

Padres ranges. 

Estimated owl sites, numbers of pairs, pair status, and distances from nearest neighboring populations of California spot46 Chapter 3 

ted owls for each of the 11 mountain areas are summarized in 

table 3H (Stephenson 1991). 

Forest Service, Pacific Southwest Region (R5) 

Most known spotted owl populations in southern California 

are on the Angeles, San Bernardino, Cleveland, and Los Padres 

NFs, with a total land base of 3,643,927 acres. The primary 

habitat types used are riparian/hardwood, live oak/bigcone 

Douglas-fir, and mixed-conifer (Chapter 5) (types described in 

Chapter 1), but in different proportions on the four NFs (table 

3I). On the Angeles and San Bernardino, most spotted owls are 

found in live oak/bigcone Douglas fir at elevations of 2,000 to 

5,000 feet and mixed-conifer (mostly Jeffrey pine) at elevations 

from 5,000 to 8,000 feet. These types cover 137,000 acres on the 

Angeles NF (20 percent of the landbase) and 308,000 acres on 

the San Bernardino NF (37 percent of the landbase). On the 

Cleveland NF most owls are found in similar habitats, including 

a total of 40,000 acres (9.6 percent of the landbase) but we 

believe that additional owls will be found in riparian/hardwood 

habitats when they are surveyed. These habitats make up 1-2 

percent of the landbase on all four NFs (USDA, FS 1986a, 1987, 

1988b, 1988d). 

On coastal portions of the Monterey District of the Los 

Padres NF, large stands of redwoods on north slopes also provide owl habitat, but population surveys there are incomplete. 

Most owls on the Los Padres NF, especially in the southern 

block, are in narrow riparian corridors, where mixed hardwoods 

are closely flanked by chaparral. 

Regional direction for the four NFs in southern California is 

to protect all known spotted owl sites and to manage the habitat 

based on local information about suitability and availability 

(USDA, FS 1984). FS surveys for occupancy and suitable habitat in southern California lags behind the Sierra Nevada ProvUSDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Figure 3A-Areas of current and potential concern for the California spotted owl. Gap areas delineate discontinuities in owl distribution; designated 

population areas have a variety of real or potential concerns, such as low population density, relatively fragmented habitat, or extensive loss of habitat 

from recent wildfires. Place names and reasons for concern are identified in tables 3G and 3K. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. Chapter 3 47 
Table 3G-Areas of concern for California spotted owls in the Sierra Nevada. 

 General location 

 Area1 (major ownership)2 Reasons for concern 

 A Eastern Shasta County A natural bottleneck between northern and California spotted owls; habitats 

 (FS, IP) are fragmented and owl densities low. 

 B Central Tulare County Apparently a natural bottleneck in north-south distribution of owls because 

 (NPS) steep terrain provides only a narrow band of suitable habitat. 

 1 Lassen County Habitat in this area is discontinuous, naturally fragmented, and poor in 

 (FS, NPS, IP) quality due to drier conditions and lava-based soils. 

 2 Northern Plumas A gap in known distribution, mainly on private lands, extends east-west in a 

 County (FS, IP, Pvt.) band almost fully across the width of the owl's range. 

 3 Northeastern Tahoe An area of checkerboard lands; much dominated by granite outcrops and red 

 NF (FS, IP, Pvt.) fir forests; both features guarantee low owl densities. 

 4 Northern Eldorado Checkerboarded lands and large, private inholdings; owl densities unknown 

 NF (FS, IP, Pvt.) on some private lands and very low on others. 

 5 Northwestern Stanislaus Has large private inholdings; owl densities unknown on most private lands. 

 NF (FS, IP, Pvt.) 

 6 Southern Stanislaus NF Burned in recent years; the little remaining habitat is highly fragmented. 

 (FS) 

 7 Northwestern Sierra NF Habitat naturally fragmented, due partly to low elevations and dry conditions; 

 (FS) fragmentation accentuated by logging. 

 8 Northeastern Kern Only small, semi-isolated groups of owls in the few areas at elevations where 

 County (FS) habitat persists at the south end of the Sierra Nevada. 

1 Location codes correspond to letters (gaps and bottlenecks) and numbers (population areas) on figure 3A. 

2 Ownership codes: FS = USDA Forest Service; NPS = National Park Service; BLM = Bureau of Land Management; 

IP = Private industrial lands; Pvt. = multiple, small, private ownerships. 

ince because threats to the population from management appeared to be less imminent in southern California. Southern 

California was not included in the monitoring phase of the 

RD&A Program, and funding for other survey efforts has been 

inadequate. 

Three hundred seventeen sites with pairs or single owls 

have been documented on the four southern California NFs 

since 1970, and 294 sites since 1986 (table 3J). Over the last 5 

years, pairs have been verified at 214 of these sites with reproduction at 107 sites. About 42 percent of known sites were 

located in the San Bernardino Mountains (LaHaye and Gutiérrez 

1990). This probably reflects a higher relative abundance of 

owls over a larger area than elsewhere in southern California. 

The NFs estimate that approximately 190 additional sites might 

currently exist. The 294 sites identified since 1986 are 85 percent of all sites located in southern California during that period. 

Bureau of Land Management, California Office 

BLM lands are relatively few and scattered in the mountain 

ranges between the coast and inland deserts of southern California. They are located down-slope from, and in more arid areas 

than, NF lands and they include drainages and riparian/hardwood forests that may provide spotted owl habitat. In the California Desert District and the Caliente Resource Area, potential 

spotted owl habitats are managed for wildlife, riparian habitat 

48 Chapter 3 

quality, water quality, and dispersed recreation. Total potential 

owl habitat, based on vegetation, in these two management units 

is 7,560 acres, but surveys have not been done (Saslaw 1991). 

One spotted owl site has been located on BLM lands in the last 5 

years, the result of survey work on adjacent NF lands. 

U.S. Department of Defense 

Camp Pendleton Marine Base occupies about 125,000 acres 

on the coastal plain between the Santa Ana Mountains and the 

Pacific Coast in northern San Diego County. The higher part of 

the Base, with a small amount of potential habitat, abuts the 

Trabuco Ranger District of the Cleveland NF, but no owls have 

been found there recently (Buck pers. comm.). Spotted owls 

were observed about the turn of the century at several locations 

in riparian habitats on the coastal plain, including San Onofre 

Creek in what today is Camp Pendleton, but none is there today 

(Bloom pers. comm.). Santa Margarita Creek is another major 

drainage bisecting the Camp that probably had spotted owls 

historically, based on remnant bigcone Douglas-fir in the upper 

reaches, but the habitat has been too degraded by fire to support 

owls now (Bloom pers. comm.). The Base has a natural resource management program, including wildlife conservation, 

but long-term effects of training activities have degraded habitat for spotted owls to an extent that very little exists today 

(Buck pers. comm.). 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Table 3H-Owl sites1

, number of pairs and status of pairs at those sites, pair status, and nearest-neighbor distances of 

California spotted owls in southern California.2

 Potential Nearest 

 Owl sites Pairs Breeding population neighbor 

Area (all years) since 1987 since 1987 (no. sites) distances3

San Diego Ranges 37 18 6 76 18 -33 miles 

Santa Ana Mountains 2 1 - 12 30 -40 miles 

San Jacinto Ranges 20 16 9 29 11 -18 miles 

San Bernardino Mountains 124 114 66 125 6 -11 miles 

San Gabriel Mountains 54 22 5 95 6 -20 miles 

Liebre/Sawmill Mountains 14 10 2 20 12 -20 miles 

Tehachapi Mountains 4 0 - 12 unknown 

Tecuya Mtn area 5 3 - 10 9 -12 miles 

Los Padres 65 32 17 100 8 -12 miles 

So. Santa Lucia Mountains 12 6 2 19 32 -45 miles 

No. Santa Lucia Mountains 39 22 1 80 45 miles 

Total 376 244 108 578 

1 See definition of "owl site" in glossary (Appendix B). 

2

 Based on Stephenson (1991) and California Department of Fish and Game database. 

3

 Where two distance values ate shown (for example, 12-20 miles), they represent the distances to the two closest 

neighboring populations. 

California State Parks 

Five units of the SP system, totalling 58,182 acres, are in the 

vicinity of the Angeles, San Bernardino, and Cleveland NFs. 

Those with significant owl habitat are San Jacinto SP (about 

12,500 acres of conifer and pine-oak forests), Cuyamaca Rancho 

SP (about 11,000 acres of conifer and hardwood forests), and 

Palomar Mountain SP (1,000 acres of conifer and hardwood 

forests) (Bakken pers. comm.). All three of these SPs have been 

surveyed for spotted owls in recent years. Based on research, 

existing habitat, and nine known sites, they could support 13 to 

15 pairs of spotted owls (Stephenson pers. comm.). 

Four additional SP units with a combined total of 300 acres 

of coastal conifer and hardwood habitats are on the central 

California coast and have potential for a limited number of 

spotted owls. These are Julia Pfieffer Burns SP, Pfieffer-Big Sur 

SP, Andrew Molera SP in Monterey County and and Gaviota SP 

in Santa Barbara County (Bakken pers. comm.). Nojoqui Falls 

County Park is a 98-acre park which is part of the Santa Barbara 

County park system not far from Gaviota SP. It has had records 

of a spotted owl site since the early 1970s in riparian/hardwood 

habitat along Nojoqui Creek (Jimenez pers. comm.). 

National Audubon Society 

Starr Ranch, a 4,000-acre wildlife sanctuary owned and 

managed by the National Audubon Society, is located on upper 

Bell Canyon in Orange County, adjacent to the Trabuco Ranger 

District of the Cleveland NF (DeSimone pers. comm.). This area 

occasionally has nonbreeding spotted owls that move onto the 

property from NF lands, but they apparently do not nest there 

(Bloom pers. comm.). 

Native American Tribal Lands 

Several Native American Tribal Reservations are located in 

southern California; these have potential spotted owl habitat and 

a few documented sightings. The Pala, La Jolla, Santa Ysabel, 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

Mesa Grande, Los Coyotes, and Cuyapaipe Reservations all 

have potential habitat that has been identified by FS biologists 

from the Cleveland NF. Sightings indicate a population of at 

least four pairs, and known potential habitat could support a 

higher number. Activities and potential threats to spotted owls 

on these lands are unknown. 

Other Private Lands 

Major activities that threaten spotted owls on private lands 

in southern California are recreation, urbanization, fire, and 

water extraction from stream systems (Chapter 13). Both the San 

Bernardino and Cleveland NFs include significant areas of private land-about 145,000 and 120,000 acres, respectively (USDA, 

FS 1986a, 1988d). The San Bernardino NF surrounds the popular and rapidly growing mountain resorts of Big Bear Lake, Lake 

Arrowhead, Forest Falls, and Idyllwild. This is causing a loss of 

owl habitat, especially near Lake Arrowhead (Stephenson 1991). 

The situation on included lands on the Cleveland NF is not so 

severe, but expanding development and recreation on these 

lands pose long-term risks for the owl. Direct risks are caused by 

continual loss of suitable nesting habitat from these activites 

which fragments the already meager quantity of suitable habitat. 

An indirect risk is the decreasing ability of the FS to use prescribed burning to reduce fuel accumulations that can lead to 

stand-destroying fires, because of the potential risk of prescribed 

fires to residential areas as well as local air pollution regulations. 

Urbanization and wildfires also affect owl habitat outside of 

and surrounding all four NFs. The Angeles and the Los Padres 

NFs do not have major problems with private inholdings, but 

they are experiencing residential development around their edges. 

Loss of habitat in these areas outside the forests reduce the 

quantity of suitable habitat around the edge of the isolated 

parcels of NF lands and occasional tracts of habitat on private 

lands. Additional impacts of urbanization are effects of fire 

management policies, recreation patterns, and diversions of waChapter 3 49 
Table 3I-Known California spotted owl sites, by habitat type, on and in the vicinity of National Forest lands in 

southern California, 1970-1991. 

National Forests 

San Total 

Angeles Bernardino Cleveland Los Padres sites Percent 

Habitat types 

Riparian/hardwood 2 4 110 116 32 

 Live oak/bigcone 48 73 26 147 41 

 Douglas-fir 

 Mixed-conifer 11 72 6 6 95 26 

 Redwood 2 2 >1 

Total Sample1

 59 147 36 118 360 100 

Reproductive sites 

Breeding pairs2 10 82 7 8 107 

1

 Data provided by NFs for all sites with available information on habitats. Actual total counts of sites for southern 

California are given in Table 3J, based on CDFG's database. 2

 Data provided by NFs for sites where young were produced. 

Table 3J-Estimated numbers of California spotted owl sites (pairs) in southern California. 

 Total known sites Sites in 

 (known pairs) reserved areas 

 Estimated 

Location 1970-1991 1987-1991 1970-1991 1987-1991 additional 

 sites 

Federal Lands 

National Forests 

 Los Padres 111 (62) 94 (61) 451 41 58-100 

 Angeles 59 (31) 56 (28) 91 7 40-80 

 San Bernardino 126 (114) 126 (114) 8 8 30-45 

 Cleveland 21 (14) 18 (11) 3 3 14 

Bureau of Land 

 Management 

 All lands 1 (0) 1 (0) 

Subtotal 318 (221) 295 (214) 65 59 142-239 

State and Local Lands 

State Parks 7 (4) 7 (1) 13-15 

Other State 1 (0) 0 

Local Parks 1 (1) 0 

Subtotal 9 (5) 7 (1) 13-15

Private Lands 

Indian Nations 4 (3) 4 (3) 

Other (by county) 

Monterey 2 (1) 1 (1) 

 San Luis Obispo 2 (1) 0 

 Santa Barbara 3 (2) 2 (1) 

 Ventura 0 0 

 Kern 5 (1) 4 (0) 

 Los Angeles 3 (2) 3 (1) 

 San Bernardino 16 (14) 16 (13) 

 Riverside 7 (6) 7 (6) 

 Orange 0 0 

 San Diego 7 (5) 4 (4) 

Subtotal 45 (32) 41 (29) 

Overall total 376 (261) 343 (244) 

1

 Data from FS were augmented by information from CDFG's database for historical sightings only. 

50 Chapter 3 USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
ter from streams for local residential use. These development; 

also accentuate the barriers separating owl populations. They 

also have the effect of reducing dispersal between blocks and the 

chance for population support from adjacent areas. 

Over the last 5 years spotted owls have been found at 37 

sites on private lands in all counties along the southern California coast except Ventura, Orange, and San Luis Obispo (table 

3J). We expect these numbers to be low because most private 

lands are at elevations lower than "traditional" spotted owl 

habitat. Some surveys have been done in the gaps between NF 

lands and on NF lands with habitats similar to those found on 

private lands. Spotted owls have been located in local situations 

and at relatively low densities. We do not expect that further 

surveys will document additional, extensive populations that 

would dramatically change our current understanding of the 

species' distribution and density in southern California. 

Areas of Concern: Southern California 

Our concern for spotted owl populations in southern California is much greater than is the case for the Sierra Nevada, 

because processes are at work there that we believe will further 

isolate populations from one another and reduce the sizes of 

individual populations. These processes are likely to reduce the 

interchange of owls between populations (Chapter 9), lowering 

our estimates of the stability of the metapopulation in southern 

California. The likelihood of local extinction resulting from 

stochastic processes will increase if local populations are further 

isolated and suitable owl habitat declines. 

Concern about the stability and persistence of the California 

spotted owl population in southern California centers on four 

factors: (1) Every population is separated from neighboring 

populations by unsuitable habitat; (2) even within mountain 

ranges, gaps occur in the known distribution of owls; (3) suitable 

habitat is highly fragmented; and (4) some areas have low crude 

densities of spotted owls. Whereas the Sierra Nevada contains 

one relatively contiguous population of spotted owls, southern 

California contains at least 11 populations separated from their 

nearest neighbors by at least 6 miles (figs. 3A, 9A; tables 3H and 

3K). The gaps, in all but two cases, contain major highways, 

often urban development, and hotter, drier, lower elevations 

unsuitable for nesting, foraging, or dispersal. 

The owl's habitat in the Sierra Nevada most often consists 

of relatively large areas with a fairly high proportion of suitable 

habitat. In southern California, even in areas with owls, the 

habitat is often restricted to narrow riparian strips separated from 

adjacent and similar habitat by open expanses of 1/2 mile or 

more of chaparral (Areas 10,11, 12, 15,17, 19, and 20 in fig. 3A, 

table 3K). The intervening areas are not amenable to changes 

that might result in their having suitable owl habitat. 

Areas 9, 10, 11, 13, 17, 19, and 20 also have low crude 

densities, given the potential habitat that might be present. Estimates of low crude densities of owl pairs and individuals in many 

of these populations may be a result of insufficient survey work. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

&RQFOXVLRQV

Diverse in character and geography, habitats of the California spotted owl are managed by several State and Federal agencies and various private landowners, most with different objectives and mandates. The FS has implemented policies to provide 

for conservation of the California spotted owl, but these policies 

have been challenged recently as a result of recommendations by 

the ISC for the northern spotted owl (see Thomas et al. 1990). 

The extensive and nearly continuous nature of Federal land 

ownership in primary habitats of the California spotted owl over 

the length of the Sierra Nevada makes it feasible to implement a 

coordinated and effective conservation strategy there. We also 

believe that lands held by commercial timber companies and 

other private owners could make a substantive contribution to 

maintenance of the spotted owl population in the Sierra Nevada. 

Private timber companies differ in their land-management objectives and policies, however, and it is uncertain the extent to 

which future management of their lands will maintain suitable 

spotted owl habitat. The final form of expected new State forestry rules cannot be fully predicted except that, at a minimum, 

they will change logging practices on some private lands and 

probably benefit spotted owls as a net effect. 

In contrast to the case in the Sierra Nevada, populations of 

spotted owls in southern California occur in relatively isolated 

blocks, primarily on mountain tops and in wooded ravines extending down the mountain sides, and most of these populations 

are small (Chapter 9). This island-like nature of owl distribution 

in southern California makes it especially challenging to contrive a conservation strategy for the bird, based solely on public 

lands. For a variety of reasons, southern California is losing 

spotted owl habitat for breeding, foraging, and dispersal. Among 

the primary reasons are residential developments in previously 

remote valleys and mountains, stand-destroying fires, increasing 

concentration of recreational activities in prime owl habitat, and 

the "mining" of water from streams within the range of the owl. 

These impacts will continue, leading to increased insularization 

of owls in the relatively small mountain ranges where they now 

occur. These are the primary risks facing continued spotted owl 

distribution in southern California. It will probably be impossible to maintain a viable population of spotted owls in southern 

California without stabilizing the amount and distribution of 

suitable habitat on public lands and without changes in land-use 

policies on private lands, especially those that adjoin public lands. 

Chapter 3 51 
Table 3K-Areas of concern for California spotted owls in southern California. 

 General location 

 Area1 (major ownership)2 Reasons for concern 

 Gaps in known owl distribution 

C East-central Kern County A gap of 30 miles in the known owl distribution; believed to contain little or no suitable owl habitat. 

 (Pvt., BLM) 

 D Tejon Pass Includes Interstate 5 and has little suitable habitat; most is grasslands and chaparral. 

 (Pvt., FS) 

 E Northern Ventura County Has little suitable habitat; most is chaparral or pinyon-juniper woodlands. 

 (FS) 

 F Western Antelope Valley High desert with no suitable habitat. 

 (Pvt.) 

 G Piru Gorge Has little suitable habitat and bisected by Interstate 5; most of the area in chaparral. 

 (FS, Pvt.) 

 H Santa Clara River Has little suitable habitat; includes State Highway 14 and associated developed areas; most of the area in chaparral and 

 (FS, Pvt.) desert scrub. 

 I Cajon Pass Consists mainly of chaparral; bisected by Interstate 15. 

 (FS, Pvt.) 

 J Banning Pass Mostly desert scrub and chaparral at higher elevations; has Interstate 10 and developments. 

 (BLM, FS, Pvt.) 

 K Anza area Mostly chaparral; includes some grasslands, limited riparian woodlands, and semirural housing. 

 (BLM, FS, Pvt.) 

 L Los Angeles Basin No owl habitat; fully urban sprawl with some grasslands and residual chaparral areas. 

 (Pvt.) 

 M Temecula area Mostly chaparral; includes Interstate 5; urbanization is increasing. 

 (BLM, Pvt.) 

 N Cuyama River Has little suitable habitat; contains chaparral and some semidesert scrub. 

 (FS, Pvt.) 

 O Northwestern San Luis May contain some suitable hardwood and riparian habitat, but very poorly surveyed. 

 Obispo Co. (Pvt.) 

 P San Joaquin Valley Has little suitable owl habitat; most land in crops, grasslands, or residual desert scrub. 

 (Pvt.) 

 Population problems 

9 Tehachapi Mountains Habitat is fragmented and known owl density is low. This small population is isolated from the population to the south 

 (Pvt.) and probably from the population to the west. 

 10 Tecuya Mountain Owl density is low and habitat occurs only in north-slope drainages. This small population is probably isolated from 

 (FS) populations to the south and east. 

 11 Ventura/Santa Barbara This is a large area with varying owl densities and distributions of habitat. Except for the San Rafael Wilderness 

 (FS) Area, populations are scattered in suitable habitat, mostly in north-slope drainages. The population here is probably 

 isolated from other populations to the northwest, northeast, and east. 

 12 Liebre/Sawmill Area has a limited population in suitable habitat on the north sides of mountains; owls are isolated from the population 

 (FS) to the southeast and probably from the ones to the north and west. 

 13 San Gabriel Mountains Large area varying in habitat distribution and owl densities; much habitat is mixed-conifer or bigcone Douglas-fir/oak. 

 (FS) This population is isolated from the population to the northwest and probably from the one to the east. 

 14 San Bernardino Mountains Contains the largest population and most extensive owl habitat in southern California; at least partially isolated from 

 (FS) the population to the west and maybe totally from the one to the south. 

 15 Mount San Jacinto Most habitat here is bigcone Douglas-fir/oak on south and west sides of the mountain. The limited population is isolated 

 (FS) from populations to the north and southwest. 

 16 Palomar Mountain The mountain-top habitat is mixed-conifer and bigcone Douglas-fir/oak. The owl population is isolated from populations 

 (FS, IP) to the northwest and northeast but is continuous with the population to the south. 

 17 Central San Diego County This population is spread sparsely through the canyons at lower elevations south from Palomar almost to the Mexican 

 (FS, NATL, Pvt.) border. It includes some smaller mountains and links Palomar with Area 18. Habitat is usually oak in canyons. Surveys 

 here have been limited and local. 

 18 Cuyamaca/LagunaMountains Areaconsistsoftwoadjacentmountaintops,bothwithsmall,localpopulations.Habitatismixed-coniferandoak-dominated 

 (FS, CA) canyons. This is southern-most population in California and only links to populations further north in San Diego County. 

 19 Santa Ana Mountains This range contains the smallest known population (two sites), both in conifer/oak habitat at heads of canyons on the range's 

 (FS) west slope. This population is isolated by the Los Angeles Basin from populations to the southeast and north-northeast. 

 20 Southern Santa Lucia This limited population exists in canyons dominated mostly by oak habitats. It is probably isolated from populations 

 Mountains (FS, Pvt.) to the southeast and northwest. 

 21 Coastal Monterey County This is the northern-most population of the California Spotted Owl on the coast. It occurs in a variety of habitats including 

 (FS, Pvt.) the southern-most groves of coast redwood. It is isolated from the northern spotted owl by 100 miles of mixed habitat, 

 including the highly developed San Francisco area. It is isolated from California spotted owls in the Sierra Nevada by 

 the San Joaquin Valley. 

1

 Location codes correspond to letters (gaps and bottlenecks) and numbers (population areas) on figure 3A. 2

 Ownership codes: FS = USDA Forest Service; BLM = Bureau of Land Management; CA = State of California; NATL = Native American tribal lands; Pvt. = 

multiple, small, private ownerships. 

52 Chapter 3 USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
References 

Bakken, Steve, Forester, Julia Pfieffer Bums State Park, California Department of Parks and Recreation, Sacramento. [Personal communications]. 

January 1992. 

Bias, Michael A.; Gutierrez, R. J. 1988. Population ecology of the California 

spotted owl in the central Sierra Nevada: preliminary results, 1987. Unpublished final report. Sacramento, CA: Department of Fish and Game, Nongame 

Bird and Mammal Section report, Job ILA.2; 24 p. 

Bloom, Pete, Research Biologist for the Western Foundation of Vertebrate 

Zoology, Santa Ana, CA. [Personal communication]. February 1992. 

Buck, Slader, Wildlife Management Supervisor, Camp Pendleton Marine 

Base, Camp Pendleton, CA. [Personal communication]. January 1992. 

Carrier, Dean, Wildlife Biologist, Resource Management International, Sacramento, CA. [Personal communication]. October 1991. 

Coburn, Mary, Wildlife Biologist, USDA Forest Service, Toiyabe National 

Forest, Sparks, NV. [Personal communication]. February 1992. 

Colclasure, Perry; Moen, Joel; Bolsinger, Charles L. 1986. Timber resource 

statistics for the northern interior resource area of California. Resource 

Bulletin PNW-135. Portland, OR: Pacific Northwest Forest and Range 

Experiment Station, Forest Service, U.S. Department of Agriculture; 32 p. 

Dawson, William R.; Ligon, J. David; Murphy, Joseph R.; Myers, J. P.; 

Simberloff, Daniel; Verner, Jared. 1987. Report of the scientific advisory 

panel on the spotted owl. Condor 89:205-229. 

DeSimone, Pete, Starr Ranch Wildlife Sanctuary Manager, Audubon Society. 

Trabuco Canyon, CA. [Personal communication]. January 1992. 

Dulitz, Dave, Forester, California Department of Forestry and Fire Protection, 

Mountain Home State Forest, Springville, CA. [Personal communication]. 

January 1992. 

Ewing, Robert A., Program Manager, Forest and Rangeland Resources Assessment Program, California Department of Forestry and Fire Protection, 

Sacramento. [Personal communication]. January 1992. 

Forsman, Eric D. 1976. A preliminary investigation of the spotted owls in 

Oregon. Corvallis: Oregon State Univ.; 127 p. Thesis. 

Forsman, Eric D. 1980. Habitat utilization by spotted owls in the west-central 

Cascades of Oregon. Corvallis: Oregon State Univ.; 95 p. Dissertation. 

Forsman, Eric D. 1981. Habitat utilization by spotted owls on the Eugene 

District of the Bureau of Land Management. Unpublished report. Portland, 

OR: Bureau of Land Management, U.S. Department of Interior; 63 p. 

Gasser, Don, Department of Forestry Property Manager, Univ. of California, 

Berkeley, CA. [Personal communication]. February 1992. 

Gould, Gordon L, Jr. 1974. The status of the spotted owl in California. 

Contract No. 39-4870. San Francisco, CA: Region 5, Forest Service, U.S. 

Department of Agriculture; 56 p. 

Gould, Gordon L, Jr. 1977. Distribution of the spotted owl in California. 

Western Birds 8:131-146. 

Grinnell, Joseph; Miller, Alden H. 1944. The distribution of the birds of 

California. Berkeley, CA: Cooper Ornithological Club, Pacific Coast Avifauna 27:203-205. 

Heald, Robert, Forest Manager, Blodgett Forest Research Station, Univ. of 

California, Berkeley. [Personal communication]. January 1992. 

Hiserote, Bruce A.; Moen, Joel; Bolsinger, Charles L. 1986. Timber resource 

statistics for the San Joaquin and Southern California resource areas. 

Resource Bulletin PNW-132. Portland, OR: Pacific Northwest Forest and 

Range Experiment Station, Forest Service, U.S. Department of Agriculture; 35 p. 

Husari, Susan. 1990. Fire ecology of the vegetative habitat types in the Lassen 

Fire Management Planning Area (Caribou Wilderness and Lassen Volcanic National Park). Unpublished report. Susanville, CA: Lassen National 

Forest, Forest Service, U.S. Department of Agriculture; 18 p. 

Jimenez, Ruben, Park Ranger, Nojoqui Falls County Park, Santa Barbara 

County, Santa Barbara, CA. [Personal communication]. February 1992. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

LaHaye, William S.; Gutierrez, Ralph J. 1990. Big Bear spotted owl study, 

1989. Unpublished final report. Sacramento, CA: Department of Fish and 

Game Contract FG7524; 15 p. 

Lloyd, J. D., Jr.; Moen, Joel; Bolsinger, Charles L. 1986. Timber resource 

statistics for the Sacramento resource area of California. Resource Bulletin 

PNW-134. Portland, OR: Pacific Northwest Forest and Range Experiment 

Station, Forest Service, U.S. Department of Agriculture; 32 p. 

MacNamara, Dave, Forester, California Department of Forestry and Fire 

Protection, Latour State Forest, Burney, CA. [Personal communication]. 

January 1992. 

Remsen. J. V., Jr. 1978. Bird species of special concern in California. Sacramento, 

CA: Department of Fish and Game, Unpublished Report No. 78-1; 54 p. 

Roberts, Cindy K.1989. California spotted owl (Strix occidentalis occidentalis) 

inventory and demographic study, Sequoia and Kings Canyon National 

Parks: preliminary results, 1988. Unpublished final report. Sacramento, 

CA: Department of Fish and Game, Job ILA.2; 24 p. 

Roberts, Kevin; Escallier, Mike; Gould, Gordon L, Jr. 1988. Spotted owl 

distribution and abundance in Yosemite National Park, 1988. Progress 

report. Sacramento, CA: Department of Fish and Game, Nongame Bird 

and Mammal Section, Wildlife Management Division, Job ILA.2; 20 p. 

Saslaw, Larry. 1991. Status report for California spotted owl on BLM lands in 

southern California and the Sierra Nevada. Bakersfield, CA: Bureau of 

Land Management. Unpublished draft supplied by author; 5 p. 

Steger, George N., Biological Technician, Wildlife, USDA Forest Service, 

Pacific Southwest Research Station, Fresno, CA. [Personal communication]. December 1991. 

Stephenson, John, Wildlife Biologist, USDA Forest Service, Palomar District, 

Cleveland National Forest, Ramona, CA. [Personal communication]. December 1991. 

Stephenson, John. 1991. Spotted owl surveys on the National Forests of 

southern California: a status report and recommendations for the future. 

San Bernardino, CA: Forest Service, U.S. Department of Agriculture; 13 p. 

Thomas, Jack Ward; Forsman, Eric D.; Lint, Joseph B.; Meslow, E. Charles; 

Noon, Bury R.; Verner, Jared. 1990. A conservation strategy for the northern spotted owl. Washington, DC: U.S. Government Printing Office; 427 p. 

U.S. Department of Agriculture, Forest Service. 1984. Land management 

planning direction. Unpublished internal planning policy. San Francisco, 

CA: USDA Forest Service, Pacific Southwest Region; 254 p. 

U.S. Department of Agriculture, Forest Service. 1988. Final supplement to the 

EIS for an amendment to the Pacific Northwest Regional Guide. Spotted 

owl guidelines. Volume 1. Portland, OR: USDA Forest Service, Pacific 

Northwest Region; 157 p. 

U.S. Department of Agriculture, Forest Service. 1987. Land and resource 

management plan. San Francisco, CA: USDA Forest Service, Pacific 

Southwest Region; 

1986a Cleveland National Forest; 139 p. 

1986b Sierra National Forest; 173 p. Draft 

1987 Angeles National Forest; 243 p. 

1988a Eldorado National Forest; 438 p. 

1988b Los Padres National Forest; 286 p. 

1988c Plumas National Forest; 498 p. 

1988d San Bernardino National Forest; 315 p. 

1988e Sequoia National Forest; 225 p. 

1990 Tahoe National Forest; 817 p. 

1991a Lassen National Forest; 334 p. Draft 

1991b Stanislaus National Forest; 458 p. 

U.S. Department of Interior, Fish and Wildlife Service. 1973. Threatened wildlife of the United States. Research Publication 114. Washington, DC; 289 p. 

U.S. Department of Interior, Fish and Wildlife Service. 1982. The northern spotted 

owl: a status review. Portland, OR: Endangered Species Program; 29 p. 

U.S. Department of Interior, Fish and Wildlife Service. 1989. The northern 

spotted owl- A status review supplementary final report. Unpublished 

report. Portland, OR.; 111 p. 

U.S. Department of Interior, National Park Service. 1988. Management policies: National Park Service. Washington, DC; 122 p. 

van Wagtendonk, Jan W. Research Scientist, USDI National Park Service, 

Yosemite National Park. [Personal communication]. January 1992. 

Chapter 3 53 
Chapter 4 

The California Spotted Owl: 

General Biology and Ecological Relations 

Jared Verner, R. J. Gutiérrez, and Gordon I. Gould, Jr. 

Understanding a species' ecology and life history and its 

role in ecosystem functions is vital to successful management of 

that species. The extent to which this understanding is comprehensive and correct is also critical. This chapter emphasizes life 

history elements and ecological relations of the California spotted owl that bear directly on planning for its management. In that 

sense, this is not an extensive, detailed treatment of the owl's 

biology and ecology. Nor does it provide a thorough analysis of 

some of the primary factors that must be considered, such as 

habitat relations, home ranges, density, and demography. Instead, because these components are so critical to conservation 

planning, and each requires extensive analyses, other chapters 

treat them in considerable detail. 

Description and 

Classification 

The spotted owl is generally mottled in appearance, the 

back is brown with irregular white spots and the underparts 

distinctly lighter, with white spots and brown barring. The facial 

disk is pale brown with concentric rings of darker brown and 

bordered by a ring of dark brown feathers. Pale "eyebrows" and 

"whiskers" merge into a conspicuous, light-colored "X" between 

the eyes and above the beak. Unlike most other owl species, which 

have yellow eyes, the eyes of spotted owls are dark brown. The 

beak is pale yellowish. Wings and tail are rounded and all flight 

feathers are dark brown with light-brown crossbars. The legs and 

toes are fully covered with short, pale-buff feathers, and the claws 

are dark brown to black. Adult males and females cannot be 

distinguished by plumage (Forsman 1981), but they are readily 

identified by voice and size (Forsman et al. 1984, Blakesley et al. 

1990). Barrows et al. (1982) indicated that sexes can be distinguished by the number of tail bars, but Blakesley et al. (1990) 

found this characteristic to be unreliable. Moen et al. (1991) noted 

that first-year and second-year adults can be distinguished by the 

shape of the tips of their tail feathers. 

Only four of the 19 species of owls occurring in North 

America are larger than the spotted owl (Johnsgard 1988). Based 

on live weights from a sample of 46 adult male and 48 adult 

female California spotted owls captured in the Sierra National 

Forest (NF) and Sequoia National Park (NP) (Steger pers. comm.), 

females averaged 22.2 ounces-13.6 percent heavier than the 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

average of males at 19.6 ounces. Weights of individuals captured more than once differed markedly, which is not surprising 

considering that some common prey items of spotted owls (for 

example, woodrats) weigh as much as a third of an adult owl's 

weight. For comparison, live weights of 65 female northern 

spotted owls captured in northwestern California averaged 23.2 

ounces; 68 males averaged 20.3 ounces (Blakesley et al. 1990). 

The American Ornithologists' Union (1957) recognized 

three subspecies of the spotted owl-California spotted owl (Strix 

occidentalis occidentalis) (Xantus 1859), northern spotted owl 

(Strix occidentalis caurina) (Merriam 1898), and Mexican spotted 

owl (Strix occidentalis lucida) (Nelson 1903). The Mexican form, 

found from southern Utah and Colorado southward into Arizona, 

New Mexico, and Mexico, is geographically isolated from the 

California and northern subspecies. The California spotted owl is 

confined to the State of California (figs. 4A, 4B, and 4C), where its 

distribution in the southern Cascade Mountains adjoins a southeastern extension of the range of the northern spotted owl. This 

contact zone is along the Pit River, east of Redding. 

Barrowclough (pers. comm.) has observed north-south clinal variation (gradual change) in morphology of northern and 

California spotted owls. Barrowclough and Gutierrez (1990) 

found no electrophoretic differences between the northern and 

California subspecies, so their study shed no light on the question of a taxonomic distinction. As in other such studies, however, only a fraction of the total genetic information was compared. The American Ornithologists' Union has consistently 

recognized two subspecies of spotted owls in California; this 

position was recently reaffirmed (N. K. Johnson pers. comm.). 

Questions about whether the northern and California spotted owls are valid subspecies, however, are essentially irrelevant 

to concerns about the status of the owls. Spotted owls certainly 

move back and forth across the Pit River, and interbreeding 

between northern and California forms undoubtedly occurs in 

that area. Subspecies, by definition, are not reproductively isolated from each other. The important management questions 

relate to conditions of populations and habitats in various parts 

of the spotted owl's overall range, not to details of its subspeciation. 

The owl may be doing well in some areas but not in others. 

Where it is not doing well, we need to consider options for 

improving its status. Generally, this will involve improving 

habitat conditions and increasing the crude densities of owls to 

increase the efficiency of dispersal (Chapters 8 and 9). Maintaining the continuity of spotted owl populations from the northern 

Sierra Nevada into the southern Cascades and northwestern 

California is an important part of assuring the overall viability of 

both subspecies (Dawson et al. 1987, Thomas et al. 1990). 

Chapter 4 55 
Figure 4A-Distribution of California spotted owls. 

56 Chapter 4 USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Figure 4B-Detail of the relative abundance of California spotted owls in the Sierra Nevada Province. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. Chapter 4 57 
Distribution 

The California spotted owl occurs in coniferous forests, 

mixtures of conifers and hardwoods, and in hardwood forests in 

the western Sierra Nevada; few locations have been documented 

east of the Sierran crest (figs. 4A and 4B). They also occur in 

conifer, conifer/hardwood, and hardwood stands in mountainous country of southern California, and in coastal mountains and 

foothills from the Santa Barbara area north at least into Monterey 

County (figs. 4A and 4C). Most owl pairs in the Sierra Nevada 

occur in the mixed-conifer forest type, but in the mountains of 

southern California they are almost equally represented in three 

major habitat types-mixed-conifer, live oak/bigcone Douglas-fir, 

and riparian/hardwood (tables 3A and 3I) (habitat types are 

described in Chapter 1 and Appendix B). 

58 Chapter 4 

Areas in the coastal mountains north of Santa Cruz appear 

to have suitable habitats. These have not been inventoried adequately for owls, but they appear similar to others in the State 

where spotted owls are known to breed. Finally, spotted owls 

may also occur in denser stands of riparian/hardwood forests, 

especially in foothills bordering eastern portions of the Central 

Valley and along the south-central California coast (fig. 4D). 

The potential spotted owl habitat in lower-elevation hardwood 

forests depicted in figure 4D includes habitat within the ranges 

of both the northern and the California spotted owl. It encompasses about 5,000,000 acres, of which about 1,100,000 acres 

are rated "high" as potential owl habitat (Greenwood and Steger 

pers. comm.), with 625,000 acres of the high-potential habitat 

within the range of the California spotted owl. Essentially none of 

this has yet been surveyed for spotted owls, so we cannot estimate 

the number of pairs, if any, that may be located there. Obviously 

surveying these habitats, on a sample basis, is a priority need. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992 

Figure 4C-Detail of the relative abundance of California spotted owls in the Southern California Province. 
Figure 4D-Statewide distribution of potential California spotted owl habitat in lower-elevation hardwood forests.' 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. Chapter 4 59 
Habitat Associations 

Quantitative details about habitat associations of California 

spotted owls are given in Chapters 5, 6, and 7. Generally, 

however, the birds occur only in habitats with substantial tree 

cover and especially with some larger, older trees present (see 

the set of color photos at the end of Chapter 5). 

Nesting Habitat 

Habitats used for nesting typically have greater than 70 

percent total canopy cover (all canopy above 7 feet), except at 

very high elevations where canopy cover as low as 30-40 percent may occur (as in some red fir stands of the Sierra Nevada). 

Nest stands typically exhibit a mixture of tree sizes and usually 

at least two canopy layers, and some very large, old trees are 

usually present. Often these have large, natural cavities, broken 

tops, and/or dwarf mistletoe brooms. Nest stands in conifer 

forests usually have some large snags and an accumulation of 

fallen logs and limbs on the ground; downed woody debris is not 

a major component of nest sites in lower-elevation riparian/ 

hardwood forests. 

Spotted owls do not build their own nests but depend mainly 

on finding a suitable, naturally occurring site. Nest heights vary 

regionally-about 38 feet in riparian/hardwood forests at lower 

elevations; about 65 and 57 feet in conifer forests of the northern 

and southern Sierra Nevada, respectively; and about 58 feet in 

conifer forests in the San Bernardino Mountains (table 5K). In 

Sierran conifer forests, nests are usually in cavities or on 

broken-topped trees or snags. Less often they are on platforms 

associated with abandoned raptor nests, squirrel nests, dwarf 

mistletoe brooms, or debris accumulations in trees (Chapter 5). 

LaHaye (1988) found an increase from north to south in the 

proportion of platform nests used by northern spotted owls in a 

study in northwestern California. Similarly, cavity nests dominate nest types of California spotted owls in the Sierra Nevada, 

but platform nests predominate in the San Bernardino Mountains (table 5I). These trends probably reflect the distribution of 

stand ages rather than latitude. 

Nest trees (details in Chapter 5) are typically large [mean 

diameter at breast height (d.b.h.) of about 45 inches for nest trees 

in Sierran conifer forests and 37 inches in the San Bernardino 

Mountains] and decadent. Among 124 nests found on NFs in the 

Sierra Nevada, 34 were in snags and 90 were in live trees. 

Eighty-two (66.1 percent) were in cavities and 19 (15.3 percent) 

were on broken tops of living or dead trees, or on dwarf mistletoe brooms. These conditions all tend to develop in older trees. 

Only 17 (13.7 percent) of the nests were on stick platforms built 

and used previously by other species (probably including goshawks, sharp-shinned hawks, ravens, and tree squirrels). Some 

owl pairs use the same nest cavity or platform repeatedly from 

year to year, some select new sites each year, and yet other 

alternate nest sites over time (Foreman et al. 1984, p. 31; R. J 

Gutiérrez pers. observ.; LaHaye pers. comm.). 

60 Chapter 4 

The species of nest trees used seems to depend on what is 

available, with 10 species of conifers and 7 species of hardwoods accounting for all nests in our database (table 5J). Locations of nest trees in the Sierran sample ranged in elevation from 

1,000 to 7,740 feet, with 86 percent found from 3,000 to 7,000 

feet in elevation. The highest elevation of nests increased from 

the northern to the southern Sierra Nevada (Lassen NF - 6,400, 

Plumas NF - 6,100, Tahoe NF -7,000, Eldorado NF - 6,340, 

Stanislaus NF - 7,200, Sierra NF - 7,500, and Sequoia NF - 7,740 

feet). The distribution of breeding spotted owls in the Sierra and 

Sequoia NFs extends down at least to 1,000 feet in elevation 

(Neal et al. 1989). The lower elevation of the study area with 

nests at 1,000 feet is bounded by a reservoir, so owls elsewhere 

may nest at even lower elevations. Few surveys have been done 

at these lower elevations. In southern California, nests ranged in 

elevation from 1,000 feet (Los Padres NF) to 8,400 feet (San 

Bernardino NF). The mean elevation of nest sites in the San 

Bernardino Mountains was about 6,000 feet. 

Roosting Habitat 

Stands used for roosting are similar to those used for nesting, with relatively high canopy cover, dominated by older trees 

with large diameters, and with at least two canopy layers. Studies of roosting northern spotted owls indicate that they respond 

to variation in temperature and exposure by moving higher or 

lower within the canopy, or around the roost tree, to access more 

comfortable microclimates (Barrows and Barrows 1978, Forsman 

1980, Barrows 1981, Solis 1983, Forsman et al. 1984). The 

structure of multistoried stands characteristic of roost sites facilitates this movement. This observed response led Barrows and 

Barrows (1978) to propose that old-growth forests are necessary 

for spotted owls to avoid heat stress. Based on the following 

observations, however, we doubt that avoidance of heat stress is 

an essential benefit of old-growth forests: (1) California spotted 

owls are relatively common in riparian/hardwood forests in 

southern California and the Sierran foothills, where ambient 

temperatures are high during summer months. (2) A female 

nested in full sunlight on an abandoned Cooper's hawk nest 

platform in the Sierran foothills. .While incubating, she was 

sometimes exposed to ambient temperatures exceeding 100 degrees Fahrenheit, and the developing young experienced like 

conditions (Steger pers. comm.). (3) Adult California spotted 

owls often roost in full sunlight or high in the canopy on hot 

days, which is not typical of northern spotted owls (R. J. Gutiérrez 

pers. observ.). We agree that the birds probably move about in a 

forest canopy to find a comfortable microclimate, but they seem 

to be able to tolerate relatively high ambient temperatures. 

Foraging Habitat 

Foraging habitats include suitable nesting and roosting sites, 

as well as more open stands, regularly down to 40-50 percent 

canopy cover, that are generally similar in structure and composition to nesting and roosting habitat. Typical conditions in 

conifer forests include: 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992 
1. A mixture of tree sizes, usually with some trees exceeding 2 feet in d.b.h., resulting in tree canopies at a wide 

range of heights in a stand but not necessarily in distinct 

layers. (Many sites depicted in the color photos at the 

end of Chapter 5 were taken in areas where spotted owls 

were known to forage.) 

2. Signs, of decadence-snags, overmature trees, downer 

woody debris (large logs are especially characteristic). 

3. The presence of hardwoods probably tends to enhance 

foraging habitat in conifer forests. 

4. Ample open flying space within and beneath the canopy 

Conditions in foraging habitat in hardwood stands in southern 

California, and at lower elevations in the Sierra Nevada, tend to 

have less downed woody debris than found in conifer forests 

and stands tend not to be multilayered. Because woodrats dominate the diets of owls in these hardwood habitats, we can infer 

that they provide good habitat for woodrats as well. Live oaks 

are common in the canopy and a variety of shrubs provide food 

(leaves, buds, flowers, and so on) for woodrats (Chapter 10). 

Home Range and 

Territoriality 

Spotted owls generally have large home ranges, defined by 

Thomas et al. (1990, p. 419) as "the area to which the activities 

of an animal are confined during a defined period of time." By 

contrast, an animal's "territory" is generally a defended area 

(Nice 1941) within its home range. Home ranges of radio-tagged, 

neighboring spotted owls overlap to varying degrees (reviews in 

Thomas et al. 1990, appendix I; Carey et al. 1992; Chapter 6). 

Observations by Forsman et al. (1984, p. 52-53) indicate that 

aggression between neighboring birds is infrequent, apparently 

confined to interactions between members of the same sex, and 

generally more pronounced when an intruder is well within the 

home range of another bird. These observations suggest a form 

of territoriality in which "...an individual or a pair may be 

dominant in the core area of its home range but not in the periphery. This tends to produce a regular dispersion by effectively 

excluding other individuals from breeding in the core without 

necessarily excluding their presence there as subordinates engaged in other activities" (Brown and Orians 1970, p. 244). 

Median, combined home ranges of members of pairs of 

northern spotted owls, estimated from radio-tagged birds and 

using minimum convex polygons as the estimator, ranged from 

1,411 acres in the South Umpqua River Valley in the Klamath 

Mountains of southwestern Oregon to 9,930 acres on the Olympic Peninsula in northwestern Washington (Thomas et al. 1990, 

p. 194). Home ranges were generally larger in Washington than 

in areas to the south. In Oregon, areas where median pair home 

ranges exceeded 5,000 acres were usually in heavily logged sites 

with a low percentage of the landscape covered by older forests 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

(Forsman et al. 1984, Carey 1985, Thrailkill and Meslow 1990). 

Home ranges of the California spotted owl exhibit similar variation in size, being measured in thousands of acres in 

higher-elevation conifer forests but only in hundreds of acres in 

foothill woodlands of the Sierra Nevada (Chapter 6). We strongly 

suspect that prey availability accounts for a major part of the 

variation in home-range size of the spotted owl (see discussion 

below in section entitled "Why Differences in Home-Range 

Size?"). 

As in other bird species, some spotted owls do not exhibit 

fidelity to an area, their movements indicating instead that they 

do not occupy a specific home range (Chapter 6). Juveniles often 

wander widely in search of a vacant home range, and similar 

behavior may occur among adults displaced for some reason 

from their former home range. They may move within or among 

the home ranges of other birds, where they await opportunities to 

join the breeding population when a territorial owl dies or deserts 

its territory. These birds are referred to as "floaters" by ornithologists (Smith 1978, 1984). Their role in avian population 

dynamics is believed to be critical (review in Thomas et al. 1990, 

p. 295; Franklin 1992), but less is known about them than any 

other subset of bird populations. This is true because floaters are 

typically quiet and secretive, avoiding contact with territorial 

birds and being less susceptible to detection by researchers. For 

these reasons, we have no quantitative information on the ratio 

of floaters to territorial birds in any population of spotted owls. 

Vocalizations 

Spotted owls communicate with a variety of hoots, "barks," 

and whistles (Forsman 1976, Forsman et al. 1984). The context 

of some of these calls is unknown, but researchers generally 

agree on the function of some of the more common calls. For 

example, the most common call is the four-note hoot, phonetically described as "hooo---hoo-hoo---hooooo." It advertizes the 

fact that an owl is on its territory and probably functions both to 

repel intruders and to attract potential mates. Given at a lower 

pitch and intensity, this same call is also used by the male to 

announce prey delivery to his mate, and in other sexual interactions. Owl biologists most often imitate this call when attempting to locate or attract spotted owls in the field. 

Another common call is the multiple-note "series," which is 

a highly variable rendition of the basic four-note hoot (Fitton 

1991). It is used by birds in an excited state. Spotted owls also 

produce whistles that usually serve to maintain contact between 

members of a pair (Forsman et al. 1984). The calls of females are 

higher-pitched than those of males, facilitating identification of 

sexes in the field. Calls of spotted owls also vary spatially and 

temporally (Ganey 1990, Fitton 1991). 

Chapter 4 61 
Pair Bond 

All available evidence indicates that spotted owls are monogamous. Forsman et al. (1984, p. 53) concluded that pair 

bonds are usually maintained from year to year primarily because (1) individuals of both sexes tend to remain within their 

home ranges, and (2) they tend to exclude other individuals of 

the same sex from their home ranges. Owls that remain throughout the year in the same home range generally maintain a solitary 

existence during the nonbreeding period, seldom even roosting 

near their mates. Mate constancy, therefore, may be "more a 

function of the attachment to a traditional home range than 

attachment to a particular mate." Observations of migrant spotted owls support this conclusion. Mates do not migrate or spend 

the winter months together. Survivors return to their former 

summer home ranges, where former pair bonds are renewed if 

both members of the pair survive (Verner et al. 1991). 

Not all pair bonds last for as long as both members of the 

pair survive, however. "Divorces" have been observed in several 

demographic studies of spotted owls, but they are rare and 

reasons for them are unknown (R. J. Gutiérrez pers. observ., 

LaHaye pers. comm., Steger pers. comm.). When an owl of 

either sex loses its mate, it may desert its old home range and 

orm a new pair bond elsewhere; it may remain on the old home 

range and acquire a new mate there; it may remain unmated for 

an extended period of time; or it may become a floater. Too few 

instances have been observed to identify a pattern, although 

LaHaye suspects that females that lost mates left their old home 

range more often than was the case for males in his study in the 

San Bernardino Mountains. 

The Annual Cycle 

Knowledge of the owl's annual cycle is essential for delineating any restrictions on projects and activities that might be 

planned within owl territories. The following account of the 

spotted owl's breeding chronology is drawn mainly from Forsman 

et al. (1984), who have provided the best quantitative information. Although it is based primarily on observations of pairs, 

nests, and broods of the northern subspecies in Oregon, we 

believe it is the same or similar for the California spotted owl. 

Estimated periods for the different stages of the cycle were based 

on unpublished information and summaries provided by National Forest biologists; by Blakesley (pers. comm.) and Steger 

(pers. comm.) for the Sierra Nevada; by LaHaye (pers. comm.) 

for the San Bernardino Mountains; and by Stephenson (pers. 

comm.) for owls at lower elevations in southern California. 

62 Chapter 4 

The Breeding Cycle 

Available evidence indicates that spotted owls are physiologically capable of first breeding at the age of 2 years (Barrows 1985, Miller et al. 1985), although rare cases of nesting by 

yearlings are known (LaHaye pers. comm., Steger pers. comm.). 

Determining the "usual" age at first breeding is complicated by 

the fact that nesting by the owls does not occur annually, even 

among older birds. As a result, a bird that has attained reproductive maturity may not nest for one or more years beyond that age, 

probably for ecological reasons. 

Here and elsewhere in this assessment we recognize five 

stages of the breeding cycle-prelaying, laying, incubation, nestling, and fledgling. The timing of these stages (fig. 4E) is especially important information for management purposes. Because 

not all birds begin nesting at the same time, the duration of each 

stage for all owls in a region, such as the conifer forests of the 

Sierra Nevada, is considerably longer than it is for a single pair. 

The breeding cycle of California spotted owls extends from 

about mid-February to mid- or late September or early October, 

when young are no longer cared for regularly by their parents 

(fig. 4E). The cycle apparently begins earlier in some places than 

in others in a given year. For example, Steger (pers. comm.) 

believes that spotted owls in the foothills of the Sierra Nevada 

initiate breeding about 2 weeks earlier than birds in the higher 

conifer forests at the same latitude. The various stages tend to 

begin about 4 days earlier in the San Bernardino Mountains than 

in the conifer zone of the Sierra Nevada (fig. 4E). As in the Sierra 

Nevada, timing at lower elevations in southern California apparently precedes that in the San Bernardino Mountains by about 2 

weeks (Stephenson pers. comm.). Reasons for these differences 

may be related to local differences in peak periods of prey abundance for the owls. Before this can be determined, however, much 

more information is needed to refine details of the owl's breeding 

cyles and periods of prey abundance in all localities. 

Prelaying Stage (duration variable) (see Forsman et 

al. 1984, p. 34) 

Members of nonmigrating pairs generally do not roost together during the winter. By late winter, however, they increasingly roost together, engage in mutual preening, and occasionally copulate in the evening. For 2 weeks or so before the date of 

first laying, paired birds typically roost together every night and 

copulate once or twice each evening. Beginning about a week 

before laying, the female spends most of her time near the nest, 

receiving an increasing share of her food from the male. These 

activities probably do not last as long for birds that migrate, 

because members of migrant pairs spend the winter in different 

locations and do not return to their breeding territories until 

shortly before laying begins. Because the prelaying stage has no 

clearly definable beginning, we have arbitrarily designated the 3 

weeks prior to laying of the first egg as the prelaying stage. 

Laying Stage (1-6 days) 

Data suggest that the peak laying period probably occurs 

from about 7 to 21 April in the San Bernardino Mountains and 

from about 11 to 25 April in conifer forests of the Sierra Nevada. 

When egg laying begins, a female spotted owl spends almost all 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
of her time in the nest, and her mate provides nearly all of her food. 

Copulation continues on a daily basis throughout the egg-laying 

stage and for up to about 4 days after incubation begins. 

The clutch size of the spotted owl is one of the smallest 

among North American owls (Johnsgard 1988). Most clutches 

contain two eggs; three-egg clutches are infrequent and four-egg 

clutches are rare (only two records-Bendire 1892, Dunn 1901). 

The interval between laying of successive eggs is 72 ± 6 hours 

(Forsman et al. 1984, p. 33), so the laying period would be 1 day 

for a clutch of one egg, 3 days for a clutch of two, and 6 days for 

a clutch of three. (A rare clutch of four eggs would presumably 

stretch the laying period to 9 days.) 

Incubation Stage (30 ± 2 days) (Forsman et al. 1984, 

p. 33) 

Incubation begins shortly after laying of the first egg and is 

done solely by the female, who may leave the nest at night for 

periods up to 2 hours during the first 2 days of incubation. 

Thereafter, she only occasionally leaves the nest for periods of 10 

to 20 minutes at night to regurgitate pellets, defecate, preen, or 

accept food from her mate. The female does not forage during the 

incubation period, receiving all her food from her mate. The male 

typically roosts within 650 feet of the nest during the daytime and 

begins to forage shortly after sunset (Forsman et al. 1984, p. 35). 

Coincident with the laying of eggs, the female develops a 

brood patch-"a feather-free area with thickened skin and a rich 

supply of blood vessels to facilitate the transfer of heat from the 

body of the incubating bird to the eggs" (Pettingill 1970, p. 355). 

The presence of a well-developed brood patch is clear evidence 

that a female has been incubating, 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

Nestling Stage (normally 34-36 days) (Forsman et al. 

1984, p. 37) 

The peak hatching period probably occurs from about 3 to 

17 May in the San Bernardino Mountains and from about 7 to 21 

May in the conifer zone of the Sierra Nevada. The female broods 

the new hatchlings almost continuously for 8-10 days, still depending on her mate to provide food for herself, and now for the 

young. By the time her young are 2-3 weeks old, the female 

begins to forage for increasingly longer periods at night-typically 1-4 hours. The male continues to bring food to the nest, but 

the female then passes the food to the young. Apparently the 

male seldom, if ever, passes food directly to nestlings. Forsman et 

al. (1984, p. 35) reported that they never observed males feeding 

nestlings. If the male brings food to the nest while the female is 

away, he simply leaves it in the nest. The female continues to roost 

in the nest until 3-6 days before the young leave it. 

Most young observed by Forsman et al. (1984) fledged (left 

the nest) when 34-36 days old, occasionally moving off the nest 

to perch on nearby limbs for a few days before leaving the nest 

permanently. Occasionally young leave their nest earlier than 

normal. Because such young are less developed physically, they 

may spend more time on the ground than young that remain in 

the nest for the full nestling period. This may increase their 

mortality rate compared to that of later-fledging young (Forsman 

et al. 1984, p. 36). 

Fledgling Stage (80-120 days) (Forsman pers. 

comm.) 

The fledgling stage covers the period after the young leave 

the nest until they become independent of their parents. The 

Chapter 4 63 

Figure 4E-Chronology of the breeding seasons of California spotted owls in conifer forests of the Sierra Nevada 

and the San Bernardino Mountains. 
peak period of fledging probably occurs from about 8 to 22 June 

in the San Bernardino Mountains and from about 12 to 26 June 

in conifer forests of the Sierra Nevada. New fledglings are weak 

fliers, often falling to the ground, where they may spend several 

hours to several days. Within about 3 days after fledging (assuming a normal nestling period of 34-36 days), most young can 

flutter or climb to elevated perches; usually in a week they can 

fly clumsily between trees. Within about 3 weeks after fledging, 

they can hold and tear meat from prey brought by their parents 

(Forsman et al. 1984, p. 37). Although adult males bring food for 

the fledglings at all ages, they generally do not give the food 

directly to the young until they have been out of the nest for at 

least 2 weeks (Forsman pers. comm.). Both parents regularly 

bring food to the fledglings and generally continue to do so until 

mid- to late September, apparently regardless of the age or capabilities of the young. Because of this, the fledgling stage may be 

relatively long or short, depending upon when a given nest was 

begun and on variations in the age of the young at fledging. 

The Nonbreeding (Winter) Period 

Activities of spotted owls during this period are primarily 

related to basic maintenance-capturing prey, securing protection from the elements, avoiding predators, preening; and so on. 

The beginning of the nonbreeding period is technically the 

date when adults quit feeding their young, although this may not 

be well-defined because feeding may continue sporadically even 

well after the young can capture and kill prey for themselves. 

Three changes in owl status indicate that 1 October is a reasonable beginning of the nonbreeding period in the Sierra Nevada, 

at least for most birds: Young are generally independent of their 

parents by late September; juveniles begin dispersal as early as 1 

October (Laymon 1988); and some adults begin fall migration 

early in October (Laymon 1988, Neal et al. 1989). The end of the 

winter period coincides with the beginning of activities characteristic of the prelaying stage. This is also a poorly defined date, 

partly because the initiation of prelaying behavior is not abrupt 

and partly because timing differs among pairs. Many pairs, 

however, have initiated prelaying activities at least by the end of 

February, and egg laying and incubation begin at some nests by 

the end of March. For planning purposes, therefore, we can 

reasonably set the end of the winter period at the end of February, at least for most birds. 

Movements 

Regular, long-distance movements of birds beyond 

home-range boundaries are of two types-migration and dispersal. Migration is an annual movement between breeding and 

nonbreeding home ranges. Natal dispersal is the movement of 

young birds from their natal territory to a site where they breed 

or at least establish a territory where breeding could occur (Green64 Chapter 4 

wood 1980). Sometimes adult birds are displaced from established territories by loss of habitat through fire, logging, or other 

major disturbances. Adults displaced by loss of habitat, or separating from a mate, may begin searching for new territories. Their 

movement to a new territory is referred to as breeding dispersal. 

Migration 

Migration is rare among northern spotted owls (Forsman et 

al. 1984), although a few individuals in the Washington Cascades have moved back and forth several miles between breeding and nonbreeding home ranges without an appreciable change 

in altitude, latitude, or habitat type (Brewer and Allen 1985). 

Some radio-tagged California spotted owls in the Sierra Nevada 

have migrated altitudinally, however, moving downslope for the 

winter period on the Eldorado NF (Laymon 1988) and the Sierra 

NF ( Neal et al. 1988, 1989, 1990; Verner et al. 1991). At least 8 

of 10 birds in Laymon's study left breeding home ranges in 

mixed-conifer forests (elevations 4,000-5,200 feet) to spend the 

winter in oak/pine woodlands (elevations 1,000-3,500 feet). The 

mean elevational displacement between breeding and nonbreeding 

areas was 2,450 feet; the straight-line distance between areas 

was 10 to 36 miles (mean = 20). The earliest migrant left its 

summer home range on 18 October, the latest on 18 November. 

Dates of return to breeding areas were not determined, because 

batteries in the radio transmitters failed during the winter. The 

owls were, however, back in their breeding territories by mid-April. 

Six of 18 radio-tagged owls on the Sierra NF migrated from 

summer home ranges in mixed-conifer forest at 5,700-6,700 feet 

in elevation to winter home ranges in oak/pine woodlands at 

1,100-2,050 feet in elevation. Departure dates ranged from 8 

October to 17 December; one female departed her summer home 

range on 8 October 1987, 8 November 1988, and 20 November 

1989. Vertical displacement ranged from 3,680 to 5,541 feet 

(mean = 4,628) and straight-line distances between areas were 

9.6 to 15.3 miles (mean 12.3). Spring return dates ranged from 

27 February to 28 March. 

None of four owls radio-tagged by Call (1990) migrated 

during his study on the Tahoe NF (elevations from 2,200 to 

5,200 feet), where he tracked from summer through early December in both 1986 and 1987. Call's tracking period in both 

years lasted well beyond the fall migration dates observed by 

Laymon, and >2 feet of snow had accumulated in his study area 

by December of 1986. 

No members of any pair in either the Eldorado NF or the 

Sierra NF study migrated together or established winter home 

ranges in the same area. In both studies, at least one migrant 

moved back and forth between winter and summer home ranges 

once or twice during the winter. Six of the 18 owls in the Sierra 

NF study whose movements were well-documented were 

year-round residents, remaining within essentially the same 

home-range boundaries during summer and winter periods. Two 

simply enlarged their summer home ranges for the winter period, and five shifted their home range for the winter period but 

their summer and winter home ranges overlapped. Nine of the 18 

were tracked through at least two annual cycles; none of these 

birds changed behavioral patterns from one year to the next (that 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992 
is, once a migrant always a migrant, and so on). The Sierra NF 

study included another 11 owls that remained as permanent 

residents in foothill riparian/hardwood forests, at elevations ranging 

from 1,000 to 4,000 feet. Home ranges of these birds were often 

shared in winter with migrants from higher elevations. 

Data available from these studies thus show individual 

variation in migratory behavior of the California spotted owl. 

Collectively, the three studies tracked 32 birds with summer 

home ranges in the higher-elevation conifer forests long enough 

to determine whether they did or did not migrate; at least 14 (44 

percent) were altitudinal migrants. Because the sample is small, 

however, we cannot sort out the reasons why some individuals 

migrated and others did not. Nor can we determine why all 10 

owls in the Eldorado NF study migrated, none of the four in the 

Tahoe NF study migrated, and only about a fourth of the 18 birds 

in the Sierra NF study migrated. It does not appear to be the case 

that habitat quality was best in the Tahoe and poorest in the 

Eldorado study area. Only 35 percent of the Tahoe study area 

was in stands of large sawtimber (≥21 inches in d.b.h.). Because 

Call (1990) did not report the proportion of the large sawtimber 

that also had ≥40 percent total canopy cover, we assume that 

something less than 35 percent had both large sawtimber and 

suitable canopy cover. The Sierra study area was only about 14 

percent large sawtimber (table 6C). On the other hand, the 

Eldorado study area, with the highest proportion of migrants, 

was 39 percent in large sawtimber that also had ≥40 percent 

canopy cover (Bias 1989). Finally, in the Sierra study area, some 

owls left areas for the winter that were subsequently used during 

the winter by other owls (Steger pers. comm.), suggesting that the 

birds that left them did so for reasons other than food shortage. 

Altitudinal migration may expose owls to added sources of 

mortality, some related to various human activities. They may 

cross roadways and be hit by a vehicle. In many lower sites, in 

foothills of the western Sierra Nevada, traditional wintering 

areas for the owls are being developed for home sites, even 

communities. And the development of residential properties, 

with resulting homes, lawns, driveways, and so on, will eliminate otherwise suitable habitat for woodrats-the staple winter 

diet of spotted owls in these oak/pine woodlands (table 4A). 

Finally, foothill woodlands are used extensively to graze cattle 

and to harvest firewood from oaks. Both of these activities can 

have negative impacts on woodrat populations and on the cover 

value of the habitat for owls. 

Table 4A -A summary of California spotted owl diets, expressed as estimated percent biomass from different 

studies. 

Sierra Nevada Southern California 

 Mixed- Riparian/ 

 Riparian/hardwood evergreen mixed 

 Sierran mixed-conifer forest woodland forest hardwood 

Lassen Sequoia/ Riverside, Los Padres 

National Eldorado National Kings Cyn. San Diego, National 

Forest Forest Nat. Prk. Sierra National Forest Orange cos. Forest 

 Spring/ Summer Summer Spring/ Spring/ Fall/ Spring/ Fall/ All Spring/ 

Diet summer summer summer winter summer winter seasons summer 

Woodrats1

 10.1 35.0 37.5 4.0 8.3 0 73.8 88.0 79.3 93.9 

Northern flying squirrel 61.1 24.5 31.3 60.0 50.1 76.6 9.6 0 0 0 

Mice2 1.1 3.0 1.2 1.8 1.5 1.5 1.4 2.6 6.0 1.2 

Pocket gophers3

 6.2 1.3 8.1 26.9 20.4 14.7 11.6 5.6 6.0 2.5 

Moles4 1.6 4.2 2.0 3.4 3.0 2.1 0.6 0.2 0.2 0 

Diurnal squirrels5

 4.4 16.5 6.8 4.0 1.9 0 0 0 0.2 0 

Lagomorphs6

 6.1 0 0 0 0.8 0 0 0 1.0 0 

Shrews and voles 2.1 2.4 0.7 0.2 1.2 0.5 1.1 0.6 0.2 0 

Bats <0.1 0.3 0 0.1 0.1 0 Trace 0 0 0.5 

Birds 6.7 12.7 12.4 2.7 12.3 4.3 1.9 3.0 7.1 1.5 

Insects 0.6 0.1 0.3 0.3 0.5 0.3 0.1 0.1 0.3 0.3 

Number of prey items 1,008 1,232 139 162 531 71 234 556 296 30 

Elevation (feet) 4,700- 3,500- 3,000- 5,300- 4,500- 4,500- 1,000- 1,000- 3,000- 1,000 

 6,800 5,000 5,500 6,900 7,000 7,000 4,500 4,500 5,500 3,600 

Source Blakesley Thrailkill Roberts Smith 

 (pers. Laymon and Bias (pers. Barrows (pers. 

comm.) (1988) (1989) comm.) Steger and Eberlein (pers. comm.) (1980) comm.) 

1

 The Lassen sample included both bushy-tailed and dusky-footed woodrats; only dusky-footed woodrats occurred elsewhere. 

2

 Most were white-footed mice (Peromyscus spp.), but some samples included minor percentage of pocket mice, jumping mice, or house mice. 3

 As many as three species in some samples. 

4

 As many as two species in some samples. 

5

 Included ground squirrels, tree squirrels, and chipmunks. 

6

 The Lassen sample included one pika; all other lagomorphs identified at all sites were rabbits (Sylvilagus spp.)

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. Chapter 4 65 
Dispersal 

Successful dispersal is essential for population viability. 

Without it, a population will slowly decline to extinction, because deceased individuals in the breeding population will not 

be replaced by recruits from dispersing juveniles or adults that 

have been displaced or have not yet secured a territory. As Miller 

(1989, p. 1-2) stated, "The distance between adjacent pairs or 

groups of breeding owls should be such that dispersal of juveniles can replace losses (deaths or emigrations) among existing 

pairs and provide for colonization of suitable, unoccupied habitats. An understanding of dispersal in juvenile spotted owls is 

thus basic to formulation of criteria for appropriate spacing of 

habitat to accommodate pairs of owls." We especially need data 

on the variability of dispersal distances, dispersal directions, and 

habitats used by dispersing birds. We also need quantitative 

information on the extent to which fragmentation of forest habitats impedes successful dispersal (Chapter 9). 

Dispersal Among California Spotted Owls 

Unfortunately, information on dispersing California spotted 

owls is nearly nonexistent. Four radio-tagged juveniles on the 

Eldorado NF (Laymon 1988) initiated dispersal from 1 to 24 

October. One was never relocated, another moved 1 mile before 

it was found dead on 1 December; the others moved straight-line 

distances of 8.8 and 11.5 miles from their natal sites. This last 

bird crossed two major rivers and a major highway before 

settling in oak/pine woodlands near the town of Columbia, at an 

elevation of 1,300 feet. Two radio-tagged juveniles on the Sierra 

NF (Steger and Eberlein pers. comm.) moved straight-line distances of 6.1 miles (radio signal lost) and 12.7 miles from their 

natal areas. The latter bird moved from mixed-conifer forest into 

oak/pine woodlands. Three color-banded juveniles in the Sierra 

study dispersed 3.4, 3.5, and 4.1 miles from natal areas to their 

first territories; four adults banded as members of pairs later 

shifted territories, moving 2.1, 2.2, 2.4, and 4.4 miles. All of these 

banded birds were relocated because they stayed within the Sierra 

NF demographic study area (Steger and Eberlein pers. comm.). 

A male color-banded as a fledgling, probably on the Eldorado 

NF, was found on the Stanislaus NF on 5 September 1990. It was 

paired and had raised at least one young. This bird was reared in 

mixed-conifer forest and later paired with a bird in the same 

forest type. The straight-line distance from its natal home range 

to its breeding territory was at least 68 miles. 

Observations of Dispersing Juvenile Northern 

Spotted Owls 

Because information on dispersal of California spotted owls 

is so meager, we rely here on studies of dispersal by northern 

spotted owls to establish quantitative information needed for this 

assessment. We believe this is a reasonable course of action, 

because the two subspecies are so closely related that we have no 

reason to expect dispersal behavior or capability to differ markedly between them. 

Over a 4-year period in western Oregon, Miller (1989) 

fitted 48 juveniles with radio transmitters and monitored their 

movements regularly. Thirty-two survived to disperse from na66 Chapter 4 

tal areas (mean = 104 days after fledging). Twenty-seven (84 

percent) initiated dispersal between mid-September and 

mid-October. Their initial movement was usually rapid, and 

"...most juveniles settled into well-defined areas for their first 

winter after the initial dispersal movements. Those...surviving 

their first winter often began moving again in late winter or early 

spring." From a subset of birds positively identified to sex, males 

dispersed an average of 16.2 miles (SD = 14.6; n = 7) and 

females an average of 20.4 miles (not significantly different-- 

SD = 6.6; n = 6). Initial directions taken by dispersing juveniles 

did not differ from a random distribution, although six of nine 

juveniles in 1983 dispersed down the McKenzie River drainage 

from the H. J. Andrews Experimental Forest. Miller found no 

significant relation between forest fragmentation and either the 

final distance moved or the number of days survived by birds in 

his study. Dispersing juveniles used a wide variety of habitats, 

but 12 of 18 birds exhibited significant selection for old-growth 

and mature forests. 

In a study of 23 dispersing juveniles in northwestern California (11 in 1983 and 12 in 1984), the birds departed from their 

natal areas from 22 September to 5 October (Gutiérrez et al. 

1985). Dispersing juveniles in 1983 moved a mean of 5 miles 

per day (range 1 to 11), compared to 1.3 miles per day in 1984 

(range 0.8 to 6.4). The difference was statistically significant. 

Directions taken by dispersing birds varied. They left their natal 

areas in random directions, with no relation between dispersal 

direction and the geographic orientation of drainages or ridges. 

During the first 80 days of dispersal, individuals moved total 

distances of 15.3 to 92.9 miles (n = 11) in 1983 and 0.7 to 62.8 

miles (n = 7) in 1984. Total distance was the sum of all segments 

between successive locations as birds were followed during 

dispersal. Total distance is greater than the straight-line distance 

between beginning and ending points. 

In addition to these more extensive radio-tracking studies of 

dispersing juveniles, we compiled results from all sources to 

estimate dispersal distances of radio-tagged juveniles after they 

left their natal areas (fig. 4F). Only one of the 54 juveniles whose 

dispersal distances are known was later found as a member of a 

mated pair, but apparently it never nested (R. J. Gutiérrez pers. 

observ.). All other birds either died (68 percent), their transmitters failed (27 percent), or they disappeared (5 percent). This 

raises a question about whether estimates of dispersal distance 

from radio-tagged birds were biased because the birds were 

wearing radios. Although radios may have affected their 

survivorship or their ability to form a pair bond, we found no 

evidence that distances moved by radio-tagged juveniles were 

affected by radio transmitters. 

If juvenile spotted owls carrying transmitters tended to die 

sooner than normal, that is, before they dispersed as far as they 

would without a radio, their dispersal distances might be underestimated. Miller (1989) observed that dispersing juvenile northern 

spotted owls tended to move quickly from their natal area to a 

point where they settled for their first winter. Our data set on 54 

dispersing juveniles includes a subset of 31 with data on the 

number of days they dispersed and the number of days they 

survived thereafter. The dispersal period was highly variable ( x 

= 128.3 days; SD = 168.5; range = 0-657 days; n = 31). The 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992 
Figure 4F-Probability of dispersing juvenile spotted owls traveling 

various distances, based on observed dispersal distances of 56 

radio-tagged northern spotted owls (Thomas et al. 1990, appendix P). 

For example, two-thirds of the juveniles would be expected to disperse 

at least 8 miles (Murphy and Noon 1992). 

survival period following dispersal, however, was more consistent ( x = 119.2 days; SD = 37.4; range = 88-156 days, with one 

outlier each at 45 days and 284 days). In this subset of 31 

dispersers, 13 quit dispersing within 46 days after leaving their 

natal territory and 11 quit dispersing after at least 83 days 

(maximum of 657 days). The mean dispersal distances of these 

two groups were 22.4 miles (SD = 15.7) and 22.0 miles (SD = 

8.1), respectively. The correlation coefficient (rho) between 

maximum dispersal distance and the number of days dispersed 

was not significant (r = -0.03; P = 0.88; 95 percent confidence 

intervals were -0.38 and +0.33; n = 31). The power of the test 

was 0.37 for rho = 0.30, 0.61 for rho = 0.40, and 0.83 for rho = 

0.50. The power for any value of rho ≥0.70 was 1.0. Consequently, the likelihood of failing to detect a statistically significant correlation that might have had biological significance was 

slight, and the likelihood of failing to detect a significant rho of 

at least 0.70, if one truly existed, was essentially nil. 

These results suggest that dispersing juveniles quickly attained their maximum dispersal distance from their natal territory. If they continued to disperse thereafter, their movements 

tended to keep them within that earlier-attained, maximum dispersal distance. 

We also computed the correlation coefficient between maximum dispersal distance and the number of days that juveniles 

carried radio tags (r = 0.02; 95 percent confidence intervals for 

the true correlation coefficient were -0.25 and +0.30). Rho was 

not statistically significant (P = 0.87). The power of the test was 

0.60 for rho = 0.30, 0.86 for rho = 0.40, and 0.98 for rho = 0.50. 

The power for any value of rho ≥0.60 was 1.0. Examination of 

scatterplots suggested no patterns that might be missed by just 

using a correlation coefficient. Additional analyses reported in 

Thomas et al. (1990, p. 306) corroborate the conclusion from the 

above results that radio transmitters did not bias estimates of 

dispersal distance. Lacking critical data from dispersing juvenile California spotted owls, we have therefore used the full data 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

set on northern spotted owls to estimate dispersal distances of 

radio-tagged, juvenile spotted owls. The function derived from 

these data (fig. 4F) was used to parameterize one model for 

assessing the sensitivity of the southern California metapopulation 

of spotted owls to various changes in its numbers and distribution (Chapter 9). 

Mortality 

Birds of most species die of undetermined causes, because 

they simply are not found in the inconspicuous places where 

they die. In addition, their bodies are often quickly consumed by 

other animals in the environment. Spotted owls are no exception. 

We have little information on their sources of mortality, certainly not enough to establish proportions or even to rank the 

causes of death. Many dead spotted owls have been examined 

carefully by veterinarians in an effort to determine the cause of 

death, but even those examinations are generally unable to pinpoint the exact cause. Instead, a variety of contributing factors is 

typically suggested. We do know, however, that spotted owls die 

from the usual variety of causes that befall most wild birds (D. 

H. Johnson pers. comm.). 

Predation 

Although spotted owls appear to have few predators, we 

know that great horned owls and goshawks occasionally capture 

and eat them: Forsman et al. (1984, p. 38) reported seeing a 

Cooper's hawk attempt to capture a recently fledged owlet and 

Forsman and Meslow (pers. comm.) reported one incident of 

predation by a red-tailed hawk. The great horned owl and the 

goshawk are both larger than the spotted owl, and all three 

species often occupy the same forested areas. Great horned owls 

tend to be more common in areas with lower tree densities than 

is the case for spotted owls, and the smaller size of spotted owls 

probably enables them to outmaneuver great horned owls in 

dense forest. Forsman (pers. comm.) suspects that great horned 

owls only opportunistically prey on spotted owls. 

Goshawks kill both adult and juvenile owls (Gutiérrez et al. 

1985, Miller 1989, D. H. Johnson pers. comm.), but spotted owls 

sometimes nest within goshawk territories and defend their young 

against attacks by goshawks (R. J. Gutiérrez, pers. observ.). We 

agree with Forsman et al. (1984, p. 38) that goshawks probably 

are not serious threats to spotted owls. 

Accidents and Starvation 

A few deaths from accidents (flying into obstacles, automobiles, and drowning) and starvation have been recorded among 

spotted owls (for example, Gutiérrez et al. 1985, Laymon 1988, 

Neal et al. 1988, 1989; R. J. Gutiérrez pers. obser., Forsman and 

Chapter 4 67 
Meslow pers. comm.). Starvation may result from low abundance or availability of prey, or from lack of hunting experience. 

Death by starvation is more common among juveniles than 

adults (Gutiérrez et al. 1985, Miller 1989, Sisco 1990, D. H. 

Johnson pers. comm.). 

Shooting 

Shooting deaths have also been documented for spotted 

owls. For example, both members of a pair of birds that occupied 

riparian/hardwood habitat in the Switzer Picnic Area in the 

Angeles NF were shot and killed with "BB" guns by two young 

boys (Stephenson pers. comm.). This was apparently just a 

thoughtless act, not a malicious effort to destroy spotted owls, 

but it does raise a question about the safety of owls in heavily 

used recreation areas. 

Diseases and Parasites 

Little is known about diseases and parasites of spotted owls, 

and nothing is known about the extent to which they contribute 

to mortality, although Forsman and Meslow (pers. comm.) observed several instances of mortality that they attributed to diseases or parasites. Gutiérrez (1989) surveyed blood parasites in 

all three subspecies, finding an infection rate of 100 percent, 

exceeding that recorded in nearly all other bird species (Greiner 

et al. 1975). Spotted owls must be adapted to these high parasite 

loads, however, because their survival rates are high even where 

infection rates are high (Franklin et al. 1990). Hoberg et al. 

(1989) examined 20 northern spotted owls for helminth parasites 

and found eight species, representing round worms, flat worms, 

and spiny-headed worms. More than 80 percent of the birds were 

infected with at least one species, and multiple infections were 

common. 

Young et al. (1992) reported two species of hippoboscid 

flies from northern spotted owls in northwestern California. One 

species was found only once among the 382 owls examined, but 

about 17 percent of the owls they examined had hippoboscid 

infestations of the other species. Fly densities on owls were 

higher in years with higher summer and fall temperatures and 

lower winter precipitation. Young et al. (1992) speculated that 

low temperatures may have depressed survival of fly pupae. In 

demographic studies in the Sierra NF and Sequoia/Kings Canyon NPs, hippobiscid flies were detected on 15 of 45 birds (33 

percent), but searching for the flies was not an objective of field 

crews, and limited evidence indicated that the flies were more 

likely to crawl to the outer surface of an owl when it was handled 

longer (Steger pers. comm.). 

68 Chapter 4 

Competition 

The barred owl, probably the closest relative of the spotted 

owl, was historically restricted to eastern North America. Gradually 

it has extended its range westward through Canada, and finally 

southward from British Columbia into Washington, Oregon, and 

more recently into northern California, as far south as the Tahoe 

NF in the Sierra Nevada (G. 1. Gould, Jr., pers. observ.; R. J. 

Gutiérrez pers. observ.). Barred owls are larger and more aggressive than spotted owls in interspecific territorial interactions, 

and they are more generalized in their selection of prey, the 

habitat types they use, and their nest site requirements (Hamer et 

al. 1989). Their recent invasion into the range of the spotted owl 

(Taylor and Forsman 1976) is a possible source of competition 

between these closely related species. Evidence available already indicates that barred owls have displaced spotted owls 

from some sites in Washington (Hamer et al. 1989). Because 

barred owls have now been reported from about 27 different 

sites in California, interactions between these species will bear 

further study. A few hybrids between spotted and barred owls 

have been observed recently. Such hybridization is not uncommon between closely related species of wild birds. The extent of 

hybridization between these two owl species is still very limited, 

however, and the eventual outcome of this hybridization will 

take many decades or even centuries to resolve. 

Diets 

Spotted owls are "perch and pounce" predators (Forsman 

1976), selecting an elevated perch from which they locate potential prey, either by sight or sound. When an owl detects a prey, it 

drops from its perch and attempts to capture the prey in its talons 

(the "pounce"). Spotted owls are agile, capturing prey in shrubs, 

trees, and on the ground. If a potential prey is in an inaccessible 

location or at some distance from the owl's perch, the owl may 

move closer before initiating its pounce. Its silent flight allows it 

to approach prey without being detected. In addition, spotted 

owls are adept at "hawking" behavior-capturing flying prey, 

primarily birds and insects. 

Spotted owls forage primarily at night. Forsman et al. (1984) 

rarely observed daytime foraging by northern spotted owls, 

concluding that it occurred only opportunistically. On the other 

hand, Laymon (1991) concluded that California spotted owls in 

his study on the Eldorado NF, in the western Sierra Nevada, 

foraged regularly during the daytime when they were raising 

young, but not otherwise. Neal et al. (1989) reported that they 

often observed spotted owls-even nonbreeders-foraging for 

insects in the Sierra NF, taking them from bark surfaces, from 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
the ground, and even catching them in the air. These observations were usually in the late afternoon or early evening (Steger 

pers. comm.). 

Marshall (1942) was the first to report on diets of the 

California spotted owl, based on stomach contents of two specimens and 23 regurgitated pellets gathered beneath roost trees at 

Whitaker Forest, Tulare County, in Sierran mixed-conifer forest 

with giant sequoias, at an elevation of about 5,500 feet in the 

western Sierra Nevada. The stomachs contained limb bones of 

frogs or toads, four bats (genus Myotis), one deer mouse, and 

four crickets ("probably Gryllus"). The pellets contained remains of 11 flying squirrels, two deer mice, and single individuals of four other mammal species, six bird species, and one June 

beetle. The relatively high proportions of flying squirrels and 

birds in this small sample are in line with several recent, more 

extensive studies of spotted owl diets in conifer forests of the 

Sierra Nevada (table 4A). 

California spotted owls eat a variety of prey. For example, 

three samples from Sierran mixed-conifer forests produced a 

combined list of at least 78 different species-at least 1 amphibian, 1 lizard, 24 birds, at least 34 mammals, and at least 18 

insects from a combined total of 2,716 prey items (Marshall 

1942--63 prey items; Laymon 1988-1,275 prey items; Steger 

and Eberlein pers. comm.-1,405 prey items). In spite of this 

diversity, only one to five species or species-groups comprised 

at least 5 percent, by weight, of the owls' diets in the eight 

samples shown in table 4A, and at least 85 percent of the total 

biomass in each sample consisted of only one to four species or 

species-groups. Either northern flying squirrels or dusky-footed 

woodrats, or both, dominated prey biomass in all samples. Preliminary results from an analysis of the spotted owl's diet in the 

San Bernardino Mountains agree-about 80 percent of the biomass was dusky-footed woodrats, and about 10 percent consisted of medium-sized mammals like northern flying squirrels 

and gophers (Smith pers. comm.). The general pattern suggests a 

prey specialist with a search emphasis on just a few species, but 

which will capture and eat a wide variety of species and sizes of 

prey as they are encountered. 

Four patterns in the results of these studies may be important: (1) Spotted owls in conifer forests of the Sierra Nevada, 

particularly above mid-elevation mixed-conifer forests (about 

4,000-5,000 feet, depending on latitude), prey mainly on flying 

squirrels. (2) Owls in the mid- to lower elevations of the 

mixed-conifer zone and the upper part of the ponderosa pine/ 

hardwood belt prey heavily on both flying squirrels and woodrats. 

(3) Both of these sets of owls consume a relatively high proportion of diurnal squirrels and/or birds, suggesting more daylight 

foraging than is the case for spotted owls elsewhere. And (4) 

spotted owls in the Sierran foothills and throughout southern 

California, even at high elevations, obtain 79 to 97 percent of 

their energy needs from woodrats. A dichotomy between flying 

squirrel and woodrat dominance of the diet is known for the 

northern spotted owl as well (review in Thomas et al. 1990, p. 

appendix J; Carey et al. 1992). Given the California spotted 

owl's diet, therefore, we need to consider ways to manage for 

habitats that will maintain thriving populations of flying squirrels in Sierran conifer forests and woodrats everywhere else. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

Although woodrats dominate owl diets in Sierran foothill riparian/hardwoods and throughout the owl's range in southern California, only about 25 percent of all California spotted owl sites 

are in habitats where woodrats are abundant (table 1B). The 

remaining 75 percent occur in Sierran conifer forests where 

flying squirrels are the primary prey species. 

Ecological Relations 

Owls and Their Prey 

Common sense tells us that no animal species can survive or 

reproduce in areas without sufficient food, but documenting 

these relations by direct field studies is usually very expensive 

and time-consuming. For owls in general, the time needed for 

such studies is typically several years, or even decades, depending on the life span of the owl species studied and the kinds of 

prey they eat. For example, owls with diets consisting mainly of 

small mammals whose populations exhibit regular, cyclic "booms" 

and "busts" in numbers, will require longer studies to cover at 

least two cycles of the prey. In spite of the high demands on time 

and resources to complete such studies, many have been reported in the literature on owls of the world. They portray a 

general picture of a marked dependency of owls on the 

availability of their key prey species-a fact widely recognized among raptor biologists in general and owl specialists 

in particular. 

Relations between owls and prey are manifested in a variety 

of ways; some species of owls are affected in several ways. A 

moderately extensive (but not exhaustive) search of the world's 

literature on owls revealed at least five common, major effects of 

prey availability on owl biology (table 4B): Owl reproductive 

rates are often positively related to prey availability. Some species of owls nest earlier when and where prey are more abundant 

and available. Some owl pairs do not even attempt to nest when 

prey are scarce. The density of breeding owls is commonly 

higher when and where prey species are more abundant and 

available. And some species of owls exhibit major movements 

(whole populations may shift) when prey become scarce in the 

area occupied. 

Several studies linking prey and spotted owls have been 

undertaken (Thomas et al. 1990), but little evidence has been 

found of relations between prey abundance and the biology of 

spotted owls. In a study in northwestern California, Ward (1990) 

found that prey abundance (mainly woodrats) was low and that it 

varied over the landscape. The owls did not necessarily forage in 

stands where woodrats were most abundant, but they hunted 

instead in areas where the availability of prey was more predictable. The strategy suggests one of optimizing search effort. Only 

the study of northern spotted owls in Washington and Oregon by 

Carey et al. (1992) has been intensive and extensive enough to 

suggest relations between the owls and their prey. Owls in 

Chapter 4 69 

western hemlock forests on the Olympic Peninsula of Washington used about 4,200 acres of old forest annually; those in 

Douglas-fir forests in southwestern Oregon used about 2,000 

acres of old forest annually; and owls in mixed-conifer forests in 

southwestern Oregon used about 1,120 acres of old forest annually. Estimates of the combined biomass of primary prey species 

taken by the owls were 5.3, 21.3, and 29.4 ounces/acre in the 

three forest types, respectively. Carey et al. (1992, p. 241) 

concluded that "Geographic variation in the areas traversed and 

amounts of old forest used by spotted owls reflected similar 

variation in the abundance and diversity of the medium-sized 

mammals that are the preferred prey of the owl." 

Understanding Spotted Owl Habitats 

Through Ecological Linkages 

A full understanding of why California spotted owls occur 

where they do, and why they do or do not reproduce well enough 

to maintain their populations, depends ultimately on comprehending haw various components and functions within forest 

and woodland ecosystems relate to the owl's ecology. Here we 

have attempted to interpret some of those relations in terms of 

the owl's key prey species, typical nest sites, and the general 

dynamics of forest and woodland ecosystems where California 

owls are known to occur and reproduce. A general graphic 

representation of these relations helps to envision how all the 

Figure 4G-Simplified, schematic representation of some important 

ecological linkages associated with California spotted owls. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

pieces fit together (fig. 4G). Assuming that presence and nesting by 

California spotted owls depends on the availability of a 

sufficient prey source, we believe that much of what is known 

about the owl's habitat associations can be better understood in 

relation to the ecology of its primary prey, especially northern 

flying squirrels and dusky-footed woodrats. Other key linkages 

relate to assuring an abundance of suitable nest sites. How does 

all of this relate to what we judge to be important attributes of 

suitable owl habitat? 

Why Big, Old Trees? 

Large, old trees are preferentially selected for nest sites by 

spotted owls (Chapter 5). For example, nest trees averaged 

larger than 40 inches in d.b.h.-much larger than the mean 

diameter of trees generally available. About one fifth of all nests 

found were in snags (dead trees) and about four fifths were in 

live trees. Two thirds of the nests were in large, natural cavities 

formed by decay at sites where branches broke off or tore out of 

the trunk of the tree, and another 20 percent were on broken tops 

of living or dead trees, or on dwarf mistletoe brooms. These 

conditions typically develop only after a tree is relatively old. In 

addition, larger trees are needed to provide large snags and 

longer-lasting components of dead, decaying wood on the ground, 

especially in the form of large logs but also in fallen limbs of 

various sizes. 

Why Downed Woody Debris? 

Functional linkages among spotted owls, their major prey 

species, the prey's food, and the general forest or woodland/ 

shrub community where these linkages occur can be traced in 

figure 4G. Some major linkages are highlighted by broader 

arrows. For example, northern flying squirrels feed extensively 

on hypogeous (underground) fungi, especially during periods 

when the ground is not covered by snow. They may even cache 

some of these fungi to be eaten after snowfall. At least two 

California studies (McKeever 1960, Hall 1991) and one Oregon 

study (Maser et al. 1985) found that flying squirrels eat primarily 

fungi and lichens. Hypogeous fungi comprised the bulk of the 

summer diet, and the winter diet was largely arboreal lichens. 

The density of flying squirrels in red fir/white fir stands in the 

Lassen NF was strongly associated with the abundance of truffles 

(fruiting bodies of hypogeous fungi), and truffle abundance was 

strongly associated with the presence of a well-developed soil 

organic layer and a large volume of decaying logs (Waters and 

Zabel 1992). These data suggest that management practices that 

decrease the soil organic layer and the number of large, decaying 

logs will reduce the capability of a habitat to support flying 

squirrels, and possibly spotted owls as well. 

Hypogeous fungi probably also comprise a major food 

source for white-footed mice (Maser et al. 1978a), an important 

prey species of the California spotted owl. Spores of the fungi 

pass unharmed through the digestive tract of these and other 

small mammals that consume them and are thus spread in fecal 

pellets over the forest floor. All hypogeous fungi are also mycorrhizal: "Mycorrhiza literally means `fungus-root' and denotes a 

symbiotic relationship between certain fungi and plant roots" 

(Maser et al. 1978b, p. 79). Trees depend on mycorrhizae for an 

Chapter 4 71 
adequate uptake of various nutrients, and the fungi benefit by 

obtaining carbohydrates produced by the trees. Interestingly, the 

spores of hypogeous fungi are spread by the small mammals that 

eat them, thus completing a loop of interdependencies in forest 

ecosystems (fig. 4G). 

As important as large, decaying logs are to functional ecosystems where spotted owls seem to thrive, logs that are positioned perpendicular to the slope of the land are most valuable. 

This is because they are best situated to intercept soil and water 

moving downslope and, as a result, to become a substantial 

water reservoir as they reach an advanced state of decay. Even 

more important is the fact that very large logs can make a vital 

contribution to the forest ecosystem for a longer period than they 

did as standing, live trees (Maser 1989). These log reservoirs 

provide refuges for many animal species during hot, dry parts of 

the year, just as they provide water to trees whose roots, aided by 

the symbiotic fungal mycorrhizae, have penetrated them. To 

ignore the role of logs in our forest ecosystems may be to lose 

those ecosystems in the long run. 

Why Snags? 

Spotted owls occasionally select snags for nest sites, either 

on their broken top or in natural cavites in the snags, cavities that 

either carried over from the live tree or were created by decay 

after the tree died. Among the 263 nests reported in our sample 

from conifer forests (table 5I), 17 percent were in snags. Snags 

provide the primary nesting substrate for many other cavity-nesting 

birds as well. Woodpeckers, which are occasionally captured 

and eaten by spotted owls (table 4A), excavate nesting and 

roosting cavities, and a variety of nonexcavating species later use 

the same cavities for nests or roosts. Of particular significance in 

the ecology of spotted owls, flying squirrels often use old woodpecker cavities for den sites. Finally, snags eventually fall and 

contribute to the accumulation of decaying wood on the ground. 

Therefore, the snag component benefits the owls both directly and 

indirectly in a variety of ways (trace arrows in fig. 4G). 

Why Multiple Canopy Layers? 

A possible ecological explanation for the prevalence of 

multi-layered canopies in habitats frequented by spotted owls is 

not clear. The structural diversity associated with these "layers" 

may contribute to a greater diversity of prey species. Perhaps, as 

Barrows and Barrows (1978) have hypothesized, the different 

layers provide opportunities for the owls to move up and down 

within the overall canopy to find the most comfortable microclimate for roosting. Or they may be important in allowing foraging owls to use perches at a variety of heights as they search for 

prey. On the other hand, multiple layers may be simply a covariate 

of some other component of the owl's habitat that is vital, such 

as the owl's prey. 

Why Dense Canopies? 

Among the most consistent habitat relations found for spotted owls is their greater use than expected of stands having 40 

percent or greater (foraging) and 70 percent or greater (nesting 

and roosting) total canopy cover (Chapters 5 and 6). Like the 

owl's association with multiple canopy layers, however, possible reasons for this are not readily apparent. It may relate to one 

72 Chapter 4 

or more of the following: (1) Denser stands tend to be cooler 

and, as proposed by Barrows (1981), they would allow the owls 

a wider range of choices for locating thermally comfortable 

roosts. (2) Denser stands provide more concealing cover, where 

the owls may be able to nest and roost with less chance of 

discovery by potential predators. It may be no coincidence that 

their plumage is speckled, as this pattern would tend to camouflage them during the daytime in a forest full of sunflecks. (3) In 

conifer forests, where flying squirrels dominate the owls' diet, 

prey may be more abundant and available in denser forest stands 

(see Chapter 10). This would not necessarily account for the 

same observed relation between canopy cover and owl habitat 

use in areas where woodrats dominate the diet, however, because 

woodrats tend to be most abundant in relatively dense stands of 

shrubs. To benefit owls, these must be intermingled with, or 

adjacent to, the hardwood stands where the owls roost and nest. 

Why Differences in Home-Range Size? 

In general, the largest home ranges of California spotted 

owls occur where flying squirrels comprise the majority of the 

owl's diet and the smallest home ranges occur where woodrats 

dominate. Home ranges of spotted owls in conifer forests of the 

Sierra NF are several times larger than home ranges <10 airline 

miles away, in foothill riparian/hardwood forests (Neat et al. 

1990). Owls in the conifer forest prey mainly on flying squirrels, 

but those in the low-elevation hardwood stands prey almost 

exclusively on woodrats (table 4A). 

The importance of these prey to the ecology of spotted owls 

has been emphasized by the bolder connecting links in figure 

4G. Although only one study of spotted owls has shown a clear 

connection between prey abundance and areas used by the owls 

(Carey et al. 1992), we strongly suspect that the approximately 

10-fold difference in observed home-range sizes of California 

spotted owls results primarily from regional differences in diet. 

Apart from common sense and the study by Carey et al., our 

strongest scientific support for this contention is the degree to 

which densities and reproductive activities of owl species throughout 

the world-at least those that have been studied well enough to 

establish the relations-are influenced by the availability of their 

prey (table 4B). Not only are woodrat populations denser than 

flying squirrel populations, often by at least 10-fold (Chapter 10), 

but also woodrats weigh nearly twice as much as flying squirrels. 

Why Do Most Pairs of Owls Not Nest Every 

Year? 

Spotted owls exhibit marked yearly variation in the proportion of pairs that nest. This has ranged from essentially no pairs 

to nearly all pairs nesting. For example, from as low as 11 

percent to as high as 70 percent of owl pairs in the Eldorado 

demographic study have nested in different years (R. J. Gutiérrez, 

pers. observ.). On the other hand, Franklin et al. (1990) reported 

little variation in the proportions of pairs nesting during a 6-year 

study in northwestern California. Much annual variation has also 

been observed in nesting success (proportion of pairs nesting 

that also fledge young) from year to year and from region to 

region, ranging from as low as 0 to as high as 100 percent 

(Forsman et al. 1984, Gutiérrez et al. 1984, Thomas et al. 1990, 

Lutz 1992, LaHaye et al. 1992). 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Figure 4H-Percentages of mean annual precipitation in California from 1987 through 1991, by water year (1 October through 30 September) and 

hydrologic region (California Department of Water Resources 1991). 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. Chapter 4 73 
Surveys of California spotted owls have been used to determine distribution, density, and other measures of "status" (see 

tables 3A, 3B, 3F, and 3H-J). These counts overestimate the 

functional owl population, however, because pairs do not occupy all sites in all years, occupied sites do not always support 

pairs, and each pair does not breed every year. A compilation of 

results from sites occupied for 4 consecutive years, adjusted to 

represent 50 known sites where owls defended a territory in any 

one of the 4 years (G. I. Gould, Jr., pers. observ.), suggests that 

only about 41 sites would be occupied in a given year, 34 would 

be occupied by pairs, and only 11 of those would produce young. 

Over the 4-year period, owls at 20 sites, usually the most consistently occupied, would produce 90 percent of the young. 

For many owl species, failure to breed in some years has 

been shown to result from low prey availability (table 4B). As 

for home-range size, even lacking definitive studies of spotted 

owls, we strongly suspect that the local prey base largely determines whether a given pair of owls attempts to nest in a given 

year, and whether it succeeds if it does make an attempt. Drought 

may be a corollary here. Nearly all of the detailed studies of the 

California spotted owl, upon which this report is based, have 

been done during an ongoing drought that began in 1987 (fig. 

4H). A variety of scenarios might occur. For example, drought 

has been observed to depress woodrat populations or not to 

affect them (Chapter 10), so the drought may or may not have 

lowered reproduction among the owls. Mild winters accompanying the drought may have increased the survival rates of the 

owls or the flying squirrels, or both. We cannot reach conclusions about these or other options. All we can do is acknowledge 

the attendant uncertainty. 

Conclusions 

California spotted owls share many attributes of their natural history with the northern spotted owl. Yet our knowledge of 

the California spotted owl's biology is meager relative to its 

more famous northern relative. For example, we are not yet able 

to set clear bounds on the range of habitats that are capabable of 

supporting self-sustaining populations of the California subspecies (Chapter 8). And we are not likely to be able to do this until 

the owls have been studied thoroughly during both wet and dry 

climatic cycles. Our only good estimate of juvenile survival rate 

for the subspecies is based on the population in the San Bernardino Mountains (Chapter 8), although we estimate that about 75 

percent of all California spotted owl pairs occur in the Sierra 

Nevada, where habitat change (by logging) is of greatest concern. Similarly, with the exception of the San Bernardino Mountain study, estimates of age-class survival and fecundity schedules are lacking or are imprecise. No studies are available that 

relate California spotted owl populations to populations of their 

prey species. Finally, we know little about the factors important 

74 Chapter 4 

in the biology of the owl's primary prey species. The work of 

Waters and Zabel (1992) on flying squirrels in the Lassen NF is 

exemplary in that regard. Their work needs to be replicated 

elsewhere, and equally comprehensive studies of woodrat ecology need to be undertaken. 

The general lack of information about nearly all phases of 

the California spotted owl's biology and ecological relations 

leads to high uncertainty about its present status. Because of this 

uncertainty, we recommend continuation of basic ecological 

studies and prudent forest management-following practices 

that will maintain future options. 

References 

Adamcik, Robert S.; Todd, Arlen W.; Keith, Lloyd B. 1978. Demographic and 

dietary responses of great horned owls during a snowshoe hare cycle. 

Canadian Field-Naturalist 92:156-166. 

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Chapter 5 

Habitat Relations of the California Spotted Owl 

R. J. Gutiérrez, Jared Verner, Kevin S. McKelvey, Barry R. Noon, George N. Steger, Douglas R. Call, 

William S. LaHaye, Bruce B. Bingham, and John S. Senser 

The California spotted owl's distribution spans a latitudinal 

range similar in extent to that of the northern spotted owl 

(Johnsgard 1988). It occurs at higher elevations than the northern subspecies, however, and portions of the overall population 

exist in "island" subpopulations on isolated mountain ranges in 

southern California (Gutiérrez and Pritchard 1990). It also occurs within sight of the Los Angeles Basin, with one of the 

largest human populations in North America. Grinnell and Miller 

(1944) first recognized that California spotted owls occupy a 

variety of forest types. The extent of this variety has been 

documented by Laymon (1988), Bias (1989), Neal et al. (1989), 

Call (1990), Call et al. (1991), LaHaye et al. (1992a), and Bias 

and Gutiérrez (1992). In this chapter we summarize the patterns 

of habitat variation and habitat use across most of the range of 

the California spotted owl. 

Study Areas and Methods 

Sierra Nevada 

Six major studies have described habitat relations of California spotted owls in four general areas spanning the length of 

the Sierra Nevada (fig. 5A). From north to south, these four 

study areas were: (1) The Lassen Study Area (Zabel pers. 

comm.), which included primarily high-elevation (5,500-7,200 

feet) forests of red fir and white fir (Rundel et al. 1977) and 

secondarily some lower-elevation habitats dominated by pines 

and Sierran mixed-conifer forests (see color photos 5-1 and 5-4 

at the end of this chapter). The area was a mosaic of selectively 

logged, clearcut, and uncut stands (old-growth). (2) The Tahoe 

Study Area (Call 1990) was primarily in mid-elevation mixedconifer forest (Rundel et al. 1977) at 2,200-5,200 feet. The past 

history of logging there resulted in a diverse patchwork of 

different stand ages, types, and densities. (3) The Eldorado 

Study Area (Laymon 1988, Bias 1989, Lutz 1992) extended 

from low- and mid-elevation mixed-conifer forest to higherelevation fir forest (1;000-7,400 feet) (color photos 5-11, 5-12, 

and 5-18). Logging history has varied there because of the 

pattern of land ownership-about 40 percent of the land was in 

private industrial forests that occupied alternate sections in a 

"checkerboard" pattern with federal lands (Bias and Gutiérrez 

1992). Laymon's study included winter observations of migrant 

owls that moved from their higher-elevation nesting habitats to 

foothill riparian/hardwood forests as low as 1,100 feet. These 

latter sites should not be construed as representative of owl sites 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

in the Sierran conifer zone. (4) The Sierra Study Area, in the 

Sierra National Forest (NF) (Verner et al. 1991), included two 

distinct habitat types-one dominated by mixed-conifer forest at 

elevations ranging from about 4,500 to 7,500 feet, the other 

dominated by hardwoods in oak-pine woodlands and relatively 

dense riparian/hardwood forests at elevations from about 1,000 

to 3,500 feet. Only this study area included breeding populations 

both in high-elevation conifer forests and riparian/hardwood 

forests at low elevations (color photos 5-21 and 5-22). 

Southern California 

Several habitat studies have been done on isolated mountain 

ranges in southern California (fig. 5A). Here we discuss only one 

major study area-the San Bernardino, which encompassed 

almost the entire owl population in the San Bernardino Mountains (LaHaye et al. 1992b). It was the largest of the isolated 

"island" populations of spotted owls in California. The San 

Bernardino Study Area included numerous habitat types (see 

Thorne 1977) because it ranged in elevation from 2,500 to about 

9,000 feet (color photos 5-34 to 5-37). 

The habitat of two isolated populations studied by Gutiérrez 

and Pritchard (1990), on-Mount San Jacinto and Palomar Mountain (fig. 5A) (color photos 5-39 to 5-42, and 5-44), had lower 

diversity than the San Bernardino Study Area and involved 

much smaller owl populations. Barrows (1980) reported observations at five roost sites of pairs or individuals on four mountains in southern California. In addition to these investigations, 

qualitative assessments of habitat have been made by U.S. Department of Agriculture, Forest Service (FS) biologists from the 

various NFs in southern California, but especially from the Los 

Padres NF (Sandburg and Winter 1989) (color photos 5-29 to 5-

32, and 5-38). Observations of owls by forest biologists were 

part of their normal duties, and did not include designation of 

specific study area boundaries. 

Photo Series 

A set of color photographs of California spotted owl habitat 

from throughout the subspecies' range augments our text descriptions of habitats. These photographs, presented as a set at 

the end of this chapter, were taken from June to September 1991 

during field trips by the Technical Assessment Team. The purpose of the trips was to become directly familiar with the full 

range of variation in the habitat occupied by this owl. 

Chapter 5 79 
)LJXUH $/RFDWLRQV RI UDGLRWUDFNLQJ DQG GHPRJUDSKLF VWXG\ DUHDV IRU &DOLIRUQLD VSRWWHG RZOV

 Chapter 5 USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Conifer Forests of the Sierra 

Nevada and Southern 

California 

Patterns of Habitat Use 

at a Landscape Scale 

California spotted owls use forested habitats almost exclusively (color photos 5-1 to 5-48). Only one record is available of 

a pair nesting in a tree not closely surrounded by a stand of other 

trees. That pair nested in a residual snag in a clearcut on the 

Sierra NF, although relatively dense stands of oaks were within 

160 feet of the nest and the young moved there shortly after they 

left the nest (Yamanaka pers. comm.). Spotted owls have occasionally foraged in relatively open areas, such as shrubfields, 

meadows, or plantations, but this is exceptional. Call (1990), for 

example, found such habitats to be used significantly less than 

expected, based on availability in the Tahoe Study Area. We 

included these observations only to illustrate that California 

spotted owls occasionally occur in habitats that do not meet all 

their life history requirements (that is, they are unsuitable). It is 

important to note that one cannot infer suitability from the 

occasional presence of owls in a habitat. 

Data at a Landscape Scale 

Each of the seven Sierra Nevada NFs provided the following information on detected owl nests (n = 148): (1) timber 

type and size of the stand in which the nest was located, based 

on verified database mapping in their Land Management Plans 

(LMPs); (2) the same information as in (1) for each stand 

polygon that bordered the nest stand and was in a timber type 

used for foraging, roosting, or nesting; and (3) a map of the 

nest and adjacent stands. Stands bordering nest stands were 

primarily M2G, M3N, M4N, M3G, M4G, R3N, R4N, R3G, 

and R4G (codes for timber strata are defined in table 1C and in 

Appendix B). The estimated number of acres in habitat types 

listed in table 5A were based on the most recent FS inventory 

data (Fiske, pers. comm.). 

Selective Use of Forest Types 

Based only on those habitat types in which nests had been 

observed and verified, we found a significant difference between 

habitats selected for nesting stands and the overall distribution of 

available habitats in Sierran NFs (table 5A, fig. 5B). A 95- 

percent Bonferroni interval test (see Neu et al. 1974, Byers and 

Steinhorst 1984) on the forest types contributing to this difference (table 5A) showed that, based on availability of different 

forest stand types, spotted owls in the Sierra Nevada nested more 

than expected in mixed-conifer stands with medium- to largesized trees [24 to 36+ inches in diameter at breast height (d.b.h.)], 

stands with a wide range of tree sizes overall (5 to 60+ inches in 

d.b.h.), and medium (40-69 percent) (stand type M4N) to high 

(70-100 percent) total canopy cover (stand type M4G). Fortyfive percent of all nest stands in Sierran conifer forests, however, 

were in the M4N and M4G habitat types. All other habitat types 

that we evaluated were used equal to or less than their proportional availability (table 5A). 

Table 5A-Distribution of California spotted owl nest habitat types compared with proportions of available habitats in the Sierra Nevada. 

95 percent Bonferroni interval 

Habitat Sample Point 

Type1

 size Lower estimate Upper Available Result 

M2G2

 2 0 0.0135 0.0390 0.0209 U = A3

M3P 10 0.0121 0.0676 0.1231 0.2369 U < A 

M3N,G 44 0.1962 0.2973 0.3984 0.2501 U = A 

M4P 10 0.0121 0.0676 0.1231 0.1418 U < A 

M4N,G 67 0.3426 0.4527 0.5628 0.2005 U > A 

R3P 2 0 0.0135 0.0390 0.0448 U < A 

R3N,G 3 0 0.0203 0.0514 0.0522 U < A 

R4N,G 10 0.0121 0.0676 0.1231 0.0464 U = A 

Total 148 

1

 See figure 5B for distribution of habitat types that were signficantly different (x2 = 79.9, 

df = 7, P < 0.05) between used and available types. 2

 Habitat codes correspond to FS timber strata labels: M = mixed-conifer, R = red fir; 2 = 

pole-sized trees (<12 inches in d.b.h.), 3 = small sawtimber (trees 12-24 inches in d.b.h.), 4 = 

medium and large sawtimber (24-40 and >40 inhces in d.b.h.); G = 70+ percent canopy cover, 

N = 40-69 percent canopy cover; P = 0-39 percent canopy cover. 3

 U = A; current evidence does not indicate a difference between use and availability. 

 U < A; used less than expected. 

 U > A; used greater than expected. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. Chapter 5 81 
Figure 5B-Nest locations (n = 148) and proportional availability of 

different timber strata (see Appendix B or table 5A for definitions of strata 

codes). 

In addition to the strata listed in table 5A and figure 5B, four 

nests were in pure stands of ponderosa pine (one each in P3N, 

P4P, P3G, and P4G) and eight nests in hardwood types. Because 

of a lack of consistency among NFs in typing these timber strata, 

we could not develop reliable estimates of their acreages available, or of their associated nests. Consequently, these nests were 

omitted from the analysis of use-versus-availablity presented in 

table 5A. Ponderosa pine types commonly include hardwood 

components, but these are not considered in timber inventories. 

As a result, a site with ponderosa pines in the overstory and an 

understory of hardwoods would be judged by its stratum label to 

be unsuitable for nesting (and possibly even for foraging) by the 

owls, even though that may not be the case. Although we lack 

the needed data to perform rigorous statistical tests, it is our firm 

biological understanding that pine stands with high canopy closure and a significant hardwood component are excellent owl 

habitat. Not only do they have all of the necessary structural 

components, but also they are in an elevational zone where the 

distributions of woodrats and flying squirrels overlap. The presence of a rich and diverse prey base, proper structural characteristics, and relatively mild climatic conditions may well make 

these sites among the highest quality owl habitat in the Sierra 

Nevada. We believe, therefore, that at least the P4G strata is 

selected for nesting by owls, in excess of its availability. 

Bias and Gutiérrez (1992) found that nesting and roosting 

owls on the Eldorado Study Area almost exclusively used mixedconifer stands with medium (M3G) to large (M4G) timber and at 

least 70 percent canopy cover (color photos 5-11 and 5-18). The 

M4G stands were more abundant on public than on private land 

in the study area, and the owls used public lands for nesting and 

roosting significantly more than they used private lands. Gutiérrez 

and Pritchard (1990) found that spotted owls on Mount San 

Jacinto used conifer and riparian/hardwood forests (color photos 

5-41 and 5-42) significantly more than expected, based on their 

availability, and that owls on Palomar Mountain used primarily 

conifer or mixed forests of conifers and hardwoods (color photos 

5-39 and 5-40). 

82 Chapter 5 

Density of Owl Sites in Relation to 

Amount of Suitable Habitat 

Based on intensive surveys for owls by NF personnel and 

others, owl densities in U.S. Geological Survey quadrangles 

(1:24,000; n = 49) in the zone of mid-elevation, Sierran mixedconifer forests were significantly, but weakly, correlated with 

the percentage of forests having medium-sized and larger trees 

and high canopy closure (R2 = 0.38, P < 0.05; fig. 5C). We 

interpreted these relations cautiously, however, because survey 

effort was not uniform among the survey units and error exists in 

the type mapping of mixed-conifer habitat by the FS (Call 1990, 

Bias and Gutiérrez 1992, G. N. Steger pers. observ.). Nevertheless, this analysis suggested that, as with nest stands, owl densities were higher in areas with a higher proportion of dense stands 

and large trees. 

Patterns of Habitat Use 

at a Home-Range Scale 

Here we report results of various studies that have compared attributes of sites used for nesting, roosting, and foraging 

with the same attributes measured at sites randomly selected 

from the surrounding forest. Only results of studies done in 

conifer forests of the Sierra Nevada and San Bernardino Mountains are reported here. Methods used to measure habitat attributes varied among the studies. Although direct statistical 

comparisons could not be made across all studies, we believe all 

methods provided objective, quantitative estimates of the attributes in local owl habitats. 

All workers except Laymon (1988) used consistent methods within their studies, and workers from three study areas 

(Tahoe--Call 1990, Eldorado-Bias 1989, and San Bernardino--LaHaye et al. 1992a) used consistent methods among the

studies. Workers from these latter three studies used a variable 

circular-plot design, using basal area prisms that resulted in an 

increasing plot size with increasing diameters of trees, an estimation procedure commonly used by foresters for estimating 

Figure 5C-Density of owl sites in the Sierra Nevada (n= 49) in relation 

to the proportion of habitat considered to be suitable for spotted owls in 

U.S. Geological Survey quadrangles (1:24,000). 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Although our conclusions from studies in the Tahoe Study 

Area were basically the same as those reported by Call (1990) 

and Call et al. (1991), some details of our results differed slightly 

because we eliminated from Call's sample of random points all 

locations with <40 square feet per acre in basal area of softwoods. Eliminating recent clearcuts and shrubfields allowed 

more meaningful biological comparisons with foraging sites. In 

addition, we subsampled the sample of foraging sites by eliminating, at random, all but one location from each owl, or owl 

pair, in a given day. 

In addition to studies that compared habitat attributes 

between random sites and sites used by owls, B. B. Bingham 

(pers. observ.) and Steger and Eberlein (pers. comm.) sampled 

nest stands in the Lassen Study Area and the Sierra Study 

Area, respectively, but not in comparison with random sites. 

In these studies, vegetation at nest sites was sampled using 

strip transects arranged perpendicular to one another and centered on the nest tree. Beginning 32.8 feet from the center of 

the nest tree, the long axes of four strip transects (32.8 by 

113.7 feet-total area sampled = 0.30 acre) radiated on four 

sides of the nest tree along perpendicular lines through the 

nest tree. No vegetation data were collected within the 32.8 by 

32.8 foot square centered on the nest tree. 

Finally, Steger and Eberlein (pers. comm.) sampled habitats, by canopy-cover class (0-39 percent, 40-69 percent, and 

70+ percent), throughout the radio-tracking portion of the 

Sierra Study Area, based on stratified random samples of 

habitat polygons in both the foothill riparian/hardwood and 

mixed-conifer portions of the study area. Their method used 

four sets of nested plots randomly located in each polygon. 

Plots were 16.4 by 328 feet for all trees and snags, 32.8 by 328 

feet for trees >35.4 inches in d.b.h., and 65.6 by 328 feet for 

snags >15.7 inches in d.b.h. 

Differences Among Methods 

Laymon (1988), Bias (1989), and LaHaye et al. (1992a) all 

used tree-centered plots for the evaluation of the nest sites, but 

they did not center random plots on trees. This method introduced a potential bias in the comparisons between nest/roost 

sites and random locations. It also had the potential to bias the 

inferences drawn concerning stand attributes based on nest locations. Data from Call (1990), B. B. Bingham (pers. observ.), and 

Steger and Eberlein (pers. comm.) did not have this bias. 

The magnitude of any bias introduced by centering on a 

large tree is related to the density of similar trees in the surrounding stand and the size of the sample plot. One way to envision the 

bias is to ask how likely it is that a random plot (of identical size 

and shape) in the same stand would sample either the nest tree or 

another tree having similar size and condition. We would expect 

samples centered on nest trees to be more strongly biased than 

roost locations-roosts are often in small trees in dense stands, 

whereas nest trees are generally larger than trees in the surrounding stand (for example, tables 5B-5D). 

Based on plot size, Laymon's (1988) data may be the most 

biased. He used small (0.17-acre), fixed plots and all of his owluse plots, including foraging locations, were tree-centered. Bias 

(1989) and LaHaye et al. (1992a) both used variable-plot methUSDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

ods with a basal area factor (BAF) of 20. This sampling technique has the advantage that plot size increases with the size of 

the nest tree. For a tree 40 inches in d.b.h., the plot size is 0.44 

acres; it is nearly an acre for a 60-inch tree. 

Estimating the Bias-We could not repeat these studies or 

obtain all the raw data for re-analysis. We could, however, 

estimate the probable magnitude of the statistical bias due to 

tree-centering and determine the extent to which it changed the 

inferences drawn from these analyses. For Bias (1989) and 

LaHaye et al. (1992a), we corrected for total basal area simply 

by removing the nest/roost tree. When prism sampling, overall 

basal area can be determined simply by counting the number of 

"count trees" in the plot. Each counted tree represents a certain 

basal area per acre, as determined by the BAF. With a BAF of 

20, for instance, each tree represents 20 square feet basal area per 

acre (BA/acre). At nest and roost sites, Bias obtained an average 

of 251 and 294 square feet BA/acre, respectively (table 5B), and 

LaHaye et al. reported 223 and 267 square feet BA/acre for nest 

and roost sites, respectively (table 5C). On average, Bias and 

LaHaye et al. were "counting" between 11 and 15 trees in each 

sample plot. Removing the nest tree from the basal area calculation would remove only one tree from each plot, a reduction of 

20 square feet BA/acre. This suggests a range from 203 to 274 

square feet BA/acre in their nest and roost stands. 

While this decrease is not large, we believe it probably 

exceeds the potential bias of including the nest or roost tree in 

the sample. For example, if the nest tree were not within the plot, 

most likely other trees would be. The basal area of these trees 

would, in part, compensate for the removal of the nest tree. To 

obtain better estimates of the true bias, we analyzed nest site data 

from B. B. Bingham (pers. observ.) and Steger and Eberlein 

(pers. comm.). These data were taken in the vicinity of the nest 

tree but did not include the nest tree. We added the nest tree and 

recomputed live tree and snag basal area. For Bingham's data, 

live tree basal area was increased by about 7 square feet BA/acre 

and was not significantly different from the original estimate (P 

= 0.18). Basal area for the largest diameter-class, however, was 

shifted by 16.4 square feet BA/acre (P = 0.002). Similarly, snag 

basal area was significantly overestimated by including the nest 

snag in the sample. For Steger's and Eberlein's data, live tree 

basal area was increased about 4 square feet/acre, not significantly different from the original estimate (P = 0.17). Snag basal 

area was increased 5 square feet/acre, not significantly different 

from the original estimate (P = 0.36). 

B. B. Bingham (pers. observ.) and Steger and Eberlein 

(pers. comm.) used fixed plots of 0.3 acres. These plots were 

smaller than the plots associated with large d.b.h. classes using a 

BAF of 20, but not greatly so. We believe these data can be used 

to make reasonable inferences concerning the magnitude of 

bias in the results of Bias (1989) and LaHaye et al. (1992a). 

Laymon's (1988) plots were about half the size of those used 

by Bingham and by Steger and Eberlein, so the potential for 

bias is therefore greater. 

Conclusions-Given their sampling methods, results from 

Bias (1989) and LaHaye et al. (1992a) should be interpreted with 

a number of cautions. The overall basal area figures and basal 

area for all trees >16 inches in d.b.h. would not be significantly 

Chapter 5 83 
Table 5B-Habitat characteristics (mean and percent Coefficient of Variation) of California spotted owl nest 

(n =11) and roostsites (n = 29), and random sites (public land, n =328) in the Eldorado Study Area, central Sierra 

Nevada (Bias 1989). 

Nest sites Roost sites Random sites 

Mean Percent CV Mean Percent CV Mean Percent CV 

Physiographic attributes 

Percent slope 26.1 46.8 30.1 58.3 32.3 61.2 

Elevation (feet) 4,900.0 18.6 4,561.0 14.6 4,728.3 18.1 

Structural attributes 

Percent cover 89.3 16.1 95.6a 3.9 79.2 32.0 

Basal area (square feet per acre) 

 Hardwoods 38.3 69.1 43.6 60.9 61.3 138.1 

 Softwoods 234.4 43.8 268.4a

 44.0 151.6 80.2 

Total live 251.0 25.1 293.7a

 37.0 173.4 70.5 

Snag 52.7a

 45.8 52.7a 72.8 37.0 134.2 

Basal area (square feet per acre, by d.b.h. class)1, 2

 3.9-4.8 5.5 - 7.4 - 3.9 - 

4.9-10.7 14.5 - 24.4 - 44.7 - 

10.8-20.6 89.9 - 90.3 - 61.9 - 

20.7-35.5 91.0 - 82.7 - 52.4 - 

 >35.5 110.9 - 95.1 - 42.6 - 

Downed woody debris (percent cover, by diameter class in inches) 

 1-11.8 inches 12.1 57.8 10.2 69.0 11.2 122.6 

 >11.8 inches 2.7 87.0 2.6 187.8 3.9 244.6 

a

 Significantly different from the corresponding random sample, at the 0.001 level of significance 

(1-way ANOVA, using Scheffe test for multiple comparisons). 1 Diameter at breast height, in inches. 

2 CVS could not be computed for these data. 

Table 5C-Habitat characteristics (mean and percent Coefficient of Variation) of California spotted owl 

nest (n = 131) and roost sites (n = 43), and random sites (n = 296) in the San Bernardino Study Area, 

southern California (LaHaye et al. 1992a).

Nest sites Roost sites Random sites 

Mean Percent CV Mean Percent CV Mean Percent CV 

Physiographic attributes 

Percent slope 51.2a

 50.8 54.7a 44.6 32.3 68.8 

Elevation (feet) 6,052.0a

 20.9 6,299.2a 18.5 6,942.3 14.7 

Structural attributes 

Percent cover 76.9a

 21.3 83.6a 15.3 52.2 50.5 

Basal area (in square feet per acre) 

Total live 222.7a

 41.7 266.7a 35.2 124.2 68.8 

Snag 21.4a

 120.9 20.9a 123.3 7.8 217.3 

 Dead-topped trees 12.2 143.3 15.7 99.1 7.0 224.6 

Hardwoods (by d.b.h. class)1

 0-5.9 5.2 263.0 10.5 182.8 14.8 245.2 

 6.0-11.8 13.1 207.1 32.7 182.3 14.8 230.3 

11.9-17.7 14.4 224.6 25.7 163.9 3.5 356.0 

 >17.7 19.6 151.0 28.8 162.5 3.1 391.6 

Total 52.31 129.5 97.61 119.5 36.2 169.0 

Softwoods (by d.b.h. class) 

 0-9.8 12.2 178.1 16.6 160.5 13.1 207.1 

 9.9-19.7 27.0 123.8 41.4 128.4 24.8 135.7 

19.8-29.5 46.2 100.3 43.6 117.4 21.8 127.8 

 >29.5 85.4 77.1 67.1 67.7 29.2 124.1 

Total 170.4a

 58.9 168.6a 68.5 88.0 85.1 

a Significantly different from the corresponding random sample, at the 0.001 level of significance (MannWhitney U test). 1

 Diameter at breast height, in inches. 

84 Chanter 5 USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 
Table 5D-Habitat characteristics of California spotted owl roost sites in summer (n =120), fall (n = 78), and winter (n = 61), and at random sites throughout summer/ 

fall (n = 120) and winter (n = 66) in the Eldorado Study Area, central Sierra Nevada (Laymon 1988). Note that all winter data were obtained in low-elevation, oakpine forests, where the birds migrated for the winter (see Chapter 4). 

 Roost sites Random sites 

 Roost sites Random sites 

Summer Fall Summer/fall in winter in winter 

Mean Percent CV Mean Percent CV Mean Percent CV Mean Percent CV Mean Percent CV 

Physiographic attributes 

Percent slope 18.1a

 36.3 21.1a 35.6 23.4 46.8 26.8a 36.4 19.9 30.6 

Structural attributes 

Percent cover 85.8a

 6.4 75.2 9.4 67.9 29.0 63.5b 12.9 29.7 32.8 

 Basal area (square feet per acre) 

 Hardwoods 20.9 79.8 12.6a

 106.5 26.1 146.0 18.7b 56.7 7.0 126.9 

 Softwoods 330.7a

 21.6 259.7a 25.9 163.8 62.6 57.1b 65.1 29.2 78.8 

 Total live1

 351.6 - 272.3 - 189.9 - 75.8 - 36.2 - 

 Snags 41.4a

 57.6 47.5a 60.8 17.4 246.4 35.7b 168.3 19.6 63.2 

 Downed woody debris (tons per 

 acre, by log diameter class)2

 0-3 inches 0.82 0.30 0.90 0.32 0.82 0.74 0.23b

 0.78 0.18 0.50 

 >3 inches 12.16a

 0.59 10.53 0.72 9.36 1.39 1.56b 1.55 0.62 1.44 

a

 Significantly different from corresponding random sites in summer/fall, at the 0.05 level of significance (Student's t test). b

 Significantly different from corresponding random sites in winter, at the 0.05 level of significance (Student's t test). 1

 The sum of softwood and hardwood basal areas. Data were not available to compute a percent Coefficient of Variation for this variable. 2

 Assumes a specific gravity of 0.4 for downed woody debris. 

biased. Basal area in the largest diameter-class would, however, 

be overestimated, by perhaps as much as 20 square feet BA/acre. 

Similarly, snag basal area would be overestimated by perhaps 10 

square feet BA/acre. Laymon's (1988) basal area data should be 

viewed with caution. For all of these studies, we have no reason 

to believe that estimates of canopy closure were affected by treecentering (for example, see table 5B). 

No important changes in inferences made from results of 

these studies were associated with these biases, although it is 

possible that the statistical significance of certain test results 

could be altered. It is important to note that in Bias (1989) and 

LaHaye et al. (1992a) the statistically significant basal area

differences from random samples were associated most strongly 

with sites of roost trees, where we do not expect tree-centering to 

have as much impact as at nest tree sites. It is also worth noting 

that the statistical results presented by Call (1990), B. B. Bingham 

(pers. observ.), and Steger and Eberlein (pers. comm.) were not 

based on tree-centered plots. 

Nest and Roost Sites 

Physiographic Attributes-Mean elevation at nest sites in 

conifer forests increased -from about 5,300 feet in the northern 

Sierra Nevada to 6,000 feet in southern California-about coincident with low- to mid-elevation mixed-conifer zones. Nests at 

the highest elevations were in red fir forests in the Sierra Nevada 

and white fir forests in the San Bernardino Mountains. No nests 

have been located in subalpine forests anywhere in California. 

Blakesley et al. (1992) reported that nest sites of northern 

spotted owls in northwestern California tended to be on steeper 

slopes and in the lower third of canyons. This pattern was true 

for the California spotted owl only in the San Bernardino Study 

Area (table 5C). Based on statistical methods for analyzing data 

from circular distributions (Zar 1984, p. 422-469), the mean 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

aspects of nesting, roosting, and random sites were not significantly different in the San Bernardino Mountains (LaHaye et al. 

1992-). Assuming equal availability of aspects, significantly 

more nest sites in the Sierra Nevada were located on north 

aspects and fewer on southwest aspects than expected (n = 148; 

P < 0.001; fig. 5D). These results should be viewed with caution, 

however, as we cannot evaluate the validity of the assumption 

that all aspects were equally available. 

Roost sites used by California spotted owls were similar in 

structure and composition to those used for nesting, although we 

know less about owl roosting habitat than nesting habitat. Some 

spotted owls have repeatedly used the same roost sites, suggesting that they have narrow selection patterns for roosting (Barrows 

and Barrows 1978, Barrows 1980, Steger and Eberlein pers. 

comm.) or that they tend to return to familiar or favorable roosts. 

Within study areas, roost and nest sites were similiar in 

elevation and slope (tables 5B and 5C), probably because the 

owls often roosted near a nest site (LaHaye et al. 1992a). All of 

the roosting California spotted owls found by Barrows (1980) 

were on north-facing slopes. Studies in the Sierra Nevada (Laymon 

1988, Bias 1989) and in the San Bernardino Mountains (W. S. 

LaHaye pers. observ.), however, did not reveal a significant 

Figure 5D-Proportions of nest sites on slopes facing different directions 

from all nest sites located on National Forests within the range of the 

California spotted owl (n = 143). 

Chapter 5 85 
difference in mean aspect between roost sites and random locations. The owls studied by Barrows (1980) were in habitats at 

low elevation, where the dense-canopied stands selected by the 

owls would be found most often on north-facing slopes. 

Structural Attributes-Mean canopy cover at nest and roost 

sites, for the three studies that measured these variables (tables 

5B-5D), was consistently higher than the canopy cover at random samples-on average, about 20 percent higher. Canopy 

closure at nest sites was not significantly higher than at random 

locations in the central Sierra Nevada (table 5B), but it was 

significantly higher in the San Bernardino Mountains (table 5C). 

Roost sites had significantly (P = 0.001) higher canopy closure 

than random sites in all three studies. 

Mean softwood basal area was also consistently higher at 

nest and roost sites in all three studies. Total live tree BA/acre 

averaged 127 square feet greater in nest and roost stands than at 

random sites. As with canopy closure, measures of basal area 

were more often significantly different between roost sites and 

random sites than between nest sites and random sites (tables 

5B-5D). In all cases allowing a statistical comparison, roost sites 

had significantly more total live tree basal area and basal area of 

softwoods than random locations. At nest sites, total live tree 

basal area and softwood basal area were consistently higher than 

at random sites in both studies reporting these results (tables 5B 

and 5C), significantly so for both attributes in the San Bernardino Study Area (table 5C). 

Tree size-class data were available for nesting and roosting 

sites in the Eldorado NF (table 5B) and the San Bernardino NF 

(table 5C) and for nest sites in the Sierra and Lassen NFs (table 

5E). In the Eldorado NF, nest sites averaged 111 square feet BA/ 

acre in trees >35.5 inches in d.b.h. Nest sites in the San Bernardino NF had, on average, >85 square feet BA/acre in trees >29.5 

Table 5E-Habitat characteristics (mean and percent Coefficient of Variation) of spotted owl nest sites in the Lassen Study 

Area (n = 24), southern Cascade region of northern California (B. B. Bingham pers. observ.), and the Sierra Study Area 

(n = 11), southern Sierra Nevada (Steger and Eberlein pers. comm.). 

Lassen Study Area Sierra Study Area 

Mean Percent CV Mean Percent CV 

Physiographic attributes 

Percent slope 25.8 51.6 7.9 66.2 

 Elevation (feet) 5,599.6 8.0 4,840.0 7.9 

Structural attributes 

Percent cover 85.2 9.8 85.6 13.8 

 Live trees1

 Basal area (square feet per acre) 

 Total 243.8 36.7 185.8 73.3 

 Stems per acre (d.b.h. class in inches)2

 5-10 93.2 72.0 96.3 73.3 

 11-20 60.3 50.8 35.9 43.7 

 21-35 17.1 73.8 16.6 57.0 

 >35 8.0 86.7 5.2 88.7 

 Stems per acre (height class in feet) 

 13-25 14.8 251.7 99.0 101.9 

 26-39 44.3 87.1 35.0 60.1 

 40-53 34.2 84.4 25.1 65.3 

 54-79 42.4 83.4 35.0 62.3 

 80-105 20.8 92.3 10.4 90.9 

 106-131 13.2 70.6 9.2 135.3 

 132-157 6.7 160.1 6.4 215.2 

 158-183 1.8 172.0 2.1 145.2 

 >183 0.4 358.7 0.6 222.5 

 Snags 

 Basal area (square feet per acre) 

 Total3

 47.5 85.0 33.0 167.2

 Large4

 21.3 119.1 19.5 131.8 

 Stems per acre (d.b.h class in inches) 

 5-10 8.8 171.4 20.2 146.5 

 11-20 7.7 185.8 5.8 109.9 

 21-35 5.9 79.5 2.5 151.8 

 >35 2.8 130.8 1.2 222.0 

Downed woody debris (tons per acre, by log diameter-class)5

11-20 inches 3.2 103.2 2.5 78.6 

21-35 inches 7.7 94.4 5.1 131. 

 >35 inches 11.4 100.7 13.5 128.5 

1

 Only live trees with d.b.h. >4 inches. 

2

 Diameter at breast height, in inches. 

3

 Snags >4 inches in d.b.h., and ≥4.6 feet tall (Lassen Study Area) or ≥6.6 feet tall (Sierra Study Area). 4

 Snags ≥15 inches in d.b.h. and ≥20 feet tall. 5

 Only logs >10 inches in diameter at the large end. Assumes a specific gravity of 0.4 for downed woody debris. 

86 Chapter 5 USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
inches in d.b.h. Nest sites in the Sierra and Lassen NFs averaged 

five and eight stems per acre >35 inches in d.b.h., respectively. 

These data, when combined with the analysis of nest-site 

preferences based on timber strata (table 5A, fig. 5B), demonstrate that a significant large-tree component is present in most 

owl nest stands. 

At nest sites, basal areas were highest in the large-tree 

classes (>24 inches in d.b.h.) and decreased through each of the 

smaller size-classes (tables 5B and 5C). This often resulted in 

multi-storied nest stands dominated by larger trees with a welldeveloped understory (for example, color photo 5-27). The vertical stratification in these stands was not as extensive as that 

observed in nest stands of the northern spotted owl in northwestern California (LaHaye 1988, Solis and Gutiérrez 1990, B. B. 

Bingham pers. observ.). 

Percent canopy cover, total live tree basal area, softwood 

basal area, hardwood basal area, and snag basal area were generally greater at owl roost sites than at random locations (tables 

5B-5D). As at nest sites, the basal area of trees at roost sites was 

concentrated in the large size-classes, but with trees of all sizes 

resulting in multi-storied canopies. 

Many of these parameters exhibited a large degree of variability, and the differences between habitat used by spotted owls 

and random locations may or may not have been statistically 

significant within a given study. The data were, however, consistent and mutually supportive among all studies. California 

spotted owls in these various studies chose to nest and roost in 

stands that were denser than average and that contained a largetree component. Most nest sites were selected in dense mixedconifer stands with average quadratic mean diameters of canopy 

trees >24 inches in d.b.h. We know of no data that contradict 

these findings. 

Size of Activity Centers 

Activity centers are areas within which owls find suitable 

nesting sites and several suitable roosts, and in which they do a 

substantial amount of their foraging. Using the sample of nest 

trees from conifer forests in the Sierra Nevada (n = 148), we 

estimated the sizes of stands containing the nest trees and the 

cumulative size of each nest stand plus all adjoining stands that 

were of a timber type used equal to or greater than its availability 

(table 5A). These adjoining stands may make important contributions to activity centers, because the owls have direct access to 

them. In some instances, nesting pairs may even spend more 

time in one or more of the adjoining stands than they do within 

the stand containing their nest. 

The mean size of nest stands was 99.9 acres (SD = 114.9 

acres) and the median was about 65 acres. The mean size of the 

nest stand plus adjacent stands was 306.7 acres (SD = 386.6 

acres) and the median was about 265 acres. Numerous roosts 

would be available in the nesting and adjoining stands. Cumulative distribution functions for these two variables provide two 

important insights (fig. 5E). The majority of nest stands were 

smaller than 100 acres, but, in contrast, the majority of nest-plusadjacent stands exceeded 200 acres. 

Based on results of radio-tracking studies, the latter variable 

provides a better estimate of the size of activity centers. An 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

Figure 5E-Cumulative distribution of nest stands, and nest stands plus 

adjacent stands, arranged by size. Adjacent stands were restricted to 

those timber types used equal to, or greater than, their availability (n = 138). 

Chapter 5 87 

     

$UHD DFUHV

estimate of the area used for foraging in an activity center may be 

approximated by the area that includes half of the nighttime 

(foraging) locations of owls during the breeding period, as estimated by the adaptive kernel method (Worton 1989). In the 

radio-tracking study in the Sierra NF (G. N. Steger pers. observ.), 

this averaged 317 acres (SD = 202; n = 9) in 1987, 296 acres (SD 

= 110; n = 7) in 1988, and 310 acres (SD = 127; n = 9) in 1989. 

These estimates were not independent from year to year, because some of the same individuals' home ranges were included 

in 2 or 3 of the years. In the radio-tracking study in the Lassen 

NF (Zabel pers. comm.), the average was 788 acres (SD = 347; n 

= 10) in 1989-90. In all cases, only birds with at least 20 

nighttime locations were used for these estimates. 

Nest Sites on Private Timberlands 

Hofmann and Taylor (1992) reported general habitat conditions at 18 nest sites on industrial forest lands in the northern (n = 

4), central (n = 5), and southern (n = 9) Sierra Nevada. Fourteen 

of the pairs nested successfully, but the report did not indicate 

the proportion of all owls found on private lands that nested or 

the proportion that fledged young. The owls nested in a variety 

of forest stands, ranging from sparse (5 percent) to high (86 

percent) cover by multi-storied stands with large-diameter trees. 

Reported values of site attributes were within ranges given 

above for nest sites in NFs. Overall, 60-65 percent of the area 

within a radius of 1,000 feet of nests had "dense" canopy cover. 

No other structural habitat features were reported. 

Taylor (1992) provided additional information on the habitat associations of owls nesting on industrial forest lands in the 

Sierra Nevada. Based on a sample of 28 nests, Taylor found 

"...a clear pattern of dominance of nest sites by medium and 

large d.b.h. stands..." and the data to clearly indicate "...that 

owls nest in areas with moderate-to-dense canopy closure" 

(Taylor 1992, p. 9). These results are consistent with those 

reported in tables 5B-5D. 
Table 5F-Structural attributes of habitats (mean and percent Coefficient of Variation, by canopy cover class) used by California 

spotted owls during radio-tracking studies in the mixed-conifer zone of the Sierra Study Area, southern Sierra Nevada (Steger and 

Eberlein, pers. comm.). 

 0-39 percent 40-69 percent 70+ percent 

 canopy cover canopy cover canopy cover 

 (n =64) (n = 35) (n = 21) 

Mean Percent CV Mean Percent CV Mean Percent CV 

Percent canopy cover 

 By densiometer 26.5a

 67.6 60.0b 23.9 73.7c 14.0 

 By aerial photo 21.0a

 49.9 52.9b 16.7 74.8c 6.8 

Basal area (square feet per acre) 

 Green trees 61.4a

 84.2 172.2b 41.3 211.3c 20.7 

 Snags 4.7a

 137.0 12.1b 76.3 13.9b 64.1 

 Hardwoods (by d.b.h. class)1

 0-4.7 inches 0.1a

 300.0 0.3 200.0 0.6b 269.2 

 4.8-10.6 inches 0.7a

 280.0 1.5b 144.1 2.0b 104.1 

 10.7-20.5 inches 1.1a

 250.0 2.0 188.9 1.6b 141.0 

 20.6-35.4 inches 0.6 323.1 1.3 313.8 2.0 233.3 

 >35.5 inches 0.1 800.0 0.6 442.9 0.0 0.0 

 Total 2.6a

 193.3 5.6b 160.9 6.2b 109.4 

Softwoods (by d.b.h. class)1 

0-4.7 inches 1.7a 137.5 3.7b 107.1 4.4b 72.4 

 4.8-10.6 inches 7.2a

 107.8 13.9b 70.8 21.2c 46.6 

 10.7-20.5 inches 15.4a

 99.2 39.5b 45.3 54.5c 38.2 

 20.6-35.4 inches 22.6a

 107.9 63.2b 60.8 74.0b 60.0 

 >35.5 inches 12.0a

 148.0 46.3b 79.4 51.0b 75.7 

 Total 59.0a

 84.2 166.5b 43.9 205.2c 21.1 

Shrubs per acre 3.7a 325.5 18.0b 293.7 8.4 250.9 

Downed woody debris (tons/acre, by log diameter-class)2

 11-20 inches 2.1a

 88.9 3.2b 75.5 3.1b 74.2 

 21-35 inches 4.0a

 102.1 4.7b 100.7 6.9b 83.7 

 >35 inches 3.3a

 213.3 9.3b 144.7 8.2b 106.1 

a,b,c`Values in the same row with different alpha superscripts are different at the 0.05 level of significance (one-way ANOVA). 1

 Diameter at breast height, in inches. 

2

 Assumes a specific gravity of 0.4 for downed woody debris. 

88 Chapter 5 USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

in d.b.h.) significantly more than expected (P = 0.01). In addition, the probability of an owl's using large timber was significantly greater than of its using medium timber (P = 0.003). 

Based on vegetation sampled at foraging locations by Call (1990, 

p. iv), a "discriminant function analysis indicated that the owls 

selected habitats with late-successional stand characteristics including mature and old-growth timber [>35 inches in d.b.h.], 

multiple vegetation strata, and high live timber basal area." The 

combined results from Laymon's and Call's studies suggest that 

spotted owls in these Sierran conifer forests tended to forage in 

stands of intermediate to older ages. 

Based on three canopy-closure classes (see table 5F), 

owls in the Sierra Study Area used stands with 70+ percent 

canopy closure significantly more and stands with 0-30 percent canopy closure significantly less than expected (Verner et 

al. 1991; also see Chapter 6). 

Physiographic Attributes-Laymon (1988) found that random sites were significantly steeper than foraging sites during 

both summer and fall in the Eldorado Study Area (table 5G), 

but Call (1990) found no difference in the Tahoe Study Area 

(table 5H). 

Foraging Sites 

Attributes of foraging habitats used by California spotted 

owls have been estimated in only two studies (Laymon 1988, 

Call 1990). In both studies, the range of habitat structures used 

by the owls was greater in foraging habitats than in nesting and 

roosting habitats. Laymon (1988, p. 100) concluded that the 

majority of spotted owl foraging locations in his Eldorado study 

were on sites with medium to large trees (in Laymon's classification, these were trees >24 inches in d.b.h.) and dense canopy 

closure (60 to 100 percent). They significantly selected for 

stands with trees >24 inches in d.b.h. and canopy cover of 40 to 

59 percent, based on availability. On the other hand, they used 

stands significantly less than expected that had (1) trees in the 

11- to 24-inch d.b.h. group and 60 to 100 percent cover, and (2) 

trees >24 inches in d.b.h. and canopy cover of 10 to 39 percent. 

Laymon (1988, p. 115) also reported that the owls in his study 

selected "foraging sites with more and larger snags." 

Call (1990, p. 30) concluded that the owls in his Tahoe NF 

study area used clearcuts, shrubfields, and plantations significantly less than expected (P = 0.01), based on availability. They 

used medium timber (11-20 inches in d.b.h.) in proportion to 

availability (P = 0.51), and they used large timber (20-35 inches 
Structural Attributes-Percent canopy cover, softwood basal 

area, total live tree basal area, snag basal area, and the amount of 

large, downed woody debris were generally greater at foraging 

than at random sites (tables 5G and 5H). Stand measurements 

reported for the Sierra Study Area were not related to specific 

foraging locations of spotted owls, so they could not be used to 

compare attributes at foraging locations with those at random 

locations. Total basal areas reported in the Sierra Study Area 

(table 5F) in stands with 40-69 percent and 70+ percent canopy 

cover were comparable to those at foraging sites reported by Call 

(1990) for the Tahoe Study Area and by Laymon (1988) for the 

Eldorado Study Area. That was not the case, however, for stands 

with 0-39 percent canopy cover in the Sierra Study Area. Conifers were the dominant component of basal area in all locations 

studied (tables 5F-5H). Point estimates for softwood, hardwood, 

and snag basal areas also were generally less in foraging than at 

roost and nest sites. 

Selection Patterns of Radio-tagged Birds 

Where significant patterns of habitat selection were observed among radio-tagged owls, results were consistent with 

those from the studies reported here. In the Lassen and Sierra 

Study Areas, some radio-tagged owls used denser stands and 

were associated with larger trees more than expected on the basis 

of availability. Only one bird used stands in the lowest canopycover class more than expected, and none did so for small tree 

classes (Chapter 6). 

Results of Call's (1990) and Laymon's (1988) studies, reported earlier in this chapter, tend to show stronger selection for 

habitat attributes by foraging owls than suggested by studies 

reported in Chapter 6. We believe this resulted from differences 

between studies in their scale of measurements. Call and Laymon 

sampled habitats at or very near actual locations where owls 

foraged. Studies reported in Chapter 6, on the other hand, characterized the entire stand in which a given owl foraged, thus 

lacking the localized scale used by Call and Laymon. 

Patterns of Habitat Use 

at a Stand Scale 

Nest Types 

All recently located nests of California spotted owls have 

been in trees, but some early records exist of nests found in other 

locations. For example, records obtained from the Western Foundation of Vertebrate Zoology (Kiff pers. comm.) included notes 

on 14 nests located between 1889 and 1947. Five were on cliff 

ledges, one was in a pigeon coop, and eight were in hardwood 

trees. The pigeon coop nest was composed of "manure," hay, 

and feathers in a deserted coop under a roof supported by four 

posts. This nest contained four eggs-only one of two clutches 

of that size ever reported for spotted owls (see Johnsgard 1988). 

Quantitative information on nest trees and nest sites were 

provided by each of the seven NFs in the western Sierra Nevada 

and the four southern California NFs. This database was supplemented with identical information from four of the major study 

areas (Lassen--Blakesley pers. comm.; Eldorado-Moen pers.

comm.; Sierra and Sequoia/Kings Canyon--G. N. Steger pers.

observ.; and San Bemardino--W. S. LaHaye pers. observ.).

Table 5G-Habitat characteristics (mean and percent Coefficient of Variation) of California spotted owl foraging 

locations in summer (n = 120) and fall (n = 79), and at random locations throughout summer/fall (n = 120) in the 

Eldorado Study Area, central Sierra Nevada (Gaymon 1988). 

Foraging sites 

Random sites in 

Summer Fall summer and fall 

Mean Percent CV Mean Percent CV Mean Percent CV 

Physiographic attributes 

 Percent slope 19.4a

 90.3 17.9a 86.9 23.4 46.8 

Structural attributes 

 Percent cover 66.8 32.0 61.6 45.5 67.9 29.0 

 Basal area (square feet per acre) 

 Hardwoods 16.6a

 172.9 9.2a 275.2 26.1 146.0 

 Softwoods 200.4a

 69.0 196.1 74.1 163.8 62.6 

Total live1 217.0 - 205.3 - 189.9 - 

 Snags 23.1 216.9 28.8 208.8 17.4 246.4 

Downed woody debris (tons per acre, by log diameter-class)2

0-3 inches 0.80 0.75 0.83 0.82 0.82 0.74 

>3 inches 11.84 1.67 13.76 a 1.43 9.36 1.39 

a

 Significantly different from the corresponding random sites, at the 0.05 level of significance (Student's t-test). 1

 The sum of softwood and hardwood basal areas. Data were not available to compute a percent Coefficient of 

Variation for this variable. 2

 Assumes a specific gravity of 0.4 for downed woody debris. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. Chapter 5 89 
Table 5H Habitat characteristics (mean and percent Coefficient of Variation) of foraging (n = 158) 

and random sites (n= 256) from six California spotted owl home ranges in the Tahoe Study Area, central 

Sierra Nevada (Call 1990). 

Foraging sites Random sites 

 Mean Percent CV Mean Percent CV 

Physiographic attributes 

Percent slope 28.4 58.1 27.4 63.0 

 Elevation (feet) 3,852.0 12.5 3,888.0 13.9 

Structural attributes 

Percent canopy cover 91.8a

 12.4 85.2 47.1 

 Basal area (square feet per acre) 

 Hardwoods 23.9 161.0 20.2 153.5 

 Softwoods 154 .9a

 52.7 129.8 52.3 

 Total live 178.7a

 43.7 150.6 44.1

 Snags 15.3a

 154.6 10.1 169.6 

 Basal area (square feet per acre, 

 by d.b.h. class)1

 3.9-4.8 inches 1.6 374.5 2.2 379.2 

 4.9-10.7 inches 25.5 119.9 27.6 116.9 

 10.8-20.6 inches 52.8 79.7 51.8 86.2 

 20.7-35.5 inches 67.1a

 81.1 49.5 94.7 

 >35.5 inches 32.1a

 104.9 19.7 149.14 

 Downed woody debris (percent cover, 

 by diameter class) 

 1-11.8 inches 5.86 82.1 4.5 103.3 

 >11.8 inches 3.26 184.8 1.7 195.9 

a

 Significantly different from the corresponding random sites, at the 0.005 level of significance 

(Student's t-test). b Significantly different from the corresponding random sites, at the 0.001 level of significance (MannWhitney U test). 1

 Diameter at breast height. 

Based on nest locations in trees, we recognize five nest types that 

are used regularly by spotted owls in the Sierra Nevada and 

southern California (table 5I). (1) Cavity nests (color photo 5-3)

are placed in natural cavities resulting from decay, usually in the 

heartwood of a large, old tree. Cavities large enough for owl 

nests can form where large branches tear out of the trunk of the 

tree (side cavities). (2) Broken-topped trees and snags (color 

photo 5-8) may develop depressions via decay, or they may 

persist without much decay but still provide a broad enough 

surface for the owls to lay their eggs. (3) Platform nests (color 

photo 5-14) are those placed on remnant platforms built by other 

species (for example, goshawks, ravens, or tree squirrels), or on 

debris accumulations in densely branching structures of trees. 

(4) Dwarf mistletoe brooms are often dense enough that they 

form a suitable nest substrate. (5) "Undefined" nest types are 

those that do not clearly fall into types 1-4 or for which we lack 

data. In spite of the variety of nest types used by these owls, the 

nest trees are still generally larger than other trees within the 

same stand (see below, table 5K). The large trees most often 

selected for nest sites by California spotted owls also exhibited 

signs of old age. Forty-three percent of the nests were in large 

decay cavities, and another 16 percent were on broken-topped 

trees or snags (for example, color photo 5-8). Many other nests 

were placed on limb deformities that supported debris platforms. 

Among 276 nests located recently, most of them in NFs in 

the Sierra Nevada and southern California, nest type varied 

90 Chapter 5 

markedly in different parts of the owl's range (table 5I). In

particular, cavity nests dominated nest types in both northern 

and southern Sierran conifer forests, but platforms were most 

common in conifer forests of southern California, perhaps as a 

result of differences in the availability of nesting substrates (see 

LaHaye 1988). Patterns on industrial forest lands in the Sierra 

Nevada were similar, but with a greater proportion of platform 

nests. Based on a sample of 16 nests, half were cavity and half 

were platform type (Taylor 1992). Northern spotted owls in 

Oregon nested primarily in tree deformities (Forsman et al. 

1984). A similar pattern was found among northern spotted owls 

in northwestern California (LaHaye 1988), but platform nests 

were more frequent in the southern part of that study area. 

Forsman et al. (1984) reported that young northern spotted 

owls from several platform nests in Oregon left their nests 

several days earlier than young from cavity nests. Because they 

were younger and less able to climb, they spent longer on the 

ground than young from cavity nests. Forsman et al. speculated 

that, as a result, these young may have experienced higher 

predation while on the ground than young from cavity nests. 

LaHaye et al. (1992a), however, found no differences in overall 

nesting success among broods produced in cavity nests, brokentopped nests, or platform nests in the San Bernardino Mountains. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Table 5I-Characteristics of nest trees used by California spotted owls, by major habitat type, based on data from all study 

areas except the Tahoe, and from all relevant National Forest files, 1986-1991. 

 Tree 

General condition Nest type1

 Nest tree 

habitat Number 

type of nests Alive Snag CA BT PL MI UN Conifer Hardwood 

Northern Sierran 83 61 22 55 9 15 4 0 79 4 

Conifer 

Southern Sierran 41 29 12 27 4 2 2 4 29 12 

Conifer 

Southern California 139 128 11 33 28 78 0 0 109 30 

Conifer 

Riparian/hardwood 13 13 0 5 3 5 0 0 0 13 

forests 

1

CA = cavity, BT = broken-top, PL = platform, MI = dwarf mistletoe, UN = undefined.

Table 5J-Tree species used as nest sites by the California spotted owl, based on data from all study areas except the Tahoe, and from all relevant National 

Forest files, 1986-1991. 

General 

habitat White Red Douglas- Bigcone Incense- Sugar Jeffrey Ponderosa Black Live White 

type fir fir fir Doug-fir cedar pine pine pine oak oak alder Other1 

Northern Sierran 22 9 21 0 7 16 2 8 3 0 0 1 

Conifer 

Southern Sierran 12 5 1 0 1 2 2 5 12 0 0 1 

Conifer 

Southern California 45 0 0 21 15 10 17 0 5 22 3 1 

Conifer 

Riparian/hardwood 0 0 0 0 0 0 0 0 3 5 2 3 

forests 

1

One nest each was in giant sequoia, Coulter pine, blue oak, tanoak, cottonwood, and California sycamore.

Table 5K-Nest stand and nest tree characteristics of California spotted owls, by general habitat type (mean ± SD),

based on data from all study areas except the Tahoe, and from all relevant National Forest files, 1986-1991. 

 Total canopy Nest tree Nest tree 

General habitat Elevation cover d.b.h.1 height Nest height 

type (feet) (percent) (inches) (feet) (feet) 

Northern Sierran 5,284 ± 922 75.4 ± 17.2 43.5 ± 14.7 96.8 ± 36.7 64.9 ± 25.7 

conifer n = 65 n = 28 n = 81 n = 75 n = 75 

Southern Sierran 5,750 ± 1355 75.5 ± 27.4 46.7 ± 19.6 95.0 ± 52.7 57.5 ± 31.0 

conifer n = 41 n = 17 n = 41 n = 40 n = 40 

Southern California 6,002 ± 1270 79.3 ± 17.7 37.0 ± 13.9 87.5 ± 33.3 56.5 ± 23.2 

conifer n = 137 n = 131 n = 139 n = 139 n = 139 

Riparian/hardwood 2,618 ± 1271 89.3 ± 10.6 29.5 ± 16.6 55.0 ± 23.5 38.4 ± 14.0 

forests n = 13 n = 6 n = 13 n = 13 n = 13

1

Diameter at breast height.

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. Chapter 5 91 
Nest Trees 

Ten species of conifers and seven species of hardwoods 

accounted for all recent nest sites reported for California spotted 

owls (table 5J). Nest trees were typically in stands with high 

canopy cover-75.4 percent in Sierran conifer and 79.3 percent 

in southern California conifer forests (table 5K; see fig. 5F). 

These estimates compared favorably with those reported above 

for nest stands (tables 5B and 5C). Eighty-three percent of all 

nests in the conifer forests were in living trees (fig. 5G) and 82 

percent were in conifers. These trees were very large, averaging 

about 90 feet tall (range 25-262 feet) and 41 inches in d.b.h. 

(range 9-81 inches), with means slightly higher in Sierran than in 

southern California conifer forests (table 5K). More than 75 

percent of all nest trees were larger than 30 inches in d.b.h. (fig. 

5H). Results from surveys of nest sites on industrial forest lands 

also showed that owls nested in large trees. Based on a sample of 

17 nests from the Sierra Nevada conifer zone, nest tree d.b.h. 

averaged 41.8 inches (Taylor 1992). 

The d.b.h. of nest trees in our current sample was significantly greater than that of conifers in general in the Sierra 

Nevada even in 1900 (χ2

 = 167, df = 6, P < 0.001; fig. 5I), 

based on trees on plots measured by Sudworth (1900) prior to 

extensive logging. Interestingly, the diameter distribution of 

white fir trees used as nest sites by owls was not significantly 

different (χ2

 = 7.469, df = 5, P > 0.10; fig. 5J) from that of 

white firs measured by Sudworth. The white fir comparison is 

appropriate because that was the most common nest tree species used by California spotted owls (table 5J). Comparison of 

the sample of nest trees from Sierran conifer forests to the 

current diameter distribution in the M4G timber strata on the 

Tahoe NF also shows extensive selection for large-diameter 

trees (fig. 5K). To the extent that the Tahoe NF sample is 

representative of this timber strata in the Sierra Nevada, selection for large trees for nesting is clearly evident. 

The significant inference from these results is that California spotted owls in conifer forests today are selecting nest trees 

from among the few remaining trees that are as large as or larger 

than average trees in 1900, before extensive logging began to 

remove the largest trees from the forest. When this pattern is 

coupled with the past and projected future trends of large trees in 

the Sierra Nevada (Chapter 13), it is reasonable to hypothesize 

large-diameter trees as a current or potentially limiting factor 

sometime in the future. That is, even if large trees are not 

currently limiting, we have reason to be concerned that they 

could soon be limiting if specific constraints on their removal are 

not invoked in the near-term. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

Figure 5F-Number of nest sites, by canopy-cover class; nests in snags 

and live trees are differentiated (n = 201). 

Figure 5G-Number of live trees and snags, by stem diameter-class, 

used for nest sites by California spotted owls (n = 288).

Figure 5H-Cumulative distribution of nest trees in the Sierra Nevada 

conifer zone, arranged by diameter at breast height (n = 122).

92 Chapter 5 
Figure 5I-Proportions of nest trees (recent) and trees available in 

stands in 1900 (Sudworth 1900), by stem diameter-class. 

Figure 5J-Proportions of nest trees (recent) that were white firs, and 

white firs available in stands in 1900 (Sudworth 1900), by stem diameterclass. 

)igure 5K-Proportion of nest trees (recent) in Sierran conifer forests (n

=122) and trees currently available in M4G stands on the Tahoe NF, by 

stem diameter-class. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

Foothill Riparian/Hardwood 

Forests 

Measurements of winter foraging sites and foraging stand 

attributes in foothill riparian/hardwood forests were available 

for the Eldorado Study Area (Laymon 1988) and the Sierra 

Study Area (Steger and Eberlein pers. comm.) (tables 5D and 

5L). Because sample sizes were small, these analyses should be 

considered as preliminary at this time. Point estimates suggest 

about the same range of values for percent canopy cover as 

observed in the conifer forests at higher elevations (compare 

tables 5D, 5F, and 5L). Basal areas of green trees and snags were 

considerably less, and shrub density was much higher in the 

hardwood type than in the conifer forest. Based on Laymon 

(1988), and a comparison of tables 5F and 5L, together with 

considerable on-site experience with these habitats, we found 

that riparian/hardwood forests dominated by oaks tended to have 

less canopy layering than most sites in the Sierran mixed-conifer 

and ponderosa pine/hardwood types (color photos 5-23 and 5- 

25). Multiple layers were present, however, in the mixed-hardwood forests in southern California, where spotted owls occur in 

narrow riparian corridors in steep-sided canyons, as in the Los 

Padres NF (color photo 5-28). Tables 5D and 5L also suggest 

less downed woody debris in the low-elevation hardwood type, 

compared to conifer forests. 

Nest sites in riparian/hardwood forests averaged about 2,600 

feet in elevation. No pattern was evident in the types of nests 

found in these forests (table 5I), but the sample size was too 

small to establish a pattern if one existed. All of the nests located 

were in hardwoods that averaged 55 feet tall (range 16-98 feet) 

and 30 inches in d.b.h. (range 13-72 inches) (table 5K). 

Collectively, these data indicate that owl habitat in the 

foothill/riparian forests was characterized by both lower live 

tree basal area and lower canopy cover than higher-elevation 

sites in the conifer zone. The high closure measured at nest 

sites in the foothill zone (table 5K) may represent a highly 

localized phenomenon-one uncharacteristic of stands in this 

area in general. Furthermore, in the foothill habitats, the average nest-tree d.b.h. was smaller (table 5K) and platform nests 

were more common (table 5I) than in the conifer zone. Given 

these contrasts, the structure of owl habitats in foothill riparian/hardwood forests appeared to be substantially different 

from that in mixed-conifer forests. We caution that no data 

exist that suggest that stand basal areas, cover classes, and 

nest-tree sizes used in the foothill/riparian zone would, if 

created in the conifer zone, provide suitable owl habitat. Indeed, owls that migrated to low-elevation sites for the winter 

moved back upslope and chose dense stands with large trees in 

the mixed-conifer forest as summer habitat (table 5D). 

Chapter 5 93 
Table 5L-Structural attributes of habitats (mean and percent Coefficient of Variation, by canopy cover class) used 

by California spotted owls during radio-tracking studies in the riparian/hardwood zone of the Sierra Study Area, 

southern Sierra Nevada (Steger and Eberlein pers. comm.). 

0-39 percent 40-69 percent 70+ percent 

canopy cover canopy cover canopy cover 

(n = 7) (n = 5) (n = 8) 

Structural Attributes Mean Percent CV Mean Percent CV Mean Percent CV 

Percent canopy cover 

 By densiometer 36.3a

 39.4 67.26 11.8 74.3b 16.3 

 By aerial photos 15.9a 66.8 59.06 11.6 80.0c 5.8 

Basal area (square feet per acre) 

 Green trees 20.0a 80.0 50.1b 51.5 65.6b 67.1 

 Snags 0.9a 175.0 4.1 87.2 5.0b 80.7 

 Hardwoods (by d.b.h. class)1

 0-4.7 inches 1.4a 124.2 4.6b 48.1 6.5b 91.9 

 4.8-10.6 inches 6.7 115.0 18.8 37.4 19.0 30.7 

 10.7-20.5 inches 5.9 97.8 12.9 61.5 20.0 113.7 

 20.6-35.4 inches 5.1 125.0 8.5 80.9 12.0 129.1 

 >35.5 inches 1.0 262.5 5.4 150.0 1.3 279.3 

Total 20.0a

 79.4 50.2b 51.4 58.7b 65.3 

Softwoods (by d.b.h. class)1

 0-4.7 inches 0 0 0 0 0.1 200.0 

 4.8-10.6 inches 0 0 0 0 0.3 300.0 

 10.7-20.5 inches 0 0 0 0 2.3 219.2 

 20.6-35.4 inches 0 0 0 0 4.4 281.0 

 >35.5inches 0 0 0 0 0 0 

Total 0 0 0 0 7.0 258.1 

Shrubs per acre 105.0 198.8 69.0 62.5 127.0 144.4 

Downed woody debris (tons per 

 acre, by log diameter-class)2

 11-20 inches 0.09 264.4 0.10 223.5 0.68 164.1 

 21-35 inches 0 0.41 223.7 0.45 187.5 

 >35 inches 0 0 0 

a,b,c Values in the same row with different alpha superscripts are different at the 0.05 level of significance (one-way

ANOVA). 1

 Diameter at breast height, in inches. 

2

 Assumes a specific gravity of 0.4 for downed woody debris. 

'LVFXVVLRQ

General Patterns of Habitat Use 

The first specimen of the spotted owl was collected by 

Xantus (1859) in the Tehachapi Mountains of southern California. This bird was likely found in the closed-canopied, riparian/ 

hardwood forests common to the Fort Tejon region. Following 

this first documented sighting by Europeans, many reports of 

California spotted owls appeared in the literature over the next 

80 years (see Grinnell and Miller 1944, Gould 1974). These 

reports and several nest records at the Western Foundation of 

Vertebrate Zoology (Kiff pers. comm.) indicated that California 

spotted owls in past decades used both conifer and hardwood 

habitats, and were found consistently in densely forested areas. 

More recent observations and studies document the habitat 

associations of this bird throughout its range and in a quantitative fashion. These results suggest some general patterns about 

94 Chapter 5 

habitat use by California spotted owls. First, they use a broader 

array of habitats than used by the northern spotted owl. Second, 

within the general habitat types selected, they use forest patches 

that are complex in structure relative to what is available (for 

example, many trees in different diameter-classes and high canopy 

closure). Third, California spotted owls appear to select remnants of the older Sierran and San Bernardino conifer forests that 

have managed to survive 200-400 years to the present time (see 

table 5M). Especially for nesting and roosting, present data from 

several different sources suggest that most California spotted 

owls select dense stands with very large, presumably old trees. 

Fourth, although habitat attributes associated with California 

spotted owls at their nest and roost sites parallel those associated 

with northern spotted owls, foraging habitat used by the California subspecies appears to be much more variable than for its 

northern relative. Indeed, the considerable range in variation 

found in habitats used by California spotted owls is well-illustrated by the photos at the end of this chapter. In spite of this 

variation, however, results of studies reported in this chapter 

firmly establish that these owls use their forested environment in 

a nonrandom fashion. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
The association between the variable habitats used by California spotted owls and their population effects is not well 

understood. For example, we do not know if the birds survive 

and reproduce equally well in each of the many habitat types 

where they are found. We strongly suspect, however, that populations in some of those habitats are "sources" (reproduction 

exceeds replacement needs within the population, so surplus 

young are produced which emigrate to other areas) and populations in others are "sinks" (where reproduction is generally 

insufficient to replace local losses, so maintenance of the population requires immigration from other areas). We also do not 

know if spotted owls in the Sierra Nevada evolved in a more 

heterogeneous environment than was the case for northern spotted owls. Spotted owls evolving in different environments may 

exhibit different adaptive responses (for example, see Gutiérrez 

and Pritchard 1990). Although we do not yet know the answers 

to some of these critical questions, we infer that human-induced 

habitat changes (for example, from logging) may adversely 

affect the owls and lead to population declines. 

California spotted owls in conifer forests exhibited significant overuse of M4G stands at the landscape scale. These are 

dense (70+ percent canopy cover) mixed-conifer forests with 

canopy trees averaging  LQFKHV LQ GEK 7KLV SDWWHUQ RI

selectivity was corroborated by comparisons with random locations of stands used for nesting, roosting, and foraging in the 

conifer forest. The vast majority of birds used sites with greater 

canopy cover, total live tree basal area, basal area of softwoods 

and hardwoods, and snag basal area than found at random sites. 

These are attributes that we would expect to find in M4G stands. 

Finally, selection of nest sites and nest trees by the owls further 

corroborates the findings from analyses at other scales of resolution. A high proportion of nest sites, especially in Sierran conifer 

forests, were in natural cavities or in broken-topped trees or 

snags. Trees used for nest sites were significantly larger and 

probably older than available in the general forest matrix, even 

in M4G stands. This pattern suggests that, currently, most nest 

trees are surviving remnants from forests of past centuries. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

Is the Owl A Habitat Specialist? 

In spite of the fact that the owl used a range of habitat 

conditions, we believe it should be characterized as a specialist. 

For example, we observed that 80 percent of all nest trees were 

located in stands with canopy cover of at least 70 percent. The 

average (and median, and modal) d.b.h. of nest trees in the Sierra 

Nevada was about 45 inches, but only 2 percent of all trees 

greater than 10 inches in d.b.h. in M4G stands on the Tahoe NF 

were in the 40-49 inch d.b.h. group. By specialist we mean an 

animal that shows a clear selection for certain habitats or habitat 

attributes. Two assumptions we invoked in our analyses were (1) 

the pattern of habitat usage we observed was not constrained by 

any other species, and (2) the use of a habitat type significantly 

in excess of its availability reflects selection. The degree to 

which these assumptions were met is unknown. It is possible, for 

example, that the California spotted owl's pattern of habitat use 

was influenced by competition with another raptor species. Also, 

the methods we used can be misleading if, for example, the 

preferred habitat is also the most common habitat. Regarding 

these two possibilities, we have no data that suggest direct 

competitive interactions with other raptor species, and the habitat used most commonly for nesting (M4G) is uncommon (about 

20 percent, based on FS inventory data) in the Sierran landscape. 

We do not claim dependence by the spotted owl upon any 

given habitat type or attribute. Further, we do not contend that 

our data, at this time, indicate that any given habitat type or 

attribute is "required," in the sense that its absence would lead to 

the owl's extinction. But we do believe that the habitat distribution patterns of the majority of owls, given the constraints of 

availability and possible biotic interactions, currently reflect optimal choices. If better choices were available, natural selection 

would act to remove those individuals making the wrong choice. 

Based on our definition of a specialist and the operational 

manner in which we identified selection, we believe we have made 

the best use of the available data to craft a set of recommendations intended to secure future options for managing the 

Chapter 5 95 

Table 5M-Core data (mean ± SD) and ages of nest trees used by California spotted owls in the San Bernardino 

Mountains; sample sizes are in parentheses beneath means and SDs. 

 Mean percent 

Tree Nest Core age of total 

Trees D.b.h.1

 height2 height2 in years radius cored 

All trees 41 ± 13.0 103 ± 32.9 65 ± 4.3 230 ± 93.8 69 ± 26.4 

 (30) (30) (30) (29) (29) 

White fir 41 ± 11.7 91 ± 25.1 65 ± 19.2 238 ± 68.2 70 ± 20.1 

 (14) (14) (14) (14) (14) 

Sugar pine 37 ± 17.6 90 ± 31.2 53 ± 22.0 217 ± 43.1 46 ± 48.2 

 (3) (3) (3) (3) (3) 

Jeffrey pine 40 ± 14.8 116 ± 37.9 68 ± 27.8 223 ± 143.1 78 ± 26.3 

 (8) (8) (8) (8) (8) 

Incense-cedar 46 ± 15.8 131 ± 29.0 83 ± 27.5 193 ± 111.0 64 ± 31.4 

 (3) (3) (3) (3) (3) 

Bigcone Douglas-fir 48 ± 16.1 115 ± 54.5 42 ± 28.3 320 ± 0.0 48 ± 0.0 

 (2) (2) (2) (1) (1) 

1

 Diameter at breast height, in inches. 

2

 Height in feet. 
species. We have based our inferences on what the majority of 

the owls selected from among choices in their habitats, and 

assumed that these choices reflected the most important attributes to retain in future landscapes. 

Recommendations 

Translating all the habitat association patterns into a general 

characterization of nesting, roosting, and foraging stands that 

should be suitable for spotted owls in conifer forests is difficult. 

It is also risky! All too often, suggestions offered tentatively are 

misconstrued to be the last and definitive word on the matter. 

Even with these misgivings, however, we offer here some tentative estimates of stand attributes, based mainly on tables 5B-5H. 

Given these caveats, the tabulation below may be a fair, initial 

representation of the range of mean values of some attributes in 

suitable habitat for spotted owls in Sierran mixed-conifer forests. Most values in the tabulation generally reflect the range of 

means from the various studies reported in this chapter. We have 

combined values for nesting and roosting stands, because (1) 

they tend to be very similar, (2) spotted owls commonly roost 

within the stands that include their nests, and (3) we believe that 

a stand suitable for nesting would also provide conditions satisfying the roosting needs of the owls. 

Nesting and Foraging 

roosting stands stands 

Percent canopy cover1

 70-95 50-90 

Total live tree basal area2

 185-350 180-220 

Total snag basal area 30-55 15-30 

Basal area of large snags3

 20-30 7-17 

Downed woody debris4

 10-15 10-15 

1

 Mostly in canopy  IHHW KLJK LQFOXGLQJ KDUGZRRGV 2

 Square feet per acre. 

3

 Dead trees  LQFKHV LQ GEK DQG  IHHW WDOO 4

 Tons per acre. 

The values for total snag basal area are high and reflect 

sampling of all, or nearly all, dead trees by various workers. We 

believe, however, that relatively small snags have little value in 

terms of spotted owl habitat. Snags need to be large enough to 

accommodate nest sites for medium to large cavity-nesting birds 

and den sites for flying squirrels. In addition, larger snags take 

longer than small snags to decompose after they have fallen. We 

consider snags that are at least 15 inches in d.b.h. and 20 feet tall 

to be near the smaller end of suitability for owl habitat. In two 

comparisons of total snag basal area with the basal area of 

"large" snags ( LQFKHV LQ GEK DQG  IHHW WDOO WDEOH (

large snags comprised 45 percent and 59 percent of the total. 

Snag values in the above tabulation could be adjusted accordingly to estimate the basal area of large snags in owl habitats. 

This limited analysis suggests that owl nest and roost sites have 

from about 19 to 31 square feet basal area of large snags, and 

foraging sites have from 7 to 17. Because we recommended an 

interim approach to management for spotted owls in the Sierra 

Nevada (Chapter 1), one based on the concept of preserving all 

options for future management of spotted owls, we recommend 

maintaining at least 20 square feet basal area of large snags 

96 Chapter 5 

wherever possible in owl habitat. Based on current information, 

this standard appears to be appropriate for maintenance of suitable nesting and roosting sites. It is at the lower end of estimates 

for nesting and roosting sites and slightly above the upper end of 

estimates for foraging sites. 

Estimates of the mass of downed wood in owl nesting, 

roosting, and foraging sites ranged from 10.5 to 24.7 tons per 

acre, with a mean of 17.4 and a standard deviation of 5.3. (Based 

on values in tables 5D-5G, and on approximations of tons/acre 

from values given in tables 5B and 5H.) Most of this was in 

pieces at least 11 inches in diameter. We believe that much of 

this has managed to accumulate because of effective fire suppression in Sierran conifer forests during the past 80-90 years, so 

these quantities probably exceed what was present during 

presettlement times. Fuels management specialists from R5 and 

from NFs in the Sierra Nevada have assured us that the fuel loads 

indicated by the average in owl habitats represent a significant 

hazard vis-à-vis the intensity of fires and the likelihood of standdestroying fires. Consequently, we believe that a compromise is 

needed between fire threats and what the data indicate to be 

typical quantities of large logs in owl habitats. We suggest a 

range of 10-15 tons per acre, in the largest logs available, and 

believe it is inadvisable to retain logs smaller than 11 inches in 

diameter to attain this level. This range is at the low end of the 

values observed in owl habitats. 

Research Considerations 

Despite the considerable amount of study already done in an 

effort to estimate habitat-use patterns by spotted owls, we still 

have limited knowledge about this critical aspect of the bird's 

ecology. This is particularly true relative to the relations between 

habitat and demographic variation. This is the case largely for 

three reasons. First, research on this subspecies was slow in 

getting started, because most of the early concern was directed at 

the northern spotted owl. Funds available for research on the 

California spotted owl have not been sufficient to allow studies 

of their prey, or even to complete a full inventory of all habitats 

where the owl is likely to occur. Second, most definitive studies 

of the California spotted owl were initiated in 1986 or later. 

Coincidentally, this was when the current drought began (fig. 

4H), making it impossible to determine whether observed results 

from owl studies should be interpreted primarily in relation to 

changing habitat conditions, to the drought, or both. Third, 

knowledge about different gradations in habitat suitability must 

be based on habitat-specific studies of the owl's demography--

whether or not they reproduce well enough in a given habitat to 

equal or exceed annual mortality rates. 

Demographic information on owl populations in a variety 

of different habitats will take many years to obtain and will 

depend on monitoring demographic trends through both wet and 

dry climatic periods. We should continue to build on existing 

demographic studies to attain some of the information needed, 

because they already have an accumulating database. We also 

need to consider a range of options for adding other demographic studies. In particular, we see great potential for demoUSDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
graphic studies on lands of some of the major commercial timber 

companies whose forest practices differ in important ways. These 

are ad hoc experiments (Bias and Gutiérrez 1992) on the effects 

of different management regimes on the owls. 

Every effort should be made to establish consistent methods, attributes to be measured, data forms, and analytical procedures to be used by all researchers on California spotted owls. 

Failing adoption of standard methods and so on, quantitative 

relations among different techniques should be developed to 

ease interpretation of data from different studies. Finally the 

efficiency and utility of different techniques should be explored 

to encourage adoption of a uniform protocol for basic information collection in the field. 

The "Achilles heel" of wildlife management is separating 

correlation and causation. All analyses of habitat described in 

this paper are based on correlational studies. That is, we observe 

a particular variable, such as canopy closure, to be consistently 

high at owl sites relative to available sites, but we do not know if 

that variable is the reason why owls are present at the site. They 

may be there because canopy closure is usually associated with a 

different stand attribute, one that we have not measured and one 

that may be too subtle to quantify. Hence, we do not know if 

changes in canopy closure will consistently result in a functional 

response by owls inhabiting a site. The fact that we have not 

demonstrated cause-and-effect relations, however, does not dilute 

the power of these documented trends. Rather, it tempers our 

recommondations about managing for functional owl habitat. 

Controlled experiments, in which one or more attributes are 

changed and subsequent functional responses of the owls are 

documented, are the best way to identity cause-and-effect relations. Experimentation in certain fields of science is the rule (for 

example, see Platt 1964). Experiments in natural ecosystems, 

however, are generally orders of magnitude more complicated 

and less controllable than experiments in fields like chemistry, 

physics, or molecular biology. Nevertheless, the widespread 

logging of forests for commodity production provides a possibility of undertaking ecological experiments on owl responses to 

changes in habitat. Experiments can occur at two spatial scales--

the landscape (that is, a collection of owl home ranges) and the 

site (within a forest patch occupied by an owl or pair of owls). 

The critical question to be asked is: Do the changes in spatial 

arrangement or the structure and composition of habitats affect 

the owl's ability to survive and reproduce? Formulation of 

specific experiments should be based on empirical information 

generated from natural history studies (see above) and the creative application of results from modeling habitat conditions 

observed in natural history studies. The success of experiments 

in discerning alternative outcomes will determine their potential 

for applying a management system to a larger landscape. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

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Barrows, Cameron W.; Barrows, Katherine. 1978. Roost characteristics and 

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Bias, Michael, A. 1989. Habitat use by California spotted owls in the central 

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spotted owls in the central Sierra Nevada. Journal of Wildlife Management. 56:584-595. 

Blakesley, Jennifer A. Wildlife Biologist, USDA Forest Service, Pacific Southwest Research Station, Arcata, CA. [Personal communication.] December 

1991. 

Blakesley, Jennifer A.; Franklin, Alan B.; Gutiérrez, R. J. 1992. Spotted owl 

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Byers, C. Randall; Steinhorst, R. K. 1984. Clarification of a technique for 

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Call, Douglas R. 1990. Home-range and habitat use by California spotted owls 

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Unpublished draft supplied by authors; 15 p. 

Eberlein, Gary P. Statistical Technician, USDA Forest Service, Pacific Southwest Research Station, Fresno, CA. [Personal communication]. November 

1991. 

Fiske, John. Regional Reforestation/TSI Program Manager, USDA Forest 

Service, Region 5, San Francisco, CA. [Personal communication]. February 1992. 

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Contract No. 39-4870. San Francisco, CA: Region 5, Forest Service, U.S. 

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718 p. 
Color Photo Section 

Habitat from throughout the 

range of the California 

spotted owl 
5-1-Foraging habitat. Note the extensive downed wood and abundant 

staghorn lichen on tree trunks. Dominant trees, in order, were red fir, 

white fir, and Jeffrey pine; some sugar pines were present but not visible 

in the photo. 

Habitat type: ---------------Sierran Red Fir 

Location: --------------------Lassen National Forest 

Elevation: -------------------6,170 feet 

Date: -------------------------24 June 1991 

Photographer: -------------John S. Senser 

5-2-Typical red fir canopy. This site was occupied by owls 5 of the 6 

years surveyed, 3 of these years by pairs. 

Habitat type: ---------------Sierran Red Fir 

Location: --------------------Eldorado National Forest 

Elevation: -------------------6,000 feet 

Date: -------------------------24 June 1991 

Photographer: -------------John S. Senser 

100 Color Photograph Section USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. Color Photograph Section 101 
5-3-Probable nest tree with adult and two juvenile owls. The nest may 

have been the cavity in the bole of the tree, above the owls. 

Habitat type: ----------------Sierran Red Fir 

Location: --------------------Tahoe National Forest 

Elevation: -------------------7,000 feet 

Date: -------------------------26 June 1991 

Photographer: -------------John S. Senser 

5-4-The forest floor in a foraging area showing a thick mat of decomposing organic matter. The fallen trees progress from recognizable 

trunks (background) to a spongy mass of organic matter (foreground). 

Habitat type: ---------------Sierran Red Fir 

Location: --------------------Lassen National Forest 

Elevation: -------------------6,170 feet 

Date: -------------------------29 June 1991 

Photographer: -------------John S. Senser 

102 Color Photograph Section USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. Color Photograph Section 103 
104 Color Photograph Section USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. Color Photograph Section 105 
5-7-Ground litter 100 feet east of a nest tree. Observe the extensive 

decomposing deadwood, which may provide shelter and nurture food 

resources (such as hypogeous fungi) for the owl's prey. 

Habitat type: ---------------Sierran Mixed-conifer 

Location: --------------------Sequoia National Park 

Elevation: -------------------7,000 feet 

Date: -------------------------10 July 1991 

Photographer: -------------John S. Senser 

5-8-Nest snag. Overstory canopy cover around the nest was about 30 

percent, and about 40 percent in the surrounding forest. The pair at this site 

produced two young in 1991; they were found roosting nearby in a dense 

stand of ponderosa pine and incense-cedar. The area was logged for large 

pines during the railroad logging era of the 1920s, selectively cut in 1980, 

and re-entered for additional salvage logging in 1989 and 1990. Today, the 

forest is composed mostly of a mosaic of height classes and trees not more 

than 70 years old (with scattered 120-year-olds). 

Habitat type: ---------------Sierran Mixed-conifer 

Location: --------------------Fiberboard Timber Company 

 (within Stanislaus National Forest) 

Elevation: -------------------4,400 feet 

Date: -------------------------8 July 1991 

Photographer: -------------John S. Senser 

106 Color Photograph Section USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
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108 Color Photograph Section USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. Color Photograph Section 109 
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5-18-Ground cover 30 feet west of a nest tree, showing a dense 

understory of bigleaf maple and conifer saplings. Note the extensive 

layer of needle/cone litter and presence of large downed limbs, such as 

the black oak limbs seen here. 

Habitat type: ---------------Sierran Conifer/Hardwood 

Location: --------------------Eldorado National Forest 

Elevation: -------------------4,200 feet 

Date: -------------------------25 June 1991 

Photographer: -------------John S. Senser 

116 Color Photograph Section USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

5-17-Canopy typical of the Sierran Conifer/Hardwood type, showing a 

60-70 percent closure of white fir and black oak. 

Habitat type: ---------------Sierran Conifer/Hardwood 

Location: --------------------Stanislaus National Forest 

Elevation: -------------------3,700 feet 

Date: -------------------------8 July 1991 

Photographer: -------------John S. Senser 
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5-22-Roost area in a streamside location. The photo shows dense 

stands of California sycamore and live oak. A roosting owl is visible on 

a limb in the upper-center of the photo. Nearby Cooper's hawk nests in 

canopies of interior live oaks provided nesting platforms in 1989 and 

1990. 

Habitat type: ---------------Sierran Foothill Hardwood 

Location: --------------------Sierra National Forest 

Elevation: -------------------1,360 feet 

Date: -------------------------9 July 1991 

Photographer: -------------John S. Senser 

120 Color Photograph Section USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

5-21-Overview of owl foraging habitat showing the live oaks that are 

typical of the area. 

Habitat type: ---------------Sierran Foothill Hardwood 

Location: --------------------Sierra National Forest 

Elevation: -------------------1,360 feet 

Date: -------------------------9 July 1991 

Photographer: -------------John S. Senser 
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5-26-Overview of a north-facing slope with typical owl habitat as it 

occurs in the central coastal region. The slope shows a mix of conifers 

(mostly coast redwood, Douglas-fir, Coulter pine, and sugar pine) and 

hardwoods (live oak, Pacific madrone, and California-laurel). Chamise, 

sagebrush, other chaparral species, yucca, and grasses are dispersed 

along the well-grazed, south-facing slopes. 

Habitat type: ---------------Central Coastal Redwood/ 

 California-laurel 

Location ---------------------Los Padres National Forest 

Elevation: -------------------3,000 feet 

Date: -------------------------13 July 1991 

Photographer: -------------John S. Senser 

124 Color Photograph Section USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

5-25-Woodrat nests in owl foraging area showing distribution of nests 

in oak woodland. 

Habitat type: ---------------Sierran Foothill Hardwood 

Location: --------------------Sequoia National Forest 

Elevation: -------------------3,300 feet 

Date: -------------------------11 July 1991 

Photographer: -------------John S. Senser 
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5-30-Roost area showing good cover composed of coast live oaks, 

California-laurel, and streamside white alder. 

Habitat type: ---------------Riparian/Mixed Hardwood 

Location: --------------------Los Padres National Forest 

Elevation: -------------------2,520 feet 

Date: -------------------------12 July 1991 

Photographer: -------------John S. Senser 

128 Color Photograph Section USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

5-29-Foraging area typical of riparian/hardwood habitat. The roost site 

is farther up-canyon (upper center of photo). The dense chaparral cover 

on the slopes blends with the thick white alder, live oak, and 

California-laurel along the stream channel, making it difficult to trace its 

course in the photo. 

Habitat type:----------------Riparian/Mixed Hardwood 

Location: --------------------Los Padres National Forest 

Elevation: -------------------2,400 feet 

Date: -------------------------12 July 1991 

Photographer: -------------John S. Senser 
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5-32-Canopy typical of riparian/mixed-hardwood habitats. Trees pictured include bigleaf maple, California-laurel, and white alder. California 

sycamore was also present but not visible in this photo. 

Habitat type:----------------Riparian/Mixed Hardwood 

Location: --------------------Los Padres National Forest 

Elevation: -------------------2,290 feet 

Date: -------------------------12 July 1991 

Photographer: -------------John S. Senser 

130 Color Photograph Section USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

5-31-Stream diversion to a reservoir for domestic use. Future water 

demands, especially in southern California, may have a marked 

negative impact on spotted owl habitat. 

Habitat type:----------------Riparian/Mixed Hardwood 

Location: --------------------Los Padres National Forest 

Elevation: -------------------2,250 feet 

Date: -------------------------12 July 1991 

Photographer: -------------John S. Senser 
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5-33-Overview of foraging habitat showing stands of bigcone Douglas-fir 

and canyon live oak on the moister north-facing slopes and various 

species of shrubs from the chaparral community on the drier, more 

south-facing slopes. 

Habitat type: ---------------Bigcone Douglas-fir 

Location: --------------------Angeles National Forest 

Elevation: -------------------3,400 feet 

Date: -------------------------19 August 1991 

Photographer: -------------John S. Senser 

5-34-Ground view 60 feet west of a nest tree. Observe the extensive 

dead wood, which can provide shelter for owl prey. 

Habitat type: ---------------Mixed-conifer/Black Oak/Live Oak 

Location: --------------------San Bernardino National Forest 

Elevation: -------------------5,920 feet 

Date: -------------------------21 August 1991 

Photographer: -------------John S. Senser 

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5-35-Within a nest grove. The dominant tree species here were 

canyon live oak, incense-cedar, white fir, and assorted pines. Canopy 

closure was about 75 percent. 

Habitat type: ---------------Mixed-conifer 

Location: -------------------San Bernardino National Forest 

Elevation: -------------------5,920 feet 

Date: -------------------------September 1988 

Photographer: -------------William S. LaHaye 

5-36-Looking north from a nest tree. Note the open character of the 

canopy but relatively dense lower vegetative cover, composed here of 

curlleaf mountain mahogany. Only two of 131 nest sites and four of 134 

owl territories found in the San Bernardino Mountains were in these 

open habitats (LaHaye et al. 1992). 

Habitat type:----------------Mixed-conifer with Pinyon Pine 

and Mountain Juniper 

Location: ------------------San Bernardino National Forest 

Elevation: -------------------8,100 feet 

Date: -----------------------21 August 1991 

Photographer: -------------John S. Senser 

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5-46-Part of the study group "hooting" their way through a beautiful 

stand of live oaks and grassland. Moments after this photo was taken, 

a male owl responded from a hardwood stand/poison oak thicket just to 

the left of this view. 

Habitat type: ----------------Hardwood Ravines 

Location: --------------------Cleveland National Forest 

Elevation: -------------------2,100 feet 

Date: -------------------------21 August 1991 

Photographer: -------------John S. Senser 

144 Color Photograph Section USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

5-45-Overview of a hardwood ravine habitat. Note the dense hardwood stands in ravines, valleys, and on north aspects, with chaparral 

and grasses dominating the dryer slopes. 

Habitat type: ---------------Hardwood Ravines 

Location: --------------------Cleveland National Forest 

Elevation: -------------------2,100 feet 

Date: -------------------------21 August 1991 

Photographer: -------------John S. Senser 
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Chapter 6 

Home-Range Size and Habitat-Use Patterns of 

California Spotted Owls in the Sierra Nevada 

Cynthia J. Zabel, George N. Steger, Kevin S. McKelvey, Gary P. Eberlein, Barry R. Noon, and Jared Verner 

Home range is an "area utilized by an individual during its 

normal activities such as food gathering, mating, and caring for 

young" (Burt 1943), as distinguished from its territory, which is 

typically defended against intrusion by other individuals of the 

same species, except a mate or a potential mate (Nice 1941). 

Home ranges of neighboring individuals commonly overlap, but 

territories are usually more exclusive. Studies of home ranges 

often require attachment of radio transmitters on animals, so 

their movements can be monitored. Many such studies have 

been done on spotted owls (recent review in Thomas et al. 1990, 

appendix I). In this chapter we report results of two radio-tracking 

studies in three different study areas, one in the northern and two 

in the central Sierra Nevada. These data provide estimates of 

home-range sizes of individual males and females, and of pairs, 

during different periods of the annual cycle. In addition, we have 

compared patterns of habitat use in home ranges in relation to 

the different habitats available to the birds. In that sense, habitat-use 

information given here augments that presented in Chapter 5. 

A fundamental difference exists, however, between the scales 

of habitat use reported here and those reported in Chapter 5. 

Studies in Chapter 5 examined habitat selection by owls at three 

scales-landscape, home-range, and stand. Stand-scale studies 

measured habitat attributes very near the point of an owl's 

activity-nesting, roosting, or foraging-and compared them 

with similar measurements at random locations in the surrounding forests. This was a fine-grained scale of analysis that addressed habitat attributes closely associated with an activity. 

Studies reported in Chapter 6 were done at a scale intermediate 

between the home-range and stand scales. Here we examined 

habitat selection at the scale of a habitat polygon (stand), a patch 

in the overall forest landscape that was similar enough within 

itself to be set apart from adjoining patches. The minimum patch 

size we recognized was 5 acres. For example, a meadow would 

be one polygon type, and an adjoining patch of forest with fairly 

uniform canopy closure and tree size-class would be another 

type. But a forest polygon could still be heterogeneous-and 

typically it is, with smaller subgroups of trees within it having 

higher canopy closure and/or larger trees than the polygon as a 

whole. If an owl were selecting for attributes at a scale less than 

the polygon size, the stand-level analyses reported in Chapter 5 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

would be much more likely to detect that selection than would be 

the results reported in this chapter. We can differentiate habitat 

selection only at the level of the entire polygon. Consequently, 

evidence of habitat selection given in this chapter is likely to be 

less conclusive than that given in Chapter 5. 

Study Areas 

Results presented here came from two study areas, one in 

the Sierra National Forest (NF) near the southern end of the 

Sierra Nevada, and the other east of Lassen National Park (NP) 

in the Lassen NF, at the northern end of the Sierra Nevada and 

the extreme southern end of the Cascade Mountains. The study 

area in the Sierra NF had one division in mixed-conifer forest in 

the Huntington quadrangle (hereafter the S-CON site) and another in foothill riparian/hardwood forests and adjoining oak-pine 

woodlands in the Patterson quadrangle (the S-OAK site). These 

were situated about 45 miles northeast of Fresno, in watersheds 

of the San Joaquin River and the North Fork of the Kings River. 

Vegetation in the S-CON site was dominated by mixed-conifer 

forests of white fir, Jeffrey pine, ponderosa pine, sugar pine, 

incense-cedar, and red fir. Elevations ranged from 5,000 to 

8,000 feet. Much of the area was selectively logged from 1880 to 

the present, with most of the old-growth conifer trees removed. 

Logging within the NF and on small parcels of private land 

within NF boundaries is now concentrated on second-growth 

timber and the few remaining stands of old-growth. The S-OAK 

site, at elevations from 1,000 to 3,000 feet, was dominated by 

blue oak, interior live oak, digger pine, and various chaparral 

species. The Lassen NF study area (the L-CON site) was dominated by red and white fir at high elevations (5,800 to 6,600 feet) 

and Jeffrey, ponderosa, sugar, and lodgepole pines at lower 

elevations (5,000 to 5,800 feet). Selective logging has been the 

predominant silvicultural method used there. 

Chapter 6 149 
Methods 

Field Operations 

Owls were captured with noose poles, mist nets, or 

fish-landing nets and fitted with backpack-mounted radio tags 

weighing 0.6-0.8 ounces. We attached the radio tags with 

cross-chest harnesses (Forsman 1983). Radio tags (AVM Instrument Co., Livermore, Calif.) had 12-inch antennae and life 

expectancies of 12 months. Owls tracked >1 year were recaptured and fitted with new radio tags. Owls were located by radio 

triangulation using the loudest-signal method (Springer 1979). 

At least three compass bearings were taken from known points 

for each owl location and plotted on 1:24,000 topographic maps. 

Error polygons (the area enclosed by the intersection of three or 

more compass bearings) at L-CON were classified as <50, <20, 

<5, or <2.5 acres. We attempted to obtain error polygons of <2.5 

acres for all observations. At S-CON and S-OAK, we obtained 

additional bearings on all birds until error polygons of <2.5 acres 

were attained. The geometric center of each error polygon was 

assumed to be the owl's location. We attempted to obtain one 

nighttime location, by radio tracking, on each of four nights per 

week and one daytime location per week by direct visual observation. All nighttime observations were considered foraging 

locations and all daytime observations were classed as roosting locations. 

Vegetation Classification 

Stands of relatively homogeneous vegetation were mapped 

at each study site and grouped into habitat types that could be 

cross-classified to U.S. Department of Agriculture, Forest Service (FS) timber stand types. Black-and-white aerial photos, 

U.S. Geologic Survey topographic maps, large-scale color aerial 

photos, and 1:24,000 FS black-and-white orthophotoquads were 

used to define vegetation boundaries. Stands were classified 

according to compositional (vegetation type) and structural (diameter size-class of dominant trees and canopy-closure classes) 

features that could be estimated from aerial photos (table 6A). 

Structural classes at S-OAK differed from those at S-CON and 

L-CON, because most trees there were oaks with relatively 

small diameters at breast height (d.b.h.) when compared to 

conifers. We assigned each stand to two canopy-closure classes: 

cover by all vegetation above 7 feet (total canopy closure) and 

cover by only the dominant trees in the canopy (dominant canopy 

closure). About 70 percent of the mapped stands (polygons) in 

each study area were field-verified for classification accuracy. 

Vegetation maps were subsequently digitized, stored in a Geographic Information System (GIS), and analyzed using 

ARCINFO software. 

150 Chapter 6 

Table 6A-Vegetation classifications used in Chi-square tests of habitat 

selection. 

Classes Characteristics 

Tree size-classes 

Open grassland No trees 

Sapling D.b.h. <5 inches1 

Pole D.b.h. 5-10 inches 

Small sawtimber D.b.h. 11-20 inches 

Medium sawtimber D.b.h. 21-35 inches 

Large sawtimber D.b.h. >35 inches 

Canopy-closure classes 

Open 10 percent closure2

Sparse 10-19 percent 

Poor 20-39 percent 

Normal 40-69 percent 

Good <69 percent 

Suitability as owl habitat 

Suitable Medium or large sawtimber, canopy 

 closure class poor or better, and total 

 closure >69 percent 

Unsuitable All other lands 

1

 Diameter at breast height of the dominant size-class, according to basal area 

2

 Canopy closure based on the dominant tree size-class. 

Statistical Analysis 

By definition, home-range estimators assume repeated use 

of an area, and a random flight path does not constitute a home 

range. For this reason, we determined whether individual owls 

exhibited site fidelity prior to calculating home-range size (Spencer 

et al. 1990). The mean-squared distance from the center of 

activity (MSD) (Calhoun and Casby 1958) was used to measure 

site fidelity. A bird displayed site fidelity if its flight path was 

less than the MSD for 975 of 1,000 simulated paths. (See Spencer et al. 1990 for simulation techniques.) 

Home-range size was computed using two estimators, the 

100-percent minimum convex polygon (MCP) (Mohr 1947 

Hayne 1949) and the 95-percent adaptive kernel (AK) (Worton 

1989, Baldwin pers. comm.). Because convex polygon areas are 

sensitive to sample size (Jennrich and Turner 1969), we used the 

95-percent AK estimates for all comparisons and statistical tests. 

We report 100-percent MCP estimates of home-range size to 

allow comparisons with other studies reported in the literature 

and elsewhere. The correlation coefficient between AK and 

MCP estimates was significant (r = 0.93, d.f. = 52, P < 0.0001). 

Telemetry data were partitioned into a breeding period (1 March-31 

August) and a nonbreeding period (1 September-28 February 

Foraging (nighttime) locations were used to estimate home ranges. 

The Chi-square goodness-of-fit test was used to test the 

hypothesis that owls used habitat types within their home ranges 

in proportion to availability (Neu et al. 1974). When this hypothesis was rejected, we used Bonferroni confidence intervals 

(at the P < 0.05 level) to determine which habitat types were 

used more or less than expected (Byers et al. 1984). Mapped 

polygons were classified by diameter size-class of the dominant 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
trees, total canopy closure, and dominant canopy closure. 

Chi-square analyses were performed for each of the three types 

of classification. In addition, the analyses were repeated after 

reclassifying polygons as suitable or unsuitable, based on current FS, Region 5 (R5), definitions of suitable spotted owl 

habitat. Suitable stands were those in which the diameter size-class 

of dominant trees was ≥21 inches in d.b.h., canopy closure of 

dominant trees was ≥20 percent, and total canopy closure was 

≥70 percent. 

Results 

Home-Range Size 

Eleven females and 10 males were radio-tracked between 

26 April 1987 and 28 February 1990 at S-CON; and six females 

and six males were radio-tracked between 28 February 1989 and 

28 February 1990 at S-OAK. Nine females and eight males were 

tracked between 25 May 1989 and 5 April 1990 at L-CON. Owls 

were monitored over periods ranging from 56 to 794 days. 

sampling intervals varied among owls because transmitters failed, 

individuals died, or owls permanently left the study areas. 

Eighteen of 21 owls at S-CON, 12 of 12 at S-OAK, and 13 

of 17 at L-CON passed the site-fidelity test. Owls that failed to 

exhibit site fidelity had few radio locations or made long movements during the breeding or nonbreeding seasons. All three 

birds at S-CON that failed the test were migrants that exhibited 

long movements (see Chapter 4). 

To compare home-range sizes among sites and between 

seasons, we excluded owls that were tracked over a period of 

less than 5 months during the 6-month nonbreeding season. Our 

sampling frequency varied among individual owls, and 5 months 

was as close to complete coverage as we could achieve because 

of irregular sampling intervals. Estimates of home-range sizes 

for the nonbreeding period were calculated for 13 owls at S-CON, 

5 at S-OAK, and 7 at L-CON that passed the site fidelity test and 

were tracked for a period of at least 150 days. The number of 

radio locations per season among these owls ranged from 21 to 

91 (x = 58.6 ± 17.0). Home-range sizes of owls that passed the 

site-fidelity test and that were tracked over a period of at least 

150 days were not significantly correlated with the number of 

radio locations. We relaxed the criteria for breeding-season 

estimates. Requiring a tracking period of at least 150 days, only 

seven owls had sufficient data to estimate a home-range size. To 

use a larger sample of owls, we excluded only owls with fewer 

than 20 radio locations within a breeding season. Fifteen of 21 

owls at S-CON, 7 of 12 at S-OAK, and 9 of 15 at L-CON met 

this criterion. The mean number of locations among these owls 

as 37.6 (± 10.6) and they were tracked over an average period 

of 116.0 days (± 39.1, range = 56-184 days). 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

Home ranges were significantly larger at S-CON than at 

S-OAK during both seasons (table 6B). A two-way ANOVA for 

these two sites indicated significant effects of study site (F = 

13.9, d.f. = 1, 50, P < 0.001) and season (F = 4.2, d.f. = 1, 50, P 

< 0.05) on AK home-range size, with an interaction effect. The 

interaction effect was due to larger home ranges in the nonbreeding 

than in the breeding season at S-CON, but home ranges at 

S-OAK were larger during the breeding season. The difference 

in home-range size between seasons at L-CON was not significant (t = 1.4, d.f. = 14, P > 0.15), but owls at L-CON had 

home-range sizes about twice those at S-CON in both seasons. 

Home-range sizes did not differ significantly between sexes at 

any site during either season. 

Owls at S-CON exhibited variable behavior during the 

winter. Individual birds either migrated, occupied nearly the 

same home range in winter as in summer, enlarged their home 

range in winter but still used most or all of the summer home 

range as well, or shifted their home range for the winter but still 

overlapped a portion of the summer home range (Chapter 4). 

Among 21 owls radio-tagged at this site, six were classified as 

year-round residents, two as enlargers, five as shifters, five as 

migrants, and three as unknown. Differences in nonbreeding 

home-range sizes among these categories of birds were significant (F = 12.4, d.f. = 3, 17, P < 0.001). Shifters had the largest 

home ranges ( x = 13,254 ± 4,984 acres), followed by enlargers 

( x = 5,960 ± 3,031), and residents ( x = 3,302 ± 781). Only one 

migrant passed the site-fidelity test, and its home range was 

9,146 acres. 

Annual home ranges were calculated for owls that passed 

the site-fidelity test both seasons, were tracked over a period of 

at least 150 days during the nonbreeding season, and had ≥20 

radio locations during the breeding season (table 6B). Owls at 

S-CON had significantly larger annual home ranges than those 

at S-OAK (t = 2.5, d.f. = 19, P < 0.05). As with breeding and 

nonbreeding home-range sizes, annual home-range sizes at L-CON 

were more than twice the size of those at S-CON. 

Seasonal home-range sizes of pairs were calculated only 

when both members of pairs passed the site-fidelity test, were 

tracked for at least 150 days during the nonbreeding period, and 

had ≥20 radio locations during the breeding period (table 6B). 

Only one pair at L-CON met these criteria, precluding further 

analysis of pair home-range data. A two-way ANOVA for S-CON 

and S-OAK indicated no significant differences between the 

nonbreeding and breeding periods (F = 1.1, d.f. = 1, 12, P > 

0.30), but pair home ranges were larger at S-CON than at 

S-OAK (F = 3.9, d.f. = 1, 12, P = 0.07). The mean proportion of 

home-range overlap between members of pairs did not differ 

significantly by study site (F = 0.3, d.f. = 1, 12, P > 0.60) or by 

season (F = 0.2, d.f. = 1, 12, P > 0.60). At both sites in the Sierra 

NF, pairs had more overlap in their areas of use during the 

nonbreeding period (x = 51 ± 18 percent) than during the breeding period ( x = 47 ± 12 percent). 

Spotted owl home ranges shifted seasonally. Overlap between breeding and nonbreeding periods, using 95-percent AKs 

of individual home ranges, was 34 ± 18 percent at S-CON, 54 ± 

5 percent at S-OAK, and 38 ± 8 percent at L-CON. 

Chapter 6 151 
Habitat Use 

Habitat Composition Within Home Ranges 

During the breeding season, 13 and 63 percent of the habitat 

types available within individual home ranges were medium and 

large sawtimber (≥21 inches in d.b.h.) at S-CON and L-CON, 

respectively (tables 6C-6E), and 91 percent of the habitat types 

available within owl home ranges at S-OAK were classified as 

old-growth. Percentages were similar during the nonbreeding 

season in all areas. The mean proportions of individual home 

ranges that were ≥40 percent dominant canopy closure varied 

among sites (table 6F); proportions were similar at L-CON and 

S-CON, but they were about two times greater at S-OAK. 

Proportions for ≥40 percent total canopy closure were similar 

among all sites. Mean percentages of home ranges that were 

"suitable" habitat using R5 definitions were 4 (S-CON), 27 

(S-OAK), and 26 (L-CON) at the three sites during the breeding 

season. 

152 Chapter 6 

Habitat-Use Patterns Within Home Ranges 

Among all of the owls during the breeding season, 10 and 

82 percent of the radio locations were in medium and large 

sawtimber at S-CON and L-CON, respectively, and 92 percent 

were in old-growth at S-OAK; 21, 70, and 99 percent of the 

locations were in these size classes at the three sites, respectively, during the nonbreeding season (tables 6C-6F). More use 

occurred in small than in medium and large sawtimber at S-CON 

87 percent and 73 percent of the radio locations during the 

breeding and nonbreeding seasons, respectively. The proportions of radio locations that were in dominant canopy closure 

≥40 percent were again similar at L-CON and S-CON, but 

nearly twice as high at S-OAK. Higher proportions of locations 

occurred in total canopy closure ≥40 percent, and results were 

similar among the sites. Use of R5 suitable habitat was low--

means of radio locations in this habitat class were 4 percent 

(S-CON), 47 percent (L-CON), and 49 percent (S-OAK) during 

the breeding season (table 6F). Use was greater than availability 

for most site-season comparisons and definitions of suitable 

habitat (fig. 6A). Use of R5 suitable habitat at S-CON was equal 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992 

Table 6B--Means (x) and standard deviations (SD) of home-range sizes of California spotted owls studied from 1987-1990 in the northern and central Sierra Nevada. 

Study sites were in conifer forest on the Lassen NF (L-CON) and in conifer forest (S-CON) and riparianlhardwood forest (S-OAK) on the Sierra NF. Estimates were 

determined using foraging locations and the 95-percent adaptive kernel method; 100-percent minimum convex polygon estimates of home-range sizes are in 

parentheses; n = number of individuals or pairs of owls. 

Total home-range size (acres) 

Birds L-CON S-CON S-OAK 

x ± SD n x ± SD n x ± SD n 

Individual birds 

Breeding 7,061.2 ± 5,992.5 9 2,366.8 ± 740.0 24 985.0 ± 745.0 7 

 (5,422.6 ± 5,194.4) (1,798.7 ± 787.2) (714.6 ± 624.4) 

Nonbreeding 11,601.0 ± 6,664.1 7 6,834.5 ± 5,138.3 18 661.0 ± 510.1 5 

 (14,676.7 ± 8,251.8) (5,943.3 ± 4,529.5) (761.7 ± 495.7) 

Annual 12,473.5 ± 7,305.5 6 5,715.1 ± 4,289.9 16 874.1 ± 644.2 5 

 (12,927.2 ± 10,132.2) (5,968.8 ± 4,639.9) (1,042.6 ± 865.7) 

Pairs-breeding period 

Total area 3,869.3 1 3,420.5 ± 858.1 8 720.5 ± 402.9 2 

 (3,014.4) (2,514.8 ± 873.6) (457.4 ± 274.4) 

Area shared 1,869.3 1,544.5 ± 364.8 397.9 ± 341.6 

 (1,164.9) (1,027.5± 317.6) (278.6± 225.8) 

Pairs-nonbreeding period 

Total area 9,871.8 1 9,730.8 ± 10,168.0 4 573.3 ± 271.0 2 

 (17,292.5) (7,201.0 ± 6,901.2) (818.8 ± 251.4) 

Area shared 1,407.9 4,021.2 ± 3,929.3 321.3 ± 160.1 

 (563.2) (3,766.0 ± 4,355.4) (297.6 ± 133.9) 

Pairs-annual 

Total area 8,253.0 ± 7,872.6 4 778.0 ± 405.8 2 

 (7,709.4 ± 7,184.0) (875.6 ± 303.8) 

Area shared 4,443.0 ± 4,626.1 447.6 ± 318.9 

 (4,492.2 ± 4,945.2) (459.9 ± 201.3) 
Table 6C Means ( x ) and standard deviations (SD) of areas, proportions of home ranges, and proportions of foraging and roosting radio locations in different tree 

size and canopy-closure classes for owls that passed the site fidelity test at the S-CON study site in the central Sierra Nevada. The 95-percent adaptive kernel method 

was used to estimate home ranges for 1987-1989 breeding and nonbreeding seasons. 

Breeding period1 Nonbreeding period1 

Proportion of Proportion of Proportion of Proportion of 

Area (acres) home range locations Area (acres) home range locations 

Habitat type x SD x SD x SD x SD x SD x SD 

Tree size-class 

d.b.h. in inches) 

No trees 9.1 35.8 0.003 0.01 0 0 736.3 1,828.5 0.058 0.14 0.031 0.08 

<5 0 0 0 0 0 0 0 0 0 0 0 0 

5 - 10 66.4 82.3 0.029 0.04 0.027 0.05 101.8 109.9 0.020 0.02 0.023 0.04 

11 - 20 1,666.0 976.1 0.839 0.17 0.870 0.22 4,028.8 2,826.9 0.671 0.18 0.732 0.19 

21 - 35 230.0 319.1 0.095 0.11 0.065 0.16 1,281.7 998.1 0.192 0.10 0.141 0.11 

>35 59.0 143.8 0.032 0.08 0.038 0.08 477.5 684.4 0.054 0.07 0.073 0.12 

Dominant canopy 

closure (percent) 

< 10 98.8 254.4 0.027 0.06 0.007 0.02 245.8 492.0 0.022 0.03 0.013 0.03 

10 - 19 164.7 154.9 0.076 0.06 0.040 0.04 568.6 509.8 0.080 0.05 0.043 0.03 

20-39 1,147.8 776.1 0.537 0.18 0.416 0.24 3,117.6 2,220.8 0.492 0.14 0.442 0.19 

40 - 69 605.4 311.0 0.354 0.22 0.537 0.25 1,945.9 977.6 0.342 0.14 0.471 0.19 

70 - 100 4.2 16.3 0.002 0.08 0 0 13.1 35.1 0.001 0 0 0 

Total canopy 

closure (percent) 

<10 32.9 70.1 0.010 0.02 0.002 0.01 124.2 215.9 0.012 0.01 0.007 0.01 

10 - 19 111.1 230.7 0.038 0.06 0.015 0.03 288.2 367.3 0.035 0.02 0.019 0.02 

20 - 39 306.5 293.2 0.137 0.07 0.076 0.08 999.4 1,172.8 0.126 0.07 0.077 0.06 

40 - 69 1,318.0 618.5 0.677 0.12 0.685 0.21 3,534.6 1,943.9 0.598 0.14 0.644 0.15 

70 - 100 252.9 188.5 0.132 0.09 0.222 0.21 944.3 560.7 0.164 0.06 0.222 0.13 

1 Breeding period--n = 24 owls with a total of 1,583 locations; nonbreeding period--n = 18 owls with a total of 1,358 locations. An individual owl may contribute

to n more than once if it was radio tracked during multiple breeding or nonbreeding seasons. 

to or less than availability, but note the small amounts of this 

type available at S-CON 

Nearly half of the owls at all three sites had significant 

Chi-square tests of habitat use for tree size-class, and nearly 

three-fourth of the owls had significant tests for use of 

canopy-closure classes. Differences in the proportion of owls 

that demonstrated habitat selection between the breeding and 

nonbreeding seasons were neither large nor consistent (fig. 6B). 

Fewer owls selected for tree size-class than for canopy-closure 

classes, and more selected for dominant canopy closure than for 

total canopy closure. Fewer owls had significant tests for habitat 

use when only foraging locations were used than when foraging 

and roosting locations were pooled. Summed across the three 

study areas (foraging and roosting locations pooled), 14 of 39 

owls had significant tests for use of tree size-class, 26 owls for 

total canopy closure, and 29 owls for dominant canopy-closure 

classes during the breeding season. During nonbreeding 

season, 14 of 29 owls at the three study sites selected significantly for three size-class, 20 owls for total canopy closure, and 

20 owls for dominant canopy-closure classes. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

We tested whether owls used "suitable" habitat more than 

expected based on FS, R5, definitions (fig. 6A). Results were 

similar for L-CON and S-OAK: seven of eight birds at L-CON 

had significant tests during the breeding season, and four of six 

birds tested significant during the nonbreeding season. At S-OAK, 

six of seven birds and four of five birds had significant tests 

during the breeding and nonbreeding seasons, respectively. On 

the other hand, at S-CON, only four of 24 owls during the 

breeding season and six of 18 owls during the nonbreeding 

season had significant tests for use of suitable habitat. 

Patterns of habitat use were weak and inconsistent among 

the subset of owls that passed the test of habitat selection (figs. 

6C-6E). Most owls at all three sites used habitat types in proportion to their availability. Patterns were clearer and stronger for 

canopy closure than for tree size-class. More than half of the 

owls used poor cover classes less than expected and many used 

canopy closure ≥40 percent more than expected. Differences 

between dominant and total cover were minor. Patterns of selection for canopy closure did not appear to be stronger during the 

breeding than during the nonbreeding season. Use of tree size-class 

Chapter 6 153 
Table 6D-Means ( x ) and standard deviations (SD) of areas, proportions of home ranges, and proportions of foraging and roosting radio locations in different tree 

size and canopy-closure classes for owls that passed the site fidelity test at the S-OAK study site in the central Sierra Nevada. The 95-percent adaptive kernel method 

was used to estimate home ranges for the 1989 breeding and nonbreeding seasons. 

Breeding period1 Nonbreeding period1 

Proportion of Proportion of Proportion of Proportion of 

Area (acres) home range locations Area (acres) home range locations 

Habitat type x SD x SD x SD x SD x SD x SD 

Tree size-class 

(d.b.h. in inches) 

No trees 6.2 10.6 0.005 0.01 0 0 0 0 0 0 0 0 

<5 0 0 0 0 0 0 0 0 0 0 0 0 

5-10 0 0 0 0 0 0 0 0 0 0 0 0 

11 - 20 67.7 138.1 0.083 0.15 0.080 0.18 7.7 11.1 0.014 0.02 0.006 0.01 

21 - 35 703.2 577.5 0.912 0.15 0.920 0.18 647.6 556.5 0.986 0.02 0.994 0.01 

>35 0 0 0 0 0 0 0 0 0 0 0 0 

Dominant 

canopy closure 

(percent) 

<10 26.4 55.3 0.021 0.03 0 0 52.6 76.6 0.055 0.04 0.024 0.03 

10 - 19 127.9 161.3 0.132 0.14 0.081 0.09 156.6 145.7 0.210 0.16 0.108 0.09 

20 - 39 102.0 167.7 0.105 0.11 0.033 0.04 125.0 244.3 0.113 0.14 0.068 0.13 

40 - 69 444.8 312.7 0.542 0.23 0.494 0.36 239.6 152.2 0.391 0.10 0.378 0.17 

70 - 100 69.4 32.4 0.194 0.24 0.391 0.36 81.8 39.8 0.231 0.23 0.422 0.32 

Total canopy 

closure 

(percent) 

<10 26.4 55.3 0.021 0.03 0 0 52.6 76.6 0.055 0.04 0.024 0.03 

10 - 19 99.0 141.5 0.093 0.09 0.069 0.08 102.8 145.7 0.112 0.08 0.060 0.06 

20 - 39 76.6 80.0 0.106 0.10 0.027 0.03 113.9 86.4 0.170 0.07 0.057 0.06 

40 - 69 455.0 400.4 0.501 0.27 0.416 0.30 268.5 293.9 0.347 0.11 0.385 0.23 

70 - 100 114.1 49.6 0.273 0.26 0.488 0.31 117.6 43.0 0.316 0.25 0.474 0.32 

1 Breeding period--n = 7 owls with a total of 498 locations; nonbreeding period--n = 5 owls with a total of 548 locations.

showed stronger patterns at L-CON than at S-CON and S-OAK. 

More than half of the birds at L-CON used small sawtimber (<11 

inches in d.b.h.) less than expected, and medium and large 

sawtimber more than expected, during the breeding season. 

These results were weaker during the nonbreeding season. 

Discussion 

Home-Range Size 

Estimates of home-range size among California spotted 

owls are extremely variable. All available data indicate that they 

are smallest in habitats at relatively low elevations that are 

dominated by hardwoods, intermediate in size in conifer forests 

in the central Sierra Nevada, and largest in true fir forests in the 

northern Sierra Nevada. 

Home-range sizes of owls in our studies at L-CON, S-CON, 

and S-OAK varied among areas and between seasons. The mean 

area used was about twice as large in the northern compared to 

154 Chapter 6 

the central Sierra Nevada, and was two to 10 times larger in 

high-elevation conifer forests compared to low-elevation oak 

woodlands in the central Sierra Nevada. Median home-range 

estimates for pairs of northern spotted owls were 3,000 to 5,000 

acres (Thomas et al. 1990)-less than half the size of pair home 

ranges of California spotted owls at L-CON (12,500 acres) but 

about the same as the pair estimates at S-CON. California spotted owls in the foothill riparian/hardwoods and oak woodlands at 

S-OAK used less than 900 acres, or approximately 20-30 percent of the area used by northern spotted owls. 

Two pairs of owls radio tracked in the San Bernardino 

Mountains used home ranges averaging more than 5,300 acres 

(100-percent MCP-LaHaye pers. comm.) (table 6G). 

Home-range sizes of pairs during the breeding period averaged 

4,569 acres on the Tahoe NF (100-percent MCP, n = 2; Call 

1990, p. 21) and 4,759 acres on the Eldorado NF [100-percent 

MCP, n = 4 (excludes two pairs with relatively few radio locations); Laymon 1988, p. 187]. These estimates, with those of 

3,869 acres for the Lassen NF (n = 1, table 6B) and 3,421 acres 

for the Sierra NF (n = 8, table 6B), give an overall estimate of 

about 4,200 acres (grand mean, unweighted for sample size) for 

home-range sizes of owl pairs during the breeding period in 

Sierran conifer forests. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
The smallest estimated use areas of California spotted owls 

(means for pairs ranging from 98 to 243 acres) were based on 

kown sizes of small stringers of dense riparian/hardwoods in 

the Cleveland, Angeles, and Los Padres NFs (table 6G). Owls in 

these stringers were not radio-tagged. Perhaps some of them 

used more than one canyon bottom, but the Forest Biologists 

who made these estimates reported that, in some cases, other 

individuals or pairs of owls occupied the riparian stringers in 

adjacent canyons. Most canyon sides above the riparian zones 

were covered by dense chaparral. We believe it is most unlikely 

that the owls can use the chaparral, as it is too dense for safe or 

effective flight. 

We strongly suspect that the large differences in home-range 

sizes reported here are related, at least in part, to differences in 

the primary prey of the owls in different localities. Consistently, 

California spotted owls with the smallest observed home ranges 

prey primarily on woodrats, but those with the largest home 

ranges specialize on flying squirrels. Woodrat densities generally tend to be much greater than flying squirrel densities, and 

woodrats weigh nearly twice as much as flying squirrels (Chapter 4). Similar relations are suggested in recent studies of northern spotted owls by Carey et al. (1992). 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

Habitat Selection 

Here we evaluate habitat quality based on use versus availability of types (Thomas et al. 1990, appendix F). We regard as 

suitable those habitats selected in excess of availability by most 

owls. Marginal habitats are seldom or never used in excess of 

availability, used in proportion to availability by many owls, and 

used less than expected by many other owls. Habitat types used 

less than expected by most owls are considered to be poor in 

quality and are classed as unsuitable habitat. 

Spotted owls in this study more consistently selected for 

high canopy closure than for large tree size-class. Chi-square 

values were consistently higher for canopy closure, and more 

owls had significant tests for selection of high canopy closure 

than for tree-size class in 18 site-season comparisons. Differences between total and dominant canopy closure were minor, 

but because more owls exhibited significant selection for high 

dominant cover than for high total cover, dominant cover may be 

a better measure of suitable habitat for California spotted owls. 

The amount of medium and large sawtimber in individual 

home ranges did not appear to be a good indicator of the amount 

of that habitat needed to sustain California spotted owls, unlike 

the case for northern spotted owls (see Thomas et al. 1990). 

Nearly half of the California spotted owls had significant tests of 

Chapter 6 155 

1

 Breeding period--n = 8 owls with a total of 479 locations; nonbreeding period--n = 6 owls with a total of 402 locations. 

Table 6E-Means ( x ) and standard deviations (SD) of areas, proportions of home ranges, and proportions of foraging and roosting radio locations in different tree 

size and canopy-closure classes for owls that passed the site fidelity test at the L-CON study site in the northern Sierra Nevada. The 95-percent adaptive kernel method 

was used to estimate home ranges for breeding (1990) and nonbreeding (1989-1990) seasons. 

Breeding period1 Nonbreeding period1 

Proportion of Proportion of Proportion of Proportion of 

Area (acres) home range locations Area (acres) home range locations 

Habitat type x SD x SD x SD x SD x SD x SD 

Tree size-class 

(d.b.h.in inches) 

No trees 515.0 913.2 0.049 0.04 0 0 1,368.4 1,241.2 0.112 0.09 0.007 0.01 

<5 9.9 17.5 0.001 0 0 0 39.3 38.8 0.003 0 0.002 0.01 

5 - 10 207.7 332.5 0.023 0.01 0.011 0.02 608.9 383.8 0.046 0.02 0.029 0.04 

11 - 20 2,659.2 3,724.8 0.300 0.15 0.172 0.11 3,502.0 2,340.1 0.247 0.12 0.261 0.12 

21 - 35 1,394.8 962.3 0.251 0.10 0.298 0.15 2,755.5 1,490.4 0.224 0.10 0.220 0.05 

> 35 2,371.9 2,287.2 0.377 0.12 0.519 0.17 4,644.8 2,164.0 0.368 0.09 0.480 0.10 

Dominant 

canopy closure 

(percent) 

<10 1,296.3 1,797.4 0.160 0.04 0.072 0.08 2,198.5 1,469.6 0.165 0.06 0.064 0.04 

10 - 19 1,598.6 2,003.9 0.204 0.04 0.142 0.07 2,859.0 1,329.1 0.223 0.05 0.131 0.13 

20 - 39 2,043.2 2,230.9 0.303 0.10 0.329 0.21 3,266.3 1,415.1 0.255 0.03 0.318 0.14 

40-69 1,607.0 1,461.7 0.251 0.11 0.350 0.17 2,436.2 1,439.3 0.187 0.06 0.318 0.12 

70 - 100 532.8 614.3 0.076 0.03 0.108 0.08 1,294.3 840.3 0.094 0.03 0.165 0.11 

Total canopy 

closure 

(percent) 

<10 679.5 1,191.5 0.067 0.05 0.033 0.06 1,088.0 985.8 0.079 0.06 0.006 0.01 

10 - 19 300.8 403.4 0.041 0.02 0.012 0.02 666.9 618.7 0.048 0.04 0.013 0.01 

20 - 39 1,595.1 1,983.9 0.211 0.06 0.105 0.06 1,983.2 1,359.0 0.145 0.05 0.117 0.04 

40 - 69 2,038.2 2,369.7 0.263 0.09 0.204 0.16 3,268.1 1,630.2 0.257 0.08 0.185 0.11 

70 - 100 2,464.3 2,063.7 0.411 0.09 0.647 0.15 5,048.4 2,205.2 0.396 0.05 0.675 0.11 
Figure 6A-Mean proportions of suitable habitat available in home ranges of California spotted owls in relation to proportions used by 

radio-tagged birds in different seasons and study sites. Study areas were in conifer forests of the Sierra (S-CON) and Lassen (L-CON) 

National Forests, and in hardwood/riparian forests of the Sierra (S-OAK) National Forest. Three categories of suitable habitat were 

tested: (1) R5 = Forest Service (Region 5) definition-medium or large sawtimber, dominant canopy closure poor or higher, and total 

closure >69 percent; (2) canopy closure of dominant trees ≥40 percent; and (3) total canopy closure ≥40 percent. Error bars are standard 

deviations (SD). The 95-percent adaptive kernel was used to delineate home ranges. 

156 Chapter 6 USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Figure 6B-The proportion of California spotted owls with significant (P ≤ 0.05) Chi-square tests for selection of habitats based 

on tree size-classes, dominant canopy closure, and total canopy closure. Study areas were in conifer forests of the Sierra 

(S-CON) and Lassen (L-CON) National Forests, and in hardwood/riparian forests of the Sierra (S-OAK) National Forest. Tests 

were done separately for breeding and nonbreeding periods using foraging locations alone, and using foraging and roosting 

locations pooled. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. Chapter 6 157 
selection for tree size-class, but most of them used all size classes 

in proportion to availability in the central Sierra Nevada. 

Patterns were stronger at L-CON during the breeding season, 

when about half of the birds used medium and large sawtimber 

more than expected. By contrast, old-growth was used in greater 

proportion than its availability, for nesting, roosting, and foraging, by most northern spotted owls in Oregon and Washington, 

and it was never used less than expected. Throughout their range 

and across all seasons, northern spotted owls consistently showed 

foraging and roosting patterns significantly associated with 

old-growth stands or mixed stands of mature and old-growth 

trees. Among California spotted owls, however, patterns of habitat use for tree size-class were weaker, and they were not consistent among study areas. 

Canopy closure ≥40 percent was used by many California 

spotted owls greater than expected, by a few less than expected, 

and by many equal to its availability. Canopy closure ≤39 

percent was used by most owls less than expected and in proportion to availability by many others. Based on these results, then, 

suitable habitat for California spotted owls appears to include 

canopy closure ≥40 percent, and habitat with ≤39 percent 

canopy closure is marginal to unsuitable. 

The R5 definition of suitable habitat does not appear to be 

appropriate across the range of the California spotted owl. Most 

owls at L-CON (79 percent) and S-OAK (83 percent) used 

R5-defined suitable habitat in excess of availability. But results 

were quite different for owls in the conifer forest at S-CON, 

where this habitat type was generally not available within home 

ranges. At S-CON, most birds used R5-defined suitable habitat 

in proportion to availability, a few used it more than expected, 

and a few less than expected. 

Habitat Selection and Population Stability 

Habitat selection by owls at S-CON was generally less 

evident than at L-CON, even though both sites were in Sierran 

conifer forests. We also determined that only 13 percent of the 

forest in the study area at S-CON was in medium and large 

sawtimber, whereas the L-CON site had 63 percent of its forest 

in those timber size classes and several of the owls there exhibited significant selection for those timber stands. During the 

breeding periods of 1987, 1988, and 1989, owl crews on the 

Lassen and Sierra NFs monitored the occupancy status and 

breeding activity of owl pairs in Spotted Owl Habitat Areas 

(SOHAs) managed for the owls. Over that period on the Lassen 

NF, an absence of pairs was confirmed in 8 of 27 cases (30 

percent) and breeding was confirmed in 8 of 27 cases (30 

percent). During the same period on the Sierra NF, an absence of 

pairs in SOHAs was confirmed in 8 of 11 cases (73 percent) and 

breeding was confirmed in only 1 of 11 cases (9 percent). 

Because we lack sufficient information to determine whether 

California spotted owls in any part of the Sierra Nevada are 

reproducing at a rate that can maintain the population (Chapter 

8), we cannot be certain that habitats used by owls either at 

L-CON or S-CON were adequate to provide for a balance 

between births and deaths. The data suggest, however, that the 

habitat available to spotted owls on the Sierra NF may be less 

adequate than that on the Lassen NF. Indeed, it may be that 

158 Chapter 6 

spotted owls on the Sierra NF cannot maintain their numbers, 

and that perhaps they are maintained in part by immigration 

from populations in the neighboring NPs. Note that the Sierra 

NF shares its northern border with Yosemite NP and its southern 

border with Sequoia/Kings Canyon NPs (fig. 4B). 

Power of Chi-square Tests 

The power of Chi-square tests of habitat selection is influenced by classification error (telemetry and/or mapping), the 

resolution of habitat classification, and by the number of locations available to estimate use. These factors may reduce the 

likelihood of detecting habitat selection when, in fact, it is 

occurring (type-II error-see White and Garrott 1986). In addition, the power of a test depends on the "effect size" (Cohen 

1988) or, in the case of habitat selection, the degree to which 

differences exist between proportions of available and used 

habitat types. The smaller the effect size, other things being 

equal (significance level, desired power), the larger the sample 

size needed to detect selection. The small number of locations in 

our data could have reduced the likelihood of detecting habitat 

selection when it occurred. Owls with significant Chi-square 

tests for selection of canopy-closure classes had a mean of 72 

(±23) radio locations, but owls with insignificant tests had a 

mean of only 57 (± 19) locations. The difference between these, 

sample sizes was significant (t test = 2.8, d.f. = 78, P < 0.01), 

indicating that small sample sizes may have been associated 

with our failure to detect habitat selection by radio-tagged owls. 

On the other hand, the difference between the number of locations between owls that passed ( x = 69 ± 24) and failed ( x = 62 

±21) tests of selection for tree size-class was not significant (t = 

1.3, d.f. = 68, P > 0.20). 

The number of radio locations approximately doubled when 

we pooled foraging and roosting locations. More owls had significant Chi-square tests of habitat use for the pooled data set 

than was the case for the foraging locations alone. The difference in Chi-square results between these two groups was apparently due primarily to differences in habitat use by foraging and 

roosting owls, and less so to the increase in sample size. Roost 

sites were more distinct from available sites than was the case for 

foraging sites. For example, 18 of 18 owls at S-CON used sites 

that had %40 percent canopy closure more than expected for 

roosting during the breeding period, and 83 percent had significant tests of habitat selection. Use of sites that had ≥70 percent 

canopy closure for roosting was similar: 16 of 18 owls had 

significant tests of habitat selection, and 11 of 18 owls used, these 

stands more than expected (G. N. Steger pers. observ.). By 

contrast, only 7 of 24 owls at S-CON had significant tests for use 

of foraging sites with dense canopy during the breeding period, 

and only seven owls foraged more than expected in sites with 

≥40 percent canopy closure. 

Effect size increases with the size of the difference between 

proportions of available and used habitat types, providing a 

useful index to identify where habitat selection is greatest. We 

found higher effect sizes for canopy closure than for tree size-class, 

and for foraging and roosting locations combined than for foraging locations alone. These results support the conclusion that the 

owls had differential use patterns between daytime roosting 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Figure 6C-The number of California spotted owls at S-CON (conifer forest study area in the Sierra National Forest) that used tree 

size-classes (diameter at breast height) and canopy-closure classes (percent) greater than, equal to, or less than expected during breeding 

and nonbreeding periods. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. Chapter 6 159 
Figure 6D-The number of California spotted owls at S-OAK (riparian/hardwood forest study area in the Sierra National Forest) that used tree 

size-classes (diameter at breast height) and canopy-closure classes (percent) greater than, equal to, or less than expected during breeding and 

nonbreeding periods. 

160 Chapter 6 USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Figure 6E-The number of California spotted owls at L-CON (conifer forest study area in the Lassen National Forest) that used 

tree diameter-classes (diameter at breast height) and canopy-closure classes (percent) greater than, equal to, or less than 

expected during breeding and nonbreeding periods. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. Chapter 6 161 
Table 6F--Means ( x ) and standard deviations (SD) of total area, proportions of home ranges, and proportions of foraging and roosting radio locations in suitable 

and unsuitable habitat for California spotted owls that passed the site fidelity test for the 1987-1990 breeding and nonbreeding period at L-CON, S-CON, and S-OAK 

study sites in the northern and central Sierra Nevada. The 95-percent adaptive kernel method was used to estimate home ranges (n = number of individual owls). 

Breeding period1 Nonbreeding period1 

Area Proportion of Proportion Area Proportion of Proportion 

 (acres) home range of locations (acres) home range of locations 

Study site Habitat type n x SD x SD x SD n x SD x SD x SD 

S-CON Total canopy 

 closure ≥40 pct 24 18 

 Suitable 1,570.7 734.1 0.813 0.11 0.907 0.10 4,478.9 2,378.6 0.806 0.11 0.893 0.07 

 Unsuitable 450.5 540.9 0.187 0.11 0.093 0.10 1,412.1 1,734.4 0.194 0.11 0.107 0.07 

 Dominant canopy 

 closure ≥40pct 24 18 

 Suitable 609.6 308.0 0.357 0.22 0.537 0.25 1,959.0 991.5 0.361 0.12 0.487 0.19 

Unsuitable 1,411.6 1,095.7 0.643 0.22 0.463 0.25 3,932.0 3,053.7 0.639 0.12 0.513 0.19 

Region 52 24 18 

Suitable 93.6 126.5 0.039 0.05 0.037 0.08 655.8 499.9 0.105 0.06 0.105 0.11 

Unsuitable 1,927.6 1,117.4 0.961 0.05 0.963 0.08 5,256.2 3,560.3 0.895 0.06 0.895 0.11 

S-OAK Total canopy 

 closure ≥40 pct 7 5 

 Suitable 569.1 365.8 0.779 0.16 0.904 0.08 386.1 256.4 0.663 0.14 0.859 0.09 

 Unsuitable 202.0 261.1 0.221 0.16 0.096 0.08 269.2 299.4 0.337 0.14 0.141 0.09 

 Dominant canopy 

 closure ≥40 pct 7 5 

 Suitable 514.5 284.5 0.740 0.17 0.886 0.10 321.3 122.0 0.623 0.21 0.799 0.19 

 Unsuitable 256.6 349.8 0.260 0.17 0.114 0.10 334.2 439.9 0.377 0.21 0.201 0.19 

 Region 52 7 5 

 Suitable 113.6 49.6 0.273 0.26 0.488 0.31 117.6 43.0 0.316 0.25 0.474 0.32 

 Unsuitable 657.5 599.5 0.727 0.26 0.512 0.31 538.0 592.8 0.684 0.25 0.526 0.32 

L-CON Total canopy 

 closure ≥40 pct 8 6 

Suitable 4,502.6 4,425.3 0.679 0.11 0.851 0.09 8,316.5 3,739.6 0.709 0.12 0.864 0.05 

Unsuitable 2,575.2 3,557.8 0.321 0.11 0.149 0.09 3,737.9 2,750.8 0.291 0.12 0.136 0.05 

 Dominant canopy 

 closure ≥40 pct 8 6 

Suitable 2,140.0 2,059.0 0.328 0.13 0.458 0.16 3,730.4 2,114.3 0.304 0.07 0.487 0.09 

Unsuitable 4,938.0 6,005.8 0.672 0.13 0.542 0.16 8,323.7 3,986.6 0.696 0.07 0.513 0.09 

 Region 52 8 6 

Suitable 1,465.5 883.5 0.262 0.10 0.468 0.13 2,238.8 971.0 0.252 0.09 0.415 0.17 

Unsuitable 5,167.5 5,269.0 0.738 0.10 0.532 0.13 7,898.8 4,339.5 0.749 0.09 0.585 0.17 

1

 An individual owl may contribute to n more than once if it was radio tracked during multiple breeding or nonbreeding periods. 2 Region 5 definition--suitable habitat = medium or large sawtimber, dominant canopy closure poor or higher, and total closure >69 percent; unsuitable habitat=all 

other types. 

locations and nighttime foraging locations-they were less selective among habitats when foraging than when roosting. 

The small number of radio locations among our owls did 

not result in low power to reject the null hypothesis for canopy 

closure. Estimates of power ranged from 70 to 90 percent for 

canopy closure. We had high likelihood of detecting habitat 

selection for canopy closure when it occurred. Our power to 

detect selection for tree size-class was lower-70 percent at 

L-CON, 59 percent at S-CON, and 29 percent at S-OAK. Thus, 

even if selection for tree size-class occurred, we had low power 

to detect it, especially in the oak woodlands at S-OAK. The 

number of radio locations for canopy closure and tree size-class 

162 Chapter 6 

was the same; the differences in power were due to differences 

in effect size. To achieve 80 percent power at an 0.05 level of 

significance, with 4 degrees of freedom, given the effect size we 

observed for tree size-class (0.20 at S-OAK), 298 radio locations 

would be needed; if the effect size were 0.30, only 133 locations 

would be needed (see Cohen 1988, p. 258). 

Effect size can also be strongly influenced by the definition 

of the available sampling frame. In general, the more widely 

defined the available frame, the greater the likelihood of demonstrating selection. By using the AK method to define the available sampling frame for each home range, we decreased the 

likelihood of demonstrating selection relative to a less restrictive 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Table 6G Estimated areas of use by California spotted owls that were not radio 

tracked. Data are tabulated by National Forest (NF),and some Ranger Districts, 

as minimum breeding-season estimates. Estimated areas of use were based on 

the sizes of drainages occupied by owls during summer surveys. 

Total area used (acres) 

Areas of use x x ± SD Range n 

Cleveland NF 

All sites 133.4 ± 70.0 38.0 - 294.0 15 

Chaparral sites 159.5 ± 85.9 38.0 - 294.0 8 

Oak/Pines sites 103.6 ± 29.4 70.0 - 155.0 7 

Angeles NF 

All sites 225.8 ± 148.1 37.0 - 689.0 71 

Saugus Ranger District 172.6 ± 156.7 48.0 - 530.0 14 

Tujunga Ranger District 235.4 ± 123.3 119.0 - 405.0 8 

Arroyo Seco Ranger District 304.6 ± 144.1 37.0 - 689.0 21 

Mount Baldy Ranger District 185.6 ± 134.3 46.0 - 496.0 14 

Valyermo Ranger District 195.3 ± 139.7 65.0 - 600.0 14 

All pairs 242.5 ± 147.4 54.0 - 600.0 38 

Singles 206.5 ± 148.7 65.0 - 600.0 33 

Los Padres NF 

All sites 89.3 ± 52.2 25.0 - 200.0 21 

All pairs 98.4 ± 50.4 25.0 - 200.0 16 

Singles 60.0 ± 51.9 25.0 - 150.0 5 

San Bernardino NF1

All pairs 5,329.0± 4,941.0 1,835.0 -8,823.0 2 

All individuals 3,450.1± 2,504.6 674.0 -6,294.0 4 

1

 Owls on the San Bernardino NF were radio tracked. Home-range estimates 

were determined using the 100-percent minimum convex polygon. 

definition of availability based on, for example, the 100-percent 

MCP. This occurred because the AK algorithm tightly fits the 

actual distribution of points with an irregular polygon. In the 

limiting case with many data points, the AK could fit just the 

used distribution of points and exclude other habitats that truly 

may have been available. 

Thus, failure to reject the null hypothesis (that habitat use = 

habitat available) in some of our tests of selection may have 

resulted from the effects of small samples and the use of the AK 

algorithm, rather than demonstrating a lack of selection. The 

extent to which this occurred is unknown. It is clear, however, 

that failure to detect habitat selection should be interpreted, for 

those tests with low power, in terms of a high likelihood of 

type-II errors. Given these limitations, it may be that confirmed 

selection for particular attributes (for example, canopy closure 

or tree size) by even a few owls should be considered strong 

evidence for selection in our studies. An incorrect inference 

would be to conclude that failure to detect significant habitat 

selection proves a lack of selection. For these reasons, we believe the most significant insights to spotted owl biology are 

provided by the habitat-use results, rather than results of tests for 

selection. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

References 

Baldwin, James A., Statistician, USDA Forest Service, Pacific Southwest 

Research Station, Albany, CA. [Personal communication]. April 1986. 

Burt, William H. 1943. Territoriality and home range concepts as applied to 

mammals. Journal of Mammalogy 24:346-352. 

Byers, C. Randall; Steinhorst, R. K.; Krausman, P. R. 1984. Clarification of a 

technique for analysis of utilization-availability data. Journal of Wildlife 

Management 48:1050-1053. 

Calhoun, J. B.; Casby, J. U. 1958. Calculation of home range and a density of 

small mammals. United States Public Health Monograph 55:1-24. 

Call, Douglas R. 1990. Home range and habitat use by California spotted owls 

in the central Sierra Nevada. Arcata, CA: Humboldt State Univ.; 83 p. 

Thesis. 

Carey, Andrew B.; Horton, Scott P.; Biswell, Brian L. 1992. Northern spotted 

owls: influence of prey base and landscape character. Ecological Monographs 65:223-250. 

Cohen, Jacob. 1988. Statistical power analysis for the behavioral sciences. 

Hillsdale, NJ: Lawrence Erlbaum Associates, Publishers; 567 p. 

Forsman, Eric D. 1983. Methods and materials for locating and studying 

spotted owls. Gen. Tech. Rep. PNW-162. Portland, OR: Pacific Northwest 

Forest and Range Experiment Station, Forest Service, U.S. Department of 

Agriculture; 8 p. 

Hayne, Don W. 1949. Calculation of size of home range. Journal of Mammalogy 30:1-18. 

Jennrich, R. l.; Turner, F. B. 1969. Measurement of non-circular home range. 

Journal of Theoretical Biology 22:227-237. 

LaHaye, William S., Research Associate, Wildlife Department, Humboldt 

State Univ., Arcata, CA. [Personal communication]. January 1992. 

Laymon, Stephen A. 1988. Ecology of the spotted owl in the central Sierra 

Nevada, California. Berkeley, CA: Univ. of California; 285 p. Dissertation. 

Mohr, C. O. 1947. Table of equivalent populations of North American small 

mammals. American Midland Naturalist 37:223-249. 

Neu, Clyde W.; Byers, C. Randall; Peek, James M. 1974. A technique for 

analysis of utilization-availability data. Journal of Wildlife Management 

38:541-545. 

Nice, Margaret Morse. 1941. The role of territory in bird life. American 

Midland Naturalist 26:441-487. 

Spencer, Stephen R.; Cameron, Guy N.; Swihart, Robert K. 1990. Operationally defining home range: temporal dependence exhibited by hispid cotton 

rats. Ecology 71:1817-1822. 

Springer, Joseph T. 1979. Some sources of bias and sampling error in radio 

triangulation. Journal of Wildlife Management 43:926-935. 

Thomas, Jack Ward; Forsman, Eric D.; Lint, Joseph B.; Meslow, E. Charles; 

Noon, Barry R.; Verner, Jared. 1990. A conservation strategy for the 

northern spotted owl. Washington, DC; U.S. Government Printing Office; 

427 p. 

White, Gary C.; Garrott, Robert A. 1986. Effects of biotelemetry triangulation 

error on detecting habitat selection. Journal of Wildlife Management 

50:509-513. 

Worton, B. J. 1989. Kernel methods for estimating the utilization distribution 

in home range studies. Ecology 70:164-168. 

Chapter 6 163 
Chapter 7 

Patterns of Habitat Use By California Spotted Owls in 

Logged Forests of the Northern Sierra Nevada 

Cynthia J. Zabel, Kevin S. McKelvey, and James D. Johnston 

The radio-tracking study area on the Lassen National Forest 

(NF) reported in Chapter 6 was coincidentally an area for which 

we were able to locate nearly complete records of logging. It was 

also an area for which we had much information about the 

vegetation. Based on these records, we have visually examined 

patterns of habitat use by radio-tagged spotted owls in relation to 

stand histories near the geographic center of our telemetry study 

area. We have assumed that a low density of owl locations in an 

area indicated an aversion by the owls to the habitat there. 

Conversely, a high density of locations was interpreted to indicate selection for a particular area. 

We have not attempted any statistical analyses of use vs. 

availability for these data, as done in Chapter 6. Logging practices were unique enough on the various study sites that we 

could not stratify them into meaningful categories for rigorous 

use-versus-availability tests. Instead, we based our evaluations 

of spotted owl use patterns on visual inspections of owl locations 

in relation to known histories of logging in stands. Such case 

studies may serve to enlarg;our general understanding of how 

the owls are affected by dift :rent stand treatments. 

Study Area 

The Lassen NF study area was located in the northern 

Sierra Nevada, east of Lassen National Park. It was dominated 

by red and white fir at high elevations (approximately 

5,800-6,900 feet) and pine (Jeffrey, ponderosa, sugar, and 

lodgepole) at lower elevations (approximately 5,000-5,800 

feet). The predominant silvicultural method there has been 

selective logging. For this chapter, we focus on the 

high-elevation, true fir zone. The condition of this forest in the 

early part of the 20th century was markedly different before 

logging began in the 1930s. Sierran mixed-conifer forest covered the mid-mountain slopes and red fir dominated higher 

elevations, except for shrub fields on the south side of Swain 

Mountain. The forest was dominated by large trees-average 

diameter at breast height (d.b.h.) of the various species was 

25-34 inches for white fir, 32-35 inches for Jeffrey pine, and 

28-32 inches for red fir (Boerker 1912, Evans 1919). 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

Methods 

The cutting history within the study area was determined 

using records and timber sale maps from the U.S. Department of 

Agriculture, Forest Service (FS) and private timber companies. 

This information was then transferred to U.S. Geologic Survey 

topographic maps. Cut-unit polygons were subsequently digitized and stored in a Geographic Information System (GIS). 

Each cut-unit polygon contained historical information, including year of cut, type of harvest (overstory removal, group selection, thinning, sanitation, salvage, partial cut), and species removed. Radio-telemetry points from daytime roosting and nighttime foraging locations of spotted owls (Chapter 6) were overlaid onto the cut-history maps using GIS. Vegetation types (table 

6A), determined from large-scale color aerial photographs and 

1:24,000 black-and-white orthophotoquads, were then overlaid 

onto the owl telemetry points and the cut-unit maps. Vegetation 

polygons were classified by diameter size-class of the dominant 

trees (structural size class) and canopy closure class of dominant 

trees. For this chapter, we classified structural size class into two 

categories: <21 inches in d.b.h. (size classes 0-3) and ≥21 

inches d.b.h. (size classes 4 and 5). Similarly, canopy closure 

was categorized as <40 percent cover (open, sparse, or poor) or 

≥40 percent cover (normal or good). Polygons were reclassified 

as suitable or unsuitable spotted owl habitat based on current FS 

definitions. Suitable stands were those in which the diameter 

size-class of dominant trees was ≥21 inches, canopy closure of 

dominant trees was ≥20 percent, and total canopy closure was 

≥70 percent. 

We present three figures for each area of interest with owl 

radio locations overlaid on each of three vegetation types--

suitable spotted owl habitat according to the FS; structural size 

class ≥21 inches in d.b.h.; and canopy closure ≥40 percent. Cut 

units and areas of owl use have been numbered in the sequence 

in which they are discussed in the results. For one area, we also 

present a 1980 orthophotograph for a visual inspection of the 

landscape. Timber-sale boundaries delineated on the figure do 

not always align with boundaries on the orthophoto. The figure 

represents sale maps or planned cuts, but the orthophoto depicts 

actual cut boundaries after logging was done. Cut units delineated on the figures are minimum representations of actual areas 

that were logged. Many areas were entered with light removal 

outside of the units we mapped, but the FS generally did not 

keep detailed records of such light cuts. 

Chapter 7 165 
Results 

We examined four areas located near the center of our owl 

home-range and habitat-use study and where much logging had 

occurred (fig. 7A). These areas are designated here as Griffith Hill 

(fig. 7B), Swain Mountain (fig. 7C), Silver Lake (fig. 7D), 

and Star Butte (fig. 7E). Most logged areas on Swain Mountain 

were little used by owls, and some heavily logged areas at 

Griffith Hill, Silver Lake, and Star Butte were avoided by the 

owls, but other logged areas were used extensively. 

Griffith Hill 

The Griffith Hill area was dominated by a strip of FS land 

approximately 3/4 of a mile wide and 2 miles long, lying between two areas of private land (figs. 7B and 7F). Canopy cover 

on FS land was mostly ≥40 percent. The FS land was characterized by selectively logged true fir and mixed-conifer forests. 

Several sites (numbers 1, 2, 3, and 4) were selectively logged in 

the mid-1960s by overstory removal and group selection. Private land south of this section (site 5), on the other hand, has 

been logged much more heavily, and canopy cover there was 

<40 percent. Site 5 was rarely used by the owls. Pine and fir were 

harvested there in 1960; in 1973 trees >24 inches were removed 

and the site was salvage-logged for merchantable timber; and 

trees of all species <20 inches in d.b.h. were thinned in the 

1970s-1980s (McCrory pers. comm.). Site 5 currently has sparsely 

scattered trees ≥21 inches in d.b.h. in the eastern section. The 

adjacent private land (site 6) is owned by the same company but, 

according to their records, it has not been entered. The Susan 

River drainage, which runs through this area to the south of Lake 

Road (site 7), was logged by overstory removal and group 

selection in 1966 and 1968. It now has canopy cover ≥40 

percent, but trees there are small and the site is classified as 

unsuitable owl habitat. 

In general, spotted owl use at Griffith Hill occurred mostly 

on FS land. The more open private land to the north and south 

were used relatively little. Several owl telemetry fixes were in 

site 6, characterized by trees ≥21 inches in d.b.h. and a canopy 

closure ≥40 percent. An owl pair nested in the fir stand covering 

the eastern half of site 1. The owls apparently did not use the 

western half of site 1, which had been heavily logged and was 

very open. The eastern half of site 1 was "group cut" (in small 

areas <1 acre), and the western half was cut using overstory 

removal (J. D. Johnston pers. observ.). 

Swain Mountain 

This area covers Swain Mountain Experimental Forest 

(SMEF) and adjacent forests (fig. 7C). Most of SMEF lies above 

5,800 feet in elevation and is dominated by forests of red and 

white fir. Numerous small patch cuts were made across SMEF in 

1958 and from 1970 to 1972. Several stands were also selectively logged during the early 1970s. The large south-facing

166 Chapter 7 

slope of Swain Mountain burned frequently in the past and was 

planted with Jeffrey pine in 1978. Today the area is dominated 

by a thick cover of ceanothus, chinquapin, manzanita, and small 

Jeffrey pines. Much of SMEF was shelterwood-logged from 

1982 to 1985. Those cuts underwent intensive site preparation 

following logging-units were either broadcast burned or 

tractor-piled and burned. Most of the residual trees there today 

are large, old-growth firs, with densities ranging from 5 to 15 

trees per acre. Several intact stands of old-growth still remain on 

SMEF. 

Spotted owls did not use the large areas of shelterwood cuts 

(site 8) or the large pine plantation on the south side of Swain 

(site 9). Figure 7C shows that most owl locations were in habitat 

classified as suitable (sites 10, 11, 12, and 13), and many were in 

the intact old-growth stands. Most owl locations on SMEF 

occurred during the nonbreeding period. One owl pair had a 

linear home range along a road just southwest of the pine plantation (site 14). The forest along this road, and to the west of it, has 

been selectively logged but is still dominated by trees ≥21 

inches in d.b.h. and canopy cover ≥40 percent. 

Silver Lake 

The Silver Lake area (fig. 7D) lies just west of the Griffith 

Hill area and northwest of SMEF. Its eastern part includes 

private land that was heavily logged in the past (fig. 7B, site 5). 

The area currently has a very open canopy and few stands with 

large trees remain. Relatively few owl locations occurred there. 

To the northwest and southwest were FS lands that had been 

selectively logged to varying degrees. Owls used this heterogeneous landscape extensively, avoiding the most open areas-- 

sites 15, 16, and 17. Those sites formed a continuous, broad strip 

of land that has been logged heavily. Stands between the private 

land (site 5) and logged sites 15 and 17 contained suitable habitat 

with large trees. Cutting occurred there in small blocks, but the 

owls regularly used the area. Several owl locations were in 

high-elevation (>6,400 feet) stands that were relatively open but 

dominated by large-diameter firs (site 18). 

Star Butte 

The Star Butte area (fig. 7E) has also been selectively 

logged to varying degrees using overstory removal and 

group-selection methods. Most of this area was extensively used 

by spotted owls, even though much of it was not classified as 

suitable spotted owl habitat. The only relatively undisturbed 

stand of old-growth fir in the Star Butte area was used for nesting 

by a pair of owls in 1990 (site 19). The more heavily logged 

areas (site 21) and the area around Star Butte (site 22) were both 

used infrequently by the owls. Site 21, on private land, was 

logged repeatedly in the 1930s, 1940s, 1950s, and 1970s. It was 

logged for culls in 1980, and it has been thinned in the past 3 

years (Briggs pers. comm.). Star Butte (site 22) had small pockets of large trees but was unsuitable owl habitat. It contains a 

6,700-foot peak with shrubfields along its upper slopes and 

small firs on its lower slopes. The area southeast of Star Butte 

had been heavily logged and appeared to be avoided by owls. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Figure 7A-Locations of study areas (7B, 7C, 7D, and 7E) on the Lassen National Forest. Radio-telemetry points from daytime 

roosting and nighttime foraging locations of California spotted owls are indicated. The boundary of Swain Mountain Experimental

Forest is delineated as a dashed line. Study area numbers correspond to descriptive figures 7B-7E, similarly numbered. 

USDA Forest Service Gen, Tech. Rep. PSW-GTR-133. 1992. Chapter 7 167 
168 Chapter 7 USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. Chapter 7 169 
170 Chapter 7 USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. Chapter 7 171 
Figure 7F-Orthophotograph of Griffith Hill study area (fig. 7B). Cut units and areas used by California spotted owls are numbered in the sequence in 

which they are discussed in the results section of this chapter. 

Site 14, near the southwest corner of SMEF, is an extension of an 

owl pair's linear home range referred to in the discussion of 

Swain Mountain. It has been selectively logged, but it was still 

suitable habitat and was heavily used by a pair of owls. Site 23, 

like site 18 (fig. 7D), is an uncut, high-elevation site that was 

used by owls even though it was unsuitable habitat. It also had 

areas of sparse, large trees where owl locations tended to occur, 

and pockets of high canopy cover were present. 

172 Chapter 7 

Discussion 

Decades of selective logging and wildfire on the Lassen NF 

have resulted in a complex and heterogeneous landscape. Unlike 

landscapes in the Pacific Northwest, where even-aged management has produced stands that differ primarily in stand age and 

size, uneven-aged management on the Lassen NF and throughout most of the Sierra Nevada has produced landscapes in which 

stands vary according to several factors. They vary in the number of times they have been entered for logging, the species of 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Figure 7G-Proportions of habitat types available in relation to the proportions used by radio-tagged California spotted owls at four study 

areas in the Lassen National Forest. Four categories of habitat were tested: (1) R5 suitable = Forest Service (Region 5) definition--medium 

or large sawtimber (≥21 inches in diameter at breast height), dominant canopy closure poor or higher, and total closure >69 percent; (2) 

structural size-class ≥21 inches in diameter at breast height; (3) dominant canopy closure ≥40 percent; and (4) logged stands. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. Chapter 7 173 
trees taken, the diameter distribution of trees removed, and in the 

total volume logged. As a result, it is more difficult to compare 

stand structure and composition in landscapes dominated by 

uneven-aged management than is the case where even-aged 

management has occurred. 

The areas we examined were predominantly logged by 

overstory removal and group-selection techniques in the late 

1960s. It is apparent from these results that spotted owls tend to 

avoid altogether some forest conditions resulting from logging, 

but that they will use other types for foraging. Logged areas used 

by owls contained predominantly large trees (≥21 inches), and 

most of them had retained enough structure and canopy closure 

to be classified as suitable owl habitat (for example, site 14). 

Owl use of sites with large trees, dense canopy cover, and with 

suitable habitat (R5 definition), was greater than expected from 

the availability of such sites at all four study areas (fig. 7G). 

Some areas of low canopy cover were used where large trees 

were present and stands of high and low cover were interspersed 

(sites 2, 3, and 13). Pockets of large trees were also used when 

they were in areas of high canopy cover (sites 4, 20, and 21). It 

appears as if spotted owls will use logged sites if pockets of 

suitable habitat are interspersed among stands with low canopy 

or small trees. We cannot, however, quantify the total acreage or 

proportion of a site that must be in such stands before owls will 

use the site. These results indicate that logging by overstory 

removal and group selection can be compatible with spotted owl 

management in the red fir zone, if logged areas are small and 

interspersed with varying size classes and levels of canopy 

closure. 

Use of logged sites was less than expected from the availability of such sites at three of four study areas (fig. 7G). Logged 

sites that were avoided by owls included large areas of sparsely 

distributed, big trees with low canopy cover (sites 5, 8, and 17), 

and large areas with dense canopy cover but small trees (sites 15 

and 16). Most of the logged areas that were not used by owls had 

not returned to suitable owl habitat. 

Spotted owls in the true fir zone used some naturally occurring sites classified as unsuitable owl habitat (see Chapter 6). 

These included areas at high elevations with large trees that were 

sparsely distributed (for example, Silver Lake site 18 and Star 

Butte site 23). In contrast, areas that were unsuitable habitat as a 

consequence of logging were often not used by owls. Examples 

include shelterwood-logged stands at SMEF (site 8) and private 

land that was heavily thinned north of Swain Mountain (site 5). 

Areas near site 18 and site 23 were used by owls even though 

they were far from the center of any concentration of owl 

locations. Yet, heavily logged areas (sites 5 and 8) were not used, 

even though they were adjacent to sites with concentrations of owl locations. 

Unsuitable habitat in the Silver Lake area (site 18) was used 

by five individual owls during the nonbreeding period. These 

owls moved upslope into the higher elevations during the fall 

and remained there throughout the winter. Similarly, four owls 

174 Chapter 7 

used the high-elevation, uncut stands in SMEF (sites 10, 11, 12, 

and 13) during the fall and winter. These owls did not fly across 

the shelterwood-logged stands during the summer, but they did 

so during the winter to access uncut, old-growth stands (sites 10, 

11, and 12). 

This is a first attempt to determine cut patterns that spotted 

owls will use. Strong patterns of aversion were apparent in the 

data, but preference patterns were not so clear. The silvicultural 

prescriptions described here did not adequately convey what 

was removed from cut units. All the units we examined had 

overstory removal and group selection, yet aerial photographs 

indicated that different volumes of timber were removed. Some 

units now have dense stands of large trees, while others have 

sparsely distributed, large trees. A micro-scale approach, with 

precise measurements of stand characteristics, may be needed to 

better understand what logging prescriptions are compatible 

with spotted owls. Attributes that may have been impacted by 

site preparation but were not considered in the analyses presented here, such as amounts of coarse woody debris, may also 

be important. Such attributes may influence prey abundance and 

distribution, and indirectly influence use of harvested areas by 

spotted owls (Chapter 4). 

These results indicate that cutting patterns other than clearcuts, 

such as heavy overstory removal, can make sites unsuitable for 

spotted owl foraging in the red fir zone. We do not know how 

these patterns may extend into Sierran mixed-conifer forests. 

Home-range sizes of California spotted owls were larger in the 

high-elevation, red fir zone (Chapter 6), which was underused 

for nesting (Chapter 5) compared to mixed-conifer forests. 

High-elevation red fir may be marginal for California spotted 

owls, and owls living there may be more sensitive to logging 

than owls in mixed-conifer forests. These results may not be 

applicable to other forests types. A similar study with good 

quantitative information on what was logged, and how, needs to 

be done in mixed-conifer forests. 

References 

Boerker, Richard A. 1912. A reconnaissance working plan, Lassen National 

Forest, the Susanville Block. Mineral, CA: Lassen National Forest, Forest 

Service, U.S. Department of Agriculture; 110 p. 

Briggs, Gaylord, Forester, Roseburg Resources Company, Anderson, CA. 

[Personal communication]. January 1992. 

Evans, O. M. 1919. Timber survey report, 1918, Rock Creek Unit, Lassen 

National Forest. Mineral, CA: Lassen National Forest, Forest Service, 

U.S. Department of Agriculture; 5 p. 

McCrory, David, Forester, Fruit Growers Supply Company, Burney, CA. 

[Personal communication]. January 1992. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Chapter 8 

Estimates of Demographic Parameters and Rates 

of Population Change 

Barry R. Noon, Kevin S. McKelvey, Daryl W. Lutz, William S. LaHaye, R. J. Gutiérrez, and Christine A. Moen 

Detailed analyses of a species' life-history structure are 

essential to understand its population dynamics, provide insights 

to effective management, and also suggest directions for future 

research. In this chapter, we present analyses of available demographic data on the California spotted owl, using life-history and 

demographic information in an approach similar to that of Mertz 

(1971), Nichols et al. (1980), Noon and Biles (1990), and Thomas et al. (1990, appendix L). Our objectives were (1) to characterize the life-history structure of the California spotted owl, (2) 

to estimate values of the demographic parameters needed to 

compute rates of population change, (3) to test the sensitivity of 

the rate of population change to each of the demographic parameters, (4) to infer which parameters may be most influenced by 

management decisions, and (5) to suggest future research priorities, as indicated by the demographic analyses. 

Methods 

Demographic studies of spotted owls were conducted at 

five locations for periods ranging from 2 to 6 years-Lassen 

National Forest (NF), Eldorado NF, Sierra NF, Sequoia/Kings 

Canyon National Parks (NPs), and San Bernardino NF (table 

8A). Density studies were conducted at four of those sites and at 

three additional sites (table 8B). The territorial behavior of 

spotted owls allows researchers to survey their populations by 

listening for and eliciting vocalizations. The usual procedure is 

to locate owls at night, then to follow up the next day with visual 

confirmation and, for demographic studies, to capture and uniquely 

Table 8A-Summary information on the five demographic study areas. 

 Total 

Approximate size Years of individuals 

Location (square miles) marking marked 

Lassen National Forest 500 1990-91 130 

Eldorado National Forest 136 1986-91 72 

Sierra National Forest 160 1990-91 82 

Sequoia/Kings Canyon 

 National Parks 130 1988-91 62 

San Bernardino 

 National Forest 730 1987-91 367 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

Table 8B- Density estimates (owls per square mile) of spotted owls in 

California. 

 Size in 

 square Crude 

 Study area miles density1 Reference 

Northern spotted owl 

Six Rivers National Forest 113 0.609 Franklin et al. 1990a 

Simpson Timber Company 94 0.990 Diller 1989 

California spotted owl 

Eldorado National Forest 137 0.259 Lutz 1992 

Yosemite National Park 156 0.518 Roberts et al. 1988 

Sierra National Forest 160 0.526 Verner et al. 1991 

Sequoia/Kings Canyon 132 0.440 Roberts 1989; Verner et al. 

 National Parks 1991 

San Bernardino National 207 0.347 LaHaye et al. in press 

 Forest 

Mount San Jacinto 67 0.466 Gutiérrez and Pritchard 

 1990 

Palomar Mountain 13 1.657 Gutiérrez and Pritchard 

 1990 

1

Estimated number of owls/total study area. Much of the area included was 

not suitable habitat, and the proportion of unsuitable may have varied markedly 

from one study area to another. 

color band each bird to allow individual identification in the 

field without recapture. The sex of adults was determined from 

their vocalizations, and reproductive status was determined by a 

protocol in which live mice were fed to the owls and they were 

then observed to determine whether they eventually carried a 

mouse to a nest site (Forsman 1983). Captured spotted owls can 

be placed into one of four age-classes: juvenile (x < 1), first-year 

subadults (1 ≤ x < 2), second-year subadults (2 ≤ x < 3), and 

adults (x ≥ 3) based on plumage characterisitcs (Moen et al. 

1991). Model structure, however, seldom has been based on all 

four age-classes because parameter estimates do not differ among 

classes, or because of insufficient data. Second-year subadults 

and adults are often collapsed into a single age-class (see below). 

Parameter Estimation 

Estimates of density (owls per square mile) assumed complete enumeration of all owls within a defined area. Usually 

when owls were counted, they were also color-banded to minimize the likelihood of double counting. Because territorial owls 

tended to vocalize regularly and to be spatially restricted within 

their home ranges, they were more likely to be detected than 

Chapter 8 175 
nonterritorial owls (floaters). To the extent that many floaters 

occurred in an area, true density may have been underestimated. 

Ideally, we would use precise and accurate estimates of 

three classes of parameters (sx, bx, and a) to compute the finite 

population growth rate, or λ: 

1. sx = probability of survival from age x to x + 1. 

Previous demographic analyses of spotted owls generally 

assumed that s differed for at least three age-classes-juveniles 

(s0), first-year subadults (s1), and adults (s). The adult class, 

defined on the basis of age at first reproduction (a = 2 years), 

thus included second year subadults. The juvenile stage ends at 

the age of 1 year for spotted owls. In our analyses we did not 

partition juvenile survival rate into preand postdispersal probabilities, as done by Lande (1988). The subadult stage for the 

northern spotted owl covers the second year of life (age 1 to age 

2) and is assumed to be an age interval in which the birds 

typically do not breed. The adult stage was thus anything older 

than 2 years (Noon and Biles 1990). Ideally, survival rates of 

these stages can be computed directly for each population modeled, using data from that population. Using program JOLLY 

(Pollock et al. 1990), we could estimate juvenile survival rate 

(s0) only for the San Bernardino study area, because data were 

insufficient for the Eldorado and Sequoia study areas. Consequently, the San Bernardino value for so was used as an approximation for the other two locations. It is similar to estimates of s0

from several studies of northern spotted owls (Franklin 1992; 

USDI, FWS 1992), and λ is not especially sensitive to variations 

in s0. The true value of s0 for these sites, however, is unknown. 

Data were also insufficient to compute separate estimates of 

s1 and s for the Eldorado and Sequoia/Kings Canyon study areas, 

and these two values were statistically indistinguishable for the 

San Bernardino study area. Therefore, in demographic analyses 

reported here, we used survival estimates for only two 

age-classes-s0 (juvenile) and s ("nonjuvenile," combining subadult and adult age-classes). The probability of survival to age x

was thus computed as lx = s0s

x -1 (by definition l0 = 1.0) (table 

8C). 

Table 8C-Spotted owl life history (age at first reproduction is 2 years). 

x1 lx

2

 bx

3

 lxbx

4

0 1.0 0 0 

 1 s0 0 0 

 2 s0s1 b s0s1b 

3 s0s1s b s0s1sb 

 4 s0s1s

2

 b s0s1s

2

b 

. . . . 

. . . . 

. . . . 

 x s0s1s

x -2 b s0s1sx - 2b 

1 x denotes age in years.

2 lx denotes the probability that an individual aged 0 (a newly hatched 

bird) will survive to enter age class x. 3 bx denotes the expected number of female fledglings produced by a 

surviving female aged x. 4 lxbx denotes the net maternity function. At this time for the California 

spotted owl, sl

 = s. 

176 Chapter 8 

2. b = fecundity, the expected number of female fledglings 

produced per female per year; bx is the fecundity of a female of 

age x.

Fecundity values were calculated by halving the number of 

young fledged per territorial female, assuming an even sex ratio 

among fledglings. Because ages of females breeding, or attempting to breed, were seldom known, we assumed a constant reproductive rate for females of all ages: bx = b (x ≥ 2 years). 

3. a = age at first reproduction. 

Apparently female spotted owls rarely breed when only 1 

year old; most do not begin until at least 2 years old (Franklin 

1992). Therefore, we used a = 2 years in demographic analyses 

reported here. 

Estimates of age-specific survival rates from study areas 

with ≥4 years of data came from capture histories of marked 

birds using Program JOLLY (Pollock et al. 1990). Because 

some studies suggest that reproduction by spotted owls may be 

affected by radio transmitters attached with back-pack harnesses 

(Paton et al. 1991, Foster et al. 1992), estimates of survival and 

reproductive rates used in this demographic analysis were based 

solely on banded birds. To increase the precision of the estimates, we estimated the minimum number of parameters that 

adequately fit the data. When sufficient data were available, we 

tested for differences between adjacent age-classes to justify 

pooling of capture histories. In most cases, however, data were 

insufficient, so we pooled values for all individuals at least 1 

year old to compute a survival rate for adults. Because the 

Lassen and Sierra studies have been underway for only 2 years, 

their data were insufficient for statistically reliable estimates of 

survival rates. For these sites, we simply estimated survival rates 

empirically, pooling data from all males and females at least 1 

year old. 

Analysis 

Standard Lotka-Leslie methods (Leslie 1945, 1948; Lotka 

1956) were applied to the estimates of vital rates (age-specific 

birth and death rates) to make inferences about rates of population change. In addition to assumptions identified above, we 

assumed (1) that reproduction was characteristic of a "birth-pulse" 

population-one with a single, well-defined, annual breeding 

period (Caughley 1977, p. 6), (2) a stable age (stage) distribution, and (3) no density dependence-a change in population 

density did not affect the values of the vital rates. This assumption was probably valid within the limits of the population 

densities modeled here. Preadult survival rate (l2) was the probability of survival from fledging (age when leaving the nest) to 

age 2 and was given by the product of s0 and s. 

Estimates of all parameters needed for a completely 

age-specific Leslie matrix (Leslie 1945, 1948) were not available for any of the populations modeled. Often such data are 

limited for long-lived species, and estimates of all the age-specific 

parameters are impossible to attain or have large sampling variances because of small sample sizes. Repeated multiplication of 

imprecise estimates in fully age-specific models is likely to lead 

to uncontrolled error propagation (Dobson and Lyles 1989). The 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Lefkovitch stage-projection matrix model (Lefkovitch 1965) is a 

useful approximation to an age-structured model. The dynamics 

of the two models are usually similar, but the Lefkovich model is 

more tractable (Boyce 1987). A stage is simply a category 

(age-class) within which birth and death rates are assumed to be 

constant. Consequently, estimating the population dynamics of 

spotted owls with a stage matrix would yield misleading conclusions only if the birds manifest an age-related decline in reproductive rate (senescence) before about 15 years old (Noon and 

Biles 1990). Given the current high estimates of s, early sensecence seemed unlikely. 

Our application here of the Lefkovich model uses only two 

stages juveniles (J) and nonjuveniles (NJ), as defined above. 

Time was expressed as an interbreeding interval of 1 year, and 

age at first reproduction was set at 2 years. Given that popula 

tions were based on census estimates corresponding to a period 

shortly after the breeding period, the Lefkovitch matrix had the 

following structure (details in Noon and Sauer 1992): 

J

N J

sb

s s

J

N J

t

t

t

t

+

+





 



 = 



 









 



 1

1 0

0 

The location of s along the diagonal of the matrix reflects 

our assumption of constant adult survival and no senescence. 

The possibility of very old owls in the population had little effect 

on our estimate of λ assuming that adult survival was estimated, 

irrespective of adult age, from an unbiased sample of all adults in 

the population (Boyce 1987). We believe this assumption was 

valid for all populations modeled here. 

The basic characteristic equation of the matrix is given by 

λ 2 - sλ – s0sb = 0. (1) 

The dominant, real-valued solution (λ 1) is an estimate of the 

annual, finite rate of change of the population. Vital rates suggest an increasing population if λ > 1.0, a stable population if λ = 

1.0, and a declining population if λ < 1.0. We urge caution in 

using the computed estimates of λ to forecast future population 

sizes or to infer sizes of historical populations. Lamdba estimated how the population was changing only during the period 

of study. Using the estimate of λ to project future population size 

would assume, unrealistically, that current estimates of vital 

rates are constant over time. 

Hypothesis Tests of Lambda 

We had estimates of fecundity and adult survival from three 

study areas and, in addition, an estimate of juvenile survival 

from one study. Based on these parameter estimates, and assuming s0 to be equal at all locations, we computed λ from the 

Eldorado NF and Sequoia/Kings Canyon NPs in the Sierra 

Nevada Province, and from the San Bernardino NF in the Southern California Province. The sensitivities (partial derivatives) of 

λ with respect to individual life-history characteristics indicate 

which parameters most affect variation in population growth 

rate (Lande 1988, Noon and Biles 1990). In addition, sensitiviUSDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

ties are needed to estimate the standard error of λ and to perform 

hypothesis tests. Sensitivities were computed by implicit differentiation of the characteristic equation (Goodman 1971, Lande 

1988). Sensitivities for the two-stage Lefkovich model are: 

s0: ∂∂s0 = sb/(2λ - s); 

s: ∂∂s = (λ + s0b)/(2λ - s); and 

b: ∂∂b = s0s/(2λ - s). 

For all areas, change in population growth rate was most 

sensitive to variation in adult survival rate. It was much less 

sensitive to variation in first-year survival rate and fecundity; 

sensitivity to these two parameters was about equal. 

Sensitivities of λ appear in the formula that approximates 

the sampling variance of the estimate of λ (Lande 1988, p. 602): 

σ 2

λ =  (∂λ /∂p)

2 σ 2

p (2) 

where p represents each of the parameters and σ 2

p their sampling 

variance. The variances of juvenile and nonjuvenile rates were 

estimated according to the methods of Jolly (1965) and Seber 

(1965). Variance of annual fecundity was based on variance 

among females across the years of study, divided by the appropriate sample size. For the power analyses (see below), we 

assumed the survival probability to have a binomial sampling 

distribution. 

Equation (2) is based only on sampling variance, but the 

data include both sampling and temporal variance. Also, equation (2) neglects possible covariances among the demographic 

parameters and fails to account for between-year changes in vital 

rates. Year-to-year changes, estimated by factoring out the temporal component of variation from the total variance estimates of 

the vital rates and λ cannot yet be estimated for the California 

data. 

Components of the sampling variance of λ were computed 

by using equation (2), based on the sensitivities and estimates of 

the vital rates. The standard error of λ, computed as the square 

root of the variance, was used to construct a confidence interval 

around λ For example, the 95 percent confidence interval on λ 

was computed as: λ ± 1.96 σλ. An estimate of λ and its standard 

error also allowed tests of hypotheses. Of most interest, relative 

to concern for the species' persistence, was whether λ was 

significantly <1.0. We tested the null hypothesis: H0: λ ≥ 1, 

versus the alternative hypothesis: HA λ < 1. The appropriate test 

statistics followed a Z-distribution, given by: Z = |( λ - 1)/σλ|. 

Tests were one-tailed with a specified probability of a type-I 

error = 0.05 (only a 5 percent chance of concluding a decline 

when none actually occurred). 

Power of the Tests on Lambda 

When a test fails to reject the null hypothesis (H0, it is 

important to estimate the power of the test, which is the probability that H0 will be rejected when a particular alternative hypothesis (HA) is true. Failure to reject a false H0 (that is, the populaChapter 8 177 
tion is in decline) is known to statisticians as a type-II error. To 

estimate the power of the test when Ho was not rejected, we used 

a method outlined by Taylor and Gerrodette (in press). The 

method involves estimating two sampling distributions, one 

based on a specified value of λ for HA, the other on the value of 

λ for H0 (λ = 1.0). Under HA we specified a λ = 0.95, that is, a 5 

percent annual rate of population decline. This rate of decline 

would result in a loss of >40 percent of the population over a 

decade. If the two distributions are completely disjoint, the 

power of the test is 1.0. If the two distributions are coincident, 

the power is 0. Power increases as the difference between the 

specified value of λ and the hypothesized λ value (for example, 

λ = 1.0) increases, or as the variance of the distributions decreases. The probability of a type-II error (β) is estimated as a 

function of the overlap between these two distributions. The 

power of the test is then 1 - β. To estimate power, we generated 

simulated distributions of λ under each hypothesis (HA and H0)

based on 16,000 randomizations. 

Results 

Density Estimates 

Estimates of crude density (number of owls/total acreage of 

study area) were available from seven areas within the range of 

the California spotted owl, four in the Sierra Nevada Province 

and three in the Southern California Province (table 8B). Densities varied from a high of 1.657 owls per square mile on Palomar 

Mountain to a low of 0.259 owls per square mile on the Eldorado 

NF. Overall, estimates of crude density for the California spotted 

owl tended to be slightly lower than those for the northern spotted 

owl (table 8B; see also Bart and Forsman in press). 

Comparison of Demographic 

Study Areas 

Demographic studies were done at five locations widely 

spaced over the range of the California spotted owl. Two studies 

have collected data for only 2 years, and the longest study has 

been in place for only 6 years (table 8D). All studies, however, 

are planned to continue for 3-5 more years. Some significant 

highlights from a comparison of the demographic attributes in 

the five areas (table 8D) include extensive among- and within-site 

variation in the proportion of nesting pairs and mean fecundity, 

and pronounced among-site variation in pair turnover rates (proportion of banded adults and subadults that are replaced on their 

territory by another bird) and nonjuvenile survival rates. Components that contributed to the observed variance in annual 

fecundity at a given site arose from variation in both the proportion of females nesting and in their productivity. Among-site 

variation in survival rates of nonjuvenile owls was particularly 

relevant, as even small changes in survivorship of adult females 

greatly affect rates of population change (Noon and Biles 1990). 

Table 8D-Summary of demographic attributes of California spotted owls. 

 Lassen Eldorado Sierra San Bernardino Sequoia/Kings 

 National National National National Canyon National 

Attributes Forest Forest Forest Forest Parks 

Years of banding 2 6 2 5 4 

Mean proportion 0.52 0.31 0.65 0.59 0.51 

of pairs nesting1 

Range of proportion 0.38-0.70 0.0-1.0 0.63-0.67 0.50-0.76 0.18-0.80 

of pairs nesting

Mean total fecundity1,2 0.48 0.94 1.02 0.59 0.53 

Range of total fecundity1,2 0.36-0.64 0.37-1.50 0.75-1.22 0.35-0.80 0.08-1.50 

Mean subadult turnover rate 0.57 0.143

 0.75 0.283 -- 

Mean adult turnover rate 0.21 0.143

 0.19 0.283 0.24 

Mean empirical survival 0.33 0.73 0.50 0.722,3 0.802,3

rate of subadult females 

Mean empirical survival 0.84 0.73 0.74 0.722,3 0.802,3 

rate of adult females 

Mean nonjuvenile survival * 0.832,3 * 0.752,3 0.892,3

1

 Weighted by yearly sample size. 

2

 Males and females combined. 

3

 Subadult and adult age-classes combined. 

* Inadequate sample. 

178 Chapter 8 USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 
Hypothesis Tests on Lambda (λ) 

Eldorado National Forest 

This study has been underway for 6 years, with estimates of 

nonjuvenile survival rates based on capture histories of 72 birds 

>1 year old, and fecundity estimates based on reproductive 

performances of 66 territorial females (table 8E). The estimate 

of λ (equation 1) was 0.947, suggesting about a 5 percent annual 

rate of population decline during the period of study (1986-91). 

This value was not significantly <1.0 (α = 0.05, P = 0.1271), so 

the statistical test did not reject the null hypothesis that the 

population is not declining (H0: λ ≥ 1.0). The test, however, had 

a power of only 0.30, setting the probability of a type-II error at 

about 0.70. The low power of the test was a result of a relatively 

small number of marked birds, and the large standard errors of 

parameter estimates (table 8E). 

Sequoia/Kings Canyon National Parks 

This study has been underway for 4 years, with estimates of 

nonjuvenile survival rates based on capture histories of 45 birds 

>1 year old, and fecundity estimates from the reproductive 

performances of 45 territorial females (table 8F). The estimate of 

λ was 0.969, suggesting about a 3 percent annual rate of population decline during the period of study (1988-91). As in the 

Eldorado study, the statistical test (α = 0.05, P = 0.2709) failed 

to reject the null hypothesis (H0: λ ≥ 1.0). In this case, the test 

had a power of only 0.30, with an 70 percent probability of 

failing to detect a real decline of this magnitude. The very low 

power of the test was a consequence of the small number of 

marked birds and, to a lesser extent, the standard errors of 

parameter estimates (table 8F). 

San Bernardino National Forest 

This study, underway for 5 years, was unique among 

demographic studies of spotted owls in sampling, almost in its 

entirety, an insular population (LaHaye et al. 1992). It provides an estimate of juvenile survival rate of 0.296, based on 

capture histories of 130 owls banded shortly after fledging 

(table 8G). The estimate of nonjuvenile survival rate is based 

on capture histories of 184 birds > 1 year old, and the fecundity 

estimate comes from reproductive histories of 328 territorial 

females. Lambda was estimated at 0.827, suggesting about a 

17 percent annual rate of population decline during the period 

of study (1987-91). The statistical test (α = 0.05, P ≤ 0.0001)

strongly rejected the null hypothesis (H0: λ ≥ 1.0) of a 

nondeclining population. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

Table 8E--Estimates of the annual finite rate of population change (λ) for the 

California spotted owl in the Eldorado National Forest, with test statistics and 

P value for the test of the null hypothesis that λ ≥ 1.0 vs. λ < 1.0. 

 Sample Standard 

Parameter Estimate size error P

Juvenile survival (s0)

1

 0.296 130 0.055 -- 

Nonjuvenile survival (s)

2 0.826 72 0.039 -- 

Fecundity (b)

3 0.470 66 0.054 -- 

Population change () 0.947 -- 0.046 0.1271 

1 LaHaye (pers. comm.). 

2

 Combined estimate for males and females, adults and subadults. 

3

 Number of female young per territorial female. 

Table 8F-Estimates of the annual finite rate of population change (λ) for the 

California spotted owl in Sequoia/Kings Canyon National Parks, with test 

statistics and P value for the test of the null hypothesis that λ ≥1.0 vs. λ < 1.0. 

 Sample Standard 

Parameter Estimate size error P

Juvenile survival (s0)

1

 0.296 130 0.055 -- 

Nonjuvenile survival (s)

2

 0.895 45 0.047 -- 

Fecundity (b)

3 0.267 45 0.059 -- 

Population change () 0.969 -- 0.051 0.2709 

1

 LaHaye (pers. comm.). 

2 Combined estimate for males and females, adults and subadults. 

3 Number of female young per territorial female. 

Table 8G-Estimates of the annual finite rate of population change (λ) for the 

California spotted owl in the San Bernardino National Forest, with test 

statistics and P value for the test of the null hypothesis that λ ≥1.0 vs. λ < 1.0. 

 Sample Standard 

Parameter Estimate size error P

Juvenile survival (s0) 0.296 130 0.055 -- 

Nonjuvenile survival (s)

l

 0.747 184 0.024 -- 

Fecundity (b)

2 0.297 328 0.087 -- 

Population change () 0.827 -- 0.035 0.0000 

1 Combined estimate for males and females, adults and subadults. 

2 Number of female young per territorial female. 

Chapter 8 179 
Discussion 

Owl Densities 

Because we were able to estimate only crude density for 

most study areas at this time, interpretations of results are tentative. This is true because crude density does not adjust for the 

amount of unsuitable habitat in a study area. If one area has a 

higher proportion of unsuitable habitat than another, but the 

amounts are unknown, estimates of crude density for the two 

areas are not comparable. With this caution in mind, density 

estimates at least suggest some questions that need to be resolved. 

The Eldorado study area is mostly forested. The apparent 

low density of spotted owls there may result from the fact that 

much of the area is in classic, alternate-section, checkerboard 

ownership between NF and private timberlands that have been 

intensively managed for timber production. For example, Bias 

and Gutiérrez (1992) reported that most roost sites and all nest 

sites in the Eldorado study area were on public lands. The private 

lands have not been generally clearcut, however, so owls probably forage in much of the private ownership. 

Various hypotheses can be advanced to account for the 

apparent high density of owls on Palomar Mountain: (1) 

Crude density may have been overestimated. We know from 

other studies, for example, that density estimates of spotted 

owls are affected by the size of the study area, with positively 

biased estimates in study areas <40 square miles (Franklin et 

al. 1990b). Based on a regression equation and a proportional 

adjustment from the asymptotic density in Franklin et al. 

(1990b, fig. 4), the density estimate for Palomar Mountain 

should be adjusted downward to about 1.1 owls per square 

mile. But this is still higher than estimates for all other areas. 

(2) An extensive fire destroyed much owl habitat on Palomar 

Mountain in 1987, 1 year before the density survey (Gutiérrez 

and Pritchard 1990). Several previously occupied owl sites 

were lost in the fire and an unknown number of additional 

birds may have been displaced. As a result, densities may have 

increased temporarily from packing by displaced birds into 

remaining habitat. (3) Home-range sizes of spotted owls in 

much of southern California are considerably smaller than 

they are in conifer forests of the Sierra Nevada or in the San 

Bernardino Mountains (Chapter 6). Over much of the owls' 

distribution in southern California, however, suitable habitat 

tends to be concentrated in small, relatively isolated ravines 

and canyons; most of the intervening habitat is unsuitable. The 

crude density of owls there could be similar to that where 

home ranges are larger but where suitable habitat is more 

continuous. This is the case, for example, between the foothill 

woodland and conifer forest segments of the Sierra NF study 

area (Verner et al. 1991). If suitable habitat is more continuously distributed on Palomar Mountain, but owl home ranges 

there are relatively small (for example, because the owls specialize on woodrats as a prey source), the observed high 

180 Chapter 8 

density at Palomar could be a real difference from all other 

density studies of the California spotted owl. 

Sources of Bias in 

Estimates of Lambda 

Juvenile Survival 

"Estimates of juvenile survival rate can be biased low if 

some juveniles leave the study area, survive a full year, and 

never return. To the extent that these three events occur, juvenile 

survival rate is underestimated and the estimates of λ are too 

low" (USDI, FWS 1992, appendix C). If emigrating juveniles do 

not survive their first year, or if they survive but are subsequently detected, then no bias occurs. To provide a statistically 

reliable estimate, we estimated s0 using the maximum-likelihood 

procedure from Program JOLLY (Pollock et al. 1990) for the 

San Bernardino study (LaHaye pers. comm.). We believe the 

estimate from that study is reliable. First, it was based on a 

sample of 130 owls banded as fledglings. Second, the study area 

covers almost the entire San Bernardino Mountain Range, and 

the population was largely isolated from other owl populations 

(LaHaye et al., 1992). Consequently, the area covered by dispersing juveniles was more restricted and the entire area was 

surveyed each year. These factors greatly increased the likelihood of reobserving banded juveniles. Finally, the estimate of s0

(0.296) is nearly the same as that for the five northern spotted 

owl study areas (s0 = 0.311) (USDI, FWS 1992) based on longerterm studies. 

Nonjuvenile Survival 

As for estimates of juvenile survival, nonjuvenile survival 

rate would be underestimated if many banded birds permanently 

left a study area and survived at least 1 year. Thomas et al. (1990, 

appendix L) investigated this potential bias by computing the 

number of emigrations per bird-year in studies of radio-tagged, 

adult spotted owls. A bird-year was defined as one adult bird 

tracked for one calendar year. From radio-tracking studies near 

Roseburg, Oregon, only one permanent emigration occurred in 

>100 bird-years. Similarly, in northwestern California, one permanent emigration occurred in 60 bird-years. These findings 

suggest that estimates of adult survival rate for the northern 

spotted owl were essentially unaffected by permanent emigration from study areas. Although we lack comparable data from 

the California spotted owl, we have no reason to believe that 

permanent emigration by adults would be more common for that 

subspecies. 

Senescence 

The effects of age-related decline in reproductive potential 

of spotted owls have been explored by Noon and Biles (1990). 

Failure to account for senescent declines could result in a significant overestimate of λ. Conversely, incorporating senescence 

when it does not occur can result in significant underestimates of 

λ. We do not know the life span of spotted owls or whether 

fecundity remains constant through life. The high, observed 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
estimates of s would not likely arise, however, if spotted owls 

lived <15 years. If reproductive senescence occurs at earlier 

ages, the decline in λ would be increasingly pronounced. 

Researchers often assume that birds do not experience senescent declines in fecundity and survival rates (Deevey 1947). 

An estimate of mean generation length (Caughley 1977, p. 124) 

without reproductive senescence and assuming s = 0.92, however, suggests the unlikely average age of mothers at the birth-pulse 

= 53 years. Given the magnitude of this estimate, we believe that 

senescent declines must occur in reproduction and/or adult survival rate. Invoking reproductive senescence at ages of 16 or 26 

years yields more reasonable estimates of generation length ( ≅ 8 

and ≅ 12 years, respectively), but it lowers the estimates of λ. 

Reproductive senescence is not likely to occur much before 16 

years; rather, we would expect strong natural selection against 

the evolution of early senescence in a species with life-history 

characteristics like the spotted owl's. 

Stable Stage Distribution 

Calculation of λ from equation (1) assumes a stable stage 

distribution. If the vital rates are constant, the convergence to a 

stable distribution is asymptotically exponential (Caswell 1989, 

p. 70). If the vital rates vary stochastically, or are time-dependent, then the assumption of a stable stage distribution is invalid. 

At this time we have insufficient data to estimate the degree of 

stochasticity in the vital rates for the spotted owl, or to determine 

if the year-to-year differences among projection matrices represent time-dependency or simply sampling variation. In the absence of this information, we assumed mean values for the vital 

rates. We can, however, examine the likelihood of introducing a 

bias into the estimate of λ by assuming constant vital rates, and 

thus a stable stage distribution. 

If the growth of a given population is time-dependent, then 

the growth of that population is described by the product of a 

sequence of projection matrices. In the special case of independent matrices (that is, independent environments), population 

growth rate is equivalent to the dominant eigenvalue (λ) of the 

projection matrix based on the mean values of the vital rates 

(Tuljapurkar 1982). Unfortunately, under stochastic conditions, 

the growth rate of the average population may be a misleading 

indicator of population stability. This occurs because the distribution of population sizes from stochastic projection matrices is 

approximately lognormal (Tuljapurkar and Orzack 1980). A 

property of lognormal distributions is that the modal or most 

likely population size will always be smaller than that based on 

mean values of the vital rates (Gerrodette et al. 1985). Under 

conditions of stochasticity, the realized trajectory of any single 

population is simply one of an infinite number of possible 

outcomes. Thus, the actual growth rate of any single population 

with stochastic vital rates is always less than or equal to the 

growth rate estimated from mean values of the projection matrix 

(Cohen et al. 1983). 

The likely consequence of our assumption of a time-invariant, mean projection matrix, therefore, was to overestimate the 

growth rate (λ) experienced by any given population, unless the 

environmental variability was negligible. In general, as the environmental variability becomes small, the average growth rate of 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

a stochastic population approaches (from below) that described 

by the dominant eigenvalue of the mean projection matrix (Caswell 

1989, p. 222). There are some exceptions to this general rule. For 

example, in nonstochastic, periodic environments, Tuljapurkar 

(1989) found that variation can increase population growth rate 

if the period of the environmental oscillation is roughly equivalent to the generation length of the organism being modeled. We 

emphasize that this is an unlikely outcome for any climatically 

induced variation in the vital rates of a given species (see, for 

example, figs. 8A and 8B). The general observation that, in 

stochastic environments, a probability of extinction exists even 

when the mean growth rate, λ, is >1.0 (Lande and Orzack 1988) 

should temper any overly optimistic interpretation of λ values. 

Tests on Lambda 

Although the null hypothesis of a stable population was not 

rejected for either the Eldorado or the Sequoia/Kings Canyon 

study area, we could not correctly conclude that these populations were stable during the periods of study. The power of our 

statistical tests was disturbingly low for both areas, and there 

was a high likelihood of failing to detect real annual declines of 5 

percent or less. The greater uncertainty that accompanied a 

smaller sample of banded birds and a shorter time series of 

capture histories is clearly illustrated by comparison with the 

northern spotted owl. The estimate of λ for the northern spotted 

owl on lands managed by the Bureau of Land Management near 

Roseburg, Oregon, was nearly identical to that for the Eldorado 

study area (λ = 0.941 vs 0.947, respectively). In the Roseburg 

study, however, the null hypothesis of a stable population was 

strongly rejected (probability of a type-I error <0.0005-USDI, 

FWS 1992, appendix C). In contrast to the Eldorado study, the 

Roseburg study was based on a larger sample of birds (589 vs 

72) and a longer time series (7 years vs 6 years). 

Given the high likelihood of accepting false null hypotheses 

of stable populations in the Sierra Nevada studies, the correct 

inference to draw from the statistical tests is that, at this time, we 

are uncertain about true trends of the Eldorado and Sequoia/ 

Kings Canyon populations during the periods of study. 

If the California spotted owl has experienced gradual declines in habitat quality in the Sierra Nevada, these effects may 

be subtle and, therefore, more difficult to detect than those 

experienced by the northern spotted owl. With the exception of 

the mid-1980s, very little clearcut logging was practiced in the 

Sierra Nevada. Even the heavy cutting that occurred on private 

timberlands near mills and railroad lines usually left some large 

trees standing that could eventually serve as nest sites in regenerating stands. In contrast, within the range of the northern spotted 

owl, most logging has been by clearcutting, and most of the 

decline in suitable owl habitat has been compressed into the 

interval since about 1950 (Murphy and Noon 1992). As a result, 

the landscape pattern for northern spotted owls is coarse-grained, 

with clear distinctions between suitable and unsuitable habitat. 

Selective logging of the largest trees from the most productive 

sites in the Sierra Nevada has resulted in significant changes in 

diameter distributions and produced a more fine-grained landscape pattern for the California spotted owl (Chapter 11). 

Chapter 8 181 
The effects of changes in forest structure and landscape 

pattern on owl demographics are unknown. If changes in habitat 

quality accompany selective logging methods, however, it seems 

obvious that they would produce less pronounced declines in the 

vital rates than clearcut logging. As a result, only very long-term 

studies involving a large number of banded birds would be able 

to detect such effects. 

We do not know the reason(s) for the significant decline of 

about 17 percent in numbers of territorial owls in the San Bernardino study area from 1987 to 1991. Much logging occurred 

there in the 1960s, but we doubt whether that disturbance can 

explain the current decline. A more plausible hypothesis involves either direct or indirect effects of the drought in southern 

California. For example, precipitation from 1987 through 1990 

averaged about 60 percent below normal at a weather station at 

the western end of Big Bear Lake, in the San Bernardino Mountains (fig. 8A) and 67 percent of normal at the east end (fig. 8B). 

Rainfall in 1991 was above normal at both of these sites, but 

almost all of it occurred in March, after the owls would have 

initiated breeding. One working hypothesis is that dusky-footed 

woodrats, the primary prey of the owls in the San Bernardino 

Mountains (Chapter 4; LaHaye pers. comm.), have experienced 

marked population declines as a result of the drought. For example, Linsdale and Tevis (1956) and Spevak (1983) reported 

depressed woodrat populations in California during droughts, 

although Kelly (1989) failed to find a decline in woodrats during 

a drought at Hastings Natural History Reservation in Monterey 

County in the late 1980s. 

If the recent drought has contributed in some way to the 

decline of the owl population in the San Bernardino Mountains, 

it suggests that the population is subject to high levels of envionmentally induced variation in its demographic parameters. 

As the population declines, individuals may be lost from marginal habitats, where survival and reproduction are possible only 

during "good" times (for example, see O'Connor 1986). Individuals that survive, and even reproduce, during the decline may 

be those that occupy better, more stable habitats, as where more 

mesic conditions prevail (for example, riparian areas). Such 

refuges would be particularly important to the species' long-term 

persistence, and any destabilization of them-by logging, water 

diversion, depression in ground water levels, or excessive development of recreational activities-could pose a significant threat 

to the species' survival. 

Future Research Needs 

Based on a three-stage model, estimates of parameter sensitivities from all three study areas suggest that λ values were most 

sensitive to estimates of adult survival rate (s), distantly followed by first- and second-year survival rates (s0, s1) and fecundity (b) (fig. 8C). The sensitivity coefficient associated with age 

at first reproduction was small (Noon and Biles 1990). The much 

steeper slope for adult (≥2 years) survival rate demonstrates the 

importance of precise estimates of adult survival to produce 

precise estimates of λ. Introducing senescence at age 16 increases the slopes for fecundity and prereproductive survival 

rates, but s is still most important (fig. 8D). For a fixed recapture 

probability, more precise estimates of s can be attained only by 

having a larger sample of banded birds. 

A comparison of figures 8C and 8D indicates that it may be 

important to know whether spotted owls exhibit senescent decline in fecundity or survival. Noon and Biles (1990) showed 

that estimates of population growth rate (λ) were strongly influenced by the age at which reproductive senescence was assumed. Without detailed information on reproductive histories 

of marked individuals, insights into senescent declines in the 

owl's vital rates cannot be detected. 

Figure 8A-Annual precipitation (inches) from 1975-91 as measured at 

the western end of Big Bear Valley, in the San Bernardino Mountains 

(Lundy pers. comm.). 

182 Chapter 8 

Figure 8B-Annual precipitation (inches) from 1975-91 as measured at 

Big Bear City, in the San Bernardino Mountains, 7 miles east of the 

weather station for results reported in figure 8A (Lundy pers. comm.) 

USDA Forest Service Glen. Tech. Rep. PSW-GTR-133. 1992. 
Figure 8C-Population growth rate (lambda) for a stage-structured 

model of the California spotted owl. The figure shows the effects on 

lambda (λ) of a proportional decline in a specific demographic rate, with 

all others held constant. Empirical estimates of mean survival rate have 

been scaled by 1/ λ to yield an initial starting point of λ = 1.0 on the 

abscissa. 

Two sources of information are relevant to a species' rate of 

population change (λ). One is the sensitivity of λ to variation in 

the vital rates as reflected in coefficients of parameter sensitivity. The other concerns those life-history attributes that show the 

most natural variation. Variation in population growth rate (λ) 

may be more closely associated with attributes that are naturally 

more variable than with attributes that are less variable but to 

which growth rate is mathematically more sensitive. We cannot 

yet estimate the magnitude of natural variation in demographic 

parameters of California spotted owls. Estimates that confound 

sampling error and true annual variation from the five demographic study areas (table 8D), however, indicate that fecundity 

exhibits the greatest annual variation. Fecundity combines two 

important components: productivity per breeding female and 

the proportion of females ≥2 years of age that breed. The latter 

is particularly variable (table 8D). 

Factors contributing to the low and variable reproductive 

rates of spotted owls also need investigation. We do not know 

whether the extensive annual variation in reproductive success is 

due to variable resource levels (prey base), variable climatic 

conditions, or some combination. Nor do we know whether 

reproductive rates or adult survival rates can be increased by 

direct management for prey populations; this is also an important research question. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

Figure 8D-Population growth rate (lambda) for an age-structured model 

of the California spotted owl.. The figure shows the effects on lambda 

(λ) of a proportional decline in a specific demographic rate, with all 

others held constant. Empirical estimates of mean survival rate have 

been scaled by 1/ λ to yield an initial starting point of λ = 1.0 on the 

abscissa. 

Finally, as suitable spotted owl habitat becomes more fragmented, it will become increasingly important to estimate dispersal capabilities of California spotted owls (Chapter 4) and to 

identify factors that affect survival during dispersal. This includes influences on the survival of juvenile birds dispersing 

from their natal territories, as well as adult birds displaced by 

habitat loss. 

Management Implications 

Implications of the demographic results for management 

involve decisions that may differentially affect adult and preadult birds. The life-history pattern of the spotted owl suggests 

that it must have evolved in an environment stable with respect 

to adult survivorship. The much greater sensitivity of λ to variation in adult than preadult survival rates indicates strong natural 

selection to maintain low adult mortality rates. Further, the low 

fecundity rate suggests that recruitment may always have been 

variable. In spite of this, high adult survivorship has allowed the 

Chapter 8 183 
spotted owl to persist through long periods of low reproductive 

output. A consequence of this trade-off is of great concern when 

considering management for spotted owls. Namely, low fecundity precludes rapid recovery from a population decline. Any 

management action that lowers adult survival rate, particularly 

when coupled with a reduction in population size, markedly 

increases the likelihood of local extinction. 

Although λ is relatively insensitive to changes in s0 and b

(figs. 8C and 8D), we cannot infer that these attributes are 

unimportant when developing management plans. Adult survival rate is relatively high and may not be amenable to further 

increases. Assuming no reduction in adult survival rate, increasing first year survival or fecundity by direct management activity may be the most feasible way to increase the growth rate of 

spotted owl populations. For example, certain silvicultural prescriptions may increase the availability of the owl's preferred 

prey and somewhat ameliorate the otherwise negative effects of 

logging. But these prescriptions are still unknown and untested. 

Given the spotted owl's life history structure, an evaluation 

of management decisions in terms of persistence likelihoods are 

possible only when viewed over the long-term (50-100 years). 

With a high adult survival rate and an apparently long life span, 

the Spotted Owl may be able to persist over the short-term even 

in the face of extensive reduction in the amount of its suitable 

habitat. Thus, significant time lags may occur in responses of 

spotted owl populations to declining environmental carrying 

capacity. By themselves, therefore, short-term survey results, 

even those including observations of breeding owls, are insufficient to allow inferences about population viability. 

Rapid rates of population decline in either deterministic or 

stochastic analyses are not surprising when the finite rate of 

increase (λ) is <1.0. Leslie-Lefkovitch projection matrices and 

life table models yield simple exponential models of population 

growth or decline (see discussion in Noon and Sauer, 1992). For 

this reason, it is inappropriate to use estimates of λ to project 

future population size without strict qualifications. Most natural 

populations presumably demonstrate density-dependence in one 

or more life history parameters. Many examples exist of bird 

species shown to exhibit density-dependent effects on vital rates. 

These include the gray partridge (Blank et al. 1967), the mallard 

(Hill 1984), the tawny owl (Southern 1970), and the European 

sparrowhawk (Newton 1988). No empirical data for spotted 

owls presently indicate a relation between population density 

and values of vital rates. Nevertheless, on the basis of a simple 

nonspatial model, Boyce (1987) argued that spotted owl populations at low densities should exhibit density-dependent increases 

in survival and reproduction that could stabilize the populations. 

Very different inferences are drawn, however, from models that 

include the additional reality of spatial structure, such as the 

184 Chapter 8 

uncertainties of successful dispersal and mate finding in spatially structured populations. Lande (1987) and Lamberson et al. 

(in press) found that owls at low population densities may 

experience negative effects of low densities that further depress 

survival and fecundity. In fact, their models indicate that if 

population densities are very low, or the amount of suitable 

habitat is greatly reduced, a threshold point exists beyond which 

the owl populations collapse to extinction. 

We believe there are implications for the management of 

California spotted owls. Future management activities, for example, must not increase the mean nearest-neighbor distances 

among suitable pair sites. Subtle factors that uniformly decrease 

habitat quality, or increase fragmentation, will act to reduce 

population density and incrementally increase the uncertainties 

associated with successful dispersal and mate-finding. 

Habitat-induced changes in vital rates, such as declines in first-year 

survival and in the proportion of breeding females, will lead to 

declines in population growth rate. If suitable habitat is allowed 

to decline and become fragmented, as for the northern spotted 

owl, the uncertainty of successful dispersal will become progressively more relevant to the subspecies' long-term population 

dynamics and likelihood of persistence. Should this pattern ensue for the California spotted owl, then the most effective way to 

assure its long-term persistence may be to create a connected 

network of habitat conservation areas (HCAs) as was proposed 

for the northern subspecies (Thomas et al. 1990). Large HCAs 

provide stable areas of high population density that promote a 

balance between pair turnover and colonization of pair sites, 

either through internal recruitment of dispersing juveniles or the 

emigration of owls from neighboring HCAs. Implementing such 

a strategy, however, would risk an increasing threat of 

stand-destroying fires in areas maintained primarily for the owls. 

Finally, it is important to recognize that rates of population 

change (λ) and the values of the owl's vital rates are ultimately 

determined by habitat quality at both local and landscape scales. 

For example, habitat quality at the scale of the home range may 

determine the survival and birth rates for an individual pair of 

owls. At a larger scale, the number of suitable pair sites and their 

spatial arrangement may determine the persistence of local populations. And, at a regional scale, providing habitat for a large 

number of local populations distributed widely across the landscape will increase overall persistence of the subspecies by 

decreasing the likelihood of populations simultaneously experiencing negative environmental effects. Thus, to ensure stable 

owl populations in the Sierra Nevada and Southern California 

Provinces will require specific management prescriptions, implemented at local to regional scales, to retain the amount and 

spatial pattern of habitat that will provide for a long-term balance between birth and death rates. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
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USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992 
Chapter 9 

Stability Properties of the Spotted Owl Metapopulation 

in Southern California 

Barry R. Noon and Kevin S. McKelvey 

Spotted owls in the Southern California Province have an 

insular population structure characterized by large (about 200 

pair sites) to small (about 2-4 pair sites) local populations distributed among discrete mountain ranges (fig. 9A, table 9A). The 

distribution of habitat "islands" is discontinuous across the 

landscape, reflecting natural discontinuities in vegetation structure and composition, in topographic conditions, and in the 

effects of extensive human-induced habitat disturbance and fragmentation. Lowland areas surrounding these mountain ranges 

are primarily desert scrub and chaparral habitats that are unsuitable for spotted owls. Relatively narrow gaps between populations, like the 6-mile separation at Cajon Pass, between the San 

Gabriel and San Bernardino populations (fig. 9A), are probably 

not complete barriers to dispersing owls. Longer separations, 

however, such as that of about 30 miles through the Los Angeles 

Basin, between populations in the San Bernardino and Santa 

Ana Mountains (fig. 9A), may present significant survival risks 

to dispersing owls such that successful colonization is very 

unlikely. The degree to which these gaps act to severely reduce 

or eliminate demographic rescue between populations is unknown. To date, however, no banded spotted owl has been 

located within any population outside that of its origin (LaHaye 

pers. comm.). The most significant gaps between owl populations are discussed in table 3K and shown on a distribution map 

in that chapter (fig. 3A). 

Even within many of the mountain ranges, the distributions 

of habitat and owl sites are discontinuous. For example, in the 

Cleveland and Los Padres National Forests (NFs), most suitable 

spotted owl habitat is patchily distributed because it is resticted 

to deeply dissected canyons dominated by oaks and surrounded 

Figure 9A-"Island" populations that comprise the southern California metapopulation of spotted owls. Estimated 

numbers of owl sites for each "island" are given in table 9A. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. Chapter 9 187 
Table 9A-Owl sites,1

 pairs and status of pairs on these sites, and nearest neighbor distances of California spotted owl 

populations in southern California (from Stephenson 1991 and Chapter 3). Compare values tabled here with the locations of 

these areas, shown in figure 9A. 

 Potential Nearest 

Area Owl sites Pairs Breeding population2

 neighbor 

 (all years) since 1987 since 1987 (no. sites) distances3

San Diego Ranges 37 18 6 76 18 - 33 miles 

 Palomar Mountain (18) 

 Central San Diego County (9) 

 Cuyamaca/Laguna Mountains (10) 

Santa Ana Mountains 2 1 - 12 30 - 40 miles 

San Jacinto Ranges 20 16 9 29 11 - 18 miles 

San Bernardino Mountains 124 114 66 125 6 - 11 miles 

San Gabriel Mountains 54 22 5 95 6 - 20 miles 

Liebre/Sawmill Mountains 14 10 2 20 12 - 20 miles 

Tehachapi Mountains 4 0 - 12 unknown 

Tecuya Mountain area 5 3 - 10 9 - 12 miles 

Los Padres Ranges 65 32 17 100 8 - 12 miles 

So. Santa Lucia Mountains 12 6 2 19 32 - 45 miles 

No. Santa Lucia Mountains 39 22 1 80 45 miles 

 Southern Monterey (14) 

 Northern Monterey (25) 

Total 376 244 108 578 

1

 See glossary in Appendix B for a definition of "owl site." 

2

 If each site were assumed capable of supporting a pair, the total population would be the number of sites times 2. 

3

 Where two distance values are shown (that is, 12-20 miles), they represent the distances to the two closest neighboring 

populations. 

by unsuitable chaparral habitat. Thus, many of these populations 

have an insular structure at both landscape and local scales. The 

present modeling study was prompted by the need for a better 

understanding of the potential effects on population persistence 

of isolation between subpopulations, and changes in habitat 

continuity within otherwise continuous habitat islands. 

Although spotted owl records in southern California exist 

from the early part of this century (Stephens 1902, Clay 1911), 

careful population studies are recent. LaHaye et al. (1992) have 

studied the demography of the owl population in the San Bernardino Mountains, and Gutiérrez and Pritchard (1990) reported on

surveys done on Mt. San Jacinto and Palomar Mountain. In 

addition, infrequent surveys in NF lands provide a cumulative 

record of spotted owl locations outside of these study areas 

(Gould et al. 1987, Stephenson 1991). 

The largest population occupies the San Bernardino Mountains (about 124 owl sites), with considerably smaller populations in the other ranges (table 3H). The cumulative total number 

of known owl sites within the southern California "archipelago" 

is estimated at 376 (table 9A), with an approximate population 

of 300-350 pairs at any point in time. The population trend is 

known for only one location. During the interval 1987-91, the 

resident, territorial population in the San Bernardino Mountain 

study area declined by about 17 percent per year (LaHaye et al. 

1992; Chapter 8). If this rate of decline were to continue, the 

188 Chapter 9 

territorial population would decline by 50 percent in about 4 

years. 

Based on theory and limited empirical data (reviewed in 

Gilpin and Hanski 1991), we believe the ultimate stability of this 

metapopulation will depend upon several factors. These include 

the persistence of one or more populations large enough to 

escape the negative effects of demographic and environmental 

stochasticity, and with demographic characteristics resulting in a 

production of excess individuals to serve as potential colonists 

for other local populations. Many of the small, isolated populations that occur in southern California are probably maintained 

by occasional immigration from these more productive source 

areas (see Pulliam 1988 and Howe et al. 1991 for a discussion of 

source-sink dynamics). In addition, given a high likelihood of 

significant environmental variation (for example, wildfires, prolonged periods of drought, or rapid urbanization) at least two 

source populations will be needed, and they should be sufficiently separated to minimize the likelihood of their simultaneously experiencing negative perturbations. 

The human population in southern California continues to 

expand into the forested mountain habitats of the spotted owl. In 

the San Bernardino Mountains, for example, the human population has grown from about 19,000 in 1970 to over 40,000 in 

1992, with 5 million annual visitors (Dib and Griffone 1991). 

Accompanying this growth is a reduction in the quality and 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
amount of forested habitat for spotted owls-a consequence of 

urbanization, highways and smaller roads, and recreational developments. Although we lack earlier estimates of spotted owl 

population sizes or densities, we nonetheless consider it likely 

that spotted owls have declined in both number and distribution 

from historic levels. 

Given the metapopulation structure of the southern California spotted owl population, recent declines within the largest and 

most contiguous population in the region, and significant threats 

to loss of suitable habitat due to rapid human population growth, 

we believe that a specific conservation strategy is warranted for 

the subspecies in this part of its range. 

This chapter has two main purposes: to explore, in a general 

sense, the stability properties of the southern California 

metapopulation structure, and to make specific recommendations for conservation of the subspecies in this part of its range. 

Toward this end, we make frequent reference to pertinent population theory and use results of simulation models to explore 

conservation alternatives. Our goal was not to estimate extinction likelihoods-we believe it is premature to conduct such 

analyses at this time. Currently, the only estimate of population 

change from this region suggests a rapid decline in the territorial 

population (Chapter 8). Under a strict assumption of constancy 

in the vital rates, projecting forward at this rate shows a rapid 

decline to extinction. Until more is known about natural fluctuations in vital rates of populations, however, such projections are 

unwarranted. 

Rather, our goal was to explore the geometry of the 

metapopulation, including both habitat and owls, to gain insights 

into what spatial arrangement of habitat would minimize future 

extinction risks. In this chapter we present the results of our 

efforts to integrate, in the form of computer simulation models, 

the species demography with variations in the amount, distribution, and quality of owl habitat. The computer simulation models 

were developed specifically to aid the development of a conservation plan for spotted owls (Thomas et al. 1990, appendix M; 

Lamberson et al. in press; McKelvey et al. in press). We used the 

results of computer simulations to test basic principles of reserve 

design (Murphy and Noon 1992) and to provide guidance about 

the necessary spatial design of a reserve system implemented on 

real landscapes. 

Methods 

Parameter estimates and model structure were based on 

information from the study of marked populations (LaHaye et al. 

1992; Chapter 8). Owls were captured during the breeding season, aged, sexed, and individually marked. Reproductive status 

was determined for all individuals using methods outlined by 

Forsman (1983). Young were counted after fledging had occurred (that is, after they left the nest). Individuals were initially 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

placed into four age classes (juvenile, first-year subadult, 

second-year subadult, adult) according to criteria developed by 

Moen et al. (1991). Because of similarity in survival rates, firstand second-year subadults were pooled with adults and collapsed into a single stage. Annual survival rates for banded 

juveniles, subadults, and adults were calculated using Jolly-Seber 

capture-recapture models for open populations (Jolly 1965, Seber 

1965) using program JOLLY (Pollock et al. 1990). 

The estimate of fecundity combines two important components-the number of females fledged per nest and the proportion of females ≥2 years of age that breed. Thus, "adult" refers to 

ages subsequent to the second year of life. Fecundity was estimated each year as the mean number of young fledged per pair, 

assuming a 1:1 sex ratio at birth. Within the adult age class, 

survival rate was assumed to be constant, and we assumed no 

reproductive senescence. For our simulation models, we divided 

the population into three stages juveniles, subadults, and adults. 

Time was expressed on an interbirth interval of 1 year, and we 

assumed an age at first reproduction of 2 years (that is, birds just 

beginning their third year of life). 

Lambda (λ), the estimated annual, finite rate of population 

change from the San Bernardino study was 0.827 (table 8G), 

indicating that the territorial population there was in precipitous 

decline. Because λ represents a simple exponential rate of change, 

using the vital rates estimated from this study to project future 

populations would be a trivial exercise. All populations would 

decline exponentially regardless of the amount and distribution 

of suitable habitat. To evaluate possible effects of variation in 

landscape geometry, we initially adjusted the survival rates by 

multiplying them by 1/ λ to yield a λ=1.0 in suitable habitat. A11 

demographic rates were again slightly increased to allow for the 

possibility of a 2 percent annual rate of population growth (λ = 

1.02). This scaling of the survival rates allowed for a growing 

(source) population in suitable habitat. Differences in model 

results based on the landscape model (see below) would, therefore, reflect the effect of differences in distribution and amount 

of suitable habitat, and not an assumption of depressed demographic rates. Based on current field studies (Chapter 8), however, the adjustments we made to the demographic rates for 

suitable habitat were optimistic. 

Dynamic Projection Models 

No natural population is exposed to purely deterministic 

forces. Thus, population dynamics of the spotted owl should be 

examined relative to both demographic and environmental 

stochasticity. Further, simple deterministic analyses are also 

limited in that they provide no insight into the dynamics of 

mobile populations in real landscapes. That is, they incorporate 

no information on the movement behavior of animals in heterogeneous environments and the uncertainties of finding suitable 

habitat and mates. Failure to account for the spatial component 

Chapter 9 189 
of both animal locations and their habitat distributions ignores 

the covariance between demographic rates and habitat variation. 

A conservation strategy for any species is ultimately described by a map that integrates information on the species' 

distribution; the distribution of current and potentially suitable 

habitat; and economic, political, and legal constraints (Thomas 

et al. 1990; Murphy and Noon 1992). To develop a general set of 

conservation recommendations for the southern populations of 

the California spotted owl, we needed insights into how the 

landscape-scale arrangement of owls and their habitat affected 

their population dynamics. The data available to us lacked detail 

and were imprecise (see Chapter 3). As a result, we could 

explore the effects of landscape geometry and variation in the 

distribution of habitat types only in a nonspecific way, and make 

general recommendations about the significance of different 

components of the landscape. 

To provide a framework to guide the conservation evaluation of the California spotted owl, we drew general inferences 

from several simulation models incorporating various degrees of 

spatial information. Our approach, which investigated the effects of variable dispersal efficiencies on the population dynamics of territorial animals occupying heterogeneous landscapes, 

had its foundation in previous work by Lande (1987, 1988). 

Lande's model was based on a hypothetical species with a 

monogamous, territorial breeding system and with obligate juvenile dispersal from the natal area. This model was directly 

applicable to the life history structure of the spotted owl. 

Nonexplicit Spatial Models 

In the following, we briefly review the structure and assumptions of two models used to assist in the design of a reserve 

strategy for the northern spotted owl (see Thomas et al. 1990, 

appendix M). We do not discuss these models exhaustively but 

simply make reference to results from those models that are 

most applicable to the conservation of spotted owls in southern 

California. Additional details can be found in Thomas et al. 

(1990, appendix M), Lamberson et al. (in press) and McKelvey 

et al. (in press). 

The Individual-Territory Model 

Model Description 

This model (Lamberson et al. in press) extended that of 

Lande (1987) by relaxing the assumption of demographic equilibrium and allowing the simulated landscape to be dynamic. 

Life history was expressed as a three-stage, two-sex projection 

model. The model assumes that all newly fledged juveniles 

disperse, and that adult birds who experience loss of their territory (for example, due to logging) also disperse. Lande's model 

allowed some likelihood of a juvenile's inheriting its natal terri190 Chapter 9 

tory. Based on the settling patterns of owls first banded as 

juveniles, however, inheritance of the natal site is a very rare 

event. Our model focuses on a landscape with a fixed spatial 

extent and with a fixed number of potential territories (or "sites"). 

Sites were either suitable for survival, mate attraction, and reproduction, or they were unsuitable. Only the suitable sites were 

capable of being occupied. The key response variable in our 

model is the occupancy rate of sites by pairs. As in Lande's 

model, this model lacks realism because of the global nature of 

juvenile and adult search-all cells had an equal likelihood of 

being sampled. 

The search process was simulated as sampling with replacement. The key equation describing search success in our model 

(see Lande 1987) was 

Pr(success) -1 - (1 - unoccupied suitable sites/total sites)m, (1) 

where m is the number of sites that can be searched prior to 

mortality (additional equations describing the model dynamics 

are given in Lamberson et al. [in press]). 

A nesting pair will annually produce young (according to a 

deterministic or stochastic likelihood), and these will disperse at 

the end of the season, the males seeking an unoccupied site and 

the females seeking a site occupied by a solitary male. Dispersal 

success is density-dependent, calculated by assuming random 

search of accessible sites. Search capabilities, together with the 

occupancy ratio of searched sites, determines the bird's potential 

for successful dispersal (consistent with Lande 1987). 

Relevant Model Results 

The most significant conclusions from Lande (1987) and 

Lamberson et al. (in press) are (1) that the occupancy of suitable 

habitat declines with a declining proportion of the landscape that 

is suitable habitat (fig. 9B), and (2) that if the amount of suitable 

habitat declines below some threshold value, the population will 

become extinct (fig. 9C, curve A). That is, because of uncertainFigure 9B-Trend in the number of pairs of spotted owls, number of 

sites, site occupancy by pairs, and juvenile survival rate based on a 

75-year simulation. The simulation assumed that 4 percent of the suitable 

owl habitat was lost per year until 14 percent remained, and that juvenile 

owls could search 20 sites. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
ties associated with search and mate finding, a species can be 

critically habitat-limited even in the presence of suitable but 

unoccupied habitat. 

New findings, beyond those reported by Lande (1987), 

resulted from the model of Lamberson et al. (in press), because 

of its dynamic and nonequilibrium nature. First, survival probability, as a function of the percent of suitable habitat, is affected 

by environmental variation. Once again the deterministic case 

shows a stairstep function, with the jump from 0 to 1 occurring at 

the threshold point (fig. 9C, curve A). Adding environmental 

variance makes the extinction threshold less abrupt (fig. 9C, 

curves B and C). Second, in a scenario involving incremental 

loss of suitable habitat (for example, by logging), the crowding of 

older owls into remaining suitable habitat is likely to produce 

temporarily high occupancy rates-much higher than expected 

under long-term stable conditions (fig. 9B). Thus, predicting 

long-term population status from short-term occupancy data can 

be very misleading. 

The Territory-Cluster Model 

The primary limitation of the individual-territory model 

was the global nature of search. In reality, organisms search 

locally and are primarily influenced by the local nature of the 

landscape rather than its global condition. The basis of the 

territory-cluster model (Thomas et al. 1990, appendix M; 

Lamberson and Noon 1992) is a fixed, rectangular array of 

circular clusters containing a variable number of owl sites (territories). Every site within a cluster, assumed to be of identical 

size, was either suitable or unsuitable (defined as in the 

individual-territory model). Clusters could be either totally or 

partially suitable-the carrying capacity of a cluster equalled the 

number of suitable sites. For a given simulation, all clusters were 

the same size (that is, they contained the same number of sites). 

As in the individual-territory model and Lande (1987), the response variable was the proportion of suitable sites occupied at 

any point in time. 

Suitable Habitat (percent) 

Figure 9C-The 250-year survival probability in relation to the percentage of suitable sites in the landscape. Curves A, B, and C represent the 

conditions of no, low, and high environmental variability, respectively. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

The matrix between clusters was assumed to be entirely 

unsuitable for owl sites. Assuming a constant percentage of the 

habitat as potentially suitable Habitat and restricting the habitat 

to clusters had two important consequences: (1) as cluster size 

increased, the distance between adjacent clusters increased predictably (fig. 9D); and (2) the dispersal angle between adjacent

clusters (see fig. 9E), and the probability of selecting this angle, 

were independent of cluster size.

The landscape simulated by the model had a "wrap-around" 

structure to exclude possible anomalous results that might arise 

from edge effects. The clusters on the right side of the grid were

treated within the model just as though they were immediately to 

the left of those on the left side of the grid. The top and bottom 

rows of clusters were treated in a similar manner. 

Sites per Cluster 

Figure 9D-Nearest-neighbor distance between clusters in relation to 

cluster size. Each curve represents a different percentage of the landscape assumed to be suitable habitat and located in the clusters. 

Pr GLVSHUVH IURP &OXVWHU  WR &OXVWHU  

2

2

2

2 arctan

rd

r

c −

θ =

Figure 9E-Method of estimating the angle of intersection between two 

adjacent clusters in the cluster model. The probability of dispersing in the 

correct direction is a function of the combined angles of intersection of the 

six neighboring clusters. 

Chapter 9 191 
The life history was expressed as a three-stage, female-only 

projection model. Initial values of the vital rates were based on 

table 8G, and survival rates were scaled by multiplying by 

1/0.827 to provide for a stable local population in suitable habitat. Based on observations of recently fledged young near the 

nest site (Marcot and Holthausen 1987), a predispersal survival 

component of 0.6 was assumed for the juvenile survival rate. 

The dispersal component of first year survival arose from properties of search efficiency and landscape pattern, thus allowing 

the possibility of a growing population. Fecundity was treated 

stochastically, based on good years and bad years for reproduction, with expected values equal to field data (table 8G). We 

assumed complete spatial autocorrelation for the reproductive pulses. 

Dispersal Dynamics 

As in Doak's (1989) model, our model distinguished between dispersal within and among clusters. All clusters were 

equally accessible in Doak's model, but we introduced the additional reality that search was spatially constrained and was, 

therefore, affected by the status of neighboring clusters. 

Within-cluster dispersal in our model was identical to the 

individual-territory model, with each dispersing owl allowed to 

sample with replacement a given number, m, of sites within the 

cluster (equation 1). To determine the allocation of search effort 

within a cluster, we assumed a random walk from a random 

starting point. By simulation, we estimated the expected number 

of steps taken by a dispersing bird before it crossed the boundary 

of a circular cluster. The assumption was that an owl could 

traverse one site in a single time step. Based on 10,000 simulations of circular clusters of various sizes, and regressing the 

number of steps taken before crossing the circle, we estimated 

the following relationship: 

Expected(m) = 0.41 * number of sites in the cluster. (2) 

For clusters with 20 sites, for example, an average of eight sites 

were searched within clusters before crossing the boundary into 

the matrix between clusters. The total number of searches across 

all clusters (m = 22) was based on the upper bound of the 90th 

percentile of the maximum dispersal distance of 56 radio-tagged 

juvenile spotted owls (Thomas et al. 1990, p. 305) and the 

assumption that each site had a diameter of about 2 miles. 

If a dispersing juvenile did not find a suitable site within its 

natal cluster (based on a fixed number of searches), it was forced 

to disperse between clusters. Between-cluster dispersal was modeled as a straight-line path moving away from the natal cluster at 

a random azimuth. Two sources of mortality were encountered. 

First, the bird must move in a direction that would intersect a 

neighboring cluster. The probability of intersection for each 

adjacent cluster was governed by the following equation (see 

fig. 9E): 

Probability(disperse from cluster 1 to cluster 2) = 2 /360 (3) 

where  = arctangent ( )./ 2

2

2

2 rdr c −

Second, if a correct direction was chosen, the likelihood of 

192 Chapter 9 

successful travel to the neighboring cluster lying a distance d 

miles away (measured edge-to-edge) was modeled by a declining exponential, 

Pr(survive to d) = exp(-kd) (4) 

The value of k was estimated by arranging the maximum 

straight-line distances attained by the 56 radio-tagged juvenile 

owls in rank order, then regressing the natural log of the cumulative proportion represented by that distance (dependent variable) 

on the associated distance and forcing the regression through 0 

(r

2 = 0.97) (see fig. 4F). Because search begins in the natal 

cluster and moves through adjacent clusters, the search success 

in this model, as in nature, is a function of the condition of the 

landscape near to the bird's natal site. 

Model Parallels to the Southern California 

Metapopulation 

The structure of the cluster model crudely approximates 

aspects of the southern California spotted owl habitat distribution (fig. 9A). Clusters can be considered analagous to local 

habitat islands, and gaps between local populations are analagous 

to the matrix-areas that allow dispersal (with inevitable survival risks) but no reproduction. Model results allowed us to 

make general inferences to (1) the relation between cluster size 

and local population stability, (2) the effects of variable spacing 

among clusters on mean pair occupancy, and (3) the most efficient ways to allocate lands to a reserve design. 

Relevant Model Results 

Because the number of combinations of model parameters 

is immense, many sensitivity analyses were done (table 9B). The 

most significant result, however, was based on the relation 

between mean pair occupancy and cluster size. For a simulation 

with 60 percent of the sites suitable within a cluster, occupancy 

did not stabilize until clusters held at least 15 sites (fig. 9F). 

Year 

Figure 9F-Mean proportion of cluster occupied in relation to time, 

based on a 100-year simulation. The number of sites per cluster varied 

from 5 to 25, and 60 percent were assumed suitable; 35 percent of the 

landscape was in clusters. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Clusters of 15 sites stabilized at about 70 percent occupancy, 

while clusters of >20 sites stabilized at about 80 percent, a figure 

representing nearly full occupancy given adult survival rates. 

Further increases in cluster size had little effect on occupancy. 

As the number of suitable sites within a cluster increased, the 

marginal difference in occupancy among clusters of different 

sizes became less pronounced. 

Based on the relation between the landscape fraction within 

clusters (habitat islands) and cluster spacing (fig. 9D), different 

levels of allocation to a reserve system can be viewed in terms of 

the distance between clusters. We found mean occupancy rate to 

be strongly affected by increased spacing between clusters for 

cluster sizes <20 sites (fig. 9G). In contrast, clusters with >25 

sites showed minimal distance effects beyond separations of 

about 20 miles (fig. 9G). Adding low levels of environmental 

variation in survival rates lowered occupancy rates about 7 

percent and suggested slightly larger cluster sizes for the same 

level of occupancy (fig. 9H). 

An alternative approach to choosing an optimal cluster size 

is to estimate the expected number of owls occupying a fixed 

amount of suitable habitat. Based on mean occupancies at 100 

years from fig. 9F, the efficiency of land use is clearly higher for 

larger clusters (fig. 9I). 

Inferences to Reserve Design from 

the Nonexplicit Spatial Models 

Results from these models suggest that providing for clusters of territories should increase the persistence likelihood of 

spotted owls, primarily by facilitating juvenile dispersal (compare Doak 1989). If habitat becomes too diffuse at a local scale, 

populations there will experience extinction events and require 

recruitment from larger source areas for recolonization (the 

Distance Between Clusters (miles) 

Figure 9G-Mean proportion of clusters occupied as a function of the 

distance between clusters. The number of sites per cluster varied from 5 

to 45, and 60 percent were assumed to be suitable. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

Distance Between Clusters (miles) 

Figure 9H-Mean proportion of clusters occupied as a function of 

distance between clusters. The number of sites per cluster was either 5 

or 25, and 60 percent were assumed to be suitable. The lower curve of 

each pair represents simulations including environmental variation in 

survival rates. 

Figure 9I-Steady-state mean occupancy rate in relation to cluster size. 

The curves represent 40, 60, and 100 percent of the sites within each 

cluster as suitable habitat. 

"rescue effect"). Given assumptions of the cluster model (

percent of sites suitable, moderate connectivity among adjacent 

clusters), clusters with >20 sites should provide for locally stable

populations in the absence of catastophic disturbance. Adding 

moderate levels of environmental variation in the survival rates, 

however, raises the desired minimum cluster size to about 25 sites. 

Once clusters reach about 35-45 sites, they attain high levels 

of local stability and become relatively independent of the distance to their nearest neighbor. Within the structure of our 

model, this occurred because almost all dispersing juveniles 

found suitable sites within their natal cluster. Given a fixed 

number of total searches (m = 22), and the allocation of within 

cluster search following equation 2, all search is expended within 

&KDSWHU  193 
Table 9B--Qualitative results of sensitivity analyses for the territory cluster 

model.

Factor varied Sensitivity1 

Within-cluster search efficiency Low for large clusters; high 

for small clusters 

Resistance to between-cluster dispersal Low for large clusters; high 

 for small clusters 

Search time outside of natal cluster Low to moderate for large 

 clusters; high for 

 small clusters 

Initial population size Low if population was in 

large clusters; high if 

population was in small clus 

 ters 

1

 Measured in terms of reduction in mean pair occupancy. 

the natal cluster when cluster size equals 54 sites. At this point in 

our model, all clusters are acting as independent subpopulations. 

Comparing these general model inferences to estimates of 

the number of sites in various habitat islands in the Southern 

California Province (table 9A) provides some significant insights. Based on current estimates of owl sites, many of these 

habitat islands have <25 sites, are isolated by >10 miles, and are 

therefore expected to be very unstable. That is, they will be 

subject to extinction events because of both demographic and 

environmental variation, or act as sink areas (Pulliam 1988). As 

such, their occupancy status will depend largely upon immigration from neighboring habitat islands. The possible dependence 

of these populations on immigration from larger source populations parallels results for insular populations of acorn woodpeckers in the southwestern United States (Stacey and Taper 

1992), and red-cockaded woodpeckers in the southern Appalachian mountains (Conner pers. comm.). 

The cluster model had several optimistic assumptions. Most 

important were that (1) no uncertainty was associated with mate 

finding, (2) only low levels of environmental variation in vital 

rates were simulated, and (3) clusters were assumed to be circular, thus minimizing the perimeter:area ratio. A circular shape 

(1) maximizes the density of suitable habitat and, given our 

within-cluster search algorithm, suggests very high search efficiencies; and (2) it affects the estimate of the probability of 

leaving the natal cluster in an appropriate direction. For both 

stability of mean occupancy and to achieve a specified occupancy rate, relaxing these assumptions would generally require 

larger cluster sizes. 

194 Chapter 9 

A Spatially Explicit 

Landscape Model 

The landscape model links an organism's survival and reproduction explicitly to its current habitat location. In reality, 

habitats range in quality from those that support demographic 

rates resulting in a net gain of births over deaths (source areas) to 

those that yield a net loss (sink areas). The latter habitats would 

be unoccupied in the absence of immigration from source areas. 

As Pulliam (1988) has argued, it may be that habitat-specific 

demographic rates are more important than age-specific rates to 

a species' long-term population dynamics. In this model, a 

population's rates of survival and fecundity will vary based on 

landscape configuration and the distribution of habitat types. 

The model, a two-sex, single-organism simulator, assumes that 

each organism must search the landscape to find new territories 

and mates. Each organism is born, moves, attempts to find a 

mate and breed, and dies. This format allows the behavior of 

each individual to be simulated by following a series of probabilistic rules. 

Male and female behavior in this model is similar to that in 

the territory-cluster model. Males search for territories to occupy. If they find a suitable nest site, they stop moving and 

become territorial. The likelihood of settling in a given site is a 

function of the habitat quality of that site. Males remain on their 

selected site until they die or the site becomes unsuitable for 

nesting. If the site becomes unsuitable, the males become 

nonterritorial and reinitiate search. Females are born and disperse from the natal site to seek unpaired, territorial males. 

When they find a territorial male, they obligately pair. Once 

paired, females remain on site until they die or the site becomes 

unsuitable for nesting. 

Paired individuals split up when one member of the pair 

dies, or the site becomes unsuitable for nesting. If the female 

dies, the male remains territorial and stays on site. If the male 

dies, the female has no site fidelity and will initiate search for a 

new mate. If the site becomes unsuitable for nesting, both members search independently for a new site. The movement of 

females after loss of their mate is supported by field data. In 

general, females move more often then males, occasionally 

leaving sites to search for new mates even when their previous 

mates are still alive. 

In the landscape model, by contrast with the previous two 

models, if a juvenile fails to locate an unoccupied, suitable site it 

does not die-rather, it becomes a floater. As a consequence, 

search efficiency has less of a direct effect on juvenile survival 

rate, but a strong effect on the dynamic movement between the 

reproductive and nonreproductive stages. To the extent that 

search inefficiency prevents pair formation, fecundity declines 

and decreases accordingly.

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Demographics 

All demographic parameters are linked to site quality. Individual mortality and fecundity are determined by the quality of 

the site occupied at the beginning of each time-step. In keeping 

with the stage-structured approach, risks are assumed to be 

constant within a stage over the course of a year. The year is 

divided into i time-steps, and the risk per step for an owl in 

stage-class j occupying habitat type k is defined as the ith root of 

the yearly risk for class j and habitat type k. 

In the model analyses described below, we allow a maximum of three habitat types: (1) suitable habitat with associated 

vital rates from table 8G, initially scaled by 1/ WR SURYLGH IRU D

stable or growing population; (2) sink habitat with reproduction 

and survival below that needed for a stable population; and (3) 

unsuitable habitat that allows survival at reduced rates, but no 

reproduction. The model structure will support additional categories of habitat quality, but available data do not support this 

degree of resolution. 

Movement 

The map is divided into a fixed array of grid cells, each 

representing one territory-sized unit. The grid is hexagonal to 

allow more realistic movement. The rate of movement depends 

on the size of the grid cell and the number of time-steps per year. 

Individual moves are restricted to adjacent cells. The mobile 

classes of the owl (nonterritorial males and females) have an 

opportunity to move at each time-step. To ensure that certain 

birds or areas of the map are not given preferential access to 

open territories or mates, the order of movement is fully randomized at each time-step. 

In its simplest implementation, movement is a random walk. 

The model, however, allows owls to search with "intelligence"; 

that is, they may favor movement through good habitat and 

avoid poor habitat. Similarly, females may move obligately to 

known territorial males, and nonterritorial males may be averse 

to crossing into defended territories. This intelligent behavior is 

modeled by giving the owls absolute knowledge of the quality of 

the cell they occupy and partial knowledge of adjacent cells. 

They have no knowledge of more distant parts of the landscape. 

This knowledge takes the form of a series of switches and 

weighting factors that condition the probability of movement 

(table 9C). 

Three boundary conditions can be specified at the map 

edges: absorbing, reflecting, and wrap-around. In addition, internal reflecting zones can be created by specifying a land type for 

which the owls show complete aversion. 

Fledglings 

Fledglings are defined for model purposes as young that 

survive to disperse. We assume both good and bad years for 

fledging and complete correlation in reproductive pulses among 

populations. If it is a good year, then the pair produces fledglings 

according to a beta random variable ranging from zero to the 

maximum clutch size. Two levels occur at which variability can 

affect the number of fledglings. If the area under the beta distribution is concentrated around the mean clutch size, the population will pulse based on the frequency of good years. When a 

good year occurs, all pairs at all sites will produce at about the 

mean number of fledglings. If the probability of a good year is 

1.0, variability in the number of fledglings will occur on an 

individual-territory basis and will depend on the parameters of 

the beta distribution. Both parameters are linked explicitly to 

habitat quality and the stage-class of the pair. Because this is a 

two-sex model, the sex ratio of fledglings is also adjustable. 

Relevant Inferences to the Southern 

California Metapopulation 

Effects of Cluster Shape 

The cluster model assumed circular clusters, but the landscape models allowed us to closely specify the shapes of clusters 

Table 9C -A summary of factors that can affect an individual's movement in the landscape model. 

Factor Based on Sex Form 

Becomes territorial Habitat quality/occupancy M Probabilistic switch 

Aversion Habitat quality M/F Weighted probability 

Site fidelity Habitat quality M/F Weighted probability 

Linear propensity Behavior M/F Weighted probability 

Territorial aversion Occupancy M Weighted probability 

Female finds male 

 (current cell) Occupancy F Absolute switch 

Female finds male 

 (adjacent cell) Occupancy F Probabilistic switch 

Global boundary - M/F 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. Chapter 9 195 
(habitat islands). To compare general model results between the 

two models, the landscape model was used to project population 

trends from a number of hypothetical landscapes with an identical number of suitable sites (McKelvey et al. in press). Other 

than suitable habitat configuration, there were no differences in 

the initial values of any model parameters. The map boundaries 

were wrap-around so that the exact location of the habitat within 

the map frame was unimportant. In these simulations, only two 

habitat qualities were simulated: habitat suitable (  DQG

unsuitable for nesting. The movement parameters deviated only 

slightly from a random walk-birds were twice as likely to 

choose suitable habitat, males treated occupied habitats identical 

to unsuitable habitat, and birds were twice as likely to move in 

the same direction as to choose a different direction. 

Model results parallel the territory-cluster model, showing 

that a clustering of suitable habitat is both more efficient in terms 

of mean population level and more stable in terms of lowered 

extinction probabilities than is a random structure. We found, 

however, that the shape of the cluster had an important effect on 

its stability properties and mean occupancy rates. A cluster with 

a low ratio of edge to area supported a larger population and was 

more stable than continuous clusters of identical area but with 

greater irregularity (compare figs. 9J and 9K). A high edge-to-area 

ratio had a negative impact on demographic stability: the rate of 

decline in a large, highly irregular cluster was similar to the 

decline rate of the dispersed cluster system, but it had a lower 

extinction likelihood. 

Source-Sink Dynamics 

Previous results demonstrated the impact of reserve shape 

when each landscape cell was either suitable or unsuitable for 

breeding. In that case, the breeding population was limited 

entirely to the suitable habitat. In a landscape containing a 

gradient of habitat qualities, source locations would produce an 

excess of individuals, and sink locations would absorb some of 

the dispersing juveniles from the sources. Consequently, when a 

possibility exists of nesting in sink locations, populations will 

occur exterior to the source habitat areas even though vital rates 

in the sinks do not allow for a self-supporting population (see 

similar conclusion in Buechner 1987, p. 71). The presence of 

sink areas adjacent to habitat clusters may adversely affect the 

stability properties of the clusters. In our simulations, the mean 

population size of the entire landscape was higher in the sourcesink system, but the mean occupancy of the reserve clusters was 

lower and the variability among the population size trajectories 

increased with time. 

In summary, the landscape model demonstrated that the 

shape of clusters is nearly as important as their size. As a result, 

the size of irregularly shaped clusters must be larger to maintain 

the same mean occupancy rate compared to clusters more circular in shape. Further, model results indicate a possibility that in 

landscapes exhibiting a gradient of habitat qualities, the presence of sink habitats adjacent to reserves may have opposing 

effects. Overall population size is increased but projected population sizes are more variable. In terms of persistence likelihoods, however, we believe that increases in population size 

associated with increases in the amount of sink habitat should 

196 Chapter 9 

more than compensate for any negative effects associated with 

increased variability in the sizes of populations within clusters. 

Application of the Landscape Model to the Southern 

California Metapopulation 

Constructing the Map 

The map, which characterized in a spatially explicit fashion 

the distribution of owl populations and their habitat islands in 

southern California, was based on the 1986-91 cumulative spotted owl survey data done primarily on public land (Chapter 3; 

Stephenson 1991). Each township (6-x-6 miles) in the Southern 

California Province was characterized according to the estimated number of owl sites it contained (fig. 4C). The collection 

of sections with owl sites defined the habitat islands and corresponded closely to locations listed in table 9A and shown in 

figure 9A. Thus, owl locations at a scale of 36 square-mile 

sections defined the location of suitable owl habitat. Each section, representing about 23,000 acres, was represented by nine 

hexagonal grid cells. This produced grid cells of about 2,500 

acres, roughly corresponding to the size of individual spotted 

owl home ranges during the breeding season in mixed-conifer 

forests of the Sierra Nevada (table 6B). If a section had >9 sites, 

the extra sites were added to an adjacent section. The initial map, 

as it appears at the beginning of a simulation, is shown in figure 

9L. 

Imposing a fixed grid-cell size on a species with geographically variable area requirements is problematic. The number of 

owl sites will be underestimated in those parts of the species' 

range where its area requirements are smaller. For those habitats 

in southern California where home ranges are apparently the 

smallest (riparian/hardwood areas; table 6G), we examined the 

distribution and spacing among known owl sites. Most were 

separated by large areas of chaparral, making it unlikely that a 

single grid cell would contain two or more home ranges. In some 

cases where a single cell could contain two or more owl sites, we 

have probably underestimated the number of sites. 

Initializing the Map 

The map was initialized, in terms of owl pairs, using the 

estimated number of suitable sites per township (36 square 

miles) (see fig. 4C). If a township, for example, was estimated to 

contain six pair sites, then six sites (grid cells) were randomly 

selected from among the nine sites representing that township on 

the map. Each simulation was initialized with about 360 pairs of 

spotted owls. 

For most simulations we used only two habitat types--

suitable and unsuitable. Suitable habitats were characterized by 

D  VOLJKWO\ ! 8QVXLWDEOH KDELWDWV WKH ODQGVFDSH PDWUL[

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Figure 9J-Landscape model simulation showing simulated landscape with suitable habitat arrayed in 

one large, regular block. Results are based on 30 simulations. The heavy line represents the mean 

population, the thin lines are one standard deviation from the mean. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. Chapter 9 197 
Figure 9K-Landscape model simulation showing simulated landscape with suitable habitat arrayed in 

one large, irregular block. Results are based on 30 simulations. The heavy line represents the mean 

population, the thin lines are one standard deviation from the mean. 

198 Chapter 9 USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Figure 9L-The southern California metapopulation "map" used as the initial condition for subsequent simulations 

based on the landscape model. In this map showing the initial condition, hexagonal cells defining the "islands" were 

assumed to be suitable habitat. Areas outside of the islands (the matrix) were unsuitable. 

were characterized in different ways according to expected survival rates (table 9D), but these sites never supported reproduction. Sink habitat was introduced in a third set of simulations; it 

KDG DQ DVVXPHG  

Simulations 

All simulations assumed stochastic fecundity rates, expressing 

both within- and among-year variation (see above). Survival 

rates only expressed within-year variation based on binomial 

sampling probabilities. All owls experienced similar pulses in 

reproduction-that is, we assumed complete spatial autocorrelation 

within the metapopulation. Simulations represented 150-year 

projections and were based on 50 replications. Various assumptions were made about the search capabilities of dispersing 

juveniles and the degree to which the matrix surrounding habitat 

islands could support owl survival (table 9D). In general, birds 

were twice as likely to move to a suitable cell than an unsuitable 

cell. This had the effect of making it less likely to leave an area 

of clustered, suitable habitat and more likely to be attracted to 

such an area. The map was static-after setting initial conditions, there were no changes in the amount or distribution of 

habitat-for all simulations. 

Each suitable site was assumed to contain a pair of owls at 

the initiation of each simulation. Therefore, the population was 

effectively above its carrying capacity at time zero (figs. 9L-9P). 

Even under optimum conditions, a suitable site would have an 

expected pair occupancy rate <1.0 because of random mortality 

events. As a result, each simulation showed an initial population 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

decline. Comparisons of results based on population trajectories 

should be made over the last 100 years of each simulation. 

Interpreting Simulation Results 

Results of simulations should not be interpreted as estimates of extinction likelihoods. We believe it is premature to 

make reliable estimates of extinction likelihoods at this time, as 

too many uncertainties exist about the long-term values of the 

vital rates and the dispersal capabilities of juvenile owls. For 

example, we do not known whether gaps between adjacent 

habitat islands are total or only partial barriers to dispersal. The 

most relevant output from the simulations are the maps, which 

summarize mean occupancy rates for each cell over the 150-year 

simulation interval. In general, areas of the landscape with very 

high occupancy rates represent potential source areas. In contrast, areas of the landscape with low occupancy rates represent 

sink areas. Interpreting the model results in a visual and spatially 

explicit way allows insights into areas of the landscape that are 

particularly vulnerable to local extinction events, as well as areas 

that represent sources for immigrants to other local populations. 

Results 

All simulations were initialized according to the spatial 

pattern of suitable habitat shown in fig. 9L. Suitable habitat 

supported demographic rates, on average, that yielded a  VOLJKWO\

>1.0. Differences among the simulations reflect different assumptions about the survival risks experienced by owls moving 

Chapter 9 199 
Table 9D-Values of the demographic rates in suitable habitat and in the matrix for different model simulations. Risks to 

dispersing birds and dispersal capability were varied among the different simulations. 

Simulation 

Suitable Source Source/sink 

Demographic rates habitat A B C D reduction Matrix Sink 

Juvenile survival 0.358 0.20 0.20 0.18 0.01 0.18 0.18 0.30 

Subadult survival 0.903 0.70 0.70 0.45 0.01 0.45 0.45 0.75 

Adult survival 0.903 0.80 0.80 0.45 0.01 0.45 0.45 0.75 

Fecundity 0.279 0 0 0 0 0 0 0.175 

Dispersal capability 22 44 22 22 22 22 22 

through the matrix among the habitat islands. Because dispersal 

abilities are largely unknown, we bracketed a range of possible 

responses, ranging from very optimistic to an assumption of 

almost complete isolation of local populations (see table 9D). 

None of the simulations assumed any absolute barriers to dispersal in any direction, with the exception of the Pacific Ocean, 

which acted as a reflecting barrier. 

Simulations A and B 

In these simulations we made optimistic assumptions about 

the survival risks experienced by dispersing birds. Survival rates 

were only slightly depressed from values experienced in suitable 

habitat (table 9D). Simulation A suggested a relatively stable 

metapopulation structure with areas of high occupancy concentrated in the San Gabriel-San Bernardino Mountain complex 

(fig. 9M). Smaller and more isolated subpopulations, such as 

Tecuya Mountain, the South Santa Lucia Mountains, and Palomar 

Mountain (fig. 9A), had significantly lower mean occupancy 

rates, indicating frequent turnover within sites with subsequent 

recolonization at irregular intervals. In general, the lower the 

mean occupancy rate of a site, the longer the interval to 

recolonization after a turnover event. 

Parameter values for simulation B were identical to A, 

except that we doubled the search capabilities of dispersing owls 

(table 9D) and the equilibrium population was about 15 percent 

higher than observed in simulation A. Results were similar 

except that occupancy rates were elevated overall, most significantly in the smaller but isolated clusters (for example, in the 

Los Padres Ranges of Ventura and Santa Barbara counties, and 

on Mount San Jacinto) (fig. 9A). Thus, sites were being recolonized at a greater rate. 

Simulation C: Increasing Dispersal Costs 

Assumptions made in the previous simulations about risks 

to movement in the matrix were very optimistic. In this simulation we relaxed those assumptions somewhat, and assumed that 

survival rates in the unsuitable matrix habitat were half those in 

suitable habitat (table 9D). Results indicated that small, remote 

subpopulations had occupancy rates in suitable sites of only 

20-40 percent (fig. 9N). That is, suitable sites in these habitat 

islands are unoccupied 60-80 percent of the time. Occupancy 

200 Chapter 9 

rates in the San Gabriel-San Bernardino complex remained high 

because, having many tightly clustered owl sites, they were 

effectively immune to costs of dispersal through matrix habitats. 

Total population size was reduced by about 30 percent, compared to simulation A, but appeared to reach an equilibrium 

within the last three decades of the simulation (fig. 9N). 

Simulation D: Population Isolation 

In this simulation, we set movement costs in the matrix very 

high so that all but the closest clusters were effectively isolated 

from each other. The effects were dramatic. Overall population 

size was reduced by about 40 percent, and the population size 

did not equilibrate until the ninth decade (fig. 9O). High per-site 

occupancy rates were restricted to relatively large clusters (>40 

sites) and with mostly contiguous sites (fig. 9O). In areas with a 

low density of suitable sites, even habitat islands with about 40 

sites, such as the Palomar Mountain/central San Diego County/ 

Cuyamaca/Laguna Mountains complex (fig. 9A), were unstable 

and had very low occupancy rates (<15 percent). All local 

populations between the western part of the San Gabriel Mountains and the San Rafael wilderness (Las Padres NF) experienced frequent local extinction events (fig. 9O). The San Gabriel/ 

San Bernardino complex remained the most stable locus of high 

occupancy within the metapopulation. 

Simulation E: Source Reduction 

The San Gabriel/San Bernardino complex consistently had 

the highest occupancy rates and was the key source population 

for immigrants into other areas of the metapopulation. Therefore, we wanted to investigate its resilience to disturbance and 

the effect that such disturbance would have on the stability of the 

rest of the metapopulation. To simulate this scenario, we randomly changed 1/3 of the suitable sites within this complex to 

unsuitable sites and assumed moderate survival costs to movement through the matrix (table 9D). 

The effect of the simulated decline in habitat within the 

largest population was pronounced. The overall size of the 

metapopulation was markedly reduced, and an equilibrium size 

had not been attained even after 150 years (fig. 9P). Comparing 

figures 9P and 9N suggests that loss of habitat within the largest 

source population would greatly reduce occupancy rates through 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Figure 9M-Landscape model simulation A, showing mean (±1 standard deviation)150-year trajectory of the metapopulation 

(inset) and the mean pair occupancy rate of habitat cells (from fig. 9L) averaged across the time interval and based on 50 

simulations (parameter values used in the simulation are described in table 9D). 

Figure 9N-Landscape model simulation C, showing mean (±1 standard deviation) 150-year trajectory of the metapopulation 

(inset) and the mean pair occupancy rate of habitat cells (from fig. 9L) averaged across the time interval and based on 50 

simulations (parameter values used in the simulation are described in table 9D). 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. Chapter 9 201 
out the metapopulation. In addition, the small local populations 

of owls immediately to the west of the complex had gone extinct 

or had occupancy rates less than 20 percent. Interestingly, the 

overall size trajectory for the metapopulation in this simulation 

showed a steeper decline than that for isolation (simulation D) 

(fig. 9O). That is, decreasing the net production of potential 

colonists from the major source population within the 

metapopulation appeared to have more adverse effects than 

increasing the degree of isolation among local populations. Collectively, these results suggest that, to a large extent, the persistence of much of the southern California metapopulation relies 

upon maintaining the habitat integrity of the San Gabriel/San 

Bernardino habitat complex. 

Simulation F: Source-Sink Effects 

For this simulation we randomly changed 50 percent of the 

suitable (that is, source) habitat (λ = 1.02) within the entire 

metapopulation (fig. 9L) to sink habitat (λ = 0.80). This simulation thus represented both a source reduction and the introduction of sink habitat. Sink habitat, unlike matrix habitat, supported both survival and reproduction, but at nonreplacement 

rates (table 9D). We assumed limited ability by the owls to 

discriminate between source and sink habitats-searching birds 

were only 10 percent less likely to settle in sink habitat. Simulation results suggested a dramatic, exponential decline to 

metapopulation extinction within about 100 years (fig. 9Q). 

Average occupancy rates over the interval were >15 percent 

only for the San Gabriel/San Bernardino complex. Even this 

habitat island was unstable, however, and could not sustain its 

population over the long-term. Given this level of source reduction (50 percent), even greatly increasing the owl's ability to 

discriminate source from sink habitats did not stabilize the 

population. 

Discussion 

Simulation results suggested that the San Gabriel/San Bernardino owl population plays a pivotal role in maintaining the 

southern California metapopulation. Unfortunately, the resident, 

territorial population of spotted owls in the San Bernardino 

Mountains has declined precipitously since 1987 (LaHaye et al. 

in press; Chapter 8). If the territorial population is in some sort of 

dynamic balance with a nonterritorial (floater) population, then 

these sorts of declines may be accommodated over the short-term 

and pose no long-term threat (see Franklin 1992). If these trends 

also characterize the other local populations, however, and were 

to persist for another 5-10 years, we believe the persistence of 

the entire metapopulation would be at risk. 

Several key factors remain unknown. For example, we do 

not know if these trends characterize the other local populations. 

If the decline in the territorial population in the San Bernardino 

Mountains is a response to region-wide, environmentally in202 Chapter 9 

duced variation, then other local populations may be responding 

in a synchronous fashion. The degree of environmental correlation within the metapopulation is also unknown. Regional rainfall patterns, however, suggest that the degree of correlation may 

be high (figs. 8A and 8B). The effects of environmental correlation on persistence among populations depends upon their life 

histories, including diet switching and changes in prey availability (for example, Horton and Wright 1944, Vogl 1967). Modeling efforts by Harrison and Quinn (1989) suggest that even 

strong environmental correlation in extinction risks will not 

have a major effect on persistence time for large vertebrates, 

except when local populations become very small-but see a 

counter argument in Stacey and Taper (1992). Our models all 

assumed a complete correlation among populations in their response to environmentally induced variation in fecundity. 

Persistence in metapopulations also depends critically on 

dispersal capabilities, distances between local populations, and 

risks involved in moving between habitat islands. The persistence of small, local populations with high turnover rates depends upon rescue by colonists from other populations in the 

metapopulation. This, too, is an area involving much uncertainty. The dispersal capabilities of California spotted owls and 

how they move through heterogeneous landscapes are mostly 

unknown. 

Given the uncertainties regarding synchrony of extinction 

risks among local populations, whether the declines in the size of 

the territorial population will continue into the future, and the 

dispersal capabilities of juvenile owls, we believe it is premature 

to estimate extinction likelihoods. Simple application of the 

current estimates of the vital rates for the San Bernardino population (table 8G), for example, would show a rapid collapse to 

extinction. Our approach has been to gain general insights into 

the management of owl habitat in terms of its quality, amount, 

and geometry. We did this by viewing population dynamics as 

emergent reflections of landscape pattern and the distribution of 

habitat types. The landscape model was particularly valuable 

because it allowed us to gain insights into the dynamics of 

individual populations related to specific locations on the landscape. 

Simulation results suggest that, in those parts of the species' 

range where suitable habitat constitutes only a small fraction of 

the landscape, populations are unstable and have low occupancy 

rates. The pattern is improved if the metapopulation contains a 

large source population. This result parallels the individual-territory 

model and predictions from Lande's (1987) modeling. That is, 

when the amount of suitable habitat becomes too small a percentage of the landscape, thresholds are encountered and populations may decline to extinction (fig. 9O). This occurs despite 

the continued presence of suitable habitat at these locations. 

The general predictions from the cluster model were also 

demonstrated by the landscape model. Specifically, when clusters (habitat islands) became too small, or suitable habitat too 

diffuse, occupancy rates dropped precipitously. Assuming moderate to strong risks to dispersing birds, only those clusters with 

more than 40 sites and largely contiguous habitat showed high 

occupancy rates (figs. 9M and 9N). For example, the isolated 

sites on the western slope of the San Gabriel Mountains dropped 

to low occupancy rates when dispersal costs were high (fig. 9O).

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Figure 9O-Landscape model simulation D, showing mean( + 1 standard deviation) 150-year trajectory of the metapopulation

(inset) and the mean pair occupancy rate of habitat cells (from fig. 9L) averaged across the time interval and based on 50 

simulations (parameter values used in the simulation are described in table 9D). 

Figure 9P-Landscape model "source reduction" simulation, showing mean (± 1 standard deviation) 150-year trajectory 

of the metapopulation (inset) and the mean pair occupancy rate of habitat cells (from fig. 9L) averaged across the time 

interval and based on 50 simulations (parameter values used in the simulation are described in table 9D). 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. Chapter 9 203 
Figure 9Q-Landscape model "source/sink" simulation showing mean ( ±1 standard deviation) 150-year trajectory of the 

metapopulation (inset) and the mean pair occupancy rate of habitat cells (from fig. 9L) averaged across the time interval 

and based on 50 simulations (parameter values used in the simulation are described in table 9D). 

The simulations also illustrated the interaction between size and 

spacing (fig. 9G). The islands near the San Gabriel/San Bernardino complex retained moderate occupancy rates even though 

they were small and dispersal risks were high (for example, the 

San Jacinto Mountains, 11 miles from the southern San Bernardino Mountains). In contrast, the population occupying the Los 

Padres Ranges, >30 miles from the western edge of the San 

Gabriel Mountains, declined to <40 percent mean occupancy 

even though this habitat island supports at least 65 pair sites. In 

general, the largest clusters with the shortest nearest-neighbor 

distances (table 9A) were the most stable and had the highest 

occupancy rates. 

A consistent finding from the simulations, and predictable 

from our simpler models, was the significance of the San Gabriel/ 

San Bernardino population to the dynamics of the entire 

metapopulation. Simulated declines in the amount of suitable 

habitat in this complex lowered occupancy rates throughout the 

entire metapopulation (fig. 9P). 

Population declines were particularly pronounced when half 

of the source habitat was changed to sink habitat and the owl's 

ability to discriminate among these habitats was limited (fig. 

9Q). As the ability to discriminate source from sink habitats 

increased, the rate of decline of the population became less 

pronounced, but with a 50 percent reduction in source habitat no 

positive equilibrium was attained. When selection is imperfect, 

some owls may settle in sink habitat and reduce the occupancy 

rate in source habitat. The result is a decline in population size, 

often with no positive equilibrium. This finding is consistent 

204 Chapter 9 

with results reported by Pulliam and Danielson (1991) based on 

a different model formulation. 

In some situations the presence of sink habitat may contrib.-

ute to metapopulation size (Pulliam and Danielson 1991, Howe 

et al. 1991). Whether sink habitat acts to enhance metapopulation 

persistence depends critically on the owls ability to discriminate 

between source and sink habitats. If some owls that would 

otherwise settle in source habitat, are attracted to, and settle in, 

sink habitat then the effects can be negative. The extent to which 

spotted owls can discriminate among habitat types in terms of 

their expected birth and death rates is unknown. It is unlikely, 

however, that habitat selection is perfect. Because search is 

finite, population trend ultimately depends upon the amount and 

distribution of source habitat, even when selection among source 

and sink habitats is perfect. 

In the case of the California spotted owl, the effects of 

landscape pattern on survival during dispersal may induce high 

levels of both spatial and temporal variation in juvenile survival 

rate. If habitat becomes more fragmented, the uncertainty of 

successful juvenile dispersal will become progressively more 

relevant to the likelihood of persistence. Even in a diffuse system with many suitable territories, pair occupancy will be low 

because of low recolonization rates. As a result, in a highly 

fragmented system, positive density-dependent growth rates usually associated with low population densities are unlikely. The 

effects of changes in landscape pattern on demographics are 

particularly significant if losses occur disproportionately in source 

habitat (fig. 9P). 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Habitat-induced population declines also occur at the scale of 

the individual pair site or territory. Territories, the basic unit for 

our models, can be thought of as small islands, each having a 

maximum of one reproducing pair. They are similar to islands in 

that (1) they have spatial dimension-they occupy a certain area of 

the landscape; (2) they have some level of habitat quality; and (3) 

when they experience local extinction (one or both members of the 

pair either dies or emigrates), they must be recolonized through 

immigration from outside the territory or by an existing, 

nonterritorial floater within the territory. 

The concept of territories as individual islands, with habitat 

quality explicitly defined by the expected values for birth and 

death rates, is key to understanding the dynamics of the spatial 

models. In all of the spatial models, search for suitable habitat is 

a sampling process. In the individual-territory model, the landscape is searched randomly and search success depends on the 

average density of suitable sites on the map. In the territory-cluster 

model, random search is confined to the clusters, which vary in 

the number and adjacency of suitable sites. Search success in 

this model depends primarily on the density of suitable territories within the cluster and secondarily upon the density of clusters in the landscape. In the landscape model, search success is 

defined at each time-step and for each owl by the density of 

suitable habitat in the territories immediately adjacent to the cell 

("territory") currently occupied. When three or more habitat 

types occur, each cell on the map is not only a location, but, in 

terms of search success, a habitat-quality state with specific 

transition probabilities controlling movement into alternate states. 

In a spatial model with search, a system composed of 

territorial clusters is more stable than a diffuse system because 

clusters produce regions where search efficiency is maximized. 

Circular clusters are the most stable because the density of 

suitable habitat is locally maximized. All other geometric forms 

will have lower search efficiency when compared with a circular 

cluster, unless clusters have strongly reflecting boundaries. In a 

cluster with a sufficient number of suitable territories, a population can recover from low occupancy because search efficiency 

will remain high. The key to stable populations within clusters is 

that they contain a sufficient number of high quality (= suitable) 

VLWHV (>@   WR VXSSRUW SRSXODWLRQ ODUJH HQRXJK WR DYRLG

local extinction due to stochastic demographic events. Surprisingly, it is possible that the presence of marginal or sink habitat 

may actually reduce population size even though some individuals may breed in these habitats (Pulliam and Danielson 1991, 

McKelvey et al. in press). 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

Management 

Recommendations 

Our recommendations for the persistence of spotted owls in 

the Southern California Province focus on the dynamics of its 

habitat at two spatial scales. First is the landscape scale. The 

arrangement of owls and owl habitat across the landscape shows 

that most of the population is concentrated in the San Gabriel/ 

San Bernardino Mountains complex. Every effort should be 

made to keep this population intact by maintaining the amount 

and spatial connectivity of suitable habitat there. Current demographic estimates suggest that the resident, territorial population 

is in significant decline. This population needs to be closely 

monitored for the foreseeable future, and attempts must be made 

to better understand the causes of the decline. 

Other smaller, local populations do not have the potential to 

provide a large number of colonists. These populations are 

important, however, in diminishing the risk that local populations would simultaneously experience adverse impacts (Den 

Boer 1968, 1981). Advantages are often gained by distributing 

populations widely across an extensive geographic area (Quinn 

and Hastings 1987, Gilpin 1990)-the degree of environmental 

correlation among local populations is reduced, as is the likelihood of simultaneous extinction events (see discussion in Stacey 

and Taper 1992). 

Local populations will continue to function as part of a 

larger metapopulation only if they remain connected through 

dispersal. If local populations become increasingly isolated by 

reduction in the sizes of habitat islands or by the creation of 

barriers to dispersal, occupancy rates decline because of a decline in the rate of demographic rescue, and the likelihood of 

extinction increases (table 3K; see discussion of problem areas 

in Chapter 3). Small, isolated populations will be the first to be 

lost to extinction, but even the largest, most continuous populations will experience increased risks if the overall size and 

spatial extent of the metapopulation is decreased. 

Options to decrease population isolation through management efforts are severely limited in southern California. Because 

the current metapopulation structure is largely the natural expression of vegetation patterns resulting from edaphic, topographic, and climatic constraints, little opportunity exists to 

enhance dispersal habitat between subpopulations. The obvious 

goal, therefore, is to avoid creating additional barriers to dispersal beyond those not amenable to management. 

The second level of concern is at the scale of the individual 

territory. It may be that the greatest threat to persistence of 

spotted owl populations is a subtle but continual decline in 

habitat quality (that is, a gradual conversion of source to sink 

habitat), on a site-by-site basis. As discussed in Chapter 8, subtle 

GHFOLQHV LQ  ZLOO EH GLIILFXOW WR GHWHFt except in the largest and 

longest-term demographic studies. Pulliam (1988) and Pulliam 

and Danielson (1991) concluded that knowledge of habitat-specific 

Chapter 9 205 
demographic rates is ultimately needed for the effective management of wild populations. We agree. We must be able to 

target for preservation those habitats needed today for the species' persistence, and learn how to manage for such habitats in 

the future. Only by understanding the relations between demographic rates and the structure and composition of vegetation at 

the stand level can we be certain of maintaining habitat that 

provides for a stable or growing population. 

Both scales of research and management-landscape and 

territory-are equally important, and both must be pursued to 

address the persistence requirements of spotted owls in southern 

California. 

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206 Chapter 9 

Harrison, Susan; Quinn, James F. 1989. Correlated environments and the 

persistence of metapopulations. Oikos 56:293-298. 

Horton, Jerome S.; Wright, John T. 1944. The woodrat as an ecological factor 

in southern California watershed. Ecology 25:341-351. 

Howe, Robert W.; Davis, Gregory J.; Mosca, Vincent. 1991. The demographic 

significance of `sink' populations. Biological Conservation 57:239-255. 

Jolly, G. M. 1965. Explicit estimates from capture-recapture data with both 

death and dilution-stochastic model. Biometrika 52:225-247. 

LaHaye, William S., Research Associate, Wildlife Department, Humboldt 

State Univ., Arcata, CA. [Personal communication]. February 1992. 

LaHaye, William S.; Gutierrez, R. J.; Call, Douglas R. 1992. Demography of 

an insular population of spotted owls (Strix occidentalis occidentalis). In: 

McCullough, Dale; Barrett, Reginald H., eds. Wildlife 2001: populations. 

New York: Elsevier Press; 803-814. 

Lamberson, Roland H.; McKelvey, Robert; Noon, Barry R.; Voss, Curtis. The 

effects of varying dispersal capabilities on the population dynamics of the 

northern spotted owl. Conservation Biology. [In press]. 

Lamberson Roland H.; Noon, Barry R. 1992. The territory cluster model: a 

spatial model for the northern spotted owl. Unpublished draft supplied by 

the authors; 21 p. 

Lande, Russell. 1987. Extinction thresholds in demographic models of territorial populations. American Naturalist 130:624-635. 

Lande, R. 1988. Demographic models of the northern spotted owl (Strix 

occidentalis caurina). Oecologia 75:601-607. 

Marcot, Bruce G.; Holthausen, Richard. 1987. Analyzing population viability 

of the spotted owl in the Pacific northwest. Transactions of North American Wildlife and Natural Resources Conference 52:333-347. 

McKelvey, Kevin; Noon, Barry R.; Lamberson, Roland H. Conservation 

planning for species occupying fragmented landscapes: the case of the 

northern spotted owl. In: Kingsolver, J.; Kareiva, P.; Huey, R., eds. Biotic 

interactions and global change. Sunderland, MA: Sinauer Associates. [In press]. 

Moen, Christine A.; Franklin, Alan B.; Gutierrez, R. J. 1991. Spotted owl 

age-class determination. Wildlife Society Bulletin 19:489-493. 

Murphy, Dennis D.; Noon, Barry R. 1992. Integrating scientific methods with 

habitat conservation planning: reserve design for the northern spotted owl. 

Ecological Applications 2:3-17. 

Pollock, Kenneth H.; Nichols, James D.; Brownie, Cavelle; Hines, James E. 

1990. Statistical inference for capture-recapture experiments. Wildlife Monographs 107:1-97. 

Pulliam, Ronald H. 1988. Sources, sinks, and population regulation. American 

Naturalist 132:652-661. 

Pulliam, Ronald H.; Danielson, Brent J. 1991. Sources, sinks, and habitat 

selection: a landscape perspective on population dynamics. American 

Naturalist 137:S50-S66. 

Quinn, James F.; Hastings, Alan. 1987. Extinction in subdivided habitats. 

Conservation Biology 1:198-208. 

Seber, G. A. F. 1965. A note on the multiple recapture census. Biometrika 

52:249-259. 

Stacey, Peter B.; Taper, Mark. 1992. Environmental variation and the persistence of small populations. Ecological Applications 2:18-29. 

Stephens, F. 1902. Owl notes from southern California. Condor 4:40. 

Stephenson, John. 1991. Spotted owl surveys on the National Forests of 

southern California: a status report and recommendations for the future. 

San Bernardino, CA: Forest Service, U.S. Department of Agriculture; 13 p. 

Thomas, Jack Ward; Forsman, Eric D.; Lint, Joseph B.; Meslow, E. Charles; 

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USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Chapter 10 

General Biology of Major Prey Species 

of the California Spotted Owl 

Daniel F. Williams, Jared Verner, Howard F. Sakai, and Jeffrey R. Waters 

Full understanding of the habitat relations of California 

spotted owls depends, in part, on knowledge of the habitat 

relations of their primary prey species. For example, the northern flying squirrel is the primary prey of the owl in conifer 

forests of the Sierra Nevada, comprising as much as 61 to 77 

percent of the total biomass eaten in some localities and seasons 

(table 4A). The dusky-footed woodrat is the primary prey in 

lower-elevation forests and woodlands of the Sierra Nevada and 

throughout all habitats in southern California, making up 74-94 

percent of the diet, by weight, in various areas. Current evidence 

indicates that suitable nest sites and the most common foods of 

northern flying squirrels are usually found together in mature 

and older forests, which may help us understand why spotted 

owls forage more often in such forests. Woodrats are typically 

associated with shrubfields, especially those dominated by 

thick-leaved, evergreen species. Spotted owls in the Sierran 

foothills and in southern California commonly occur in forests 

and woodlands with a light to moderate shrub understory, or that 

adjoin more extensive stands of chaparral. In addition, 

radio-tracking studies of spotted owls in the Sierra NF have 

shown that their home ranges in Sierran mixed-conifer forests 

are measured in thousands of acres, but those in foothill riparian/ 

hardwoods are measured in hundreds of acres (Neal et al. 1990). 

This difference is probably related, at least in part, to the facts (1) 

that woodrat densities are generally several times higher than 

flying squirrel densities and (2) that woodrats weigh nearly 

twice as much as flying squirrels. 

Here, to expand a general understanding of spotted owl 

ecology, we present brief biological descriptions of several species of small to mid-sized mammals that most commonly occur 

in diets of the California spotted owl (see Chapter 4). 

Dusky-Footed Woodrat 

Distribution and Habitat 

Dusky-footed woodrats occur in the Pacific Coast region 

from the Oregon side of the Columbia River to northern Baja 

California. Within the range of the California spotted owl, they 

inhabit coastal, piedmont, and montane chaparral and forest 

communities. Evergreen or live oaks and other thick-leaved 

shrubs are important habitat components throughout this woodrat's 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

geographic range (see color photo 5-29). They are most numerous where shrub cover is dense, and least abundant in open areas 

(Fitch 1947). They are one of few small mammal species of 

chaparral habitats that flourish in old, dense stands (Quinn 1990). 

Habitats that are unsuited or poorly suited for dusky-footed 

woodrats include open grasslands or fallow, weedy ground; 

sparsely wooded forests; woodlands solely of conifers or with 

little shrub understory; and pure stands of chamise, manzanita, 

or ceanothus (Linsdale and Tevis 1951). 

In the Sierra Nevada, this woodrat occurs generally below 

5,000 feet in elevation (lower in the north-about 3,300 feet at 

Mt. Lassen and 4,000 feet in Yosemite National Park (NP), and 

higher to the south-rare at 6,000-6,500 feet in the Kern River 

drainage). It occupies foothill riparian/hardwoods in the northern San Joaquin Valley. The highest capture rates of woodrats in 

the foothills of the west-central Sierra Nevada were in chaparral, 

woodland, and forest communities with a mix of overstory trees 

and shrubs (table 10A). These results appear to agree with those 

of Sakai and Noon (1992a), who indicate that dusky-footed 

woodrats in northwestern California are most abundant in brushy 

stands of sapling/early-aged poletimber. In the southern Sierra 

Nevada (Kern County), in a chaparral community of ceanothus 

and interior live oak between 2,560 and 3,200 feet in elevation, 

woodrats were most often trapped around patches of rock gooseberries, and their nests were common where gooseberry thickets 

encircled rock outcrops or dead snags (Lawrence 1966). 

In the San Bernardino Mountains, the dusky-footed woodrat 

occurs on both the Pacific and desert slopes, ranging from about 

1,600 feet on the Pacific slope and 3,800 feet on the desert slope 

up to at least 8,000 feet on both sides, where it is the primary 

prey species of the spotted owl (LaHaye pers. comm.). Grinnell 

(1908) found most nests in California scrub oak and pinyon 

associations and few along willow-lined canyons. They occur in 

big sagebrush/pinyon-juniper woodlands in the New York-Providence mountain chain in eastern San Bernardino County. 

In the San Gabriel Mountains, dusky-footed woodrats occur 

on both the Pacific and desert slopes, exhibiting the same 

elevational distribution as in the San Bernardino Mountains. In 

the coastal sage belt, they are restricted to areas close to intermittent streams supporting tall shrubs or small trees. Nests are built 

mostly of white sage in isolated clumps of lemonade sumac. 

Population densities -generally are low in this community 

(M'Closkey 1972). Higher numbers are found in adjacent canyons on densely vegetated slopes. At higher elevations on the 

desert side, favored spots for nests are thickets of chokecherries, 

mountain whitethorn, and currants. In the pinyon-juniper woodlands, both conifer species were used for nest sites; but CaliforChapter 10 207 
Table l0A-Captures of small mammals in snap traps (Museum Specials and Victor rat traps) 

in various habitats and seral stages in the western Sierra Nevada, California, based on 

sampling in Amador, Calaveras, Mariposa, Nevada, Placer, and Tuolumne counties. Values 

are captures per trap night1

 (adapted from Williams and Johnson 1979). 

Species captured 

Habitat and Dusky-footed Brush California Deer Pinyon 

 Seral stage woodrat mouse mouse mouse mouse 

Annual grassland 0 0 0 0.0910 0 

Chaparral 

 Grass/forb 0 0 0 0.2340 0 

 Light shrub 0.0039 0.0133 0 0.0261 0.0011 

 Dense shrub 0.0098 0.0031 0 0.016 0.0004 

Oak/digger pine 

 Seral 0.0007 0 0 0.0074 0 

 Mature 0.0052 0.0319 0.0007 0.0059 0 

Ponderosa pine 

 Seral 0.0011 0.0015 0 0 0.0011 

 Mature 0.0007 0 0 0.0020 0 

Mixed-conifer 

 Seral 0.0048 0.0045 0 0.0003 0.0003 

 Mature 0.0030 0.0052 0 0.0015 0 

Riparian/hardwood 

 Low elevation 0.0044 0.0044 0 0.0022 0.0011 

 Mid-elevation 0 0 0 0.0007 0 

1

 Data from 33 transects; total trap nights = 19,824. 

nia scrub oak seemed to be preferred wherever it occurred. They 

sometimes build no visible nests where talus is available (Vaughan 

1954), although careful examination usually reveals clipped 

branches adjacent to crevice or tunnel entrances (Sakai and 

Noon 1992a). 

In central coastal areas, dusky-footed woodrats appear to 

prefer closed woods on drier sites, including a high percentage 

of live oaks with a mixed shrub understory (California coffeeberry 

and poison oak are the most prevalent shrubs). North-facing 

slopes meet these conditions best in the area around Hastings 

Natural History Reservation (hereafter, Hastings Reservation) at 

about 1,500-2,500 feet in elevation, where intermittent streams 

with willows also provide high-quality habitat (Linsdale and 

Tevis 1951). Overhead branches and downed logs often provide 

woodrats with a means of traveling above ground level; this 

appears to be an important structural component of the habitat for 

some populations (as at Hastings Reservation) but not for others. 

Woodrats radio-tagged by Sakai and Noon (1992b) sometimes moved in the evenings as far as 165 feet into old-growth 

forests adjoining their home ranges in shrublands. Sakai and 

Noon did not determine how long these woodrats remained in 

the old-growth, or what they did there, but generally they were 

back in their nests in the shrublands by the following morning. 

Two radio-tagged woodrats dispersed through old-growth forest 

from their natal home range into another shrubfield, in one case 

a distance of at least 650 feet. Such movements by woodrats 

208 Chapter 10 

would make them more available as prey for species like spotted 

owls that frequent these older forests. 

Patterns of Abundance 

Reported densities of dusky-footed woodrats range "from just 

a few animals to >40 per acre in early pole-timber stages, to 

perhaps 0.4 to 1.2 per acre in large saw timber and old growth" 

(Thomas et al. 1990, p. 207). In a corridor of habitat measuring 

approximately 100 by 1400 feet along an intermittent stream, 

Linsdale and Tevis (1951) trapped about 30 different individuals per 

month in one year and about 66 per month in another. These results 

suggest woodrat "densities" of about 9.3 and 20.6/acre-more than 

a two-fold difference between years. Densities in undisturbed 

habitats ranged from 2.1/acre in open woodlands of canyon live 

oaks and scattered Pacific madrones in the Santa Cruz Mountains, Santa Clara, County (Merritt 1974), to 18.3/acre in a 

riparian/hardwood forest of red alders, willows, and elderberries 

in coastal Sonoma County (Wallen 1982, Carraway and Verts 

1991). Farther inland in Sonoma County, densities were 8.1/acre 

in late summer and 5.7/acre in winter in an undisturbed, riparian 

deciduous woodland dominated by red alder, California boxelder, 

and willows (Cranford 1977). In a study on the San Dimas 

Experimental Forest in the San Gabriel Mountains (Horton and 

Wright 1944), mean densities of woodrat houses were 4.6/acre 

in an area primarily of chaparral and oak woodland, 1-4/acre in 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
chamise chaparral, 10/acre in riparian/hardwood communities, 

and 10/acre in unburned oak/chaparral and mixed chaparral 

above 4500 feet; a golden oak woodland with nearly complete 

canopy cover and almost no shrub understory had almost no 

woodrats. Chew et al. (1959) found 16 dead woodrats per acre in 

a burned canyon bottom dominated by oaks and California 

sycamores in south-coastal California. 

Various studies have reported effects of habitat change on 

densities of dusky-footed woodrats. Woodrats declined significantly during a prolonged drought in the Santa Monica Mountains, Los Angeles County (Spevak 1983), and Linsdale and 

Tevis (1951) reported depressed numbers during a drought at 

Hastings Reservation. On the other hand, Kelly (1989) reported 

a dramatic population increase during a serious drought at Hastings 

Reservation in 1988. He attributed this to a large acorn crop in 

the autumn of 1987, possibly augmented by mild weather conditions. Removal of poison oak from the understory depressed 

population density in the Berkeley Hills, Alameda County (Vestal 1938); flood, browsing, and trampling of the understory by 

ungulates reduced woodrat numbers at Hastings Reservation 

(Linsdale and Tevis 1956). Following complete removal of 

shrubs from study plots in chaparral cover in coastal dunes near 

Morro Bay, San Luis Obispo County, a woodrat population 

declined sharply in the first 2 years following treatment and 

disappeared entirely by the third year. Areas undisturbed by fire 

had higher densities of woodrats than burned areas (Lee 1963, 

Gambs and Holland 1988). Studies by Wirtz et al. (1988), in an 

area of montane chaparral that burned in southern California, 

established that preburn densities of woodrats had not yet been 

reached 4 years after the burn. Postburn densities were higher in 

areas of light and normal burn than in areas with hot burns where 

all plant material was destroyed. 

A suspected outbreak of plague in 1966-67 decimated woodrat 

populations in foothills of the southern Sierra Nevada, the Tehachapi 

Mountains, and the Coast Range (Murray and Barnes 1969). 

Cranford (1977) reported significantly larger home ranges 

for males than females (0.59 vs 0.48 acre) in a riparian woodland 

bordered by grasslands and surrounded by redwood forest. Kelly 

(1989) found the same situation in riparian/hardwoods in Monterey 

County. Sakai and Noon (1992b) found a similar difference, 

though not statistically significant, among woodrats in shrubfields 

dominated by brushy tanoaks with an overstory of Douglas-fir in 

northwestern California. Depending on the spacing of nest clusters (color photo 5-25), females often shared portions of the 

same home range; consequently home ranges of breeding males 

can overlap those of several females. 

Diet 

The herbivorous dusky-footed woodrat apparently obtains 

most or all of its water from its food. It eats parts of a wide 

variety of plant species, but the water-rich leaves of thick-leaved 

shrubs found throughout the woodrat's range are probably the 

most important source of food. The bulk of the diet consists of 

leaves and the terminal shoots of twigs, with seasonally important food sources consisting of flowers, fruits (nuts, seeds, fleshy 

fruits, and so on) and fungi. Bark, wood, and other organic 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

materials are also eaten occasionally. Fruits, fungi, and leaves 

are often cached within nests. 

Linsdale and Tevis (1951) found that acorns and fruits of 

California coffeeberry were most numerous in caches examined 

at Hastings Reservation. Leaves and other parts of coast and 

canyon live oaks, California blackberry, chamise, California 

coffeeberry, buckbrush, and Jim brush were the major plants 

eaten. Somewhat less common in the diet were valley, blue, and 

black oaks; California wild rose; toyon; poison oak; Pacific 

madrone; and mountain whitethom. Parts of 56 other plant 

species were consumed (amounting to about 10 percent of the 

total diet), while 470 plant species (87 percent of the flora at 

Hastings Reservation) were not found in the diet of this woodrat. 

In Joshua Tree National Monument, about 75 percent of the 

material in food caches in nests .consisted of shrub live oak and 

about 25 percent was California juniper (Cameron 1971). These 

proportions were the same even where the dusky-footed woodrat 

occurred together with the desert woodrat. The main foods in 

southern California coastal sage communities were lemonade 

sumac (fruit, seeds, vegetative parts), California buckwheat (vegetative parts), white sage (seeds, flowers, vegetative parts), and 

California scrub oak (leaves, fruits) (Meserve 1974). 

Weights 

Specimens from the western Sierra Nevada ranged in weight 

from 7.2 to 8.6 ounces (Grinnell and Storer 1924). Winter and 

summer weights in samples from foothill areas in San Diego 

County averaged 7.7 ± 0.7 and 6.5 ± 0.4 ounces, respectively 

(Stallone 1979). Sakai and Noon (1992b) reported a mean weight 

of 7.8 ± 0.14 ounces for a pooled sample of both sexes and all 

ages (n = 366) in Humboldt County. Adult males there averaged 

10.7 ± 0.14 ounces (n = 101), and adult females averaged 8.5 ± 

0.09 ounces (n = 133). At Hastings Reservation (Linsdale and 

Tevis 1951), adult males averaged 8.8 ounces (range 7.1-10.6) 

and adult females averaged 8.4 ounces (6.4-12.5). These rodents 

exhibit marked individual and seasonal variation in weight (table 

10B). Immature woodrats weighing <5.3 ounces were trapped 

Table 10B-Weights (mean and range in ounces) of all ages of male dusky footed 

woodrats captured each month during a year at Hastings Natural History Reservation, Monterey County, California (adapted from Linsdale and Tevis 1951). 

Month n Mean Range 

January 68 8.9 7.0 - 12.0 

February 67 9.3 4.9 -13.6 

March 94 9.2 5.4 - 13.0 

April 106 9.0 2.6 - 13.3 

May 77 8.5 3.2 - 13.2 

June 83 7.8 2.7 - 11.6 

July 97 7.0 1.9 - 11.3 

August 54 7.1 1.9 - 11.4 

September 92 7.9 3.2 - 13.3 

October 58 8.0 3.5 - 11.3 

November 66 8.6 3.5 - 12.7 

December 91 9.0 3.7 - 12.9 

Chapter 10 209 
only between April and September. Immatures weighing from 

3.5 to 7.0 ounces were trapped in all months but were taken most 

often in June and July. 

Nests 

Nests of sticks and other woody debris are typically located 

on the ground, occasionally in trees (color photo 5-23) or dense 

shrubs where support for the structure is available, and sometimes in rock crevices and abandoned human structures. Linsdale 

and Tevis (1951) summarized nest locations at Hastings Reservation as follows: 

Most often Bases of coast live oaks, California coffeeberry, 

willows, poison oak, California buckeye, 

California-laurel (bay). 

Less often Against logs, in rock outcrops (probably be 

cause of a lack of appropriate rocks or insuffi 

cient cover plants in areas studied-rocks are 

important in the San Gabriel Mountains and the 

southern Sierra Nevada), hollow cavities in trees 

(perhaps because of rarity and difficulty of de 

tecting such nests), and among limbs of trees (trees 

with the right configuration of large and 

small branches to support nests may be pre 

ferred over the ground). 

The structure of the plant community where nests were 

located at Hastings Reservation was described by Linsdale and 

Tevis (1951) as a mixed woodland with a mosaic of dense shrubs 

and trees, forming a complete and complex (multilayered) canopy: 

Most often Closed woodlands consisting predominately of 

coast live oaks (59 percent; n = 100). 

Less often Dense shrubs (28 percent), especially where 

California coffeeberry and poison oak were 

most abundant. 

Rare (<10 pct) Lone coast live oak trees (5 percent); live oak 

savanna (8 percent) (blue oak and valley oak 

savannas were not used for nests). 

Nests are an important part of woodrat population dynamics. Nest clusters, occupied by related individuals, are common 

in favored habitats. Females, unlike males, stay in or near their 

natal area throughout their life, where related females breed in 

the same vicinity, living close together in kin clusters but in 

separate houses (Kelly 1989). In addition, individuals tend to 

cluster in favored habitat patches; consequently, such favored 

areas tend to become "crowded" over time (Linsdale and Tevis 

1951). Vogl (1967) reported one adult per nest, but Sakai and 

Noon (1992a) have occasionally captured two adults per nest in 

northwestern California. 

Reproduction and Development 

Linsdale and Tevis (1951) found that 70 percent of the 

woodrats in their study area at Hastings Reservation survived 

less than 1 year, 27 percent survived 2 years, and 3 percent 

survived at least 3 years. Reproduction occurred in all months at 

Hastings, with the fewest pregnancies in December and the most 

in February. The number of juveniles appearing outside the nest 

210 Chapter 10 

was greatest in July and least (0) in January and February. 

Females were polyestrous, producing one to five litters per year, 

with one to four young per litter (mean about 2.5). 

Forest Management 

Fires, shrub removal, logging, and other human and natural 

disturbances generally reduce the suitability of woodrat habitat. 

Selective cutting of trees that opens the canopy and promotes 

growth of shrubby understory probably enhances habitat after 

several years, as do other logging techniques that promote successional stages with a complex mix of over- and understory 

trees and shrubs (Hooven 1959). The short-term effect, however, 

probably would be to reduce habitat suitability for woodrats. 

Although studies by Sakai and Noon (1992b) indicate that 

woodrats sometimes move from shrubfields into the edges of 

old-growth forests, it cannot be argued that logging to create 

openings would result in a net benefit for spotted owls in the 

conifer zone of the Sierra Nevada. First, available data from 

radio-tracking studies indicate that spotted owls seldom forage 

in shrubfields (Neal et al. 1989, Sisco 1990, Solis and Gutiérrez 

1990, Zabel et al. 1992); and second, dusky-footed woodrats are 

generally uncommon as high as the mixed-conifer zone in the 

Sierra Nevada, where most logging currently occurs. Flying 

squirrels are the dietary staple of spotted owls in forests at these 

higher elevations, so logging there is more likely to have a 

negative effect on owl prey (via flying squirrels) than a positive 

one (via woodrats). 

In forests below the Sierran mixed-conifer zone, small-scale 

logging operations might benefit spotted owls by enhancing 

woodrat populations. Although this needs further study, smaller 

sales might benefit spotted owls if done in areas adjacent to 

forested stands where the owls are known to forage. In such 

cases, woodrats that occasionally wander from their shrubby 

home ranges into the adjoining forest could become available as 

prey for spotted owls. 

Woodrats do not survive fire well, especially very hot burns 

(Wirtz et al. 1988), and they are slow to recolonize burned areas 

(Longhurst 1978, Wirtz et al. 1988). Consequently, aggressive 

fuels management programs in chaparral country can benefit 

woodrat populations, especially in southern California where 

home ranges of owls in riparian/hardwood forests are closely 

surrounded by thick stands of chaparral (Chapter 5). 

Northern Flying Squirrel 

Distribution and Habitat 

The northern flying squirrel is a medium-sized, nocturnal 

rodent that nests in trees in a great variety of forest communities 

over a broad, continental distribution. In California they occur in 

the North Coast, Klamath, southern Cascade, Sierra Nevada, and 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Transverse Ranges. They are distributed throughout forested 

regions of the Sierra Nevada but apparently are more common in 

the mixed-conifer and red fir forests of the Pacific Slope than in 

the drier forests of the east slope. They are generally found 

above about 4,000 feet elevation in the Yosemite region and 

down to about 3,000 feet or lower in the northern Sierra Nevada, 

and in protected canyons and on north-facing slopes farther 

south. A single record from Chico is probably exceptional, but it 

suggests that flying squirrels may sometimes occur near the 

floor of the Central Valley in riparian/hardwood forests. Isolated 

populations also occur in the San Bernardino and San Jacinto 

mountains and probably in the San Gabriel Mountains of southern California. 

Unfortunately, little published information is available on 

habitat associations or population levels of flying squirrels within 

the range of the California spotted owl. In the Sierra Nevada, 

common tree species associated with flying squirrels are black 

oak, white fir, and red fir. In the Lassen area, McKeever (1960) 

found flying squirrels in stands of ponderosa pine, lodgepole 

pine, and mixed stands of red and white fir. According to Waters 

and Zabel (1992), populations of flying squirrels have been 

located in second-growth stands of white fir at high elevation 

(about 6,300 feet) in the Lassen NF (color photo 5-1). These 

squirrels often travel and forage on the ground, so elements of 

the forest understory also are probably important in determining 

the suitability of their habitat. 

In the San Bernardino Mountains, flying squirrels occur in 

mixed-conifer forests between about 5,200 and 7,500 feet in 

elevation (color photo 5-35). White fir and black oak are the 

principal tree species associated with these squirrels in the San 

Bemardinos (Grinnell 1908, 1933; Williams 1986). On a ridge 

south of Big Bear Lake, Summer (1927) caught 22 flying squirrels over the course of several months-all in white firs. 

Stand size was an important attribute of suitable flying 

squirrel habitat in mature mixed-evergreen forests dominated by 

Douglas-fir on the Six Rivers NF. Northern flying squirrels were 

found in 60-80 percent of stands larger than 50 acres, on about 

15 percent of stands of 25-50 acres, and on <10 percent of stands 

smaller than 15 acres (Rosenberg and Raphael 1986). 

Sites with northern flying squirrels in isolated populations 

in the southern Appalachians varied markedly in plant community structure and composition (Payne et al. 1989). Occupied 

sites were commonly on north-facing slopes or in montane 

islands of conifer forests with cooler, mesic environments. Density of overstory trees varied from 364 to 1336 per acre; density 

of snags ranged from 11 to 138 per acre; and understory cover 

ranged from 35 to 86 percent. In a study of the southern flying 

squirrel in central Virginia, Sonenshine and Levy (1981) concluded that areas with few shrubs or vines as ground cover were 

unsuitable as habitat. An oak or oak-associated canopy with an 

understory of dense shrubs was optimal habitat. Presence of the 

squirrels was strongly correlated with shrub density. Although 

major differences exist in the distribution and habitats of northern 

and southern flying squirrels, the study by Sonenshine and Levy 

suggests that understory may be as important as tree canopy in 

determining habitat suitability. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

Although Doyle (1990) captured similar numbers of flying 

squirrels in riparian and upland habitats, she concluded that 

riparian habitats were, nonetheless, superior to upland sites for 

flying squirrels. Waters and Zabel (1992) have found relatively 

high densities of flying squirrels in forest stands on the Lassen 

NF that are not near running water. 

Patterns of Abundance 

No published data are available on population densities, age 

structure, or reproduction in the Sierra Nevada or the mountains 

of southern California. A summary of most available literature 

indicated that "typical squirrel densities reported for mature and 

old-growth forests are 0.4 to 1.2 animals per acre" (Thomas et al. 

1990, p. 205). Carey et al. (1992) found that flying squirrel 

density in southwestern Oregon was significantly greater in 

old-growth Douglas-fir stands (mean density = 0.8/acre) than in 

managed second-growth stands (mean = 0.4/acre). On the other 

hand, Rosenberg and Anthony (in press) failed to show significant differences between flying squirrel densities in old-growth 

(mean = 0.9/acre) and second-growth Douglas-fir stands (mean = 

0.8/acre). Waters and Zabel (1992) found that average flying 

squirrel density was about 43 percent higher in late-seral red fir/ 

white fir stands on the Lassen NF (range = 0.7-1.5/acre) than in 

red fir/white fir stands that were about 100 years old (range = 

0.6-1.0/per acre). They have also found that flying squirrel density is strongly associated with the abundance of truffles-fruiting 

bodies of underground (hypogeous) fungi. 

Diet 

The diet of northern flying squirrels, at least as determined 

in California studies, consists primarily of truffles and arboreal 

lichens (McKeever 1960, Hall 1991, Waters and Zabel 1992), 

although they are known to eat a variety of other foods including 

seeds, nuts, insects, bird eggs and nestlings, and tree sap 

(Wells-Gosling and Heaney 1984). Maser et al. (1985) found 

that 90 percent or more of foods eaten in Oregon were fungi and 

lichens-hypogeous fungi accounted for more than 80 percent 

of the summer diet, and lichens comprised more than 50 percent 

of the diet year-round in northeastern Oregon. At Sagehen Creek, 

in the eastern Sierra Nevada, Hall (1991) found that spores of 

hypogeous fungi were the most common items found in feces 

and stomach samples year-round, but suspected that samples 

taken during deep snow cover indicated that the squirrels may 

store hypogeous fungi for consumption during winter months. 

Lichens and gill fungi were most prevalent during periods when 

snow covered the ground at Sagehen Creek. Hall considered 

arboreal lichens to be a very important winter food source for 

flying squirrels in areas with much snowfall (also see McKeever 

1960). From studies of captive-reared animals, Laurance and 

Reynolds (1984) determined that winter diets consisting almost 

wholly of lichens may be more a matter of necessity than of 

preference for northern flying squirels. The captives selected 

pine seeds over lichens, moss, algae, and cones and branch tips 

from ponderosa pine. 

Chapter 10 211 
Weights 

Size varies significantly in a north-south cline along the 

Pacific Coast (Wells-Gosling and Heaney 1984), with the largest individuals in Alaska and British Columbia and the smallest 

in California. Weights of flying squirrels captured in Yosemite 

NP ranged from 3.62 to 5.76 ounces (Grinnell and Storer 1924). 

Juveniles captured in August and September in the Lassen NF 

averaged 2.89 ounces, and adults averaged 4.34 ounces (Waters 

and Zabel 1992). In a study by Witt (1991) in Douglas County, 

Oregon, the mean weight of adults captured between September 

1983 and June 1984 was 4.7 ± 0.1 ounces (range = 3.7-6.5 

ounces, n = 164). Generally the mean weight of adults was 

highest in January (mean = 5.0 ounces), dropping steadily to 

April, remaining stable from April through August (mean = 4.4 

ounces), and increasing again through December. 

Nests 

Northern flying squirrels use several den sites; Carey (1991) 

found individuals in Oregon that used as many as seven. Two 

types of nests or dens are common-those located among branches 

of trees (for example, stick nests built by birds or other tree 

squirrels, clumps of dwarf mistletoe, and moss), and those located in natural cavities in trees and snags or abandoned woodpecker holes (Wells-Gosling and Heaney 1984). In Oregon, live 

conifers with cavity nests averaged 49 inches in d.b.h.; snags 

with cavity nests averaged 35 inches in d.b.h. (Carey 1991). 

Nests in such cavities are probably important in areas with cold 

winters (Cowan 1936, Weigl 1978), although Waters and Zabel 

(1992) have found populations of flying squirrels in high-elevation 

(about 6,300 feet) stands of second-growth white fir where few 

snags or cavities occurred. We do not know how commonly 

flying squirrels build their own nests. 

Reproduction and Development 

Litters commonly consist of two to four young, rarely one to 

six (Wells-Gosling and Heaney 1984). In Yosemite NP, females 

with two to four embryos were found in June (Grinnell and 

Storer 1924). Young are born between mid-June and mid-August 

in Oregon (Carey 1991). Weaning occurs at an age of about 60 

days. Carey reported that young in an Oregon study were not 

weaned until mid-October to mid-November; they either dispersed in autumn or spent the winter in the nest with their 

mother. Young can walk and begin to leave the nest when about 

40 days old. Most yearling females did not breed, and about 25 

percent of the adult females did not breed in a given year. 

Although several authors have suggested that more than one 

litter is produced per year (Grinnell and Storer 1924, Witt 1991), 

a single litter is probably more common, at least throughout the 

Sierra Nevada (Waters and Zabel 1992). In the Sierra Nevada, 

two "half-grown" young were captured on 31 October, and a 

"quarter-grown" young was found on 16 September (Grinnell 

and Storer 1924, p. 214). 

212 Chapter 10 

Forest Management 

Habitat features that most strongly influence flying squirrel 

abundance include: sufficient trees to enable efficient locomotion; nest and den site substrates (cavity-bearing trees and snags), 

and truffle and arboreal lichen biomass. Although flying squirrels 

can glide at least 155 feet (Mowrey and Zasada 1984, J. R. Waters 

pers. observ.), forestry practices that create openings wider than 

about 120 feet probably have a negative effect on flying squirrel 

locomotion. Tree height is also important. Flying squirrels cannot 

glide as far from small trees as they can from tall ones. 

As cavities provide important nest and den sites, efforts 

should be made to leave cavity-bearing trees and snags. In areas 

lacking potential nest sites, it may be possible to increase flying 

squirrel populations by adding nest boxes. 

Truffles and arboreal lichens are the most important food 

types for flying squirrels throughout California and in the Pacific 

Northwest. Arboreal lichens are especially important as a winter 

food resource. Forest practices that reduce truffle and lichen 

biomass will probably negatively impact flying squirrel abundance. Ongoing research by Waters and Zabel (1992) indicates 

that truffle biomass is strongly associated with the presence of a 

well-developed soil organic layer and the volume of decaying 

logs (color photos 5-7, 5-18, and 5-34). Forest practices that 

negatively impact those parameters, such as broadcast burning 

and bulldozer piling after logging (Harvey et al. 1980), will 

reduce the capability of the forest to sustain flying squirrels. 

Data from Waters and Zabel (1992) also show that arboreal 

lichens (in the genera Letharia and Bryoria) commonly eaten by 

flying squirrels are much more abundant in older red fir/white fir 

forests than in younger forests. 

Management of conifer forests in the Sierra Nevada for 

flying squirrels should emphasize retention of large snags and 

older trees, and nonintensive site-preparation techniques. 

Pocket Gophers 

Distribution and Habitat 

Two species of pocket gophers occur within the main 

geographic range of the California spotted owl. Mountain pocket 

gophers range from the Mt. Shasta Region southward in the Sierra 

Nevada to at least the northern boundary of Tulare County (Hall 

1981). They generally occur from above 6,900 feet in the Sierra 

NF (5,600 feet in the Stanislaus NF and Yosemite NP) to slightly 

above timberline (Grinnell and Storer 1924; D. F. Williams, pers. 

observ.). They are found throughout subalpine areas of both 

Sequoia and Kings Canyon NPs (Graber, pers. comm.), generally 

at elevations above 8,500 or 9,000 feet that are not frequented by 

spotted owls. They are most common in deeper, drier soils around 

meadow margins, but they occur everywhere 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
except on bare rock and within closed-canopied, mature and older 

forests with little or no herbaceous ground cover (Ingles 1952). 

Southwestern pocket gophers occur in the western Sierra 

Nevada at elevations below the mountain pocket gopher, and in 

the lowlands, mountains, and deserts of western and southern 

California and northern Baja California. They probably range to 

timberline in the southern Sierra Nevada, from Tulare County 

southward. Distribution records of pocket gophers above 6,900 

feet in the southern Sierra Nevada are unavailable, although one 

or the other species surely is found there (Hall 1981). Southwestern pocket gophers are most common on open ground with 

well-drained soils supporting grasses and forbs, but they can be 

found everywhere except on bare rock and in closed-canopied, 

mature and older forests. 

Great Basin pocket gophers occur on the eastern slopes of 

the Sierra Nevada and on the Modoc Plateau. We do not discuss 

them in detail here because they probably occur mostly outside 

the breeding range of the California spotted owl, and because 

features of their population dynamics, habitat, and diet that are 

of importance to spotted owls probably do not differ from those 

of other pocket gophers. Generally, the ranges of these three 

species of pocket gophers do not overlap. 

Patterns of Abundance 

Mean density of mountain pocket gophers in favored meadow 

habitats was estimated at 10/acre in autumn over a 4-year period 

(Ingles 1952). The lowest estimates were in summer and autumn 

1950 (4/acre), the highest in summer 1949 (19/acre). Biomass of 

mountain pocket gophers fluctuated from a low of 27.7 ounces/ 

acre in spring 1948 to a high of 46.3 ounces/acre in summer 

1949; this had dropped to an estimated 11.8 ounces/acre by the 

following summer. 

In favored habitat at the San Joaquin Experimental Range 

(SJER), Madera County, the density of breeding adult southwestern pocket gophers averaged about 2.0/acre over five breeding seasons. Numbers of young produced by these adults averaged 2.3/acre over four breeding seasons (estimates from figures 

in Howard and Childs 1959, p. 340). Near Bass Lake, Madera 

County, in a Sierran mixed-conifer forest (about 4,500 feet), the 

density on a plot that included cutover forest and meadows was 

4.6/acre, but Storer et al. (1944) believed this to be only half or 

less of the actual population. 

General Life History Features 

Pocket gophers are solitary and territorial, normally not 

ranging beyond the boundaries of their territories. They are most 

active early in the morning and late in the day, near sundown; at 

the highest elevations, most activities occur in late afternoon and 

evening. They are fossorial creatures, digging and living in 

underground tunnels, and creating many shallow, foraging tunnels about 5 inches below ground level. Their burrow entrances 

are plugged with dirt except when the gophers are pushing dirt 

from excavations to the surface, foraging on plants around the 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

burrow entrance, or searching for mates. During excavation and 

while foraging on plants at the surface, they usually expose no 

more than the anterior half of their body at the burrow entrance. 

Most burrowing activities occur during the cooler, wetter 

months in western California. At lower elevations, little or no 

burrowing occurs during the dry summer period, when gophers 

retreat to their few, deeper tunnels and plug the shallow ones. 

They may subsist mainly or entirely on cached food during this 

period. At higher elevations, when snow covers the ground, 

pocket gophers come to the surface and burrow through the 

snow to reach food plants, often packing these tunnels with dirt 

from underground excavations. 

Burrow systems of neighbors typically are discrete. Any 

interconnections that may be accidently established apparently 

are kept plugged with dirt. Hearing may play a role in preventing 

encroachment by neighbors (Ingles 1952). 

Soil, plant cover, and seasonal flooding are the principal 

factors determining habitat suitability and density of these pocket 

gophers. Areas with waterlogged soil and sites of seasonal flooding 

are unsuitable as permanent habitat. At SJER, at an elevation of 

about 1,000-2,000 feet in the western foothills of the Sierra 

Nevada, the strongest correlation with abundance of pocket 

gophers was soil depth (Howard and Childs 1959). Gophers 

were not found living in soils shallower that about 12 inches, and 

were most abundant in soils at least 24 inches deep. Areas with 

the deepest soils showed the highest above-ground productivity 

of herbaceous plants. 

Mean weights of female pocket gophers were greater in 

sites with deep soils compared to sites with shallow soils, but 

differences were not statistically significant. Pocket gophers 

living in irrigated fields are significantly larger and heavier than 

their genetically identical neighbors in natural communities 

(Howard and Childs 1959, Patton and Brylski 1987). Areas 

supporting an abundance of grasses and forbs, especially species 

forming underground rhizomes, corms, tubers, bulbs, and other 

storage organs, provide the greatest habitat values for food. 

Areas with dense or complete canopy cover of woody shrubs 

and trees provide the poorest habitats for pocket gophers. 

Diet 

Pocket gophers eat a variety of plants, favoring herbaceous 

over woody material. Food not immediately consumed is cached 

in underground larders. Much of the information on diet comes 

from examination of these caches. Most species of grasses and 

forbs known to occur in a foothill oak-pine savanna at SJER 

were found in caches of southwestern pocket gophers. Seeds, 

tubers, bulbs, rhizomes, and acorns also were found in the 

caches (Howard and Childs 1959). Mountain pocket gophers 

near Huntington Lake, on the Sierra NF, also ate a wide variety 

of plants. During snowless months, caches of corms and roots of 

meadow bitterroot and golden brodiaea were found in caches. In 

winter, mountain pocket gophers cached mountain whitethorn 

leaves in snow tunnels, and the parts of willow stems covered 

with snow were frequently gnawed (Ingles 1952). 

Chapter 10 213 
Weights 

Microgeographic, seasonal, and annual variations occur in 

weights of adult pocket gophers. Season, sexual activity, and 

habitat quality have major influences on size and mass of adults 

(Howard and Childs 1959, Daly and Patton 1986, Patton and 

Brylski 1987). Mean weights of mountain pocket gophers in a 

Sierra Nevada meadow ranged from 2.2 (lowest summer average) to 3.1 ounces (highest spring average). Nonbreeding animals in autumn averaged from 2.2 to 3.0 ounces in different 

years (Ingles 1952). Weights of southwestern pocket gophers at 

SJER varied as follows (Howard and Childs 1959, fig. 11). 

Females 8-10 months of age--mean = 2.2 ounces (range 1.8-2.6).

Females 20-22 months of age--mean = 2.6 ounces (range 2.2-3.1).

Males 8-10 months of age--mean = 3.2 ounces (range 2.3-8.8).

Males 20-22 months of age--mean = 4.3 ounces (range 3.9-4.7).

Mean weights of trapped animals were highest in spring and 

lowest in summer and autumn. 

At Hastings Reservation, modal weights of trapped males 

were between 4.0 and 4.4 ounces, with a range of about 2.1 

(juveniles) to 7.7 ounces (largest adults). Modal weights of 

trapped females were between 3.2 and 3.9 ounces, with a range 

from about 1.9 (juveniles) to 4.7 ounces (largest adults) (Daly 

and Patton 1986, fig. 3). 

Reproduction and Development 

Mountain pocket gophers begin breeding in May or June 

and young are born in June to August. Only one litter of three or 

four young per year is the norm. From about mid-July to early 

September, young disperse over the ground surface until a suitable site is found (Ingles 1952). Often burrow systems established by dispersers are in marginal or unsuitable habitats, such 

as shallow, sterile, granitic soils, or in small plots of higher 

ground surrounded by waterlogged soil. Some adults and young 

of the previous year apparently disperse in winter through snow 

tunnels. Dispersing adults are predominately males. 

Based on studies by Howard and Childs (1959), southwestern pocket gophers at SJER commenced breeding in January, 

considerably earlier than is the case with the mountain pocket 

gopher. Most females were first pregnant the last 2 weeks of 

February; mean litter size there was 4.6. The young dispersed 

from March to May, although both young and adults occasionally moved over the ground at other periods of the year. Females 

born in January sometimes produced litters in April or early May 

of the same year. Most females produced only one litter per year, 

but a few had two. Between 50 and 75 percent of the females in 

January 1950-1954 were young of the previous breeding season 

(9-11 months old). 

In a mixed-conifer forest at an elevation of about 4,500 feet, 

near Bass Lake, Madera County, scanty data suggest that young 

are born in early July and that some females may have two 

litters, similar to populations of this same species at lower 

elevations (Storer et al. 1944). 

In a montane woodland community at Hastings Reservation, southwestern pocket gophers began breeding after onset of 

214 Chapter 10 

the winter rainy season, usually by January (Daly and Patton 

1986). The breeding population was composed of animals at least 

7-8 months old. Most females probably had only a single litter. 

The breeding season of southwestern pocket gophers in the 

San Bernardino Mountains, at an elevation of about 7,500 feet 

near Bear Lake, is probably similar to that of mountain pocket 

gophers in the Sierra Nevada (Grinnell 1908). 

Estimated survival of southwestern pocket gophers to 1 

year of age ranged from about 5 to 40 percent for males and 15 to 

50 percent for females between 1949 and 1953 (Howard and 

Childs 1959). Less is known about mountain pocket gophers, 

but 34 percent of the individuals in autumn populations in the 

Sierra NF were young of the year, and little turnover was detected in the spring breeding populations of 1949 and 1950 

(Ingles 1952). 

Forest Management 

Generally, actions that tend to benefit pocket gophers would 

tend to lessen overall habitat suitability for spotted owls, so we 

would not recommend any active management to increase the 

amount of suitable habitat for gophers. Natural and man-made 

openings in the forest will undoubtedly occur with sufficient 

regularity to assure that these burrowing mammals will continue 

to be available as prey for California spotted owls. 

White-Footed Mice 

Distribution and Habitat 

Five species of white-footed mice (genus Peromyscus) occur within the range of the California spotted owl. Indeed, 

white-footed mice are nearly ubiquitous in terrestrial habitats 

and often one or another species in this group is the most 

abundant small mammal. They exhibit considerable geographic 

variation in habitat associations, so results of studies on a given 

species in one locality should not be too broadly applied. Because of its marginal occurrence with the California spotted owl, 

we do not include the cactus mouse in this review. 

Brush Mouse 

Brush mice range throughout most of the area inhabited by 

California spotted owls, although they are absent from most of 

the inner coastal ranges (Diablo Range) of central California 

south of Suisun Bay and north of the Transverse Ranges in Kern 

and Santa Barbara Counties. They are relatively scarce above 

3,500 feet in the northern portion of the western Sierra Nevada 

(Grinnell et al. 1930), but may occur higher in chaparral and 

other shrub associations on south-facing slopes (Jameson 1951). 

They occur up to about 5,100 feet at the level of the Sierra NF. 

On the Pacific slopes of the San Gabriel Mountains, brush mice 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
occur mainly between 1,600 and 6,000 feet, where they show 

decided preferences for rocky sites in oak woodlands, riparian/ 

hardwood communities, and mixed-species chaparral (Vaughan 

1954). Brush mice climb readily and are often seen or captured 

in trees. They are not known to hibernate or to enter torpor. 

Brush mice may construct nests in hollows in trees or in 

ground burrows. They are closely associated with oaks and 

rocky sites. At Pinnacles National Monument in the Gabilan 

Range, Monterey County, brush mice comprised only 5 percent 

of the white-footed mice captured in a complex mix of woodland, chaparral, and grassland communities. They were significantly associated with poison oak and medium-sized rocks (10-50 

inches), and they showed a significant negative association with 

grass (Fellers and Arnold 1988). Elsewhere they are typically 

the most common white-footed mouse where rocks and oaks 

occur together in oak woodlands and forest communities below 

the mixed-conifer zone. In the central Sierra Nevada (Yosemite 

NP), oaks and proximity to water were commonly associated 

with brush mice (Grinnell and Storer 1924). Other researchers 

have not verified a dependence on surface water; possibly exposed rocks in canyon bottoms and shrubby growth along streams 

provide suitable habitat in otherwise inhospitable surroundings. 

At SJER, an area with no permanent streams, brush mice were 

the most common species of white-footed mouse, preferring 

rocky areas sheltered by oaks (Quast 1954). They were also the 

most common species in the La Panza Range of San Luis Obispo 

County, with nearly equal abundance in blue oak woodland, 

mixed chaparral (chamise, ceanothus, and scrub live oak), and 

the ecotone between these communities along canyon bottoms 

(Murray 1957). 

California Mouse 

California mice occupy chaparral and woodland communities in western California and northern Baja California, south of 

San Francisco Bay on the Coast, and from Mariposa County 

southward in the Sierra Nevada foothills. Their elevational distribution in the Sierra Nevada is generally from the lower half of 

the ponderosa pine forest downslope to the mid-elevation, oak-pine 

woodlands and chaparral. Elevational limits in the mountains of 

southern California are generally below about 4,900 feet, with 

an exceptional record at 7,900 feet in the San Jacinto Mountains 

(Grinnell 1933). California mice climb readily and are frequently 

captured in traps set in shrubs and trees (Meserve 1976a, 1977). 

They may become torpid on a diurnal cycle when deprived of 

food (Hudson 1967). 

California mice have more specialized habitat requirements 

than brush and pinyon mice, preferring broadleaved woodlands 

and mixed chaparral and being more limited in their elevational 

and latitudinal distributions. Within their geographic range, they 

are closely associated with the distribution of both dusky-footed 

woodrats and California-laurel (bay), although both associates 

occur much farther north than California mice (Merritt 1974). 

Plant communities inhabited include valley foothill hardwood 

and oak-pine woodlands, various chaparral associations, and 

riparian deciduous. Within preferred habitat, California mice are 

often the most abundant small mammal species. At Pinnacles 

National Monument, California mice accounted for 10 percent 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

of all captures of white-footed mice. They were absent from 

areas of extensive grass and large patches of chamise chaparral. 

Variables most strongly associated with the presence of this 

species were hollyleaf cherry, medium-sized rocks, and Chinese 

nests (a wildflower) (Fellers and Arnold 1988). They are among 

the commonest rodent species in mixed-chaparral communities 

in the San Gabriel Mountains below about 4,900 feet (Vaughan 

1954, Wirtz et al. 1988). In coastal sage communities, they are 

generally limited to thickets of large shrubs and small trees in 

riparian/hardwood stands (Vaughan 1954). M'Closkey (1972) 

captured only six residents in coastal sage scrub and their mean 

duration on plots was less than half that of cactus and deer mice. 

He believed their occurrence on the study plot was due to 

previous flooding of their preferred habitat along washes where 

trees and large shrubs were found. 

California mice often nest in abandoned or occupied stick 

nests of dusky-footed woodrats. They also may nest in hollows 

in trees, snags, or logs, and they construct stick nests of their 

own, often under fallen logs and smaller downed woody material (Merritt 1978). They apparently do not burrow readily; 

many researchers have proposed that their distribution and abundance are limited by availability of suitable nesting sites (Merritt 

1974, 1978). 

Deer Mouse 

Deer mice occur throughout the range of the California 

spotted owl, and in most plant communities, from marshes and 

grasslands at or below sea level, through woodlands and forests, 

to above timberline in the mountains. Within this broad area, 

however, they are generally common only in riparian/hardwood 

and grassland communities at lower elevations, and riparian, 

forest, and meadow communities from the mid-elevation 

mixed-conifer zone upslope through lodgepole and subalpine 

pine forests. Deer mice probably are the most terrestrial of the 

white-footed mice considered here (King 1968; Meserve 1976a, 

1977). Meserve (1977) seldom found them in shrubs or trees in a 

southern California community of coastal sage, even though 

they can climb readily and are taken in traps set in brush and 

trees. Torpor under natural conditions is unknown for deer mice. 

Deer mice typically nest in ground burrows, hollow logs, or 

talus. Nests are less frequently located in hollows of trees and 

snags. They are generally much less common than brush and 

pinyon mice in ponderosa pine, oak-pine woodlands, foothill 

and montane hardwood forests, and chaparral on the western 

slopes of the Sierra Nevada (for example, tables l0A and l0C; 

Quast 1954), in the coast ranges, and in mountains of southern 

California. Within these communities, they are most often found 

in riparian deciduous associations, wet and dry meadows, and 

grass/forb seral stages. In the northern Sierra Nevada, deer mice 

were significantly more abundant in forested than in shrub stages 

of mixed-conifer forest (Jameson 1951). In coastal woodland 

and chaparral communities, deer mice are uncommon and usually associated closely with riparian/hardwood communities or 

large openings dominated by annual grasses and forbs. At Pinnacles National Monument, deer mice comprised 20 percent of 

the white-footed mice captured in a complex of grassland, oak, 

pine, and chaparral communities. Most captures were on burned 

Chapter 10 215 
Table 10C-Captures of small rodents that may be prey of California spotted owls, by successional stage (after Verner and Boss 1980)1 in forest communities 

of the western Sierra Nevada, Sierra NF (D. F. Williams pers. observ.). Total adjusted sampling effort was 18,200 trap days (one trap day = one pitfall trap 

set for 24 hours). Trapping was simultaneous in all forests and successional stages. Values are actual captures, except "catch rate," which is the number captured 

per trap day. Captures were standardized to represent equal sampling effort in the various habitat types. (Most habitat types/stages were sampled on two transects 

of 10 traps each, set for 7 days in 1980 and 28 days in 1982. LTB in ponderosa pine, GF in mixed-conifer, and LTC in red fir forests were sampled only on single 

transects, so numbers of actual captures there were doubled.) 

Habitat type/stage 

Forest community and Catch 

 mammal species GF SSS PMA PMB PMC LTA LTB LTC RH Total rate 

Ponderosa pine 

Southwestern pocket gopher 1 1 2 0 0 2 0 -- 1 7 0.0013 

 Deer mouse 12 0 0 0 1 1 2 -- 1 11 0.0020 

 Brush mouse 3 2 3 0 0 1 8 -- 10 27 0.0048 

 Pinyon mouse 0 6 4 6 13 13 0 -- 0 42 0.0075 

 California vole 2 0 0 0 1 0 0 -- 1 4 0.0007 

Total 18 9 9 6 15 17 10 -- 13 91 0.0188 

Mixed-conifer 

Southwestern pocket gopher 0 0 1 1 0 0 0 0 0 2 0.0003 

 Deer mouse 2 21 14 3 6 7 12 11 29 105 0.0167 

 Brush mouse 0 0 0 0 1 0 0 1 2 4 0.0006 

 Pinyon mouse 0 0 1 2 0 0 0 0 0 3 0.0005 

 Long-tailed vole 0 12 4 1 0 0 1 0 7 25 0.0040 

Total 2 33 20 7 7 7 13 12 38 138 0.0221 

Red Fir 

Mountain pocket gopher 6 1 1 0 0 0 0 2 4 15 0.0024 

 Deer mouse 40 30 9 15 12 14 15 10 42 187 0.0300 

 Long-tailed vole 5 6 0 5 1 1 5 0 40 63 0.0100 

Total 51 37 10 20 13 15 20 12 86 265 0.0424 

1

 GF = grass/forb; SSS = shrub/seedling/sapling; PMA = pole-medium tree with <40 percent canopy cover; PMB = pole-medium tree with 40-69 percent 

canopy cover; PMC = pole-medium tree with >69 percent canopy cover; LTA = large tree with <40 percent canopy cover; LTB = large tree with 40-69 percent 

canopy cover; LTC = large tree with >69 percent canopy cover; RH = riparian/hardwood community in corresponding forest zone. 

areas and in a grassy field. Variables positively associated with 

their occurrence were chamise and bird's foot trefoil (together), 

and yerba-santa. Percent cover of oak leaf litter was negatively 

associated with deer mice (Fellers and Arnold 1988). 

In the San Bernardino Mountains, brush mice outnumbered 

deer mice on mixed-conifer plots at elevations between 5,800 

and 7,000 feet, but deer mice were more abundant on mixed-conifer 

plots at 7,600 feet (Kolb and White 1974). Grinnell (1908) found 

deer mice common only in big sagebrush on the desert slopes 

and high ridges, and in montane forests above 6,900 feet. Deer 

mice were not captured by Spevak (1983) at four sites in the 

Santa Monica Mountains, Los Angeles County, in chaparral, 

riparian, and coastal sage associations. Yet Price and Kramer 

(1984) caught small numbers in a variety of microhabitats in a 

coastal sage community in Riverside County. M'Closkey (1972) 

also captured deer mice in a coastal sage community in Orange 

County and noted that they were the most general in habitat 

preference, being found along the moisture gradient from grasslands to woodlands. In mixed montane-chaparral communities 

of the Pacific slopes of the San Gabriel Mountains, deer mice 

were absent from chaparral that had not been burned for several 

years but were present on all burned plots, with peak numbers 

occurring 2 years after burns (Wirtz et al. 1988). 

216 Chapter 10 

Pinyon Mouse 

Pinyon mice also occur throughout the area inhabited by 

California spotted owls, but generally range below the 

mixed-conifer forests on the western slopes of the Sierra Nevada. They are associated with oak-pine woodlands and chaparral communities, usually with one or more species of conifers 

such as juniper, pinyon pine, or digger pine. They are less 

common in ponderosa pine habitats, where they most often 

occur on hotter, drier slopes in association with chaparral or in 

more mature stages of forest. In southern California they are 

generally uncommon or only locally distributed on the Pacific 

slopes of the mountains. Vaughan (1954) found none on the 

Pacific side of the San Gabriel Mountains. In the San Bernardino 

Mountains, Grinnell (1908) captured them only at two localities 

on the Pacific side. One was a south-facing slope at 6,500 feet 

vegetated with Coulter pine, chamise, deer brush, curlleaf mountain mahogany, and California scrub oak-plants typical of the 

upper-elevation chaparral of the Pacific side, and pinyon pine 

and western serviceberry-plants more typical of the desert side. 

The other was a south-facing slope where pinyon mice were 

taken between 5,100 and 5,500 feet among a mix of plants of 

upper-elevation chaparral and lower-elevation forest (ponderosa 

pine, white fir, and black and canyon live oaks). Chaparral plants 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
predominated. Pinyon mice readily climb and are frequently 

taken in traps set in shrubs and pygmy conifers (for example, 

juniper, pinyon pine, digger pine, Coulter pine). 

Pinyon mice nest in ground burrows or hollows in trees. 

They apparently do not become torpid. In the central coastal 

chaparral, woodland, and forest communities, they are common 

wherever shrub or tree cover is found, especially with one or 

more species of pygmy conifers or dense stands of chamise or 

ceanothus chaparral (or a mix of these and other shrubs). Pinyon 

mice were the most abundant small mammal trapped in chamise 

chaparral at Hastings Reservation (Bradford 1976). At Pinnacles 

National Monument, pinyon mice accounted for about 66 percent of the white-footed mice captured in a variety of oak, pine, 

grass, and chaparral associations. They were found in all community types but grassland. Large rocks and various shrub species were significantly associated with occurrence of pinyon 

mice. Grasses and forbs indicated the poorest habitats (Fellers 

and Arnold 1988). In the Sierra Nevada, they are mostly limited 

to areas with dense brush or tree cover below about 4,600 feet, 

and are usually abundant only where oaks do not dominate. At 

SJER, pinyon mice were commonly associated with moderate to 

dense stands of brush, particularly buckbrush, and rocks. Likewise, in the southern Sierra Nevada (Kern County), pinyon mice 

preferred stands of buckbrush and rock outcrops (Lawrence 

1966). In the La Panza Range of San Luis Obispo County, they 

were more than four times as common in a chaparral community 

of chamise, buckbrush, and coast live oaks as they were in a blue 

oak/digger pine woodland (Murray 1957). None was taken in 

chaparral communities in over a decade of studies on the San 

Dimas Experimental Forest, on the Pacific slope of the San 

Gabriel Mountains (Wirtz et al. 1988). 

Patterns of Abundance 

Results of trapping suggest some differences in habitat 

affinities of white-footed mice in the western Sierra Nevada 

(tables l0A and lOC). Deer mice were captured in most habitat 

types and seral stages, exhibiting apparent specialization at low 

elevations (where brush and pinyon mice co-occur) in sites 

dominated by grasses and forbs. Their habitat associations increase markedly with increasing elevation to the point that they 

are ubiquitous in the red fir zone, where no other species of 

white-footed mice occur. Pinyon mice were generally confined 

to sites with shrubs, mixtures of shrubs and small trees, or shrubs 

and widely scattered trees in all habitats sampled except the oak/ 

digger pine type, yet other studies (Quast 1954, Block et al. 

1988) found them to be common in this woodland type, where 

they were associated with shrubs. Brush mice were relatively 

uncommon in most habitats, except those at lower elevations 

that had large trees in the canopy. California mice were essentially missing from the sample, although study sites were either 

too far north for this species or generally in habitat types where 

we would not expect to find them. 

Any interpretation of habitat use reported in tables 10A and 

10C should note differences in elevation and forest composition.

Sierran mixed-conifer sites reported in table 10A, for example,

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

were mostly on north-facing slopes of canyons 2,000-3,900 feet 

in elevation, where Douglas-fir occurred with ponderosa pine, 

sugar pine, incense-cedar, or black oak. Studies reported in table 

IOC were farther south, at elevations of 5,200-6,500 feet. 

Douglas-fir was rare there, and most sites had white fir mixed 

with the other species listed above. 

Densities of white-footed mice within the range of the 

California spotted owl generally fluctuate between lows in December and January to highs in July and August. At higher 

elevations in mixed-conifer and fir forests, annual peaks in 

densities may be delayed into August or September. Densities of 

different species vary from <1 to >30 mice/acre. Various density 

estimates have been reported in the literature; these are summarized below (animals/acre), but readers should be mindful of the 

problems with estimating densities of small mammals and the 

variety of methods used. 

Brush mice-1.0-15.3 (Zeiner et al. 1990); 3.0-15.2 (Sierran 

mixed-conifer, Bass Lake area, Storer et al. 1944); 1.3 

(Lake Tahoe area, Storer et al. 1944). 

California mice-31.2 (xeric chaparral, central California, 

Merritt 1974); 37.2 (mesic oak-laurel forest, central California, Merritt 1974); 0.1-0.8 and <0.8 (southern California coastal-sage scrub, MacMillen 1964 and M'Closkey 

1972, respectively). 

Deer mice-4.0-10.0 (White et al. 1980); 4.9-14.2 

(mixed-conifer forest,Sierra Nevada, Bass Lake area, 

Storer et al. 1944); 19.1 (Lake Tahoe area, Storer et al. 

1944), <0.4-1.6 (southern California coastal sage, 

M'Closkey 1972). 

Pinyon mice-1.0 (oak-laurel forest, central California, Merritt 

1974); 34.8 (xeric chaparral, Merritt 1974). 

General Life History Features 

White-footed mice are nocturnal and active throughout the 

year. Some species become torpid under food or water deprivation, but others do not. They nest in ground burrows, talus, 

hollow logs, and in hollows in trees. They readily climb in brush 

and trees (scansorial). Arboreal tendencies differ among species, 

however, with brush and California mice being the most scansorial 

of the species occurring in California, and deer mice the least. 

White-footed mice are not highly territorial except near their 

nest, but territoriality differs among species. The California mouse 

is the most territorial species, living in male-female pairs with 

nearly nonoverlapping home ranges (Ribble and Salvioni 1990). 

Diet 

White-footed mice are omnivorous, feeding on seeds, fruits, 

fungi, flowers, foliage, insects and other arthropods, carrion, and 

other animal matter (Zeiner et al. 1990). Specific studies suggest 

that the four species considered here are largely opportunistic in 

choice of diet, although differences are seen where species' 

ranges overlap (for example, Jameson 1952; Meserve 1976a, 

1976b). Insects (especially larvae and pupae), seeds, fruits, and 

fungi probably comprise the bulk of their diets. California mice 

Chapter 10 217 
eat large quantities of California-laurel seeds, the thick, hard 

coats of which cannot be cracked by pinyon mice (Merritt 1974). 

In a coastal sage community, California mice ate mostly shrub 

fruits, seeds, and flowers and smaller quantities of grass seeds 

(Meserve 1976a). 

Reproduction and Development 

Reproduction varies geographically, altitudinally, and annually. Females appear to be seasonally polyestrous, with most 

births occurring between March and October, but some females 

may be pregnant in any month. The most prolonged reproductive seasons are found in populations at or near sea level and on 

the lower slopes of coastal ranges. The shortest reproductive 

seasons occur at the highest elevations in montane communities. 

The breeding season of brush mice peaks in April and May; 

a secondary peak in June to August seems to depend upon the 

previous crop of acorns. Litters average three to four young; 

females probably average near two litters per year, although they 

can have four. Females born in spring can breed in the summer 

of their first year. 

Although California mice may breed year-round in coastal 

areas, most breed between March and September. Litter size 

averages two to three (MacMillen 1964, Merritt 1978), with up 

to three to four litters per year. Females born early in the year 

breed late in the reproductive season of the same year, although 

California mice mature more slowly than the other white-footed 

mice considered here. 

Deer mice may breed year-round, depending upon climate, 

but most reproduce between March and October. Litters are l 

arger at higher elevations and latitudes, but probably average 

four to six for populations within the range of the California 

spotted owl. Numbers of embryos for 46 females from the Sierra 

NF, most captured between mid-June and mid-August in 

mixed-conifer and red fir forests, averaged 5.2 ± 1.43 (range = 

2-9). Mean litter size for 11 females in a southern California 

coastal-sage community was 4.3 ± 1.3 (MacMillen 1964). 

Females may have two to four litters during the breeding 

season-fewer at higher elevations and latitudes. Young born in 

spring breed later in the same summer or autumn. 

Pinyon mice breed mainly from May to September, averaging two to three young per litter. Females may breed when 3 

months old. 

Little information is available on dispersal by these species 

within the geographic range of the California spotted owl. Dispersal probably commences soon after weaning, but individuals 

may leave their natal homes over a protracted period, depending 

upon circumstances such as survival of the mother, population 

density, and food abundance. Time to weaning varies geographically and by species. For brush and deer mice, weaning probably 

averages about 25 days (range of reported values, 22-37 days, 

summarized by Zeiner et al. 1990). California mice are weaned 

in an average of 35 days, although some litters may not be 

weaned until 44 days (McCabe and Blanchard 1950, Merritt 

1974). Clark (1938) reported 50 days as the period before weaning in pinyon mice, although other studies found considerably

218 Chapter 10 USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992.

shorter periods-about 25 (McCabe and Blanchard 1950) or 30 

days (Douglas 1969). 

Weights 

The deer mouse is the smallest of the four species considered here. Adults (excluding pregnant females) weigh about 

0.53-0.91 ounces (Layne 1968); the mean for a Sierra Nevada 

sample was 0.70 ± 0.01 ounces, n = 144. Grinnell and Storer 

(1924) listed a range of 0.45-0.74 ounces for deer mice caught in 

the central Sierra Nevada. 

Adult brush mice ranged in weight from about 0.77 to 1.19 

ounces in the Yosemite region (Grinnell and Storer 1924). The 

mean of a mixed-age sample of brush mice in the Sierra NF 

varied from about 0.74 to 0.95 ounces (table 1013). 

Breeding adult pinyon mice from central coastal California 

averaged 1.19 ± 0.11 ounces (Merritt 1974). The range of weights 

in the Yosemite region was 0.82-1.44 ounces (Grinnell and 

Storer 1924). Samples of breeding adults from the Sierra NF 

averaged about 0.98-1.09 ounces. 

California mice are the largest of the white-footed mice in 

California. Most reported adult weights range from about 1.33-1.75 

ounces (Grinnell and Storer 1924, Layne 1968, Jameson and 

Peters 1988). Merritt (1974) reported mean weights of breeding 

adults as 2.0 ± 0.28 ounces in central coastal California. A 

population mean for all ages and sexes captured year-round in a 

coastal-sage community in southern California was 1.20 ounces 

(MacMillen 1964). 

Table l0D-Variation in weights (in ounces, mean ± SD, with range 

and sample size below) of three species of white footed mice during summer 

(June-August, Sierra National Forest, Fresno County) and winter (January, San 

Joaquin Experimental Range) (D. F. Williams pers. observ.). 

 Brush Deer Pinyon 

Season Sample mouse mouse mouse 

Summer Adults 0.99 ± 0.11 0.74 ± 0.13 1.00 ± 0.07 

 (0.74 - 1.33) (0.46 - 1.12) (0.77 - 1.30) 

 n = 39 n = 208 n = 31 

 Young 0.51 ± 0.08 0.43 ± 0.09 0.54 ± 0.12 

 (0.41 - 0.63) (0.21 - 0.56) (0.39 - 0.77) 

 n = 5 n = 113 n = 12 

 Pooled 0.93 ± 0.18 0.63 ± 0.19 0.87 ± 0.24 

 (0.42 - 1.33) (0.21 - 1.12) (0.39 - 1.30) 

 n = 44 n = 321 n = 43 

Winter Adults 0.78 ± 0.07 0.54 ± 0.07 0.90 ± 0.13 

 (0.67 - 0.95) (0.39 - 0.70) (0.70 - 1.09) 

 n = 31 n = 14 n = 9 

 Young 0.61 ± 0.03 0.49 ± 0.07 0.57 ± 0.08 

 (0.56 - 0.67) (0.35 - 0.60) (0.49 - 0.67) 

 n = 12 n = 13 n = 5 

 Pooled 0.73 ± 0.10 0.51 ± 0.08 0.78 ± 0.20 

 (0.56 - 0.95) (0.35 - 0.70) (0.45 - 1.09) 

 n = 43 n = 27 n = 14 
Management 

Clearcutting or similar tree harvest and brush thinning or 

removal generally result in increased numbers of deer mice. 

Wildfires and controlled burns that reduce shrubs and small trees 

and increase cover of grasses and forbs also enhance populations 

of deer mice in woodland, forest, and chaparral communities 

(Jameson 1951, Quast 1954, Lawrence 1966, Spevak 1983, 

Fellers and Arnold 1988, Wirtz et al. 1988) (also see table 10C).

On the other hand, numbers of other species of white-footed 

mice in lower-elevation mixed-conifer, ponderosa pine, oak-pine 

woodland, and various hardwood and chaparral communities 

would be reduced or eliminated by clearcutting, brush removal, 

or fire. Management that promotes increased abundance of oaks 

would increase numbers of brush mice. In the San Gabriel 

Mountains, in mixed-chaparral communities on the San Dimas 

Experimental Forest, brush mice were rare on plots in chaparral 

that had not been burned for 28 years, but increased in abundance after burns. Brush mice increased to six times their preburn 

density on areas with normal burns and 14 times on areas with 

hot burns. Brush mice recolonized burned areas sooner than 

California mice (Wirtz et al. 1988). In coastal northern California (Hopland Field Station, Univ. of California), both brush and 

pinyon mice were adversely affected by converting chaparral to 

grassland, but positively affected by converting old, decadent 

chaparral to young- and intermediate-aged stands (Longhurst 

1978). California mice were the only common Peromyscus in 

the mature montane chaparral at San Dimas Experimental Forest. They were the slowest of the white-footed mice to recolonize mixed montane-chaparral communities in the San Gabriel 

Mountains after burns. Captures after bums were greater on 

normal burns than hot burns, and postfire densities were generally greater than preburn densities (Wirtz et al. 1988). Management that promotes increased cover and vertical complexity of 

chaparral and woodlands, increased abundance of California-laurel 

and dusky-footed woodrats, and increased numbers of potential 

nests would enhance populations of California mice. 

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Sierra Nevada. Gen. Tech. Rep. PSW-37. Berkeley, CA: Pacific Southwest 

Forest and Range Experiment Station, Forest Service, U.S. Department of 

Agriculture; 321-438. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

Williams, Daniel F. 1986. Mammalian species of special concern in California. 

Administrative Report 86-1. Sacramento, CA: Department of Fish and 

Game, Wildlife Management Division; 112 p. 

Williams, Daniel F.; Johnson, Dennis F. 1979. Small mammal communities of 

the western Sierra Nevada foothills. Unpublished report. Folsom, CA: 

Bureau of Land Management, U.S. Department of Interior; 33 p. 

Wirtz, William O., II; Hoekman, David; Muhm, John R.; Souza, Sherrie L. 

1988. Postfire rodent succession following prescribed fire in southern 

California chaparral. In: Szaro, Robert C.; Severson, Keith E.; Patton, 

David R., tech. coords. Management of amphibians, reptiles, and small 

mammals in North America. Gen. Tech. Rep. RM-166. Fort Collins, CO: 

Rocky Mountain Forest and Range Experiment Station, Forest Service, 

U.S. Department of Agriculture; 458 p. 

Witt, Joseph W. 1991. Fluctuations in the weight and trap response for Glaucomys 

sabrinus in western Oregon. Journal of Mammalogy 72:612-615. 

Zabel, Cynthia J.; McKelvey, Kevin; Paton, Peter W. C.; Bingham, Bruce B.; 

Noon, Barry R. 1992. Home range size and habitat use patterns of northern 

spotted owls in northwestern California and southwestern Oregon. Unpublished draft supplied by authors; 40 p. 

Zeiner, David C.; Laudenslayer, William F., Jr.; Mayer, Kenneth E.; White, 

Marshall, eds. 1990. California's wildlife. Vol. III. Mammals. Sacramento, 

CA: Department of Fish and Game; 407 p. 

Chapter 10 221 
Chapter 11 

Historical Perspectives on Forests of the Sierra Nevada 

and the Transverse Ranges of Southern California: 

Forest Conditions at the Turn of the Century 

Kevin S. McKelvey and James D. Johnston

Preface 

Let me say a word of thanks to the members of the forestry force 

who acted as my escort. I wish to thank other gentlemen also, but 

particularly the members of the forestry force. l am, as you gentlemen probably know, exceedingly interested in the question of forest 

preservation. I think our people are growing more and more to 

understand that in reference to the forests and the wild creatures of 

the wilderness our aim should be not to destroy them simply for the 

selfish pleasure of one generation, but to keep them for our children and our children's children. 1 wish you, the Forest Rangers 

and also all the others, to protect the game and wild creatures and 

of course in California, where the water supply is a matter of such 

vital moment, the preservation of the forests for the merely utilitarian side is of the utmost, of the highest possible consequence; and 

there are no members of our body politic who are doing better 

work than those who are engaged in the preservation of the forests, 

keeping nature as it is for the sake of its use and of its beauty. 

Theodore Roosevelt, 9 May 1903. 

The Sierra Nevada has been impacted by western civilization for more than 150 years and heavily impacted for at least the 

past 100 years. Impacts include mining, logging, the grazing by 

both sheep and cattle, and changes in fire patterns-notably fire 

suppression in the twentieth century. Site-specific data from the 

1800s are scattered and not comprehensive, but several events 

occurred around the turn of the century that led to reasonably 

detailed and quantitative accounts of the condition of the land 

and forests. General Grant, Sequoia, and Yosemite National 

Parks (NPs) were established in 1890. Forest reserves were set 

aside between 1891 and 1909 and renamed as National Forests 

(NFs) in 1907. After creation of the forest reserve system, the 

United States Geologic Survey (USGS) initiated the first systematic surveys of the forest reserves as part of a series of annual 

reports. For a brief period, from about 1897 to 1902, these 

surveys were very thorough and often included maps of topography, timber volume, logging intensity, and species distribution. 

Surveys covering most of what is now the Eldorado and Stanislaus 

NFs and Yosemite NP were published in 1900 (Fitch 1900a, 

1900b; Marshall 1900; Sudworth 1900a). Similar inventories 

were done by Leiberg (1902), mainly in what is now the Tahoe 

and Plumas NFs. Additional information concerning what are 

now the Sierra and Sequoia NFs and Sequoia/Kings Canyon 

NPs was obtained by analyzing Sudworth's (1900b) unpublished field notes. 

Areas in southern California were also surveyed around the 

turn of the century. Leiberg (1899a, 1899b, 1899c) examined the 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

San Jacinto, San Bernardino, and San Gabriel reserves in 1897. The 

San Jacinto Quadrangle was reexamined in 1900 and Barnard 

(1900) produced a 30-minute topographic map (fig. 11A). 

This chapter is largely an analysis of these USGS survey 

efforts, with ancillary data from other sources. The men involved with the surveys were professionals whose job it was to 

assess the condition of the newly created reserves. Their studies 

appear to be reliable and accurate, providing quantitative assessments of forest stands that are of great historic interest and 

importance today. 

Sierran Forests at the 

Turn of the Century 

Nonaboriginal Human Impacts 

Logging in the Middle and Southern Sierra Nevada 

Logging was used mainly to support local markets, including the growing towns and communities of the Sacramento 

and San Joaquin Valleys (Ayers 1958). All major towns were

associated with mines, and nearly all timber cut supported the 

mines-for housing employees, timbers to keep tunnels from 

collapsing, processing ore, and transporting processed ore to 

market by rail (Sudworth 1900a). "Large mines" consumed 

between 2,000 and 3,500 cords annually (fig. 11B). Fuel needs 

of stamp mills-machines or mills for pulverizing ore-were 

impressive. The Empire Mine, for instance, had a 30-stamp mill 

that consumed 11 cords of wood in a 10-hour period (Bohakel 

1968) or, assuming constant operation, 9,600 cords a year. 

Most logging before 1900 occurred at low elevations on 

lands adjacent to mines, because wagons were the chief means 

for transporting timber (Sudworth 1900a, Leiberg 1902) (fig. 

11C). Sudworth mapped cut-over and partially cut regions (fig. 

11D, table 11A). With the exception of a large area of cutting that

extended up the American River, Forest Service (FS) lands and 

many higher-elevation private lands had not been logged (fig. 11E). 

Laudenslayer and Darr (1990) provided an analysis of cutting during 

that period by the Michigan-California Timber Company. 

The partially cut lands, especially those more distant from 

Placerville, were largely exploited for the shake market (Sudworth 

Chapter 11 225 
Figure 11A--Areas surveyed by Leiberg (1899a, 1899b, 1899c, 1902), Marshall (1900), Fitch (1900a, 1900b), and Sudworth (1900a). 

226 Chapter 11 USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Figure 11B-Large mining plant at 

Angels Camp, where great quantities of 

yellow pine cord wood were consumed. Thousands of cords of this 

4-foot cordwood were needed to keep 

each mill going throughout the year. 

Hundreds of mills operated up and down 

the Sierra Nevada during this era, processing ore for gold and other minerals. Wood also was the primary fuel for 

heating and cooking in homes and businesses, as well as providing energy to 

run the steam engines, which powered 

winches, sawmills, and locomotives. 

Location: -------In foothills just below the 

Stanislaus National Forest, central Sierra Nevada. 

Elevation:-------About 2,000 feet 

Date: ------------Probably 1899 

Source:---------Sudworth 1990a 

Photographer: -----Unknown 

Figure 11C--A common method of

hauling yellow pine logs to mills was by 

horse and wagon. Before railroads 

entered the area, transportation of raw 

logs and lumber had definite limitations. Mills, which remained close to 

the timber source, tended to be smaller 

and more mobile. When the local timber was used up, the mill was disassembled and moved to another area. 

Location:-------Stanislaus National Forest, central Sierra Nevada 

Elevation: ----About 4,000 feet 

Date: ----------Probably 1899 

Source: -------Sudworth 1990a 

Photographer: ----Unknown 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. Chapter 11 227 
Figure 11D--Areas of logging activity from maps by Marshall (1900), Fitch (1900a), and Sudworth (1900a). Culled 

areas were selectively logged. Most of the cutting in this area was associated with the town of Placerville. 

228 Chapter 11 USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Table 11A--Areas sampled by Sudworth in the Sierra Nevada at the 

turn of this century (Sudworth 1900a) (see figs. 11A and 11D).

Area (square miles) 

Areas Logged Culled1

 Virgin2

Thirty-minute quadrangles 

 Placerville 122 162 152 

 Jackson 175 18 0 

 Pyramid Peak 0 148 642 

 Big Trees 89 104 564 

 Markleeville 0 0 322 

 Dardanelles 0 0 659 

Forest Reserves 

 Stanislaus 0 0 641 

 Tahoe 0 3 130 

Total 386 435 3,190 

Percent of total 10 Il 79 

1 Culled areas were selectively logged. 

2

 Lands listed as grazing or nonforest were omitted. 

1900a). Shakes were the most valuable product at the time, 

maintaining a market value well above dimension lumber into 

the twentieth century. On the Stanislaus NF in 1912, for instance, shake prices were $5.00 to $7.50 per thousand (Graves 

1912), approximately the same price reported by Sudworth.. 

Shakes were produced only from the choicest sugar pines, and 

only from select portions of the bole-no more than 40 percent, 

according to Sudworth. For this reason, shake cutting always 

resulted in partial removal of standing timber, and it always left 

much waste. Following a similar pattern, giant sequoias were 

split to produce grape-stakes (fig. 11F). 

Logging in the Northern Sierra Nevada 

Ore deposits extended into higher elevations in the northern 

Sierra Nevada than in the middle and southern parts. Consequently, placer and hard-rock mining and logging were more 

extensive there than to the south. In particular, areas north of 

Nevada City and the Truckee Basin were heavily cut (fig. 11G). 

Again, access was key and the presence of the Southern Pacific 

Railroad allowed transportation to more distant markets. Lake 

Figure 11E--Chronology of cutting on lands of the Michigan-California Timber Company. These lands lie within the area surveyed by Sudworth in 1900. 

Cutting prior to 1910 occurred within a very limited acreage. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. Chapter 11 229 
Figure 11F--This ancient giant sequoia, named the Mark Twain Tree in 1888, sprouted in 550 A.D.; it was 1,341 

years old and 90 feet around the base at the time of this photo. Smith Comstock erected a sawmill just 250 yards 

west of this tree. Giant sequoia wood made poor lumber, so it was split into grapestakes (center) and posts (left) 

to supply California's rapidly expanding vineyards. 

Location: --------------Big Stump, Sequoia/Kings Canyon National Parks 

Elevation: -------------5,760 feet 

Date: -------------------1891 

Source: ----------------Steve Anderson Collection, Hume Lake Ranger District, Sequoia National Forest 

Photographer: -------C. C. Curtis 

Tahoe also provided convenient access, allowing large quantities of timber to be taken from areas adjacent to the lake (Leiberg 

1902, McKeon 1984). In addition, a 4-mile-wide strip following 

the railroad between Reno and Sacramento was heavily logged 

for locomotive fuel (fig. 11G; Palmer 1992). Gold-bearing deposits at mid-elevations (Clark 1966) caused mines to be scattered throughout forests of the region. The Yuba River Basin, 

north of Nevada City, was logged to supply large mines at Grass 

Valley and Nevada City (elevations 2,400 and 2,600 feet, respectively) as well as at Sierra City (elevation 4,200 feet). 

Leiberg (1902) reported that 1,386,890 acres in the northern 

Sierra Nevada were logged between 1850 and 1902, and that 

2,337,930 acres were uncut. Logging intensity there varied from 

5 to 99 percent of total volume removed. Leiberg estimated that, 

on the average, only 50 percent of the volume was removed from 

logged stands. He provided two means to evaluate the spatial 

extent and intensity of logging. The maps used to prepare figure 

11G gave volume estimates, both in the "culled" areas and in areas 

that were not entered. Those maps were supported by detailed 

written descriptions of the forest on a drainage-by-drainage basis. 

230 Chapter 11 

Analysis of cutting patterns in the North and Middle Forks 

of the Feather River, for instance, clearly shows that most cutting occurred at elevations below current FS boundaries. Oroville 

was the primary destination of lumber from these drainages, and 

the only method for transporting the timber was by wagon, 

restricting the trip to 40 miles or less (Leiberg 1902). Most cut 

areas reported by Leiberg were within 20 miles, straight-line, of 

Oroville or along lower slopes adjacent to the Central Valley 

north of there. For example, Big Bend Mountain (about 15 

miles) was heavily cut over and Chino Creek (about 20 miles) 

was reportedly logged. According to Lefberg (1902, p. 58), 

however, in an area of about 15 square miles immediately north 

of Chino Creek "...lies a block of heavy forest, the heaviest in the 

basin. It is of ,the yellow-pine type, but contains an unusually 

large proportion of sugar pine... Both the yellow and sugar pine 

in this heavy block of timber are of exceptionally large size and 

of old growth. Much of the sugar pine runs above 5 feet basal 

diameter, with clear trunks 40 to 60 feet in height." This block of 

timber was clearly desirable but just beyond reach of the 

wagon-based logging of 1902. For this reason, cutting in these 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Figure 11G--Areas of logging activity from maps by Leiberg (1902). Leiberg did not, as did Sudworth (1900a), differentiate 

between cut-over lands and culled lands. In many areas cutting appears to have been very light. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. Chapter 11 231 
drainages did not extend far into what is now the Plumas NF (fig. 

11G). Areas of NF land that were entered tended to be at extreme 

limits of the transportation capabilities and would therefore have 

been subjected primarily to high-grading of sugar pines. This 

assertion is supported by an inventory done by the Plumas NF in 

1910, which estimated that 870,506 acres of the Plumas were in 

a forested condition but only 11,983 acres (1.3 percent) had been 

"cut over" (Moore 1913). 

On the other hand, 57 percent of the Yuba river drainage 

was described as being cut. In areas adjacent to towns or major 

rivers, Leiberg stated that most trees were removed. On other 

lands, cutting was much more selective. 

Results of inventories by Leiberg and Sudworth, supported 

by the Plumas NF plan of 1913 and records of the 

Michigan-California Timber Company, show that cutting at the 

turn of the century was spatially limited to areas near population 

centers and major transportion routes. With notable exceptions 

of the Truckee Basin, Yuba River Basin, and lands immediately 

adjacent to the Southern Pacific Railroad, logging occurred at 

low elevations with relatively little within NF boundaries in the 

Sierra Nevada prior to the turn of this century. Limited by 

transportation capabilities, much of the logging on what are now 

NF lands was reduced to light high-grading of the most valuable 

trees. The NF most heavily impacted was the Tahoe. There the 

railroad, combined with mining at higher elevations, caused 

significant logging in what is now FS land. Even in the Tahoe 

NF, however, about half of the forest had not been entered at the 

turn of the century. 

Grazing by Sheep 

Unlike logging, which impacted limited acreage in the Sierra Nevada prior to 1900, the entire Sierran range appears to 

have been intensely overgrazed for decades, beginning in the 

early 1860s when a severe drought killed most of the cattle in 

California (Vankat 1970, Ratliff 1985, Ewing et al. 1988). According to Ratliff, not only did the drought cause a shift from 

cattle to sheep, but it also initiated the practice of summer 

grazing in alpine meadows of the Sierra Nevada (fig. 11H). In 

1862, California ranges supported 3 million sheep, about 40 

percent of which grazed the Central Valley. Grazing peaked in 

1876, when more than 6 million sheep grazed state-wide (Ratliff 

1985, Ewing et al. 1988). Sheep grazing was still intense in 

1900, when an estimated 200,000 animals grazed during summer and fall in the Sierra Reserve (Vankat 1970). Descriptions 

of the impact of grazing on vegetation are particularly intense in 

much of the literature of that period: 

There are practically no grasses or other herbaceous plants. 

The forest floor is clean. The writer can attest the inconvenience of 

this total lack of grass forage for in traveling over nearly 3,000,000 

acres not a single day's feed for saddle and pack animals was 

secured on the open range... Barrenness is, however, not an original sin. From a study of long-protected forest land in the same 

region and from the statements of old settlers, it is evident that 

formerly there was an abundance of perennial forage grasses 

throughout the forests of this territory... It would seem that this 

bare condition of the surface in the open range has been produced 

only through years of excessive grazing by millions of sheep--a 

constant overstocking of the range (Sudworth 1900a, p. 554-555). 

232 Chapter 11 

The trampling of thousands of sheep pastured on these slopes 

during the summer and fall reduces the soil, to a depth of 6 or 8 

inches, to the consistency of dust. Rain washes this dust into creeks 

and rivers, and heavy winds lift it up and carry it away (Leiberg 

1902, p. 15, concerning the red fir type). 

The great obstacle to the explorer is not the danger of crag or 

chasm, but the starvation threatening his animals, through the 

destruction of the fine natural meadow pasturage by sheep (Russell 

Dudley 1898, professor of botany, as quoted by Vankat 1970, p. 20). 

The soil being denuded of grass is broken up by thousands of 

sheep tracks, and when the rains come this loose soil is washed 

down the mountainsides into the valleys, covering up the swamps 

and meadows, destroying these natural reservoirs (1894 report by 

Acting Superintendent of Sequoia and General Grant NPs, as quoted 

by Vankat 1970, p. 20). 

The Kern River drainage was ...almost impassable to the 

traveler, to such an extent is every living thing eaten off the face of 

the earth and trampled underfoot by the hundreds of thousands of 

sheep which each year roam over that territory (1893 report by 

Acting Superintendent of Sequoia and General Grant NPs, as quoted 

by Vankat 1970, p. 29). 

Clean surface; sheep grazed and burned. No reproduction. 

10,000 feet elevation (Sudworth 1900b, p. 10). 

The last quote above, referring to an area at the headwaters 

of the Kings River, now in Sequoia/Kings Canyon NPs, was 

typical of site-specific examples in Sudworth's notes. The seasonal migration of sheep from the Central Valley up to alpine 

meadows in mid-summer meant that even very remote locations 

experienced the impact of overgrazing. 

Recurrent themes typify the writings of different observers. 

Grazing at an intensity that produced significant soil erosion was 

noted in both the northern and southern Sierra. Removal of all 

grass was repeatedly mentioned in the context of travel being 

inconvenient because pack animals could not graze. In many 

cases, this was contrasted with the contention that grasses were 

abundant in forests prior to grazing. Although we suspect that 

many early writers harbored an anti-sheep bias, the numbers of 

sheep mentioned casually (hundreds of thousands, millions) 

appear to be accurate. 

Burning by Sheep Herders 

Sheep herders also burned extensively to encourage growth 

of grasses and forbs and to remove fuel and young trees from the 

forest floor. All accounts mention this, but we cannot be certain 

about the extent to which sheep-related fires contributed to 

overall fire frequency. John Muir, writing in 1877, felt that 90 

percent of fires were caused by sheep herders, but his estimate 

was probably exaggerated (Vankat 1970). It is clear, however, 

that the pattern and intent of fires set by sheep herders differed from 

those set by Native Americans. The intent was to improve grazing 

on high-altitude pasturage and to remove obstacles that impeded 

movement by sheep. Consequently, sheep herders gave special 

attention to burning large, downed fuels (Sudworth 1900a, Vankat 

1970) and to burning mesic areas to stimulate forage production. 

We consider it likely that this intensity of grazing, combined 

with repeated burning by sheep herders, had a severe impact on 

herbaceous vegetation and on patterns of forest regeneration in the 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Figure 11H--Sheep grazing in the Sierra Nevada.

Location: --------------Northside of Crabtree Meadow, Sequoia National Park 

Elevation: -------------10,400 feet

Date: -------------------29 July 1890

Source:-----------------Vankat (1970) Photographer: -----Unknown

Sierra Nevada. In the absence of competing vegetation, regeneration was rapid and dense when sheep and fires ignited by sheep 

herders were no longer prevalent in montane forests. According to 

Leiberg (1902, p. 15), such areas were common 

...varying from 1 acre to 3 or 4 acres, scattered along the ridges 

from Webber Peak to the Rubicon River, on which the grass and 

weeds have been so thoroughly eaten out that even the sheep have 

abandoned them. On such tracts, left undisturbed for four or five 

years, Shasta firs [red fir] cover the ground to the number of 10,000 

to 15,000 trees to the acre. 

Leiberg (1902, p. 43) also reported that stands freed from burning for 15 years 

...may be seen in the Mohawk Valley, on areas in the central 

portions of the basin of the West Fork of Feather River and in the 

northern portion of the Truckee Basin. These sapling stands, composed of yellow pine, red [Douglas-] and white fir, and incense 

cedar, singly or combined, are so dense that a man can with 

difficulty force his way through. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

Sudworth (1900a, p. 553) found a "general lack of herbaceous growth and irregular reproduction of timber species [and 

a] general absence of small-sized timber intermediate between 

seedlings and the large timber" in unfenced forests. In "fenced 

and otherwise protected forests," he found "uniform abundance 

of herbaceous growth," "plentiful reproduction of timber species," and "presence of intermediate sizes of small timber." 

Vankat (1970) measured the ages of numerous trees in 

Sequoia NP, finding that a major regeneration pulse occurred at 

approximately the time sheep were removed from the Park (fig. 

11I). Vankat also found that particular areas (for example, meadows) and species (for example, big sagebrush) had not recovered 

from grazing by the late 1960s. We can reasonably infer that the 

intensity of sheep grazing for nearly 40 years in the Sierra Nevada 

impacted stand structure and regeneration patterns, producing lasting changes in mixed communities of grasses, forbs, and shrubs. 

Chapter 11 233 
Figure 11I--Periods of significant regeneration on sites within Sequoia 

National Park. This graph was generated by combining tables found in 

the appendices of Vankat (1970). 

Stand Structure 

The best data on stand structures at the turn of this century 

came from a set of 1/4-acre plots measured by Sudworth (1900a, 

1900b). Data from 22 plots were presented in his formal document (1900a) for the USGS on the Stanislaus and Tahoe reserves, and data from 26 additional plots were included in his 

notes on the Sierra Reserve (1900b). Here we report analyses of 

all plots in the USGS report but only 20 of the 26 plots in 

Sudworth's notes. Four of those not analyzed were in groves of 

giant sequoias (Sudworth was particularly interested in the big 

trees), and two were at high elevation and contained large components of foxtail pine. These plots represented uncommon 

forest types and were not considered of general interest. Sudworth 

did not describe the methods he used to select plots, but their 

primary purpose was to support the maps he created. His maps, a 

standard product in USGS surveys of that period, subdivided the 

entire area surveyed (about 3,000,000 acres) into subunits based 

on similarity in timber volumes. Sudworth often referred to his 

plots as being "representative" of a particular area. He probably 

used scattered plots to calibrate ocular estimates. He reported 

only trees 11 inches or more in diameter at breast height (d.b.h.) 

in the USGS report and only 12 inches or more in d.b.h. in his 

notes. He mentioned regeneration in his notes, but the meaning 

of the term was not quantified. 

Several uncertainties are associated with analyses of data 

from the turn of the century. In particular, estimates of volume 

per acre, either at the stand level or derived from plot data, 

should be viewed with caution. At a stand level, volumes were 

calculated using scaling rules based on the limits of merchantability. Because small trees (for some species defined as trees 

<16 in d.b.h.) and tops were not used, and because saw kerfs 

were wider than today, the volume reported on a per-acre basis 

in 1900 would, therefore, be considerably less than if the same 

acreage were cruised today. This underestimation (by current 

standards) was significant. For the San Jacinto Reserve, for 

instance, Leiberg (1899a, p. 356) estimated the total volume at 

234 Chapter 11 

98 million board feet (MMBF). He noted, however, that "The 

sawmills in the reserve... handle the timber in the most economical manner possible, utilizing the trees far up in the crown, where 

the diameter dwindles to 8 inches or less. Worked up in this 

manner, the quantity of available merchantable timber would 

amount to at least 200,000,000 feet..."--a full doubling of the 

volume estimate. 

Volume estimates based on the plots could be derived directly from the data, but this too is risky because the criteria used 

for plot selection are unknown, and volume appears to have been 

very heterogeneously distributed in the forest. Fitch (1900a) 

noted that volume averaged 80-140 thousand board feet (MBF) 

per acre in selected areas of the Yosemite Quadrangle, but the 

overall average was between 30 and 40 MBF. Given the small 

number of plots available for analysis, coupled with the highly 

variable stocking levels present at the time, use of volume 

statistics derived from plot data to infer volume-per-acre for the 

forest system is not justified. For this reason, we have not used 

volume estimates to compare current and historical forest conditions. Instead we have used the proportions of stems in particular 

diameter classes, or the proportions of basal area by species. 

Diameter Distribution 

Stands described by Sudworth (1900a) were very large and 

very old. The average yellow pine, for instance, was reportedly 

150-180 feet tall, 3-4 feet in d.b.h., and 250-350 years old. 

Although Sudworth did not measure trees less than 11 inches in 

d.b.h., available evidence indicates that trees in those smaller 

classes were uncommon, though patches of very small regeneration appear to have been present, based on photos and text in 

Sudworth's report and on discussion by Leiberg (1902) of regeneration following removal of sheep. Most stems exceeded 25 

inches in d.b.h., and many extremely large specimens were 

measured (fig. 11J). Of the major timber species, sugar pine, 

Douglas-fir, and white fir occurred only as very large trees (fig. 

11K). Sugar pines were a minor stand component, but most of 

them were in the very large diameter classes. A comparison of 

that distribution with the largest diameter stands in Sierran forFigure 11J--Distribution of trees in forests of the Sierra Nevada in 1900. 

All species were lumped by diameter class and all of the plot information 

(Sudworth 1900a, 1900b) was combined. Many of the trees with diameters at breast height <15 inches were lodgepole pines. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Figure 11K--Diameter distribution of trees in the Sierra Nevada, by species, for published plot data 

(Sudworth 1900a). Sugar pine, Douglas-fir, and white fir were present only in very large diameter classes. 

ests today shows that far more of the stand basal area in the 

forests of 1900 was concentrated in very large trees (fig. 11L). 

Species Composition 

Species composition was apparently mixed at the turn of the 

century, with all major timber species represented. White fir and 

incense-cedar were widely distributed and present in large numbers (table 11B, fig. 11M). Pines did not dominate the forests, 

either in number or in volume. Volume estimates by species at 

about the turn of the century, available from several sources 

(table 11C), supported the plot-level data of Sudworth (1900a, 

1900b). Note that many estimates made at that time underestimated the volume in true fir, because those trees were not 

considered merchantable unless they were at least 16 inches in 

d.b.h., whereas pines were merchantable when 12 inches or 

more in d.b.h. (Leiberg 1902). For his part, Sudworth did not give 

forest-wide volume estimates, but noted that the elevational range 

of white fir was between 3,800 and 7,500 feet, and that it formed 

30-45 percent of the stand in areas where it was most abundant. 

When compared with the current species composition in 

Sierran forests, the composition at the turn of the century was 

reasonably similar (compare fig. 11N and table 11C). Comparing the forest-wide estimates made by the Plumas NF in 1913 

(Moore 1913) with current estimates from mixed-conifer timber 

strata (timber strata codes are defined in table 1C and in Appendix B), it appears that true fir and incense-cedar have increased 

and that pines have decreased (fig. 11O). This is probably an 

overestimate of the shift, however, because other strata, such as 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

Diameter at Breast Height (inches) 

Figure 11L--Basal-area distributions of trees in forests of the Sierra 

Nevada for 1900 and current stands. The 1900 distribution was based on 

information presented in figure 11J; the current distribution was based on 

Forest Service Region 5 inventory data from timber strata for the largest 

size-classes (4, 5, and 6). 

Chapter 11 235 
Table 11C-Estimates of the proportional volumes of conifers at various times

and locations in the Sierra Nevada. 

 Yellow pine1 Shasta fir 

 type, turn of type, turn of Plumas NF, M4G strata,2

Species this century this century 1913 1980-1990 

Sugar pine 20 0 16 12 

Yellow pine 38 21 39 15 

Incense-cedar 0 0 29 13 

Douglas-fir 21 0 14 16 

White fir 19 0 24 44 

Red fir 12 75 18 0 

Western white pine 0 3 0 0 

Mountain hemlock 0 2 0 0 

1 Yellow pine and Shasta fir types from Leiberg (1902). These data were for 

standing "mill timber" cruised using "Michigan practice"-trees >8 inches in 

d.b.h. and having  IHHW RI FOHDU OXPEHU LQ WKH WUXQN %RWK LQFHQVH-cedar and 

lodgepole pine were listed as being present in the stands, but they were not 

included in the mill estimates of timber volume. "Yellow pine" is either 

Jeffrey or ponderosa pine. 2

 Values shown for M4G strata were averaged over all Sierran National 

Forests. M4G strata were mixed-conifer stands, with trees  LQFKHV LQ

diameter at breast height, and >69 percent canopy closure (see table 1C). 

236 Chapter 11 

Figure 11M-Proportions of individuals and frequencies of occurrence 

among plots of tree species, based on plot data from Sudworth 

(1900a). 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Figure 11N--Species composition (1980-1990) in the most widespread mixed-conifer strata (JP =Jeffrey pine, PP = ponderosa pine, SP 

= sugar pine, WF = white fir, IC = incense-cedar, and DF = Douglas-fir). For the strata labels, M = mixed-conifer forest, "3" means that the 

dominant tree diameter-class lies between 12 and 24 inches; "4"indicates that it lies between 24 and 40 inches. "P" indicates poor stocking--

canopy closure 0-39 percent; "G" indicates good stocking-canopy closure >69 percent. These data were derived from Forest Service 

Region 5 inventory data. 

the ponderosa pine type, have far fewer firs and a greater percentage of yellow pine. It is reasonable, however, to infer from 

these data that the proportion of fir (basal area or volume) has 

increased by perhaps 10-20 percent, while the proportion of 

yellow and sugar pines has decreased by a similar amount. We 

are surprised that this trend has not been stronger, given the 

preference for logging yellow and sugar pine and the expected 

successional patterns of the forest. The stand structure at the turn 

of the century was often quite open, and became more scattered 

subsequent to heavy logging (figs. 11P and 11Q). These open 

stand conditions may have favored pine regeneration and helped 

to produce the species composition we see today (figs. 11R and 

11S). Compare figures 11P and 11Q with figures 11R and 11S to 

observe these changes. The trend toward the more shade-tolerant 

fir will be enhanced by selective removal of other species, by fire 

suppression, and by maintenance of the very dense stand conditions that exist in many areas of the Sierra Nevada today. The 

trend will, to a certain extent, be counteracted by infestations of 

the fir engraver beetle, to which these stands appear to be very 

susceptible. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

Figure 11O--Volume estimates, by species, in the Plumas National 

Forest in 1910 (Moore 1913) and from current inventory data. Mixedconifer (MC) strata M3G, M4P, and M4G were combined for this analysis. 

See figure 11 N for species codes and a description of the timber strata 

classifications. 

Chapter 11 237 
Figure 11P--Left half of 

two-photo historic panorama. Between 1924 and 1930, this area 

was owned and intensively 

logged by the Yosemite Lumber 

Company, then either sold to or 

exchanged with the Stanislaus 

National Forest. Typical of railroad logging days, about 70 percent of the timber was removed 

and about 30 percent left. An 

ample number of snags, cull logs, 

tops, and limbs were left scattered to decay naturally. 

Location: ----Looking west from 

the railroad grade, 

1 mile south of 

Camp 16, into 

Moss Creek Canyon, Stanislaus 

National Forest 

Elevation:-----6,257 feet 

Date: ----------1930 

Source: -----USDA Forest Service, Regional 

Photo Archives, 

San Francisco, CA 

Photographer: ---Unknown 

Figure 11Q-Right half of two-photo 

historic panorama. This photo was 

labeled "old" cutting (1929) on the left 

background, and "new" cutting (1930) 

on the right foreground. Note the 

area left-center (arrows) where soils 

were intensively impacted by skidding. Today this area still has mostly 

brush and young black oaks (fig. 

11S). Other areas, where some residual conifers were left and skidding 

had less impact, show a more rapid 

natural regeneration. 

Location: --Looking northwest from 

the railroad grade over 

an unnamed tributary to 

Moss Creek, Stanislaus 

National Forest 

Elevation: --6,257 feet 

Date: --------1930 

Source:------USDA Forest Service, 

Regional Photo Archives, 

San Francisco, CA 

Photographer: -Unknown 

238 Chapter 11 USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Figure 11R-Left half of 1991 comparative photo. Today the area shown 

in figure 11P shows a dense 

mixed-conifer forest of white fir, ponderosa pine, incense-cedar, sugar 

pine, and scattered black oaks. This 

regeneration is all natural. It has been 

classified as owl foraging habitat. 

Location: ----Same as previous historic photo (fig. 11P), but 

looking more tothe right. 

Elevation: -----6,257 feet 

Date: -----------December 1991 

Photographer: ----John S. Senser 

Figure 11S-Right half of 1991 

comparative photo. Arrows denote the same general areas 

seen in figure 11Q. Crane Flat 

Lookout (on the upper right 

peak) and the railroad grade 

are hidden by dense second 

growth. The light vegetation on 

the background slopes and left 

foreground is black oak in autumn foliage. Insect kills dominate the patch of gray conifers. 

Location: ----Almost the exact 

perspective as 

figure 11Q 

Elevation: ----6,257 feet 

Date: ----------December 1991 

Photographer: ----John S. Senser 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. Chapter 11 239 
Cutting History in 

the Sierra Nevada 

The rate of volume removal, in general terms, serves as an 

index of the level of logging disturbance on the land. We developed a cutting history for the Sierra Nevada from 1869-1990 

(fig. 11T). This was based largely on county records from 

1947-1990 available from the California Department of Forestry 

and Fire Protection (1947-78) and the California State Board of 

Equalization (1979-90), and from statewide totals by species 

prior to 1947 (May 1953). Prior to 1947, therefore, we assumed 

that all volume generated by species common in the Sierra 

Nevada was taken only from the Sierra Nevada. That species 

group included ponderosa and sugar pines, incense-cedar, and 

white fir. Volume estimates from the early period were probably 

inaccurate for several reasons. The assumption that the state's 

entire harvest of sugar and ponderosa pine came from the Sierra 

Nevada would tend to bias these estimates high. On the other 

hand, the volume estimates do not accurately account for fuelwood, 

shake production, and extensive wastage left in the woods (May 

1953, Laudenslayer and Darr 1990); all of those factors would 

tend to bias the volume estimate low. We do not know the extent 

to which these contrasting biases would tend to offset each other. 

By 1913, logging contracts were similar to those used today 

by the FS. Maximum stump height was 18 inches, and the 

merchantable top was set at 8-10 inches in diameter. The planned 

rotation period in the Plumas NF was set at 200 years, and the 

maximum cut was set at 132 MMBF (Moore 1913). Local use of 

wood for fuel apparently declined in the early part of the twentieth century. The proposed cut for the Stanislaus NF in 1912 

allocated only 164 MBF for "free use" (Graves 1912). Because 

of these restrictions, the market volume after 1910 probably 

reflected the actual cut reasonably well. 

Logging in the Sierra Nevada increased until about 1950, 

with a significant dip during the Great Depression. Logging 

levels declined slightly after 1950, remained fairly constant until 

the 1982 recession, then increased again to the point that 1990 

levels were near the historic peak. This pattern differs from that 

in north-coastal California, where the rate of logging increased 

dramatically after World War 11 and then declined (fig. 11U). 

Prescriptions used to cut timber in the Sierra Nevada have 

also differed from those of the coast. Logging in the Sierra 

Nevada prior to the 1980s seldom used a clearcutting prescription. If a tree had no market value, it was simply left standing. 

Even through the 1970s, when a policy shift toward clearcutting 

occurred, it accounted for most of the volume taken only from 

1983 to 1987 (fig. 11V). By the end of the decade, cutting was 

increasingly concentrated in salvage operations (fig. 11W). Because early cutting, even the relatively heavy cutting done on 

private lands, seldom involved clearcuts (figs. 11P-11S), disturbance patterns in the Sierra Nevada are very different today from 

those that have characterized the habitat of the northern spotted 

owl. In most of the lands occupied by the northern spotted owl, 

heavy clearcutting in recent years has generated a patchwork of 

uncut "old growth" and new plantations-a spatially heterogeneous system. In the Sierra Nevada, virtually all stands have 

been entered and trees have been selectively removed. Both 

private (Laudenslayer and Darr 1990) and public owners tended 

to apply similar harvest rules on large blocks of land. Because 

tree removal has targeted larger stems, and because these large 

trees appear to have been unevenly distributed on the landscape, 

forming groves (Fitch 1900b), it is likely that their removal 

caused a decrease rather than an increase in landscape-level 

forest heterogeneity. 

Figure 11T-Market volume of timber from the Sierra Nevada. Volume 

before 1947 was based on data supplied by May (1953); volumes for 

1947-1990 were based on records kept by the California Department of 

Forestry and Fire Protection (1947-78) and the California State Board of 

Equalization (1979-90). 

240 Chapter 11 

Figure 11U-Market volume of timber from California's North Coast. 

Volumes were derived from the same sources as those in figure 11T. 

Volume before 1947 was based on state-wide totals, assuming that all 

redwood and Douglas-fir came from coastal forests. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Figure 11V-Logging patterns by prescription for National Forests in the 

Sierra Nevada, 1982-1990 (from sold-sale records, Forest Service 

Region 5). 

Figure 11W-Acres entered for sanitation-salvage logging on National 

Forests in the Sierra Nevada, 1982-1990 (from sold-sale records, Forest 

Service Region 5). 

Discussion of Sierran 

Forest Conditions 

Because grazing and burning occurred simultaneously and 

in the same areas, we cannot separate their effects. As Sudworth 

(1900a) noted, however, destruction of a perennial grass community did not likely result from fire alone. The ability of intense 

grazing to interfere with regeneration is well known, and was 

documented by both Sudworth (1900a) and Leiberg (1902). The 

bare soils prevalent at the turn of the century probably provided 

an excellent seed bed for abundant tree regeneration, a process 

discussed by both Sudworth and Leiberg and supported by 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

Vankat (1970). The pattern of burning by Native Americans 

probably differed from that of sheep herders, because their goals 

differed. Both groups sought short-tern and long-term alteration 

of the forest. Native Americans probably used fire to herd game 

and to improve wild food crops (Vankat 1970, Lewis 1973, 

Anderson 1991). Sheep herders, on the other hand, sought to 

improve grazing and to remove obstacles to the passage of 

sheep. 

We do not expect, therefore, that the forest described at the 

turn of the century was in any sense pristine. Although most 

current FS lands were uncut then, the forest floor and regeneration structure were impacted repeatedly by intense grazing. Moreover, even if that forest were relatively untouched, it would not 

necessarily have looked like a pristine forest that would exist 

today. Many of the trees in older forests in 1900 were established 

in the 1600s or before, and grew during a period characterized by 

extended droughts. The periods from 1750 to 1820 and from 

1860 to 1880 were very dry in California (fig. 11X) (Fritts and 

Gordon 1980, Fritts 1991). The forest that Sudworth (1900a) 

described was largely established before those droughts began. 

Drought, combined with grazing and fire, created a forest 

dominated by very large, old trees and with very little ground 

cover. We believe the forests described at the turn of this century 

were less heterogeneous than the forests influenced by aboriginal and lightning fires. The latter were probably typified by 

heavily stocked areas on more mesic sites, more trees in intermediate size-classes, and more large-diameter logs and other woody 

materials on the ground (sheep herders specifically targeted 

large woody debris for burning). 

Cutting in the Sierra Nevada increased steadily over time, 

reaching a peak after World War II. Since then, with the exception of the 1982 recession, cutting has remained at fairly constant levels. The pattern of partial cutting that typically removed 

only the largest and oldest trees from a stand, coupled with 

abundant regeneration that followed removal of sheep from the 

forests and the initiation of fire suppression, resulted in a shift in 

diameter distribution of trees between the forests of 1900 and 

1990. Available evidence suggests that species composition, 

measured as basal area or volume, has not yet been substantially 

altered by these practices, but observation suggests that much of 

the current regeneration consists of true fir and incense-cedar. 

The mixed-conifer zone of the Sierra Nevada, therefore, has 

few or no stands remaining that can be described as natural or 

pristine. To various degrees, the forest system has been changed 

from one dominated by large, old, widely spaced trees to one 

characterized by dense, fairly even-aged stands in which most of 

the larger trees are 80-100 years old. This forest appears to be 

unstable. It is highly susceptible to drought-induced mortality, as 

competition for water weakens trees on drier sites. It is impacted 

by massive bark beetle infestations. And it is very flammable 

(Chapter 12). Its trajectory into the future is largely unknown, 

but stand structure can be expected to change markedly over the 

next 100 years. 

Chapter 11 241 
Figure 11X-Patterns of precipitation in the western United States from 1600 to 1970 (Fritts 1991, 

figure 7.4), based on tree-ring analyses. The solid line is based on data from the western United 

States, the small dots are based on data from a larger grid covering the entire United States, and 

the large dots in the twentieth century are based on measured precipitation data. 

242 Chapter 11 USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Southern California Forests 

at the Turn of the Century 

The Forest Reserve system in southern California consisted 

in 1897 of the San Bernardino, San Gabriel, San Jacinto, and 

Trabuco Canyon reserves. Today, the San Bernardino NF includes parts of the San Bernardino, San Jacinto, and San Gabriel 

(eastern edge) reserves. The Angeles NF contains most of the 

San Gabriel Reserve, and the Trabuco Canyon Reserve, which 

covered a portion of the Santa Ana Mountains, has been largely 

incorporated into the Cleveland NF. Leiberg (1899x, 1899b, 

1899c) surveyed the first three reserves in 1897, and Bamard 

(1900) resurveyed the 30-minute San Jacinto Quadrangle in 

1900 and provided a map similar to those created by Sudworth 

and Leiberg for the Sierra Nevada. 

Surveys included acreage outside of current FS boundaries. 

The San Bernardino and San Jacinto reserves, for instance, 

included many areas that are now privately owned, managed by 

the Bureau of Land Management, or incorporated into Indian 

Reservations. For this reason, total acreages are larger and the 

proportions of reserves listed in a nonforested condition are 

greater than would be indicated by current inventories. Most of 

the timbered acreage has, however, been retained by the FS, so 

statistics associated with forested lands at the turn of the century 

can be compared with those derived from modern inventories 

(table 11D). Given the differences in classification methods, the 

forested acreage statistics are remarkably similar. 

Table 11D-Acres of forested lands in southern California, as estimated in 

1899 and in 1987 and 1988. 

Year of estimate 

 1987 and 

Reserve 18991 19882

San Gabriel 

Forested 100,000 145,438 

Productive3 25,000 26,687 

Total acres 650,000 651,874 

San Bernardino 

and San Jacinto 

Forested 441,000 389,892 

Productive3 122,500 160,631 

Total acres 1,474,000 649,900 

1 San Bernardino Reserve (Leiberg 1899a); San Jacinto Reserve (Leiberg 

1899b); San Gabriel Reserve (Leiberg 1899c). Leiberg did not know the exact 

boundaries of the reserves. 2

 Data from the Angeles National Forest correspond to the San Gabriel 

Reserve (USDA Forest Service 1987); data from the San Bernardino National 

Forest correspond to the San Bernardino and San Jacinto Reserves (USDA 

Forest Service 1988). 3

 Productive lands were defined as Class 1 lands (both accessible and capable 

of producing timber) by Leiberg, and lands capable of producing >20 cubic feet/ 

acre/year for estimates by the USDA Forest Service (1987). 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

Species Composition in 1897 

Leiberg (1899a, p. 356) estimated species composition in 

the three reserves that he surveyed (figs. 11Y, 11Z, and 11AA). 

The methods used to obtain these data were not stated in detail, 

but Leiberg wrote: "These estimates were based on the customary method of scaling standing timber," suggesting that his 

estimates were based, at least in part, on measured plot data. The 

survey effort in 1897, however, appears to have been less intense 

than in 1900 or 1902. Because less time was spent in the area, the 

extent to which vegetation patterns were derived from inference, 

instead of direct observation, would increase. Therefore, tree 

species that are heterogeneously distributed-especially minor 

species-easily could be either over- or under-represented. 

The general pattern in 1900 was one of low-elevation forests 

dominated by either ponderosa pine or bigcone Douglas-fir, with 

white fir entering as a major stand component above 6,000 feet. 

Lodgepole pine was present in quantities only above 8,500 feet. 

Nonaboriginal Human 

Disturbance by 1900 

Logging occurred in the San Bernardino Mountains as early 

as 1865 (Leiberg 1899b), but was limited in extent. By 1897, 

logging was still limited to an area north of San Bernardino; it 

did not extend appreciably into the eastern or northern portions 

of the San Bernardino Reserve. Logged areas mapped in the San 

Jacinto Reserve were very small, lying primarily in the upper 

basin of the North Fork of the San Jacinto River and in an area 

adjacent to the town of Idyllwild, in the Strawberry Creek drainage. The total area logged in the San Jacinto covered "not more 

than 1 square mile" (Barnard 1900, p. 575). No extensive timber 

cutting had occurred in the San Gabriel Reserve by the end of 

last century (Leiberg 1899c). 

Little evidence exists of extensive grazing in the forests of 

southern California at the turn of the century. Leiberg (1899b, p. 

360) estimated that no more than "a few hundred head of stock" 

were regularly pastured in the San Bernardino Reserve, although 

several thousand head were grazed there in 1897 because of a 

drought. Leiberg did not state exactly what a "head of stock" was 

but, judging from his comments on the San Jacinto Reserve, he 

probably meant cattle. In the San Jacinto, he stated that "An 

unknown number of cattle and horses-probably 1,500-2,000 

head-graze in the reserve" (Leiberg 1899a, p. 354). Grazing 

was not discussed for the San Gabriel Reserve. There is, however, evidence of extensive grazing by sheep in the San Bernardino and San Jacinto Mountains in the late 1800s. Minnich 

(1988) reported that sheep grazing occurred over much of the 

San Bernardino Mountains during the period from 1860 to 1898, 

to such an extent that many areas were grazed to a bare-earth 

condition. In particular, as many as 30,000 sheep were grazed in 

the area around Little Bear Valley and an additional 30,000 were 

grazed in the Santa Ana River drainage (Minnich 1988, p. 39- 

40). As with reports of grazing in the Sierra Nevada, the extent 

of destruction that resulted from sheep grazing may have been 

exaggerated because of the biases of early observers. 

Chapter 11 243 
Figure 11Y-Species composition in the San Bernardino Forest Reserve in 1897 (Leiberg 1899b). Hardwood species, with 

the exception of oaks, were not used when Leiberg calculated percent contribution, by species. 

Figure 11Z--Species composition in the San Jacinto Forest Reserve in 1897 (Leiberg 1899a). Hardwood species, with

the exception of oaks, were not used when Leiberg calculated percent contribution, by species. 

244 Chapter 11 USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Elevation (feet) 

Figure 11AA--Species composition in the San Gabriel Forest Reserve in 1897 (Leiberg 1899c). Hardwood 

species, with the exception of oaks, were not used when Leiberg calculated percent contribution, by species. 

The San Gabriel Mountains are extremely steep, so they 

were not logged or grazed extensively. They were (and still are) 

very susceptible to burning, however. In particular Leiberg (1897c, 

p. 369) noted the destruction of bigcone Douglas-fir stands by fire: 

Among the non-commercial species of trees the bigcone fir 

has suffered the most. Large tracts once covered by it have 

been totally laid waste and much of what remains is partially 

burned or scarred... It is not improbable that a considerable 

Figure 11AB-Logging intensity in San Bernardino and Los Angeles 

Counties, 1947-1990 (California Department of Forestry and Fire Protection 1947-78, California State Board of Equalization 1979-90). 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

portion of the areas now grown up in brush were in the past 

covered with a forest of bigcone fir. The very numerous small 

groves and individual trees of the species rising from the sea 

of chaparral would lead one to infer that they represent 

remnants of a more extensive forest. It is also noteworthy that 

the worst-burnt areas in the three reserves examined are to 

be found in the San Gabriel Reserve in the region of the most 

extensive mining operations. 

Cutting History in 

Southern California 

We have reconstructed the history of the removal of timber 

volume from San Bernardino and Los Angeles counties. Volume removed from 1947 to 1990 was based on data from the 

California Department of Forestry and Fire Protection (1947-78) 

and the California State Board of Equalization (1979-90). The 

San Bernardino NF had a very active timber program after 

World War II, cutting 27.4 MMBF in 1963 alone (fig. 11AB). 

Altogether, 362.3 MMBF of timber have been removed from 

these counties since 1947. 

Discussion 

Changes induced by nonaboriginal man in the southern 

California forests appear to have been slight by the turn of the 

century. Some logging and grazing had occurred by then. Leiberg 

described fires as being widespread but, with the exception of 

bigcone Douglas-fir stands in the San Gabriel Mountains, he 

apparently did not believe that forest structure or the balance 

Chapter 11 245 
between brush and conifer forests was being changed by the fire 

patterns. Leiberg had no way to ascertain the extent to which 

burning patterns at that time were unnatural. 

In the period after World War II, forest structures would 

have been significantly altered where timber was logged. Because logging would have been concentrated on sites with higher 

productivity, it undoubtedly impacted spotted owl habitat, though 

we cannot determine the extent of that impact. In general, the 

proportion of the area supporting conifer forests appears to have 

been reasonably static over the last 90 years. No evidence supports the idea of either spreading or shrinking acreage of chaparral in southern California over that period. 

References 

Anderson, Kat. 1991. From tillage to table: the digging and burning of Indian 

potato plants among California Indian tribes as examples of sustainable 

horticulture. In: Blackburn, Tom; Anderson, Kat; symposia organizers. 

Native Americans as environmental managers. The seventh annual California Indian Conference. Rohnert Park, CA: Sonoma State University; 12 p. 

Ayres, R. W. 1958. History of timber management in the California National 

Forests, 1850 to 1937. Washington, DC: U.S. Department of Agriculture, 

Forest Service; 86 p. 

Barnard, E. C. 1900. San Jacinto Quadrangle, California. In: Annual Reports 

of the Department of the Interior, 21st Annual Report of the U.S. Geological Survey, Part 5. Washington, DC: Government Printing Office; 575. 

Bohakel, Charles A. 1968. A history of the Empire Mine at Grass Valley. 

Grass Valley, CA: Nevada County Historical Society; 24 p. 

California Department of Forestry and Fire Protection. 1947-78. Production of 

California timber operators. State forest notes published annually. Sacramento: California Department of Forestry and Fire Protection. 

California State Board of Equalization. 1979-90. California timber harvest by 

county. Annual report. Sacramento: California State Board of Equalization, 

Timber Tax Division. 

Clark, William B. 1966. Economic mineral deposits in the Sierra Nevada. In: 

Geology of Northern California. San Francisco: Bulletin 190, California 

Division of Mines and Geology; 209-216. 

Ewing, Robert A.; Tosta, Nancy; Tuazon, Raul; Huntsinger, Lynn; Marose, 

Robin; Nielson, Ken; Motroni, Robert; Turan, Sarah. 1988. Rangelands: 

growing conflict over changing uses. Sacramento: Forest and Rangeland 

Resources Assessment Program, California Department of Forestry and 

Fire Protection; 348 p. 

Fitch, C. H. 1900a. Sonora Quadrangle, California. In: Annual Reports of the 

Department of the Interior, 21st Annual Report of the U.S. Geological 

Survey, Part 5. Washington, DC: Government Printing Office; 569-571. 

Fitch, C. H. 1900b. Yosemite Quadrangle, California. In: Annual Reports of 

the Department of the Interior, 21st Annual Report of the U.S. Geological 

Survey, Part 5. Washington, DC: Government Printing Office; 571-574. 

Fritts, Harold C. 1991. Reconstructing large-scale climatic patterns from treering data: diagnostic analysis. Tucson, AZ: University of Arizona Press; 286 

p. 

Fritts, Harold C.; Gordon Geoffery A. 1980. Annual precipitation for California since 1600 reconstructed from western North American tree rings. 

Agreement No. B53367. Sacramento: California Department of Water 

Resources; 45 p. 

Graves, Henry S. 1912. Letter to the Secretary of Agriculture. Washington, 

DC: U.S. Department of Agriculture; 3 p. 

246 Chapter 11 

Laudenslayer, William F.; Darr, Herman H. 1990. Historical effects of logging 

on the forests of the Cascade and Sierra Nevada ranges of California. 

Transactions of the Western Section of the Wildlife Society 26:12-23. 

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Annual Reports of the Department of the Interior, 19th Annual Report of 

the U.S. Geological Survey, Part 5. Washington, DC: Government Printing 

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Leiberg, John B. 1899b. San Bernardino Forest Reserve. Preliminary report. 

In: Annual Reports of the Department of the Interior, 19th Annual Report 

of the U.S. Geological Survey, Part 5. Washington, DC: Government 

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Leiberg, John B. 1899c. San Gabriel Forest Reserve. Preliminary report. In: 

Annual Reports of the Department of the Interior, 19th Annual Report of 

the U.S. Geological Survey, Part 5. Washington, DC: Government Printing 

Office; 367-371. 

Leiberg, John B. 1902. Forest conditions in the northern Sierra Nevada, 

California. Professional paper No. 8, Series H, Forestry 5. Washington, 

DC: U.S. Department of the Interior, U.S. Geological Survey; 194 p. 

Lewis, Henry T. 1973. Patterns of Indian burning in California: ecology and 

ethnohistory. Ramona, CA: Ballena Press Anthropological Papers No. 1; 101 

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Marshall, C. H. 1900. Mount Lyell Quadrangle, California. In: Annual Reports 

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May, R. H. 1953. A century of lumber production in California and Nevada. 

Forest Survey Release No. 20. Berkeley, CA: Pacific Southwest Forest and 

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McKeon, Owen F. 1984. The railroads and steamers of Lake Tahoe. The 

Western Railroader. San Mateo, CA: Francis Guido, Publisher; 22 p. 

Minnich, Richard A. 1988. The biogeography of fire in the San Bernardino 

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Moore, Barrington. 1913. Forest plan, Plumas National Forest. Quincy, CA: 

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Palmer, Lex. 1992. Metal mining on California's National Forests: a portrait of 

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World War II. Unpublished draft supplied by author; 195 p. 

Ratliff, Raymond D. 1985. Meadows of the Sierra Nevada of California: state 

of knowledge. Gen. Tech. Rep. PSW-84. Berkeley, CA: Pacific Southwest 

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Sudworth, George B. 1900a. Stanislaus and Lake Tahoe Forest Reserves, 

California, and adjacent territory. In: Annual Reports of the Department of 

the Interior, 21st Annual Report of the U.S. Geological Survey, Part 5. 

Washington, DC: Government Printing Office; 505-561. 

Sudworth, G. B. 1900b. Notes on the regions in the Sierra Forest Reserve, 

1898-1900. Unpublished draft supplied by author; 141 p. 

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USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Chapter 12 

Fire and Fuels Management in Relation to Owl Habitat 

in Forests of the Sierra Nevada and Southern California 

C. Phillip Weatherspoon, Susan J. Husari, and Jan W. van Wagtendonk 

Over the millennia, fire has influenced the structure and 

function of most forested ecosystems and, consequently, most 

spotted owl habitat in California. Fire is a force that we must 

understand and attempt to deal with--as a shaper of ecosystems, 

as a tool, and as a potential destroyer of habitat-if we are to 

manage intelligently for the California spotted owl. In this chapter we discuss some fire management considerations related to 

protecting and possibly enhancing owl habitat. We concentrate 

on Sierran mixed-conifer forests--the most important and extensive habitat type statewide for the spotted owl, and on live 

oak/bigcone Douglas-fir forests--one of the most important

habitat types in southern California (table 1B). We include brief 

discussions of three additional types-Sierran red fir and southern California mixed-conifer and riparian/hardwood.

Sierran Mixed-Conifer 

Sierran mixed-conifer forests contain an estimated 62 percent of all California spotted owl sites in California (table 1B). 

They are characterized by mixtures of white fir, ponderosa pine, 

sugar pine, incense-cedar, black oak, Douglas-fir, and giant 

sequoia. The forest type occurs throughout middle elevations of 

the Sierra Nevada (Tappeiner 1980; table 1B). 

An appreciation of the concept of fire regimes is helpful in 

understanding the role of fire in mixed-conifer forests. A fire 

regime is an expression of the frequency, severity, and extent of 

fires occurring in an area (Agee 1990). It is a function of the 

frequency with which fuels are dry and continuous enough to 

carry fire at a time when ignition sources are available, and thus 

is strongly related to climate. In general, the longer the fire-free 

interval, the more severe the fire that follows, because more 

fuels have accumulated in the interim. 

Prior to European settlement in the mid-1800s, Sierran 

mixed-conifer forests were characterized by a short-interval, 

low- to moderate-severity fire regime. As a result of human 

activities since the mid-1800s (Chapter 11), including a policy of 

fire suppression initiated soon after the beginning of the twentieth century, this fire regime has been changed to one of less 

frequent but substantially more severe fires. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

Presettlement Fire Regimes 

Several studies have shown that mixed-conifer forests burned 

rather frequently during the centuries preceding European settlement. Lightning and, in many areas, Native Americans provided 

the ignition sources. The most comprehensive body of work on 

fire history in the Sierra Nevada has been done in the giant 

sequoia groves (Swetnam et al. 1991, Swetnam pers. comm.). 

These studies documented fire occurrence for 1,500 years and 

indicated a mean fire interval of 5 to 10 years since 1300 AD for 

the five groves studied. Maximum fire intervals recorded were 

20 years. Other studies (Kotok 1933, Wagener 1961, Kilgore 

and Taylor 1979, Warner 1980, Skinner 1991) suggested that the 

mixed-conifer type had a mean fire interval in the range of 5 to 

30 years. Mean fire intervals varied in response to site and 

environmental factors that affected ignition source, fuel accumulation, fuel moisture, and burning conditions. Thus, more 

mesic sites (for example, moist canyon bottoms, northerly slopes, 

and higher elevations) and sites protected from winds burned 

less often than relatively xeric and/or exposed sites (Kilgore and 

Taylor 1979, Teensma 1987). (The relatively mesic sites may 

tend to be more closely associated with nesting and roosting 

habitat used by the spotted owl.) Furthermore, climatic fluctuations on a time scale of centuries (fig. 11X) were reflected in 

more frequent fires during drier periods (Swetnam et al. 1991). 

Frequent fires in the mixed-conifer type maintained surface 

fuels at fairly low levels, and kept understories relatively free of 

trees and other vegetation that could form fuel ladders to carry 

surface fires into the main canopy. This effect of frequent fires, 

together with widespread, heavy grazing by sheep after the 

mid-1800s (Chapter I1), probably accounts for the common 

reports by early observers that forests of the Sierra Nevada were 

open and parklike (Sudworth 1900, Biswell 1989). Because fuel 

accumulation was limited, most fires were of low to moderate 

severity (Sudworth 1900, Kilgore 1973, Biswell 1989). 

High-severity crown fires usually could not be sustained over 

large areas (Show and Kotok 1924, Kilgore 1973, Kilgore and 

Taylor 1979). On the other hand, crown fires that affected small 

areas .(ranging in size from a single tree, to groups of trees, to 

perhaps several acres) probably were relatively common and an 

important influence on stand structure. These patches of high 

fire severity, interspersed within a "matrix" of low-severity fires 

(Stephenson et al. 1991), occurred in areas with heavy fuel 

accumulations, sometimes reinforced by steep slopes or extreme 

weather conditions. This complex fire regime, along with other 

agents of disturbance (for example, group kills of trees by bark 

beetles), produced a variable, irregular patchwork of even-aged 

Chapter 12 247 
groups, most from less than an acre to several acres in size 

(Show and Kotok 1924; Rundel et al. 1977; Bonnicksen and 

Stone 1981, 1982; Biswell 1989). Consequently, a relatively 

fine-grained pattern of variability, modified by topography, existed at a landscape scale. 

Openings created by fires and other disturbances provided 

conditions favorable for regeneration and growth of 

shade-intolerant and relatively fire-resistant trees and other plants. 

These species include ponderosa pine, giant sequoia, and California black oak, which is only moderately resistant to top-kill 

by fire but sprouts vigorously. They were able to regenerate 

successfully in the presence of frequent fires because of the fuel 

dynamics of openings in which they became established: A 

typical scenario may have begun with the death of a small 

group of trees-by bark beetles, locally-intense fire, or other 

causes. The resulting concentration of fuel was reduced by one 

or more fires. Mineral soil was exposed, and competing vegetation (including reserves of dormant seeds stored in duff and soil) 

was reduced. Given a good cone crop and favorable soil 

moisture and other conditions, seedlings became established. 

Subsequent fires in the vicinity burned only lightly, if at all, 

through the opening because of the local lack of an overstory to 

provide sufficient litter to carry the fire. By the time the new 

regeneration produced enough litter to carry a fire of appreciable intensity through the opening, some of the young trees 

were large enough to survive the fire (Kilgore 1973, Biswell 

1989). Fire-resistant species would have comprised a disproportionate number of the survivors. 

The more shade-tolerant and fire-sensitive species (white fir 

and incense-cedar) regenerated beneath overstory trees as well 

as in openings. Periodic fires, however, kept their numbers 

relatively low, especially in the understory. The more mesic sites 

generally experienced longer fire intervals and thus permitted 

more individuals of fire-sensitive species to survive and grow 

(Kilgore and Taylor 1979). 

Twentieth Century Fire Regimes 

Twentieth century fire regimes bear little resemblance to 

those of presettlement times, largely because of human activities 

since the mid-1800s. One marked difference has been in the 

annual acreage burned. For example, using a conservative mean 

fire interval of 20 years for the 586,000-acre Eldorado National 

Forest (NF), we would expect a mean of 29,000 acres to burn 

annually. In fact, 13,944 acres burned during the entire period 

from 1970 through 1990-an average of only 664 acres per year. 

On the 1,168,500-acre Plumas NF, 85,000 acres burned in the 

same 21-year period-an average of 4,048 acres per year, although about 58,000 acres would be expected to burn there each 

year with a mean fire interval of 20 years. 

We must get beyond the number of ignitions and acres 

burned, however, to evaluate relations of wildfires to spotted 

owl habitat. A study of lightning fires by vegetation type in 

Yosemite National Park (NP) showed that over 50 percent of the 

2,000 fires ignited between 1930 and 1983 occurred in the 

mixed-conifer zone (van Wagtendonk 1986). Forest Service 

248 Chapter 12 

statistics do not categorize fires according to vegetation types. 

Most NFs, however, do have maps that can provide useful 

information about the spatial and temporal distribution of fires 

during the twentieth century. Figures 12A and 12B, for example, 

show the distribution of all lightning fires and all large fires 

(100+ acres) on the Lassen NF since 1900. Lightning-caused 

ignitions were scattered over the entire forest. Fires that escaped 

initial attack and grew large, however, were not evenly distributed. Large fires were grouped in Sierran foothill areas and on 

the east side of the crest. Similar patterns occurred farther south 

in the Sierra Nevada. In the 1920s and 1930s, many fires occurred in Sierran foothills and at lower elevations in the 

mixed-conifer belt-the same general areas where early timber 

operations (Chapter 11) and widespread burning for range improvement were concentrated. Fires east of the Sierran crest 

accounted for much of the balance of the large fires this century. 

Over 50 percent of the nearly 85,000 acres of forested land 

burned on the Plumas NF since 1970, for example, have burned 

east of the crest. Within these two broad zones that dominate 

forest fire statistics in the Sierra Nevada, owls use significant 

portions of the lower-elevation forests on the western slopes, at 

least down through the ponderosa pine/hardwood type to the 

upper digger pine/blue oak type, but they are rare in the eastside 

forests. In these two zones, fires more often move rapidly beyond the initiating stage and defy initial attack because of flashy 

fuels, drier conditions, and exposure to high winds. 

By comparison, success of initial attack on wildfires evidently is greater in areas of owl habitat within the Sierran 

mixed-conifer type. Countryman's (1955) description of fuel 

conditions within old-growth stands applies in large measure to 

fuel conditions within many mixed-conifer stands used by the 

California spotted owl. These stands are less flammable under 

most conditions, because the dense canopies maintain higher 

relative humidities within the stands and reduce heating and 

drying of surface fuels by solar radiation and wind. The reduction of wind velocity within closed stands discussed by Countryman is supported by wind reduction factors identified by Rothermel 

(1983) for stands with closed canopies. Windspeed at mid-flame 

height for fires burning in surface fuels is approximately one-tenth 

of the windspeed 20 feet above the stand canopy. 

As fuels accumulate, however, fires that do escape initial 

attack -usually those burning under severe conditions-are 

increasingly likely to become large and damaging. Success in 

excluding fire from large areas that were once regulated by 

frequent, low- to moderate-severity fires has simply shifted the 

fire regime to one of long-interval, high-severity, stand-replacing 

fires (van Wagtendonk 1974, Kilgore 1973, Parsons and 

DeBenedetti 1979, Agee and Edmonds 1992). Forests where 

owls are found more often than expected (Chapter 5) satisfy the 

structural requirements outlined by Rothermel (1991) for the 

propagation and spread of crown fires-heavy accumulations of 

dead-and-downed fuels, conifer reproduction and other ladder 

fuels, and continuous forests of conifer trees. Crown fires occur 

when these features are combined with dry fuels, low humidities, high temperatures, steep slopes, strong winds, and unstable 

atmosphere. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Figure 12A--Fire history of the Lassen National Forest from 1900 through 1939. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. Chapter 12 249 
Figure 12B--Fire history of the Lassen National Forest from 1940 through 1990. 

250 Chapter 12 USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Two recent examples of severe, stand-replacing wildfires in 

owl habitat in the mixed-conifer type were the Stanislaus Complex (1987) on the Stanislaus NF and the Stormy Fire (1990) on 

the Sequoia NF. Both of these fires burned areas with known 

owl sites. Numerous other large fires have burned in the general 

elevational band where owl habitat is found. Although these 

areas had not been surveyed for owls prior to burning, we assert 

that they were also home to owls prior to fire. Based on such 

fires, we suggest three scenarios for severe wildfires in Sierran 

or southern California mixed-conifer types: 

Scenario l: A fire initiates low on the slope, typically in 

brushfields, and spreads into the mixed-conifer zone. The fire 

does not drop to the forest floor because heavy fuel accumulations and fuel ladders, combined with steep slopes, promote 

either spread through crowns or widespread torching of trees. 

This is the expected model for areas of mixed-conifer forest 

lying directly upslope from large brushfields and for imbedded 

habitat in brushfields in southern California forests. Such fires 

bum for one to several burning periods (see glossary). Fire 

behavior is controlled by topography and fuels. Generally the 

fire is contained within one or several drainages. Control lines 

are constructed at the break in slope. Regular occurrence of this 

type of fire results in gradual loss of habitat. 

Scenario 2: Many lightning fires are ignited simultaneously 

and fire-fighting resources are quickly exhausted. All resources 

are devoted to initial attack or to protection of structures and 

communities threatened by fires that escape initial attack. Fires 

become large and burn under changing weather and fuel conditions, until enough resources can be gathered and organized to 

effect control. In such cases fires burning in uniform fuels 

become so large that suppression remains unsuccessful until 

weather conditions moderate, spread ceases, and firelines can be 

constructed around the entire perimeter. Groups of lightning 

fires in 1977 and 1987 were examples of this scenario. 

Weatherspoon and Skinner (1992) found that the effects of such 

unconstrained fires depended on the fuel conditions they burned 

through, and corresponded well with prior management actions, 

including fuel treatments. 

Scenario 3: A human-caused fire starts under severe fire 

weather conditions. Fire spread continues through the duration 

of the wind event. Examples of synoptic weather patterns associated with such events are detailed by Schroeder et al. (1964). 

Examples are east winds in the northern Sierra Nevada and 

Santa Ana conditions in southern California. 

A study of lightning fires and spotted owl territories in 

Yosemite NP showed that owls can and do exist with extensive fires of varying intensities where forest structure has been 

affected relatively little by human activities. Fifty-six owl 

sites were confirmed in the Park during surveys in 1988 and 

1989 (Gould pers. comm.), and an additional 45 locations 

were identified as probable owl sites in unsurveyed areas of 

the Park where the habitat appears to be suitable for the owls 

(Steger pers. comm.). Among the 56 confirmed sites, six had 

been burned by prescribed natural fires during the 8 years 

prior to the surveys when owls were located there (fig. 12C). 

Over 800 acres of habitat within a 1,200-foot radius of confirmed owl sites had been burned, and an additional 1,300 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

acres had been burned within a 2,000-foot radius. Crown fires 

were extensive in one case, although the nest site itself was 

under-burned. The remaining fires were primarily low to moderate in intensity, with only occasional torched areas. 

Effects of Fire Suppression 

The structure and composition of Sierran mixed-conifer 

forests have been affected profoundly by fire suppression policies begun in the early 1900s. This and other forest types with 

short-interval fire regimes have been changed more by suppression than types with longer fire intervals because more cycles of 

fire and associated fire effects have been excluded in the 

short-interval regimes. 

As frequent fires of low to moderate severity ceased to be a 

dominant ecological force, shade-tolerant and fire-sensitive tree 

species (especially white fir) increased dramatically in abundance, particularly in small to medium size classes. Previously 

much less common except in the cool and moist extremes of the 

type, multiple-canopied stands consisting largely of these 

shade-tolerant species are now common (Parsons and DeBenedetti 

1979, Bonnicksen and Stone 1982, van Wagtendonk 1985). 

Regeneration of pines, black oak, and other shade-intolerant 

species has declined, except in areas opened by wildfires or 

management activities. "Selective" cutting has reinforced the 

changes in stand structure and composition brought about by fire 

exclusion (color photo 5-10). The patchwork of small, even-aged 

aggregations that characterized the mixed-conifer type before 

1900 has become less distinct (Bonnicksen and Stone 1982). 

Consequently, stands have become more complex when viewed 

vertically, but less complex and more homogeneous in terms of 

areal arrangement. 

With fire suppression, fuels on the forest floor (including 

coarse woody debris) have accumulated far beyond their normal 

levels (Parsons and DeBenedetti 1979). The increased prevalence of white fir in the understory has created hazardous fuel 

ladders, linking surface fuels to upper canopy layers (color 

photos 5-5, 5-10, 5-12 to 5-14). Increases not only in quantity, 

but also in horizontal and vertical continuity, of fuels have 

substantially increased the probability of large-scale, catastrophic 

fires (Kilgore and Sando 1975, van Wagtendonk 1985). 

Unnaturally dense stands mean more competition for available water, and therefore greater moisture stress. During periods 

of drought, notably the current one (fig. 4H), extensive mortality 

is the result-some directly from drought stress, much from 

stress-induced bark beetle outbreaks. These stands may be more 

prone to damage from defoliating insects and various root and 

stem diseases as well. The dead and dying trees also add greatly 

to the already high fuel loads, thereby increasing intensity and 

rate of spread in the event of a wildfire. More snags and large 

woody fuels are likely to increase fire spotting and suppression 

difficulty, and greater heating damage to soils may result from 

consumption of more large materials. Moreover, opening of the 

canopy as a result of tree mortality permits more solar radiation 

and wind to reach the ground, resulting in warmer and drier fuels 

(Countryman 1955), which ignite more easily and support more 

intense, faster-spreading fires. 

Chapter 12 251 
Figure 12C--Known and probable spotted owl sites and lightning fires in Yosemite National Park from 1930 through 1989 (based on data from Gould 

pers. comm., Steger pers. comm., and J. W. van Wagtendonk pers. observ.). 

252 Chapter 12 USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Implications for Spotted Owl 

Management 

Several changes at least partly attributable to fire suppression (for example, increased stand density, greater development 

of middle and lower canopy layers, more snags, more coarse 

woody debris) have been associated with biologists' perceptions 

of suitable owl habitat. Other such changes may be detrimental 

to habitat quality. Excessively dense understories may impede 

foraging and, to the extent that diversity of tree species-to 

include pines and oaks-is important, continued exclusion of 

fire may be degrading habitat quality. Much uncertainty still 

clouds our understanding of which stand attributes are critical 

for owls and which are only incidental. Nevertheless, it is possible that fire exclusion in Sierran mixed-conifer forests has led 

to a net improvement in spotted owl habitat there. 

If owl habitat has improved as a result of fire suppression, 

such improvement may well be illusory and short-lived. Fire is 

inevitable in these forests, and the probability of catastrophic 

fire-certainly one of the greatest threats to owl habitat-increases as surface fuels and ladder fuels continue to accumulate 

(Kilgore and Sando 1975, van Wagtendonk 1985). Overly dense 

stands are subject to extensive mortality from drought and insects, including loss of the most desirable large, old trees. 

Another possibility is that owls were highly successful in 

presettlement stand structures resulting from the unimpeded 

functioning of natural processes, including fire. For Sierran 

mixed-conifer forests, such a landscape probably consisted of a 

complex array of mostly small, even-aged aggregations and/or 

stands representing a wide range of age- and size-classes. Compared with current stand structures, stands would have been less 

dense, and groups of different-sized trees would have been 

separated more horizontally into even-aged aggregations with 

less vertical diversity within groups. 

In either case, a management policy characterized as 

"hands-off plus fire exclusion" (allow forest succession to proceed uninterrupted by periodic natural disturbances) would likely 

lead to degraded and depauperate, rather than healthy and 

biologically diverse, ecosystems. A more prudent and conservative policy for the spotted owl, as well as for other species 

and ecosystem components, would be to use our understanding of natural ecosystem processes (including fire) to guide 

management (see management options later in this chapter 

and in Chapter 13). 

Fuels 

Nesting and roosting areas in Sierran mixed-conifer forests 

exhibit structural characteristics (Chapter 5) that affect fire behavior. These stands have large trees, closed canopies, and 

multiple layers. Vertical and horizontal structures are continuous, encouraging the movement of fire into tree crowns. Such 

movement is discouraged if lower canopy layers are removed, 

interrupting the upward spread of fire. 

The limited quantitative data on surface fuels at spotted owl 

sites (see tables in Chapter 5) suggest that loadings of largeUSDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

diameter (greater than 11 inches) woody fuels are variable, but 

generally moderately heavy when compared to the range of 

loadings of natural fuels described by Blonski and Schramel 

(1981) for mixed-conifer forests in the Sierra Nevada. The guidelines in Chapter 1 for retention of at least 10-15 tons/acre of large 

downed wood represent the low end of the range of values 

observed in owl habitat-a loading probably acceptable from a 

fuels standpoint in most situations. Residues less than 11 inches 

in diameter probably are less essential to owls. Removal of 

smaller fuels, especially those less than 3 inches in diameter, 

results in reduced fire intensities, lower rates of spread, and 

lower resistance to control. 

Fire and Fuels Management Options 

Chapter 1 includes general guidelines for fuels management 

as part of its recommendations for interim management of "Other 

Forested Public Lands" in the Sierra Nevada. An alternative 

approach for interim management of "Selected Timber Strata" 

presented in Chapter 13 also includes guidelines for fuels management. In addition, Chapter 13 offers suggestions for fuels 

management as part of a set of potential long-term strategies to 

manage for owl habitat in Sierran mixed-conifer forests. Management activities described in Chapters 1 and 13 involve various kinds of cuttings. Because of the additional fuels created by 

these cuttings, and the warmer and drier microclimate at the 

forest floor resulting from stand openings, adequate treatment of 

slash fuels is essential. Otherwise, wildfire hazard is likely to be 

greater than before the activity (Countryman 1955, Weatherspoon 

and Skinner 1992). Several options are normally available to 

treat fuels, and revenues from harvested timber often will fund 

part or all of the planned fuels management. 

In this section, we discuss a general approach for using 

prescribed burning to manage fuels (in addition to any other 

benefits that might accrue) in areas of nesting and roosting 

habitat where commercial harvesting of timber or other wood 

products will not be done. Such areas might include "Protected 

Activity Centers" (Chapter 1) and any other areas of owl habitat 

from which logging has been excluded administratively or legally. They might also include "Selected Timber Strata" in 

locations where harvesting is not economical for various reasons, given the constraints outlined in Chapter 1. Effects of 

prescribed burns should be carefully monitored as part of adaptive management experiments designed to improve our understanding of owl habitat and our ability to manage for it (Chapters 

2 and 13). 

Considering the significant and increasing risk of 

stand-destroying wildfire, as discussed earlier, we recommend 

that prescribed burning in these areas be given a high priority. 

Without substantial increases in funding for prescribed burning, 

the likelihood of losing large acreages of owl habitat to severe 

wildfires will increase over time. Additional prescribed burning 

would produce more smoke at a time when regulation of air 

quality is becoming increasingly restrictive. Consequently, the 

benefits of underburning-for owl habitat, for other ecosystem 

values, and for reducing smoke from wildfires-and associated 

Chapter 12 253 
tradeoffs in air quality will need to be clearly defined and 

articulated. This should be coupled with increased utilization of 

woody material and greater use of nonburning methods of fuels 

management in timber harvest areas, to the extent that they make 

sense ecologically and managerially. The prescribed burning 

recommended here should be coordinated, on a broad scale, with 

fuels management activities described in Chapters 1 and 13 to 

ensure the most effective and efficient use of dollars and other 

resources for protecting owl habitat. In many areas, succession 

and fuel accumulation have progressed to the point that prescribed burning is impractical as a first treatment. Outside "Protected Activity Centers," such areas may require some combination of understory thinning and mechanical treatment of fuels, or 

removal prior to burning, to ensure that fire intensities remain 

within an acceptable range. We suggest the following priorities 

for prescribed burning within nonharvested areas of owl habitat 

in the Sierran mixed-conifer type. 

First Priority 

All hazard-reduction objectives of prescribed burning involve reducing amounts and continuity of fuels. Our recommended first priority is to isolate (that is, disrupt fuel continuity 

around) known nest and roost sites within "Protected Activity 

Centers," using a band of prescribed bums. We recognize that 

this strategy poses some small risk to existing birds, but these 

risks can be at least partially mitigated. To the extent possible, 

burns should be concentrated, at least initially, on south-facing 

slopes (aspects from southeast through west) and ridges surrounding or adjoining nest/roost sites. Compared with more 

northerly slopes, these aspects have drier, more flammable fuels 

likely to support severe wildfires under a wider range of burning 

conditions (given effects of fire suppression). On the other hand, 

southerly slopes offer several advantages for prescribed burning: 

(1) They dry out earlier in the year, sometimes allowing spring 

burns to be done with minimal construction of firelines (thus 

lower cost) at times when fire will not spread onto adjacent 

northerly slopes. (2) Stands on south-facing slopes tend to be 

more open, with less dense understory to provide troublesome 

fuel ladders. (3) They tend to have a higher proportion of 

ponderosa pine, which not only resists fire damage rather well 

but also provides litter that can carry fire at the cool, moist end of 

a prescription-frequently desirable conditions for a first burn. 

After many decades of fuel accumulation, initial prescribed 

burns can result in excessive consumption of duff and coarse 

woody debris. This is undesirable for at least two reasons: First, 

duff and coarse woody debris are important substrates for hypogeous fungi-prime food for flying squirrels (Chapters 4 and 

10); and second, long-duration heating from extended smoldering of duff can damage tree roots and root crowns, sometimes 

causing delayed mortality even in large, old trees (Thomas and 

Agee 1986, Swezy and Agee 1991, Sackett pers. comm.). Conducting initial prescribed burns in the spring, when the moisture 

content of duff and large fuels is high, will help to minimize the 

problem of excessive duff consumption. The more flammable 

surface fuels-litter and small woody debris-can still be largely 

consumed. An additional advantage of low consumption of duff 

254 Chapter 12 

and large woody fuels is a reduction of atmospheric emissions. 

As adaptive management experiments continue over time, efforts should be made to align prescribed burns more closely with 

natural fire regimes in terms of the frequency and season of 

burns. In the meantime, however, effects of initial spring prescribed burns should be carefully monitored to assure that species diversity is retained and that birds and other wildlife are not 

disturbed during the breeding season. 

In stands with many small understory trees and low crowns, 

burning should be initiated carefully, using small test burns at 

first. Flame lengths and their effect on torching of small trees 

should be closely monitored, and firing patterns should be adjusted as necessary to keep flame lengths within prescribed 

limits (Martin and Dell 1978, Kilgore and Curtis 1987, 

Weatherspoon et al. 1989). An initial prescribed burn will usually create some interruption of vertical fuel ladders (after scorched 

needles drop to the ground). Many small trees will be killed, 

however, and often more dead fuels will be created than consumed. Such understory burns, therefore, carry an implied commitment that the stand will be reburned at least once, generally 

within 10 years or so, to clean up fuels created by the first 

prescribed burn and to reduce vertical fuel ladders further. 

Second Priority 

After nest and roost sites have received some protection, a 

more general program of prescribed burning should be initiated. 

The objective is to break up fuel continuity on a larger scale and 

to begin to restore fire as a natural process in the ecosystem. 

Southerly slopes and ridgetops would be favored for burning, as 

suggested above. Followup burns would be implemented as 

needed to effect and maintain a meaningful interruption of fuel 

continuity and reduction in wildfire hazard. Burns should be 

well-distributed, creating a mosaic of broad "fuelbreaks" covering one-third or more of the total area. Such a program would be 

expected to limit substantially the size and severity of subsequent wildfires. Special emphasis should be given to prescribed 

burning that reinforces natural and constructed barriers to fire, 

such as rocky outcrops, bare ridges, roads, and constructed 

fuelbreaks. These provide relatively defensible areas from which 

firefighters can safely implement fire suppression strategies. 

As indicated earlier, the approach discussed in this chapter 

assumes no commercial harvesting (including biomass harvesting of small trees). Outside "Protected Activity Centers" or 

similarly restrictive areas, however, initial prescribed burns in 

some stands with dense understories and dangerous fuel ladders 

may be facilitated by felling many of the small trees before 

burning. To limit intensity and consumption, and thus damage to 

the residual stand, the prescribed burn could then be done before 

the newly felled trees have fully dried. 

Third Priority 

For long-term protection, prescribed burning should be applied throughout owl habitat in the mixed-conifer zone with a 

frequency distribution similar to presettlement fire intervals. 

Northerly aspects and other areas not previously included in the 

prescribed burning program should be incorporated. Limitations 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
on such a program and factors to consider include: (1) availability of resources; (2) need for improved information from research and monitoring activities concerning habitat attributes, 

fire history, and fire effects; (3) need for prescribed fire to 

maintain and improve owl habitat in the absence of other silvicultural tools; (4) need for fire to provide other ecosystem 

values; and (5) smoke management and air quality implications 

of a widespread prescribed burning program (including information about estimated background levels of smoke during 

presettlement fire regimes, and tradeoffs between increased smoke 

from prescribed fires versus reduced smoke from wildfires). If 

the second priority program is carried out successfully-coupled 

with appropriate fuels management in adjoining harvested areas 

(Chapters 1 and 13)---occurrence of large, catastrophic wildfires 

within owl habitat will be of less concern. The focus of prescribed fire for this third stage will then shift generally from 

hazard reduction to reestablishment of an important ecosystem 

process. Long-term sustainability of owl habitat on a landscape 

scale should result. 

Red Fir 

Fire Regimes 

The relatively few studies of natural fire regimes in the red 

fir type have indicated mean fire intervals ranging between 

about 40 and 100 years (Kilgore 1981, Pitcher 1987, Taylor and 

Halpern 1991). Higher-elevation areas tend to have less frequent 

fires because biomass (fuel) accumulates more slowly and weather 

conditions that will support a fire occur less often. Despite 

longer fire intervals than in the mixed-conifer type, fire has been 

the dominant disturbance factor associated with episodes of 

regeneration in much of the red fir type. A range of severities and 

frequencies of fire has led to a complex pattern of various patch 

sizes and tree ages (Pitcher 1987, Taylor and Halpern 1991). 

Compared with lower-elevation forest types, fire suppression 

has had less effect on the red fir type. Suppression activities 

began later in red fir forests, and fewer fires would have burned 

there even without suppression. In addition, because red fir 

forests are largely monospecific and red fir is relatively shade 

tolerant, the successional trend toward more shade-tolerant species in the absence of fire (a concern in the mixed-conifer type) 

is not a factor in the red fir type. 

Despite many lightning-caused ignitions in the red fir type 

(Maupin pers. comm.), such fires seldom have gotten large on 

lands where a suppression strategy is used, because initial attack 

is almost always successful. The behavior of natural fire has 

been extensively documented in Yosemite and Sequoia/Kings 

Canyon NPs, where much of the red fir type is included in the 

prescribed natural fire zone. To date, no evidence suggests that 

the prescribed natural fire programs in the NPs have had an 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

adverse impact on the owls found there. Lightning fires in the 

red fir type usually burn with relatively low intensity and spread 

slowly over long periods, with occasional episodes of rapid 

spread during periods of severe fire weather. Crown fire is 

unusual in this type except under rare high-wind events, 

partly because sparse understory vegetation (Rundel el al. 

1977) provides limited fuel ladders. Some examples of crown 

fire, however, were observed during the 1987 fire season. 

Torching also has been observed when human-caused fires 

burned into the red fir forest under extreme conditions. 

Sixteen percent of the fires recorded in Yosemite NP between 1930 and 1983 occurred in the red fir zone, even though it 

comprises only 8 percent of the park. The majority of those fires 

were single trees, although larger fires occurred when red fir was 

mixed with chaparral (van Wagtendonk 1986). 

Fuels 

Color photo 5-1 provides an excellent illustration of typical 

fuel conditions in red fir. Fuel bed characteristics are quite 

different from those in the other habitat types. The short needles 

form a dense litter layer, which is further compacted by a heavy 

snow pack. This litter burns slowly, with relatively low intensities. Fuel ladders are nearly always interrupted. Dead-and-downed 

fuel loadings may be heavy in older stands, but fuel moistures 

remain high during most years in these high-elevation stands. 

Fire Management Options 

Fire may be allowed to play its natural role in much of the 

red fir type. Large sections of red fir forest found in wilderness 

areas should be evaluated for inclusion in wilderness 

fire-management programs that emphasize the role of fire as a 

natural process. 

Southern California MixedConifer 

Fire Regimes 

The natural role of fire in the southern California 

mixed-conifer community is similar to that described for the 

Sierran mixed-conifer community. Studies of fire history in the 

Los Padres NF (Talley and Griffin 1980) and the San Bernardino 

Mountains (McBride and Laven 1976, McBride and Jacobs 

1980) suggest slightly longer mean fire intervals in these forests 

than those found in the Sierra Nevada. This may be explained by 

the lower incidence of lightning storms there, as well as smaller 

contiguous areas of mixed-conifer vegetation. In the southern 

California mixed-conifer type, as in the Sierra Nevada, the early 

Chapter 12 255 
fire regime was typified by frequent low- to moderate-severity 

fires, which burned over long periods under a variety of fuel and 

weather conditions (Minnich 1988). 

Fire suppression has been effective in reducing the number 

of large fires in the mixed-conifer type in the mountains of 

southern California. An analysis of fire records by McBride and 

Laven (1976) indicated that fire suppression has been effective 

in sharply reducing the number of acres burned on the San 

Bernardino NF, for example, from an annual acreage of 5,890 

acres during the 1940s to 3,774 acres during the 1960s. 

Talley and Griffin (1980) found evidence that twentieth 

century fire regimes result in infrequent fires of an intensity 

causing widespread mortality in pine stands that survived the 

frequent low- to moderate-severity fires of the past. Coniferous 

forests, which once burned more frequently than the chaparral 

stands below them, now burn at a frequency similar to that in the 

chaparral. 

Examination of fire records indicates that most large wildfires in mixed-conifer forests in southern California fall under 

Scenarios 1 and 3, described for the Sierran mixed-conifer type. 

Fire Management Options 

Current practices of protecting all pairs of owls in this 

type should be continued. The fire management options described for Sierran mixed-conifer forests apply in southern 

California as well. It is essential, however, that fuels also be 

managed in the chaparral communities surrounding the conifers, to avoid catastrophic wildfires that spread from below 

into the mixed-conifer zone. 

Live Oak/Bigcone DouglasFir Forests 

The live oak/bigcone Douglas-fir community provides habitat 

for an estimated 41 percent of owl sites in southern California 

(tables 1B and 3I). Within the lower portions of its elevational 

range, this community occurs near streams in moist, shaded 

canyons and draws, where aspects are mostly north and east. As 

elevation increases, it occurs on other aspects and is less restricted to canyons (McDonald and Littrell 1976). Typically, 

bigcone Douglas-fir comprises a scattered overstory of single 

trees or small groups of trees, and tree-sized canyon live oaks 

form a relatively continuous midstory canopy. Shrubs and herbaceous plants are largely absent in the understory except beneath openings and along the margins of stands (McDonald 

1990). Stands of live oak/bigcone Douglas-fir usually intergrade 

with chaparral along their margins. 

256 Chapter 12 

Fire Regimes and Fire Effects 

McDonald and Littrell (1976, p. 319) described "the overall 

pattern of the bigcone Douglas-fir-canyon live oak community 

[as] that of a stable, self-perpetuating, somewhat exclusive community, with tendencies toward the climax or even postclimax 

successional stage." The community evidently is not well-adapted 

to frequent fires. Bigcone Douglas-fir is unusual among conifers 

in its ability to sprout from a fire-scorched crown (color photo 

5-39), but it is weakened from repeated fires that deplete energy 

reserves (McDonald 1990). It does not survive "torching" of the 

crown (consumption of foliage by flames). Seeds contained in 

cones in torched crowns almost certainly would not survive to 

provide regeneration. Indeed, natural regeneration of the species 

is very slow following severe fires (Minnich 1980). Young 

bigcone Douglas-firs are moderately shade-tolerant, and establishment of natural regeneration is often most successful in the 

partial shade provided by the canyon live oak canopy (McDonald 

1990). Young trees grow slowly in the understory, however, and 

are easily destroyed by surface fires. 

Oaks in general are not noted for their resistance to fire 

damage. Even among oaks, however, canyon live oak is unusually sensitive to damage and top-kill by fire (Minnich 1980, 

Plumb 1980). Its thin, dry, flaky bark ignites easily, often carrying fire several feet up a bole and commonly burning completely through the bark to expose the underlying wood. Frequently, heat kills enough cambium to girdle the bole, even with 

low-intensity surface fires (Plumb 1980). The relatively closed, 

compact canopy architecture also tends to trap heat from a fire 

beneath the canopy, thereby increasing crown scorch. On the 

other hand, canyon live oak sprouts vigorously from the root 

crown following top-kill by fire, and frequently after sublethal 

damage as well. 

The unusual susceptibility of canyon live oak to top-kill by 

fire suggests that stands with many large, old trees (seemingly 

high-quality owl habitat-Chapter 5) probably have not experienced fires of any significance for many decades (Minnich 

1988). The species grows more quickly to tree size in more 

mesic habitats, where fire intervals also are generally longer. 

Canyon live oaks that escape burning long enough to attain tree 

stature, in turn, tend to bestow a fire-retardant quality on the 

stands in which they grow (Minnich 1980). Possible reasons 

include: (1) chaparral shrubs or other understory plants that 

might provide a flammable fuel ladder into tree crowns are 

virtually absent; and (2) litter tends to be meager beneath canyon 

live oak and it is not particularly flammable, perhaps in part 

because of dry ravel on steep slopes that promotes mixing of 

litter and soil. Minnich (1977, p. 447) described fires spreading 

from chaparral into live oak/bigcone Douglas-fir stands that 

"usually incinerated the outermost fringe of stands. With further 

progress into the grove, the pattern of total combustion graded 

into a hot surface fire which only scorches the oaks as the 

distance between the tree canopy and ground fuels increases." 

Where wind and other burning conditions sustain a crown fire 

through the stand, however, restoration of prefire stand conditions will likely take a long time. Without intervention, canyon 

live oak sprouts will come up in a sea of seedling chaparral 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
shrubs. For many decades, the new community will be more 

likely to reburn than the old one-until the oaks again attain tree 

stature and shade out the shrubs. Meanwhile, bigcone Douglas-fir 

probably will be very slow to return, particularly if no surviving 

seed trees are nearby. 

Fire and Fuels Management Options 

We do not recommend prescribed burning within live oak/ 

bigcone Douglas-fir stands (see Plumb 1980). From the standpoint of wildfire hazard, these stands are relatively nonflammable, as described earlier. They probably will not support a 

stand-destroying crown fire except under extreme conditions in 

which prescribed burning or other surface fuel reduction will 

have made little difference in fire behavior. Under less extreme 

conditions, the stand probably will support a moderately intense 

surface fire, which would top-kill many or most of the live oaks 

and allow many of the bigcone Douglas-firs to crown sprout. 

Restoration of suitable owl habitat would take place sooner than 

after a crown fire. If prescribed burning were done, it would be 

difficult and expensive in many stands because of the steep, 

broken terrain. Moreover, given the discontinuous, 

low-flammability fuels occurring in many stands, moderately 

severe burning conditions probably would be required to carry 

fire and reduce fuels significantly, and those conditions might 

produce levels of damage to canyon live oaks approaching those 

that prescribed burning was intended to prevent. To the extent 

that the canopy is opened by top-kill of oaks during prescribed 

burning, chaparral shrubs probably would invade the stand, 

making it more likely to burn severely in the future. 

If the decision is to use prescribed burning, it should be 

concentrated on gentler slopes where access is easier, fuels are 

more continuous, and probable damage from wildfires is 

greater. Minnich (1980) observed 37 percent survival of 

bigcone Douglas-fir following wildfires on slopes less than 20 

degrees, but more than 90 percent survival on slopes greater 

than 40 degrees. 

A better fuels management strategy to protect live oak/ 

bigcone Douglas-fir stands may be to concentrate prescribed 

burning in chaparral near these stands. Highest priority should 

be given to the more flammable chaparral types and to more 

decadent chaparral with higher dead-to-live fuel ratios, which 

would support more intense wildfires and thus be more likely to 

carry a crown fire into adjacent trees. Similarly, higher priority 

should be given to chaparral near live oak/bigcone Douglas-fir 

stands with more continuous surface fuels and those on gentle to 

moderate slopes (as opposed to very steep, broken slopes and 

canyons). Movement of fire into live oak/bigcone Douglas-fir 

should be minimal if prescribed burns are planned for relatively 

moderate burning conditions (which should be suitable for burning decadent chaparral or relatively flammable chaparral species) and in such a way that slope and wind direction favor 

movement of the fire away from the live oak/bigcone Douglas-fir 

stand. Prescribed burning of decadent chaparral should improve 

owl foraging habitat because of increased production of woodrats 

(more succulent and nutritious foliage in the new growth) and 

improved access to woodrats (see Chapter 10). 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

In the event of a severe wildfire in live oak/bigcone 

Douglas-fir, active measures should be taken to restore owl 

habitat. Because prospects are poor for natural regeneration of 

bigcone Douglas-fir, nursery-grown seedlings should be planted 

as soon as possible. Competing shrubs and other vegetation 

should be controlled adequately to ensure survival and rapid 

early growth of the bigcone Douglas-fir seedlings. Some thinning of sprouting clumps of canyon live oaks might speed their 

return to tree size. 

Southern California Riparian/ 

Hardwood 

Accumulations of dead-and-downed woody fuels are generally low in this type. Fire behavior depends on understory 

composition, which can be variable (color photos 5-29 to 5-32, 

5-45 to 5-47). Areas with a grass understory burn rapidly with 

low to moderate intensities. Effects are generally benign. Stands 

with a shrub understory show great variability in fire behavior 

and effects, depending on species composition and abundance of 

shrubs. These stands should be examined on a case-by-case 

basis, because they are of great importance, relatively limited in 

acreage, and vary in their vulnerability to wildfire. Management 

should focus on maintaining a closed canopy of trees. In some 

stands, prescribed burning or other fuels treatment may be needed 

to prevent overstory mortality from wildfire. Fire also may be 

necessary in some situations to regenerate overstory trees, such 

as oaks. 

Considerations for Fire 

Suppression in Spotted 

Owl Habitat 

The wildland fire agencies have demonstrated the efficacy 

of initial attack in excluding wildland fire from some types of 

spotted owl habitat (Sierran mixed-conifer, red fir, and southern 

California mixed-conifer). Such suppression efforts are essential 

to prevent large, severe wildfires, while concurrently making 

efforts to condition stands to reduce wildfire hazard. Continued 

success depends on maintaining sufficient initial attack resources 

to protect the habitat. Wiitala (1991) proposed a way to quantify 

the risk of experiencing unacceptable fire events in areas that are 

highly valued, but for which we have no method of assigning a 

monetary value for input into the loss portion of cost-plus-loss 

Chapter 12 257 
equations. A similar analysis should be developed for California 

owl habitat, to justify the initial attack organization needed for 

its protection. 

A consideration in suppression of wildfire in high-value 

areas is the trade-off between efficient suppression that minimizes the fire size and the damage that can result from aggressive 

suppression action. The concept of appropriate suppression 

response has been articulated by Mohr (1989) and Mohr and 

Moody (1991), emphasizing techniques that effectively and efficiently suppress fires while minimizing direct impacts. 

Selecting suppression strategies at initial attack, extended 

attack, and project fire level also requires weighing the advantages of backing off to the exterior of high-value areas to minimize suppression impacts, against the amount of damage that the 

fire is expected to cause within the selected perimeter. The fire 

manager has considerable latitude in selecting a strategy, ranging from aggressive control to confinement. Selection of suppression alternatives should be based on projected or observed 

fire behavior to predict fire effects. This analysis will be 

fire-specific because of variation in such factors as fuel loading, 

fuel moisture, weather, and topography. Interaction among wildlife biologists or resource advisors, fire behavior analysts, and 

operations personnel is essential to develop a balanced display 

of the suppression costs and resource damage of alternatives. 

Successful implementation requires preplanning by fire managers in consultation with biologists. 

Fire suppression must be done in a manner that reflects a 

high regard for public and firefighter safety. The following 

additional factors should be considered when managing wildfires that start in or threaten spotted owl habitat: 

1. The degree of involvement of canopy layers, based on the 

intensity of surface fire and the amount of torching predicted in 

each canopy layer. 

2. The expected amount of consumption in litter and duff 

layers. 

3. The relative patchiness of the burn, and the percent of 

various fire intensities projected within the fire perimeter. 

4. The expected amount of downed-log volume to be consumed, especially logs greater than 9 inches in diameter. This 

evaluation should include both rotten and sound logs. 

5. Location of the fire relative to known nest sites. 

6. Timing of the event relative to the owl's breeding season. 

7. The quantity and quality of habitat in the area of the fire. 

If the fire is burning in an area where suitable owl habitat is rare, 

greater effort would be taken to minimize habitat destruction by 

the fire. 

8. Expected impact on the owl's prey base. 

258 Chapter 12 

Conclusions 

Fire has been a dominant force in shaping the forested 

ecosystems that provide habitat for the California spotted owl. 

The various habitat types used by the owl differ in terms of (1) 

the influence of historical fire regimes on their structure, composition, and function; (2) the extent to which the habitat types 

have been altered by human activities since European settlement; and (3) the risk of substantial habitat loss by severe 

wildfires. In this chapter we have summarized fire management 

considerations relating to protection and possible enhancement 

of owl habitat, with emphasis on the Sierran mixed-conifer and 

the live oak/bigcone Douglas-fir forest types. 

The Sierran mixed-conifer type is the State's most important and extensive habitat type for the California spotted owl. 

Human activities since the mid-1800s-especially sheep grazing, fire suppression, and "selective" cutting-have profoundly 

affected the structure and composition of these forests. Changes 

include a marked increase in the density of shade-tolerant understory trees. The vertical fuel ladders thereby created, along with 

substantial increases in surface fuels, have greatly increased the 

potential for stand-replacing crown fires. Severe wildfire in 

Sierran mixed-conifer forests may represent the greatest threat to 

current owl habitat. 

The wildland fire agencies must maintain an effective suppression organization to minimize the damage from such fires. 

Problems resulting from many decades of over-zealous suppression cannot be resolved overnight by allowing wildfires to run 

their course in highly hazardous fuel complexes. Concurrently, 

however, managers must pursue aggressive, environmentally 

sound fuels management programs to reduce wildfire hazard in 

and around owl habitat. As described in this chapter, prescribed 

fire has an important role to play in reducing hazard and enhancing a variety of ecosystem values associated with the natural 

functioning of fire. In addition, fuels management methods (including but not limited to prescribed fire) should be considered 

integral components of silvicultural approaches to managing 

owl habitat, such as those discussed in Chapters 1 and 13. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
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USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Chapter 13 

Projected Trends in Owl Habitat 

Kevin S. McKelvey and C. Phillip Weatherspoon 

Here we attempt to synthesize many of the analyses presented in other chapters in this document, and to look ahead to 

the future. As such, this chapter is necessarily more speculative 

and conclusions expressed here should not be viewed as hard 

and prescriptive. We hope, however, to highlight areas of concern for future owl habitat, and for the health of the forest 

ecosystem more generally. Finally, we propose alternate forest 

management methods that we believe have merit. We do not 

consider the management methods described here to be an 

exclusive set to be prescribed forestwide. Rather we view them 

as potential experiments which, if effective, will alleviate many 

of the shortcomings that we perceive in current management 

systems as they relate to owl habitat. This report covers a wide 

range of forest types and topographic regions. We will focus 

almost exclusively on the Sierran mixed-conifer forest type, as 

it contains an estimated 82 percent of the spotted owl sites in 

the Sierra Nevada and 62 percent of all known California 

spotted owl sites. 

The Forest, Past and Present 

Vegetation Dynamics 

Forests of the Sierra Nevada have been impacted by 

nonaboriginal man over the last 150 years. The first major 

perturbation, on lands that now are managed by the Forest 

Service (FS), was the grazing of millions of sheep (Chapter 11). 

This grazing pressure and the fire patterns associated with its 

promulgation impacted regeneration and the grass/forb community. This pattern probably amplified a structure already maintained by frequent fires-one of open stands dominated by large, 

old trees (Chapter 12). It also resulted in excellent seed beds for 

tree regeneration by creating bare-soil conditions over much of 

the forest (figs. 13A and 13B as examples; Chapter 11). 

With removal of sheep and fire, stands experienced ingrowth of conifers, dominated by shade-tolerant species such as 

white fir. Stands became dense, and a combination of logging 

and natural attrition of the old forest led to a decline in the 

number of large, old trees. Past logging activities that concentrated on removal of the large, valuable trees broke up the patchy 

mosaic of the natural forest, and this too encouraged development of dense regeneration patterns with very little horizontal 

heterogeneity (figs. 11P-11S; Chapter 12). These developments,

particularly in Sierran mixed-conifer and ponderosa pine/hardwood forests, reduced large-diameter trees in many areas to a 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

small, remnant population. This condition can be seen by comparing owl nest locations, which generally are in larger, denser 

stands in today's forest (color photos 5-9 to 5-16 as examples), 

to forest conditions about 1900 (figs. 13A to 13D as examples). 

These changes have not occurred to the same degree in the 

red fir as they have in mixed-conifer and ponderosa pine types at 

lower-elevations. The red fir type differs from the mixed-conifer 

in several particulars, including in its natural disturbance regime. 

Fires are less frequent, occurring at intervals of 40+ years (Taylor and Halpern 1991) rather than the 5- to 30-year return periods 

observed in mixed-conifer stands (Chapter 12). The forest consists of blocks of various sizes, often with a high degree of 

horizontal heterogeneity (color photos 5-1 to 5-6). In many areas 

fire was the dominant disturbance factor responsible for this 

mosaic (Pitcher 1987). In addition, windthrow created small 

gaps, and red fir regeneration often established more or less 

continuously between disturbance events (Taylor and Halpern 

1991). This led to a complex horizontal pattern with the juxtaposition of blocks of various ages and sizes. Removal of fire from 

the red fir type has had less impact than in the Sierran mixed-conifer 

type because of the longer mean fire interval. Many red fir stands 

would not have burned in the twentieth century even if the 

natural fire patterns were intact. Red fir also occurs in mostly 

pure stands at higher elevations. On these sites, red fir appears to 

be a self-perpetuating, climax species (Barbour and Woodward 

1985, Taylor and Halpern 1991), and the successional trend 

toward more shade-tolerant species that characterizes the 

mixed-conifer zone is not a factor. 

Red fir was also not extensively logged until recent years. 

Most logging of red fir in the Swain Mountain area, for instance, 

occurred after 1960 (Chapter 7). Partial cutting has not broken 

up the large-diameter groups and, in many places, the forest has 

never been logged (color photos 5-1 to 5-6 as examples). The 

pattern in red fir is more that observed in the Pacific Northwest-recent, often even-aged harvest intermingled with blocks 

in reasonably pristine condition. In logged areas, regeneration 

records maintained by the FS R5 show that survival of planted 

seedlings is low (Fiske pers. comm.; fig. 13E). 

Fire Patterns 

Fire patterns in the Sierran mixed-conifer zone have changed 

radically in the twentieth century. The annual acreage burned 

may have declined by two orders of magnitude when compared 

with historic levels (Chapter 12). This, in turn, has led to historically unprecedented buildups in fuels and to stand structures that 

are prone to crown fires (Chapter 12). Because of these conditions, fires that escape initial suppression efforts-usually those 

occurring during extreme weather conditions-tend to become 

large, stand-replacing events. 

Chapter 13 261 
Figure 13A--Sugar and ponderosa pine in Foresthill block showing size of timber and open character 

of forest. Note extensive, branchless tree trunks and the absence of underbrush on the forest floor. 

Date: ----------------------7 July 1911 

Source: -------------------Tahoe National Forest Historical Photo 

Photographer: ----------USDA Forest Service 

Figure 13B--A woman and her dog following an old road through a stand of old-growth Jeffrey pines.

Date: ----------------------About 1920 

Source: -------------------Lassen National Forest Historical Photo 

Photographer: ----------USDA Forest Service 

262 Chapter 13 USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Figure 13C--Pure stand of cedar along Mill Creek above Morgan Springs.

Date: ----------------------About 1920 

Source: -------------------Eldorado National Forest Historical Photo 

Photographer: ----------USDA Forest Service 

Figure 13D--Near Strawberry, a virgin forest including sugar pine, 

ponderosa pine, fir, and incense-cedar. Observe the downed, rotting 

tree and open character of the forest floor. 

Date: ----------------------About 1920 

Source: -------------------Stanislaus National Forest Historical 

 Photo 

Photographer: ----------USDA Forest Service 
Figure 13E-Regeneration survival in plantations, by species, based on 

3-year postcut inventories. Survival rates were averaged over the years 

1986-1991 (Fiske pers. comm.). 

Owl Habitat in Current Forests 

Spotted owls in the Sierra Nevada have used all of the dense 

(>40 percent canopy closure) mixed-conifer stands at or greater 

than expected levels for nesting (Chapter 5). The only stands 

used significantly greater than expected, however, were dense 

stands with large-diameter trees [>24 inches in diameter at 

breast height (d.b.h.)] in the overstory. Within these stands, snag 

densities were higher than in the general forest matrix, and the 

average nest tree exceeded 40 inches in d.b.h. Most of the nest 

trees appeared to be old-growth remnants of stands present at the 

turn of the century (Chapter 5). These stands tend to be very 

heavily stocked with trees in smaller diameter classes (color 

photos 5-9 to 5-16, for example). 

We have few data concerning nest stand selection in the red 

fir type. The nests that we observed on our field trips were in 

uncut blocks of timber (color photos 5-1 to 5-6, for example). 

Future Trends 

Human Population Trends 

The human population in both the Sierra Nevada and southern California is growing rapidly (fig. 13F). Of particular concern is population growth in the foothill regions of the mid-Sierra 

Nevada. In these areas (Nevada, Placer, El Dorado, Amador, 

Calavaras, and Tuolumne Counties), human populations have 

increased by 50 percent over the last 10 years and are expected to 

continue to grow at that rate through the year 2000. From 1980 

264 Chapter 13 

to 1990, they were among the top 10 California counties in 

growth. By the year 2000, the combined population of these 

counties is estimated to exceed 670 thousand (Rountree 1992). 

A large proportion of this population will be living in widely 

dispersed housing "in the pines." Impacts from this type of 

development on spotted owl habitat are unknown. We do know, 

however, that spotted owls currently occur in these foothill areas 

and that this type of housing pattern makes management and 

monitoring very difficult. Any resulting negative impacts on the 

owls will be difficult to detect and even more difficult to control. 

While much of the population growth in southern California 

is in the basins, towns in the San Bernardino Mountains, such as 

Big Bear Lake, are expected to grow 50 percent by 2010 (Ruth 

Figure 13F-Human population trends in the Sierra Nevada and southern California. Only counties containing spotted owls were included in 

this analysis (data from Rountree 1992). 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
pers. comm.). Impacts of increased population on the spotted 

owl in southern California will most likely be more direct than 

they are in the mid-Sierra Nevada. Because many owls in southern California depend on narrow riparian/hardwood forests, pressure on them will increase as waters are diverted to a variety of 

human needs. 

Fire Trends 

As the human population increases, human-caused wildfires can also be expected to increase. The presence of so many 

houses within the forest will shift the emphasis of fire suppression even further from one of saving forests to one of saving 

property. Fuels will also continue to accumulate. Especially as a 

direct result of our current drought, recent bark beetle infestations will contribute a major pulse of new fuels over the next few 

decades. The likely result is a gradual increase in the number and 

acreage of large, stand-replacing wildfires. 

Cutting Trends: Land Management 

Plans 

According to Land Management Plans (LMPs) collectively 

for National Forests (NFs) of the Sierra Nevada, 65 percent of 

the forested acres are classified as suitable and available for 

timber production (USDA, FS 1986-1991b). If we exclude from 

the forested base the acres that are physically unsuitable because 

they cannot produce timber in commercial quantities, cannot be 

successfully regenerated, or have unstable soils, 74 percent of 

the lands that can potentially produce timber will be harvested in 

some manner (table 13A). On many acres, harvest will be light. 

Table 13A-Categorization of forested acreage from the Land Management 

Plans. 

Thousands of acres 

National Physically Suitable and Suitable and 

Forest Forested unsuitable1 available2 unavailable3

Lassen 825.1 0 596.3 228.8 

Plumas 1,102.4 145.7 898.9 57.8 

Tahoe 681.6 31.7 528.5 121.4 

Eldorado 458.0 63.2 307.6 87.2 

Stanislaus 784.6 308.3 328.5 147.8 

Sierra 562.9 26.8 328.9 207.2 

Sequoia 679.0 40.0 345.0 294.0 

All Forests 5,093.6 615.7 3,333.7 1,144.2 

1

 Includes acreage not capable of producing commercial timber, in which 

irreversible damage would be caused if timber were harvested, and that cannot 

be regenerated in 5 years after harvest. 

2

 Includes lands capable of producing commercial timber that have not been 

withdrawn from timber management, lands that can be regenerated within 5 

years after logging, and lands that can be managed for timber production 

without irreversible damage. Suitability here refers to areas that could 

physically sustain timber yields over time, not necessarily spotted owl 

suitability. 

3

 Includes lands that are physically suitable but that have been withdrawn 

from timber management for dedication to other uses (for example, Wilderness 

Areas, Spotted Owl Habitat Areas, and so on). 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

Seventy-two percent of the timber volume removed from these 

lands will be taken through even-aged systems, the most common even-aged system being the clearcut (table 13B). Of the 

528,474 acres of suitable timber land on the Tahoe NF, for 

instance, 68 percent will be managed for even-aged silviculture 

(24 percent long rotation, 44 percent short rotation) (USDA, FS 

1990). On the Plumas NF, 52,000 acres are scheduled for 

even-aged cutting per decade and 8,000 acres for selection cutting methods (USDA, FS 1988b; table 13C). 

Even-Aged Logging and Owl Habitat 

Clearcuts, seed-tree, and shelterwood cutting methods all 

have the same goal: produce even-aged stands. In this regard, 

seed-tree and shelterwood systems can generally be thought of 

as two-stage (sometimes three-stage) clearcuts. In nearly all of 

these cutting systems, all of the original stand will be removed 

before the new stand is scheduled to be cut. 

Table 13B-Harvest volumes from the forest Land Management Plans. 

Millions of board feet 

National Seed tree/ 

Forest Clearcut shelterwood1 Other2 ASQ3

Lassen4 40.6 23.5 31.9 96.0 

Plumas 144.2 31.3 90.0 265.5 

Tahoe 64.7 48.2 29.4 142.3 

Eldorado 58.4 55.4 23.4 137.2 

Stanislaus4 3.9 67.6 16.5 88.0 

Sierra' 25.5 32.5 30.0 88.0 

Sequoia' 64.8 3.1 29.1 97.0 

All Forests 402.1 261.6 250.3 914.0 

1

 Includes removal sales as well. 

2

 Includes all other sale types that enter into the calculation of ASQ. 

3

 Annual sale quantity. 4

 These data are given in cubic feet in the LMPs. 

Table 13C--Number of acres scheduled for logging, annually, during the first 

decade of draft and final Land Management Plans for Sierran National Forests, 

by prescription. 

Acres per year 

National Seed tree/ 

Forest Clearcut shelterwood1

 Selection2 Intermediate3

Lassen 1,600 1,000 1,400 10,200 

Plumas 4,000 1,200 800 16,286 

Tahoe 2,046 1,657 162 3,500 

Eldorado 2,084 1,836 948 2,700 

Stanislaus 510 2,386 8,7284 1,500 

Sierra 1,550 1,170 2,970 4,000 

Sequoia 1,734 128 742 6,727 

All Forests 13,524 9,377 15,750 44,913 

1

 Includes acres of overstory removal. 

2 Both group- and individual-tree selection. 

3

 Includes stand improvement, thinning, salvage, etc. 

4

 This figure represents the acres to be examined rather than the acres to be treated. 

Chapter 13 265 
In terms of owl biology, the primary impact of traditional, 

even-aged logging practices lies in the creation of simple stand 

structures and, perhaps more importantly, the removal of all 

large, old trees from vast areas of the forest. Even if silvicultural prescriptions are modified so that snags and live culls are 

left at the first cutting, no provision is made for a predictable 

recruitment of replacement trees for these relics when they fall. 

This, in turn, will lead to a loss of large-diameter, downed 

woody materials. Log slash can create much small-diameter 

woody debris, but it cannot replace the large logs. In an even-aged 

system, these old-growth features can be recreated only by an 

extreme extension of the rotation interval. Even if the rotation 

were extended to 150 years, for instance, no trees would match 

the average age of the forest at the turn of the century (Chapter 

11). Decadent features in stands are functions of age, not just 

d.b.h. (fig. 13G), and any animals that depend on decadent 

features (cavities, broken-tops, snags), or the large woody debris that they create, will simply drop out of these forests (see 

Chapters 4, 5, and 10 for examples specific to the spotted owl 

and its prey species). The rate of conversion to even-aged systems in the western Sierra Nevada is estimated by the LMPs to 

be 229,000 acres per decade. 

Even on lands set for selection logging (about 80,000 acres 

per decade), we have no guarantee that harvest prescriptions 

will leave many of the large, old trees. Ideally, stands managed 

for individual-tree selection are harvested in a manner that 

brings the diameter distribution in the stand into conformity 

with an idealized distribution characterized by a declining exponential function (in forestry referred to as an inverse "J"). 

The number of large trees within the stand is dictated by the 

slope of this function (fig. 13H) and the specified intercept 

representing the number of trees in the smallest diameter class 

(or, alternatively, the designated diameter of the largest tree). In 

selection systems, timber is removed from all diameter classes, 

as required to maintain this diameter distribution. Little evidence exists, however, that historic patterns of partial cutting 

have followed classic single-tree selection theory. "Selective" 

harvest in the Sierra Nevada has, in the past, primarily targeted 

the large trees. This system, sometimes called "pick and pluck," 

Figure 13G-Increasing decadence as a function of stand age, based on 

data from Bingham and Sawyer (1992). 

266 Chapter 13 

Figure 13H-Theoretical, uneven-aged distributions. Uneven-aged 

management allows for retention of very large trees. "Q" values of 1.2--

similar to those measured in owl nest stands---will leave two to three trees 

per acre 40 inches in diameter at breast height on a site. 

will not produce the simple structures that characterize even-aged 

methods, but its effect on the presence of large, old trees is 

similar. If the large trees are removed and no stocking control is 

performed on the smaller stems, replacement trees in these 

diameter classes will be produced very slowly, if at all, and they 

will consist primarily of the more shade-tolerant species. Even 

with classical single-tree selection, a gradual loss of 

shade-intolerant species would be likely. 

A large proportion of the future forest, as projected by the 

LMPs, will very probably be split between areas of even-aged 

plantations and areas of dense, and increasingly smalldiameter stands. 

A Different Paradigm 

Two important questions that the FS needs to answer are: 

(1) What kind of forest structures does it want to create and 

perpetuate? (2) Are the life history requirements of spotted owls 

and other species associated with late-seral forest conditions met 

by those forest structures? Fundamental to these questions is the 

paradigm that guides NF timber management. Insights to this 

paradigm can be gained by studying the LMPs produced by the 

Sierran NFs. The LMPs are as much statements of goals, priorities, and paradigms as they are implementation strategies. The 

scheduling appendix for timber removal on Stanislaus NF, for 

example, states these goals clearly: 

A regulated forest should be regarded as an objective that 

may never be fully attained... However, the concept of regulation as 

an objective, is the tool used to control present harvest levels and 

plan future harvests to assure sustained yield within reasonable 

limits. 

A forest consisting of wild and unmanaged stands with highly 

variable stocking levels and age-class distribution is made to approach regulation through harvest and scheduling regeneration 

over a period of time called the conversion period. During the 

conversion period an attempt is made to meet three criteria: 

1. Obtain the maximum yield of timber possible. 

2. Provide for essentially a non fluctuating yield over the 

conversion period. 

USDA Forest Service Gen. Tech. Rep: PSW-GTR-133. 1992. 
3. Provide a balance of age-classes and stocking levels capable 

of producing the forest's full potential timber growth at the 

end of the conversion period (USDA, FS 1991b, appendix D, 

D-3). 

This description closely follows the ideal of the "fully 

regulated forest" as it is presented in forest management text 

books (for example, Davis and Johnson 1987, chapter 14). By 

this definition, the regulated forest differs in fundamental ways 

from a wild forest, and the current status of the forest is seen as 

being in a conversion period in which wild stands are brought 

into regulated conditions through harvest and regeneration. 

Really only one general template is used to define a fully 

regulated forest-the diameter distribution, forest wide, should 

fit an "inverse J" distribution form, with optimized spacing 

(Husch et al. 1982). This pattern can be achieved through a 

series of large blocks (even-aged clearcutting), blocks smaller 

than 2 acres (group selection), or on an individual tree basis 

(individual-tree selection). All three models are generic, derived from European forestry in the late 1800s (Davis and 

Johnson 1987) and used for timber production in Europe, New 

Zealand--and in the Sierra Nevada. The purpose of the classic,

fully regulated forest, in any of its permutations, is to allow for 

an even flow of wood products, not necessarily to mimic 

natural stand conditions and processes: 

The essential requirements of a fully regulated forest are that 

age and size classes be represented in such proportion and be 

consistently growing at such rates that an approximately equal 

annual or periodic yield of products of desired sizes and quality 

may be obtained in perpetuity. A progression of size and age 

classes must exist such that an approximately equal volume and 

size of harvestable trees are regularly available for cutting (Davis 

and Johnson 1987, p. 540). 

Most other LMPs share a single emphasis concerning their 

paradigm for timber management. No certainty exists that ecosystem processes, whether they are the population dynamics of a 

single species or the successional trends of a multi-species complex, will be maintained in a fully regulated forest. The concept 

of forest regulation has traditionally not been defined in these 

terms, and the long-term ecological consequences of the paradigm are unknown. 

Recent discussion has concerned a significant shift in the 

basic forestry paradigm, from one that stresses the production 

of commodities (or amenities) to one in which the maintenance of ecosystem processes is the primary goal (National 

Research Council 1990). This new paradigm "...involves a 

shift in management focus from sustaining yields of competing resource outputs to sustaining ecosystems" (Kessler et al. 

1992, p. 221). Toward this objective, the goal of management 

activities is to maintain, protect, and, where necessary, to 

create natural forest structures. Logging is practiced only to 

the extent and in a manner that it does not impair ecological 

processes or environmental assets. This system does not guarantee an even flow of wood products from the forests. The 

efficacy of silvicultural practices is evaluated by biological 

rather than market-based criteria. 

Under this paradigm, logging would be explicitly designed 

to achieve or maintain ecological goals. These goals might, for 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

instance, include remedial actions such as the generation, within 

a topographically defined unit such as a drainage basin, of a 

pattern of stand structures that mimic historical stand conditions. 

Because those structures were very different from the stands that 

presently exist, much manipulation of vegetation will need to 

occur before the goal is achieved-and active management will 

subsequently be needed to maintain it. A great deal of sophisticated silviculture will be required. What represents a major 

paradigm shift is that the silviculture will not be constrained by 

the equal volume/equal entry area requirements that are definitional to the concept of the fully regulated forest. This is not to 

say that forest regulation and a sustainable yield of forest products are necessarily incompatible with ecosystem functioning. 

Rather, it is a question of priorities: The new paradigm "...would 

involve a view of forest lands-including soils, plants, animals, 

minerals, climate, water, topography, and all of the ecological 

processes that link them together-as living systems that have 

importance beyond traditional commodity and amenity uses" 

(authors' emphasis) (Kessler et al. 1992, p. 222). 

Under this paradigm, the answer to the first question (the 

type of forests we want) is that we wish to create forests in which

natural processes are fully functional and stable. The answer to 

the second question (how spotted owls fit into this structure) is 

the subject of the remainder of this chapter. 

Current Stand Structures at Spotted 

Owl Nests 

Nest stands have definable properties-high canopy closure, a considerable snag component, and the presence of large, 

old trees that are used by the owls for nest sites. To generate and 

maintain these stand structures we need to be more specific. 

Data on the silvicultural characteristics of owl nest stands is, 

however, surprisingly sparse. 

To evaluate the diameter distributions of owl nest stands, 

we obtained diameter distributions for areas immediately adjacent to 24 nest trees in the mixed-conifer zone of Lassen NF and 

11 nest trees in the Sierra NF. These data were based on plots 

Figure 13I-Log-linear display of the mean number of trees/diameter 

class, based on 24 nest sites in the Lassen National Forest. 

Chapter 13 267 
Figure 13J-Log-linear display of the mean number of trees/diameter 

class, based on 11 nest sites in the Sierra National Forest. 

centered on the nest tree and forming a cross with the nest tree at 

the center. All trees 15.7 inches in d.b.h. were sampled within an 

area of 0.296 acre. Smaller trees were sampled within four 

subplots 32.8 feet square. To avoid any possible bias by the 

inclusion of a large, central tree, the nest tree itself was excluded 

from density calculations. When the natural log (In) of the mean 

tree density (stems/acre) for the combined plots in these two 

areas was regressed on d.b.h., the resulting pattern was linear 

(figs. 13I and 13J), suggesting a classic "inverse J" form-the 

idealized distribution for uneven-aged management-with a "q" 

value between 1.18 and 1.21 [here q is based on 2-inch d.b.h. 

groups (Daniel et al. 1979)]. This pattern, by itself, is not sufficient to suggest that uneven-aged management is warranted. 

Stands with differing structures will tend to conform to the 

inverse J when they are combined. Viewed individually, nest 

stands in Lassen NF formed three logical groups (figs. 13K and 

13L). Eight of the stands lacked the larger diameter classes and 

conformed closely to the log-linear model. Thirteen of the stands 

Figure 13K-Nest sites on the Lassen National Forest divided into 

logical groupings based on stand structure. Data presented are the 

mean values for each group. The groups consisted of 8, 13, and 3 sites, 

respectively. 

268 Chapter 13 

Figure 13L-Diameter distributions for the three types of nest stands 

measured in the Lassen National Forest. Both the "reserve form" and the 

"large trees" groups had more large-diameter stems than an idealized, 

uneven-aged stand structure would specify. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Figure 13M-Nest stand structures in the Sierra National Forest. None 

of these stands had a large proportion of large-diameter stems, and 

several were dominated by small-diameter trees, 

generally conformed to the uneven-aged structure in the smaller 

diameter classes, but not in the large diameter classes. Three 

stands had most of their trees in the larger diameter classes. Nest 

stands on the Sierra NF also grouped logically into three classes 

(fig. 13M). Six of them generally fit an uneven-aged distribution 

form, but the others did not. Stands at two of the sites were made 

up primarily of stems <12 inches in d.b.h.. 

Most of these stand structures could be produced through a 

variety of management methods. Partial removal of the over 

story in the past and subsequent ingrowth probably led to the 

formation of many of these stands. We have no reason to believe 

that uneven-aged management, if properly applied, could not be 

used to maintain these structures-but the system should be 

sensitive to maintaining the large trees, and perhaps modified to 

generate "reserve form" stands. Reserve form stands are charac 

terized by an inverse "J" distribution in the smaller diameter 

classes and a normal distribution in the larger diameter classes. 

In all stands lacking frequent large stems, at least one large tree 

was present the nest tree, which was not included in the calculations. 

The simple premise that forest structures similar to owl 

habitat can probably be created and maintained through silviculture does not answer a fundamental question. In the long run, are 

these the types of stands we wish to maintain? We have many 

reasons to doubt whether these stand types represent either a 

necessary or ideal template as an owl nest site. Perhaps the most 

important reason is that dense stands characterized by single 

tree-sized openings would have been unusual in mixed-conifer 

forests before the turn of this century. Dense stands would have 

existed, particularly in riparian areas and at higher elevations 

(figs. 13C and 13D), but they would not have been widely 

distributed across the landscape. A second reason is that these 

stands are unstable-the stand structure is likely to be altered 

quickly and unpredictably due to the probability of stand-replacing 

fires or insect and disease outbreaks. We do not know whether 

owls inhabited the more open stands that dominated much of the 

landscape in the past (figs. 13A and 13B). Because such stands 

are rare today, we are unable to infer anything from current owl 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

locations. We do, however, need to begin to explore potential, 

alternate stand structures. These structures should be chosen to 

better mimic natural stands, to maintain tree species diversity, 

and to be more resilient to wildfires. 

Some Potential Management 

Systems for Mixed-Conifer 

Ecosystems 

General Considerations 

A fundamental assumption underlies management of owl 

habitat, as well as much of forest management in general: Ecosystems are inherently dynamic; they do not stand still. Changes take 

place both rapidly (through a variety of natural and man-made 

disturbances) and slowly (through climatic change and natural 

successional processes), and occur at many spatial and temporal 

scales. Changes have occurred in the past, and they will occur in 

the future even if we "do nothing." Given this assumption, 

management recommendations should consider provision of adequate amounts and distribution of suitable owl habitat both in 

the short-term and in the long-term. For the short-term, Chapter 

1 details management recommendations to be applied during an 

interim period of at least 5 years. These recommendations are 

intended to provide some degree of protection to existing owl 

habitat and to maintain future options for whatever long-term 

management strategy may be adopted at the end of the interim 

period. Below, we offer an example of one way in which the 

short-term (interim period) recommendations in Chapter 1 could 

be implemented in stand types shown to be selected for nesting 

by the spotted owl. These short-term practices and resulting 

stand conditions are not designed to be sustainable over long 

periods, however. The remainder of this section, therefore, offers potential silvicultural strategies that might be useful for 

producing and maintaining owl habitat over the long-term. 

An Example of a Short-Term Approach 

Recommendations in Chapter 1 set limits for cutting large 

trees-and managing other stand components. By definition, management practices cannot exceed these limits-for example, cut 

more large trees than specified-and still conform to the recommendations. In some situations, however, management objectives may be better achieved by not taking the recommendations 

to their limits. Following is one example of such an approach to 

interim management of "Selected Timber Strata" (P4G, M4G, 

M4N, M5G, M5N, M6-codes for timber strata defined in table 

1C). (Compare item #1 under "Other Forested Public Lands" in 

Chapter 1.) Objectives that this example could help to meet 

include: (1) provide for a shorter recovery period for nesting/ 

Chapter 13 269 
roosting habitat; (2) keep some stocking in middle and lower 

canopy layers, both to retain existing multiple-canopy character 

of the stand and to ensure quicker replacement of large trees; (3) 

provide for some thinning in middle and lower canopy layers to 

promote growth of trees in these layers into desirable larger size 

classes; and (4) provide some quantification of fuels management treatments, including reduction of vertical fuel ladders 

(Chapter 12). Managers should recognize that this example (like 

any other that places additional constraints) is more restrictive 

than the basic interim-period recommendations in Chapter 1. 

Thus, in exchange for the potential benefits indicated above, 

timber volumes (and associated revenues) will generally be 

lower, and costs of treating submerchantable stems and other 

fuels will generally be higher. (Basal area limits and other 

quantitative data in this example are only approximations. They 

are not based rigorously on Chapter 5 or any other real data 

pertaining to owl habitat. If an approach similar to this is to be 

used, it should be based on the best and most nearly site-specific 

data available.) 

A. Enter a stand for harvesting only once before a long-term 

strategy for managing the California spotted owl has been implemented on public lands. 

B. For stands with a Dunning and Reineke (1933) site index 

(SI) of ≥60, enter the stand for harvesting only if total basal area 

of live trees ≥5 inches d.b.h. is greater than 200 square feet per 

acre. Harvesting will not reduce total basal area to less than 200 

square feet per acre or canopy closure to <40 percent. For stands 

with SI <60, the corresponding basal area limit is 160 square feet 

per acre. To the extent possible, a mix of tree species should be 

retained. 

C. Remove no live tree ≥30 inches in d.b.h. 

D. For stands with SI ≥60, limit cutting in the 21- to 

30-inch d.b.h. class so that the combined basal area of live trees 

in d.b.h classes 21-30 inches and 30+ inches is no less than 120 

square feet per acre. A wildlife biologist should be involved in 

training tree markers to identify potential nest and roost trees in 

the 21- to 30-inch d.b.h. class so that those trees (including live 

culls) will be retained as part of the residual basal area. In stands 

currently having less than 120 square feet per acre basal area in 

those two size classes, no cutting of trees 21-30 inches d.b.h. will 

take place. For stands with SI <60, the corresponding basal area 

limit is 100 square feet per acre. 

E. For stands with SI ≥60, limit cutting in the 11- to 20-inch 

d.b.h. class so as to retain a basal area of at least 60 square feet 

per acre in that diameter class. In stands currently with basal area 

<60 square feet per acre in that d.b.h. class, no cutting of trees 

11-20 inches d.b.h. will take place. If the stand is entered for 

harvesting, and if current basal area in the 11- to 20-inch d.b.h. 

class is >80 square feet per acre, this diameter class will be 

thinned to a basal area of 60 to 80 square feet per acre. For stands 

with SI <60, the corresponding range of basal areas would be 40 to 

60 square feet per acre. 

F. Limit cutting in the 5- to 10-inch d.b.h. class so as to 

retain a basal area of at least 20 square feet per acre in that 

diameter class. In stands currently having <20 square feet per 

acre basal area in that d.b.h. class, no cutting of trees 5-10 inches 

in d.b.h. will take place. If the stand is entered for harvesting, and 

270 Chapter 13 

if current basal area in the 5- to 10-inch d.b.h. class is >30 square 

feet per acre, this diameter class will be thinned to a basal area of 

20 to 30 square feet per acre. Trees cut in the 5- to 10-inch d.b.h. 

class will be removed from the stand. Utilization of these trees is 

encouraged. 

G. If the stand is entered for harvesting, and if canopy 

cover in trees 0-4 inches in d.b.h. is, greater than 20 percent, 

trees will be felled, crushed, masticated, or otherwise rearranged to reduce canopy cover in that size class to no more than 

20 percent. The surface fuel bed resulting from these trees, as 

well as slash from logging of larger trees, should be treated 

mechanically or with prescribed fire to reduce wildfire hazard 

to an acceptable level. The emphasis should be on reducing 

vertical and horizontal continuity of fuels and associated risk of 

crown fires, especially in the vicinity of large trees. Fuel treatments and logging activities should be designed to minimize 

disturbance of duff and coarse woody debris. In most cases this 

will preclude machine piling of slash. Where prescribed burning is used, it should be done when lower duff and large woody 

debris have high moisture contents to minimize consumption of 

these materials. 

H. Follow guidelines in Chapter 1 for retention of snags 

and downed wood. 

Some Potential Long-Term Strategies 

A full discussion of management activities that may be 

appropriate to manage for owl habitat over the long-term is 

beyond the scope of this chapter. Instead, our purpose is to 

provide a sampling of ideas and considerations to stimulate 

thinking. Innovative managers and resource specialists may be 

able to use some of these ideas as a starting point in developing suitable management regimes to fit their local conditions 

and needs. On public lands, initial (interim period) implementation of the long-term strategies described here would be 

compatible with recommendations (Chapter 1) for "Other Timber Strata" used for nesting by owls but not significantly 

selected in relation to availability. We encourage managers of 

private lands to explore these approaches as well. Treatments 

should be viewed as ongoing management experiments (adaptive management, Chapter 2). Effects of treatments on stand 

structure and key ecosystem components should be carefully 

monitored, and owl habitat models should be tested. These 

experiments would incorporate information from monitoring 

and research activities into feedback loops that would serve to 

improve both our management practices and our knowledge of 

what constitutes suitable owl habitat. 

Although the scenarios discussed below describe generalized target stand structures and associated management practices 

primarily at the stand level, great flexibility exists for distributing variations and combinations of these structures across the 

landscape and through time. In these scenarios, silviculture would 

be viewed as the art and science of shaping stands and landscapes to meet management objectives-spotted owl habitat in 

this case. Timber volume would be an output of, rather than a 

driving force for, the silviculture undertaken to meet management objectives. Protection of current or future habitat would 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
continue to be a primary concern. Accordingly, appropriate fuels 

management would be integrated with silvicultural activities. 

We focus here on good-quality nesting and roosting habitat 

for the spotted owl as the "target" conditions for management 

activities. Foraging habitat appears to be more variable and less 

restrictive (Chapter 5), and its requirements should be met 

more easily and with a wider range of management practices. 

In contrast, successful management to produce, maintain, improve, and protect nesting and roosting habitat may require 

significant changes from conventional management practices. 

Some institutional barriers may need to be overcome. It will be 

more complex and expensive but should provide new and 

stimulating professional challenges for a variety of specialists 

to exercise creative thinking and pursue interdisciplinary objectives and activities. 

We make several simplifying assumptions about attributes 

of suitable nesting/roosting habitat (Chapter 5) to help define 

target stand structures and associated management practices: (1) 

high canopy closure; (2) stand basal area and canopy closure 

distributed among two or more size classes of trees; (3) diversity 

of tree species within the stand; (4) "adequate" numbers of large 

live trees; (5) "adequate" numbers of large snags; and (6) "adequate" quantities of duff and large woody debris. 

Accepting these six assumptions places limits on the range 

of stand structures that can be targeted by management. Both 

classical, even-aged silviculture and the classical, 

single-tree-selection form of uneven-aged silviculture have difficulties in meeting one or more of these assumptions, for 

reasons discussed below. We believe that two other kinds of 

stand structures-mosaics of small, even-aged groups or aggregations, and two- or three-storied stands-hold greater promise 

for producing and maintaining suitable owl habitat over the 

long-term. We recommend that these two structures, together 

with their associated silvicultural and fuels management practices, receive emphasis in long-term, adaptive management experiments concerned with owl habitat. Both can be considered 

intermediate between even- and uneven-aged (single-tree selection) management. But they can be thought of as representing 

two ends of a continuum,, with many intermediate structures 

differing in density and spatial arrangement of age/size-classes 

to help meet various specific objectives and to increase diversity across the landscape. Even- and uneven-aged (single-tree 

selection) methods should be included in these experiments, 

but at a reduced level. 

Even-Aged Silviculture 

The requirement for two or more size-classes of trees (assumption #2 above) probably could be met with even-aged 

silviculture, but it would involve significant modifications from 

conventional practice. The natural segregation of crown classes 

could be emphasized by "thinning from the middle"-that is, 

thinning in codominant and intermediate trees, thereby promoting the separation of dominant from suppressed crown classes 

and increasing growth in the dominants. Suppressed trees probably would not survive long enough to be of much value for owl 

habitat unless stand density were reduced below usual target 

stocking levels, or the lower canopy level consisted largely of 

USDA Forest Service Gen. Tech. Rep. PSW=GTR-133. 1992. 

shade-tolerant species. The latter scenario probably would be 

more practical and sustainable in mixed-species stands. 

Assuming that such an approach eventually would meet 

needs for a two- or more-storied stand, the rotation age necessary to meet tree size and decadence requirements (assumptions 

#4 and #5) probably would be much longer than called for in 

currently practiced even-aged management. As compared with 

alternative silvicultural methods, the rime from plantation establishment to achievement of conditions suitable for owl nesting 

and roosting is likely to be much longer. This long time period 

would necessitate development and/or retention of suitable replacement habitat in the interim. 

Whatever may be the disadvantages of even-aged silviculture with respect to owl habitat, even-aged plantations in the 

Sierra Nevada will continue at some level for the foreseeable 

future, if for no other reason than because severe wildfires will 

continue to occur. Even-aged plantations, therefore, should be 

included in owl-related management experiments. 

Uneven-Aged Silviculture Using Single-Tree 

Selection 

To meet the need for tree species diversity (assumption #3 

above), stand openings must be large enough to permit regeneration of shade-intolerant species such as ponderosa pine. This 

requirement generally is not met with the single-tree selection 

form of uneven-aged silviculture, at least where openings are 

mostly the size of individual large trees rather than groups of 

trees. Furthermore, retention of the smallest size-classes of 

trees well distributed through a stand-a necessity for sustaining this stand structure through time-creates dangerous fuel 

ladders and makes prescribed burning or other fuels management treatments essentially impracticable. As described earlier, 

many owl nesting stands had roughly an inverse J-shaped diameter distribution characteristic of uneven-aged stands. Composition of the smaller size classes, however, was strongly 

weighted toward shade-tolerant species, especially white fir. 

This resulted from many decades of fire suppression, augmented by partial cutting and preferential harvest of pines. In 

the absence of stand openings by cutting or by natural agents of 

disturbance such as fire and. insects, these stands will become 

increasingly dominated by shade-tolerant conifers. Single-tree 

selection should be included at a reduced level in management 

experiments to evaluate changes in structural attributes, owl 

use, managerial difficulty, and costs of implementation. Combining single-tree selection with group selection (discussed below) may work to maintain some vertical structure while permitting regeneration of shade-intolerant species. 

Mosaic of Small, Even-Aged Groups 

One kind of structure that may have promise for production 

and long-term maintenance of owl habitat is a multi-aged mosaic 

of small, even-aged groups or aggregations. Groups would generally range in size from about 2 acres down to a quarter-acre, or 

possibly less. Probably this type of structure best approximates 

presettlement stand structures (Chapter 12), thus warranting 

serious consideration. Openings would be sufficiently large to 

permit regeneration of shade-intolerant as well as shade-tolerant 

Chapter 13 271 
species. Multiple size classes in general would be separated 

horizontally rather than vertically, but in sufficient proximity to 

satisfy this attribute of suitable owl habitat. The horizontal separation of size classes also would confer some degree of resistance to crown fires (Chapter 12). 

Group selection cuttings, or modifications thereof, would 

be used to create and maintain this structure over time. Keeping 

track of a large number of small openings and groups for management purposes, long considered a major obstacle to the use of 

group selection, should be significantly easier with the advent of 

geographic information systems and satellite-based global positioning systems. Treatments certainly would be more complex 

and more expensive than with even-aged management, however, especially on steeper slopes. 

Viewed from the standpoint of area regulation-approximately equal areas maintained in each of several age-classes-a 

given "stand" of, say, 20-100 acres under steady-state conditions 

might contain three to six or more different age-classes. Each 

age-class would comprise many small, variable-sized aggregations and occupy a total area roughly equal to the area of the 

entire stand divided by the number of age-classes. "Rotation 

age"-the age at which the oldest aggregations would be regenerated-could be as long as needed to meet and maintain targets 

for large and/or decadent trees and snags. Periodic entries preceding regeneration cutting for a given age-class could be used 

to adjust stand structure to meet desired habitat attributes. These 

intermediate treatments might include thinning to speed development of large trees or to alter species composition, creating 

snags by girdling or other means, or wounding selected trees to 

induce decay. In practice, these intermediate treatments would 

take place within the various age-classes (aggregations) in the 

stand when the oldest age-class is being regenerated. As an 

example, if six age-classes and a 240-year "rotation" were selected, entries could be made every 40 years to regenerate one-sixth 

of the stand and conduct appropriate intermediate cuttings in the 

remainder of the stand. Or 20-year entries could be made, but 

regeneration cuttings would be made only every other entry. 

Successive age-classes would be separated by about 40 years. 

In groups to be regenerated, all trees could be removed or, 

especially in larger groups, scattered live trees and/or snags 

could be retained. To facilitate fuels treatment and reduce damage to the surrounding stand, cut trees should be felled as much 

as possible into the newly created opening. Site preparation/ 

fuels treatment methods used on clearcuts should be usable in 

these small openings, although they are likely to be much more 

expensive. One promising possibility may be jackpot burning of 

slash concentrations in the opening at a time of year when fire 

would not spread into the adjacent stand, thereby minimizing the 

need for firelines. Shortly thereafter, the rest of the stand could 

be underburned during somewhat drier conditions. Alternatively, 

the rest of the stand could be underburned at the same time as the 

openings. Local trials would help define a workable regime. In 

any case, we recommend that fuels be treated after each entry 

into the stand to reduce chances of severe wildfire. Various 

fuels-treatment methods may be appropriate for a given area. 

Prescribed underburning, however, has the advantage that it 

272 Chapter 13 

would begin to restore the natural role of fire and associated 

processes in the ecosystem (Chapter 12). In the scenario described earlier, with successive age-classes separated by 40 

years, the youngest (40-year-old) trees probably would be large 

enough to tolerate an underburn without excessive mortality, 

assuming early vegetation management to permit relatively rapid 

early growth. 

Openings could be regenerated either naturally or artificially, and with or without vegetation management. Even with 

planting and vegetation management, growth of tree seedlings 

would be slower in an opening typical of group selection than in 

a larger opening because of competition for site resources from 

large trees surrounding the opening. Without planting and some 

control of nonconifer vegetation, however, development of conifers could be delayed for several decades. Fuels treatment 

would be complicated as well. 

Development of a mosaic of small groups could be initiated 

in a wide range of stand conditions-for example, an older 

plantation, a variable-aged young-mature stand, or an old stand 

becoming excessively unbalanced in terms of size-class distribution or species composition. 

Two- or Three-Storied Stands 

Another kind of structure that might be suitable for production and maintenance of owl habitat is a two- or three-storied 

stand. It differs from the even-aged aggregation structure in that 

each age/size-class would be more or less uniformly distributed 

throughout the stand (although many variations in spatial arrangement would be possible). In a two-storied stand, the upper 

canopy would be sufficiently open to permit regeneration of 

shade-intolerant species in the understory. If a third canopy layer 

were to be managed, both of the upper two canopy layers would 

need to be thinned enough to allow regeneration and growth of 

multiple species. Typically, this kind of structure would be 

initiated with a shelterwood cutting. After regeneration is established, the overstory would be retained indefinitely-a practice 

referred to as irregular shelterwood-instead of being removed 

as occurs with even-aged management. Understocked stands, 

traditionally a high priority for clearcutting, could instead be 

underplanted, leaving most of the overstory in place. An overstory infected with dwarf mistletoe could be underplanted with 

species other than the one(s) infected. 

If desired, this kind of structure could be initiated relatively early in the life of a plantation by having a heavy commercial thinning double as a shelterwood-type regeneration cutting. The cut would be followed by site preparation/fuels treatment and underplanting with the desired mix of species. Throughout the "rotation" of such a stand, thinnings could be applied as 

needed to maintain desired size classes and species. These should 

be followed by prescribed burning or other fuels treatments. 

Snags could be created as needed. Once created, the stand 

would never be devoid of large trees: each regeneration cutting 

would be accompanied by retention of some trees in one or two 

overstory layers. Thus a relatively short period of time would 

elapse between a regeneration cutting and restoration of a 

desired stand structure. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Fuels treatments, including use of prescribed burning, should 

not be particularly difficult for a two-storied stand. Initial site 

preparation/fuels treatment before establishment of the understory would be the same as for a shelterwood cut. Subsequent 

treatments would be comparable to those for an even-aged 

plantation. Separation of canopy layers normally would be sufficient to keep wildfires out of overstory crowns. A three-storied 

stand could be somewhat more problematical, in terms of maintaining adequate stocking of shade-intolerant species, protecting the small understory during fuels treatments, and keeping 

wildfires out of the overstory. In all these respects, a three-storied 

stand would begin to approach conditions in a single-tree selection stand. 

Conclusions 

We believe that the dynamic trends in forest structure and 

fuels profiles that are occurring on NFs in the Sierra Nevada are 

cause for concern. The most troubling aspects are the loss of old, 

large-diameter trees and associated woody debris, a shift toward 

more shade-tolerant species, the buildup of fuels associated with 

mortality in the small diameter classes, and the continued presence of abundant ladder fuels that enable crown fires to occur. 

We do not believe the management directions elucidated in the 

current LMPs alleviate these trends; in fact, single-tree selection 

systems are likely to accelerate them. Even-aged systems can 

reduce fuel loadings and encourage the growth of shade-intolerant 

species, but they do so at the expense of the remnant large trees. 

We believe that other options exist that could deal directly with 

these concerns. Management plans should focus on addressing 

undesirable trends, designing potential solutions, and proceeding experimentally to implement those plans on the landscape. 

Owl habitat can be described in terms that are compatible 

with silvicultural methods. In this chapter we have presented 

preliminary examples of how this process might proceed. The 

FS can easily gather basic stand-level statistics from known owl 

nest stands throughout the range of the California spotted owl. 

To date, most of the data collected are either very coarse (for 

example, at the timber strata level) or based on ocular estimation 

and, therefore, are not as reliable as we would like. 

Looking to longer-term solutions, we need to begin changing the forest structure back to a form more akin to historical 

patterns: to generate fire-resistant structures with small-scale 

horizontal heterogeneity and a significant large-tree component. 

The silvicultural systems suggested here provide for the maintenance of large trees and complex stand structures, while signifiUSDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

cantly decreasing the risk of catastrophic wildfire. Through 

management experiments, we need to ascertain which, if any, of 

these stand structures may also be suitable for spotted owls. 

References 

Barbour, M. G.; Woodward, R. A. 1985. The Shasta red fir forest of California. 

Canadian Journal of Forest Research 15:570-576. 

Bingham, Bruce B.; Sawyer, John O. 1992. Canopy structure and tree condition in young, mature and old-growth Douglas-fir/hardwood forests. Unpublished draft supplied by authors; 21 p. , 

Daniel, Theodore W.; Helms, John A.; Baker, Fredrick S. 1979. Principles of 

Silviculture. New York: McGraw-Hill; 500 p. 

Davis, Lawrence S.; Johnson, K. Norman. 1987. Forest Management. New 

York: McGraw-Hill; 790 p. 

Dunning, D.; Reineke, L. H. 1933. Preliminary yield tables for second growth 

stands in the California pine region. Technical Bulletin 354. Washington, 

DC: U.S. Department of Agriculture; 23 p. 

Fiske, John. Regional Reforestation/TS1 Program Manager, USDA Forest 

Service, Region 5, San Francisco, CA. [Personal communication]. February 1992. 

Husch, Bertram; Miller, Charles l.; Beers, Thomas W. 1982. Forest mensuration. New York: John Wiley and Sons; 402 p. 

Kessler, Winifred B.; Salwasser, Hal; Cartwright, Charles W. Jr.; Caplan, 

James A. 1992. New perspectives for sustainable natural resource management. Ecological Applications 2:221-225. 

National Research Council. 1990. Forestry research: a mandate for change. 

Washington, DC: National Academy Press; 84 p. 

Pitcher, Donald C. 1987. Fire history and age structure in red fir forests of 

Sequoia National Park, California. Canadian Journal of Forest Research 

17:582-587. 

Rountree, Carl. 1992. Background report: Land use trends & projections 

within the potential habitat of the California spotted owl. Unpublished 

draft supplied by author; 7 p. 

Ruth, Larry. Lecturer in Forestry, Wildland Resources Center, Univ. of California, Berkeley, CA. [Personal communication]. March 1992. 

Taylor, Alan H.; Halpern, Charles B. 1991. Structure and dynamics of Abies 

magnifica forests in the southern Cascade Range, USA. Journal of Vegetation Science 2:189-200. 

U.S. Department of Agriculture, Forest Service. Land and resource management plan. San Francisco, CA: USDA Forest Service, Pacific Southwest 

Region; 

1986 Sierra National Forest; 173 p. Draft 

 1988a Eldorado National Forest; 438 p. 

 1988b Plumas National Forest; 498 p. 

 1988c Sequoia National Forest; 225 p. 

 1990 Tahoe National Forest; 817 p. 

 1991a Lassen National Forest; 334 p. Draft 

 1991b Stanislaus National Forest; 458 p. 

Chapter 13 273 
Appendices 
Appendices 

Marlene B. Verner, Compiler 

Appendix A-Technical 

Assessment Team and 

Chronology of Activities 

Technical Assessment Team 

Core Group 

Thomas W. Beck, Forest Biologist, Stanislaus National Forest, Sonora, 

California. 

Gordon 1. Gould, Jr., Nongame Wildlife Biologist, California Department of 

Fish and Game, Sacramento, California. 

R. J. Gutiérrez, Professor, Department of Wildlife, Humboldt State University, 

Arcata, California. 

Kevin S. McKelvey, Wildlife Biologist, Pacific Southwest Research Station, 

USDA Forest Service, Arcata, California. 

Barry R. Noon, Research Wildlife Biologist and Project Leader, Pacific Southwest Research Station, USDA Forest Service, Arcata, California. 

Jared Verner, Team Leader, Research Wildlife Biologist and Project Leader, 

Pacific Southwest Research Station, USDA Forest Service, Fresno, 

California. 

Internal Support Group 

Office Manager and Editorial Assistant 

Marlene B. Verner, Secretary, Pacific Southwest Region, USDA Forest 

Service, Fresno, California. 

Photographer 

John S. Senser, Archaeologist, Stanislaus National Forest, USDA Forest Service, Mi-Wuk Village, California. 

Silviculturist and Fuels Management Specialist 

C. Phillip Weatherspoon, Supervisory Research Forester and Project Leader, 

Pacific Southwest Research Station, USDA Forest Service, Redding: 

California. 

Fuels Management Specialist 

Susan J. Husari, Regional Fuels Management Specialist, Pacific Southwest 

Region, USDA Forest Service, San Francisco, California. 

Editor 

B. Shimon Schwarzschild, Technical Publications Editor, Pacific Southwest 

Research Station, USDA Forest Service, Albany, California. 

Cartographer 

Jean Ann Carroll, Cartographer, Pacific Southwest Region, USDA Forest 

Service, San Francisco, California. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

Consultants and Advisors 

National Forest System 

Edward Whitmore, Acting Deputy Regional Forester, Pacific Southwest Region, USDA Forest Service, San Francisco, California. 

Christine Jauhola, Director of Fisheries and Wildlife Management, Pacific 

Southwest Region, USDA Forest Service, San Francisco, California. 

John Fiske, Regional Program Manager, Reforestation and Timber Stand 

Improvement, Pacific Southwest Region, USDA Forest Service, San 

Francisco, California. 

Mary Sue Fisher, Fish and Wildlife Budget Program Manager, Pacific Southwest Region, USDA Forest Service, San Francisco, California. 

David Solis, Spotted Owl Program Manager, Pacific Southwest Region, USDA 

Forest Service, San Francisco, California. 

John Robinson, Assistant Spotted Owl Program Manager, Pacific Southwest 

Region, USDA Forest Service, San Francisco, California. 

Dennis Caird, Regional Logging Engineer Specialist, Pacific Southwest Region, USDA Forest Service, San Francisco, California. 

Lori Walsh, Program Analyst, Pacific Southwest Region, USDA Forest Service, San Francisco, California. 

John Stewart, California Spotted Owl Coordinator, Pacific Southwest Region, 

USDA Forest Service, San Francisco, California. 

Colleen Pelles, Wildlife Biologist, Pacific Southwest Region, USDA Forest 

Service, San Francisco, California. 

California Department of Forestry and Fire Protection 

Greg Greenwood, Range Ecology Specialist, California Department of Forestry and Fire Protection, Sacramento, California. 

Robert Motroni, Wildlife Biologist, California Department of Forestry and 

Fire Protection, Sacramento, California. 

Bureau of Land Management 

James Decker, State Fisheries Biologist, Division of Lands and Renewable 

Resources, USDI, Sacramento, California. 

Larry Saslaw, District Wildlife Biologist, Division of Lands and Renewable 

Resources, USDI, Bakersfield, California. 

National Park Service 

David Graber, Research Biologist, Sequoia/Kings Canyon National Parks, 

Three Rivers, California. 

U.S. Fish and Wildlife Service 

Michael Horton, Wildlife Biologist, Endangered Species Office, USDI, 

Sacramento, California. 

Liaison for Environmental Groups 

Daniel Taylor, Western Regional Representative, National Audubon Society, 

Sacramento, California. 

Sami Yassa, Research Associate, Natural Resources Defense Council, San 

Francisco, California. 

Liaison for Timber Industry 

Steve Self, Wildlife Biologist, Sierra Pacific Industries, Redding, California. 

Robert Taylor, Director of Wildlife Ecology, California Forestry Association, 

Sacramento, California. 

Chronology of Activities 

4-6 June 1991--Sacramento 

The Core Group met to set up a tentative calendar; schedule field trips; 

identify and schedule needed workshops; assess the information needed 

from each National Forest, such as maps and other spotted owl data; 

discuss other needs, such as a reference library and computer hardware 

and software. 

11 June--Sacramento 

 Arranged for office furnishings and equipment.. 

Appendices 277 
12 June-Berkeley 

Meeting to discuss editorial needs and related matters. 

18 June-Sacramento 

Review of timber sales in planning stage or under contract. 

24-29 June-First field trip 

On-the-ground inspection of California spotted owl habitat and research 

study areas in the Sierra Nevada, including nesting, roosting and foraging sites. The first two days were spent on the Eldorado National Forest 

(NF) and Fruit Growers Supply Company's lands; the third day was on 

the Tahoe NF, taking us to one of our highest elevation nests at 7,000 

feet; the fourth day was spent in visiting Sierra Pacific Industries' lands 

on the Tahoe NF; the fifth day was on the Plumas NF; and the sixth day 

was spent on the Lassen NF. 

8-13 July-Second field trip 

This week was a continuation of visits to California spotted owl habitat 

and research study areas. The first day was spent on the Stanislaus NF 

and Fiberboard Industries' lands; the second day was on the Sierra NF; 

Sequoia National Park was visited on the third day; the fourth day was 

spent on Sequoia NF; the fifth and sixth days were spent on the Los 

Padres NF visiting the southern part and northern part, respectively. 

22-26 July-Sacramento 

The Team invited Forest Biologists and Conservation Biologists to attend up-coming meetings. 

26 July 

Meeting with fire specialists. Informal discussion of fire history and 

current fuel conditions in the Sierra Nevada and southern chaparrel areas. 

26 July 

Meeting to discuss information needs on timber resources. 

29-30 July-Sacramento 

29 July 

Met with Sierran Forest Biologists (from the Lassen, Plumas, Tahoe, 

Eldorado, and Lake Tahoe Basin Management Unit) to apprise them of 

the September 27-28 Biology Workshop to be held in Sacramento. 

Additional invitations went to the Sierra, Sequoia, Angeles, San 

Bernardino, Cleveland, and Los Padres NFs. 

5-9 August-Sacramento 

Technical Assessment Team still contacting people with regard to the 

upcoming Biology Workshop. 

5 August 

Session with biologists from Tahoe NF. 

6 August 

Meeting with Northern Spotted Owl Recovery Team biologist's subcommittee to review Dr. McKelvey's spatially explicit population simulation model. Recommended structural changes and methods of parameterization. 

8 August 

Mapping session with biologists from the Tahoe NF. 

9 August 

Planning session with cartographers. 

19-22 August-Third field trip 

The first day was spent on the Angeles NF; the San Bernardino NF was 

visited on the second day; the third day was divided between Mount San 

Jacinto and Palomar Mountain; and the fourth day was spent on the 

Cleveland NF in the Laguna Mountains. 

26-30 August-Sacramento 

26 August 

Meeting with cartographers to discuss options for producing maps. 

27-28 August 

Biology Workshop with 55 participants from all the NFs in Region 5 to 

discuss the California spotted owl's current status, habitat use and distribution, and future trends of the owl in each National Forest. 

30 August 

Meeting with Steve Self, Sierra Pacific Industries, Redding, California. 

Presentation and discussion of Sierra Pacific Industries' owl plan--

approach, habitat descriptions, implementation, guidelines, and so on. 

278 Appendices 

8-13 September-Sacramento 

Formulating working library and gathering data. 

10 September 

Meeting with Jonathan Bart, Northern Spotted Owl Recovery Team 

Leader, Portland, Oregon. Reviewed recommendations on parameter 

values for Kevin McKelvey's simulation model. 

10-11 September 

Conservation Biology Workshop with 36 participants from numerous 

State and Federal agencies, universities, and private industries throughout the U.S.; discussed a variety of issues and concepts from conservation biology as they might relate to planning for the California spotted owl. 

23-27 September-Sacramento . 

25 September 

Meeting with History Group to discuss objectives of their contribution to 

the CASPO report. 

1 October-Sacramento 

Meeting with timber staffs from all Sierran NFs to discuss the kinds of 

timber inventory information that could be provided, and how soon. 

Resolved details on instructions to National Forests on maps needed and 

tabular information to accompany some maps. 

7-11 October-Sacramento 

7 October 

Meeting to discuss fuels management. 

8 October . 

Information exchange with representatives of environmental organizations. 

11 October 

Bureau of Land Management presented their potential owl habitat information. 

12-13 October-Fourth field trip 

Team visited Roseburg Resources Company's lands to gain additional 

firsthand information on owl habitat on industry lands. 

14-18 October-Sacramento 

17 October 

Silviculture Workshop; all-day session with 24 agency, academic, and 

industry silviculturists participating, along with members from the Core 

Group. 

28 October-1 November-Sacramento 

30 October 

Fifth field trip. Visited Michigan-California Timber Company's lands 

and compared current forests with old, historic photos of forests and land 

management from the past 80-100 years. 

4-8 November-Sacramento 

Compiling information for library, mapping, and general data. 

18-22 November-Sacramento 

19 November 

Meeting with Fuels Management Specialists to have a full review of all 

habitat photos taken on the field trips. Discussed their use in depicting 

spotted owl habitat. Also discussed dead-and-downed woody material 

and forest stands with regard to fire hazards. 

20 November 

Reviewed progress of the History Group. Viewed and picked possible 

habitat photos for use in Chapter 5. 

21 November 

Met to review cumulative effects analysis used by Region 5 to determine 

impacts of green timber sales on California spotted owl habitat. 

2-6 December-Sacramento 

2 December 

Meeting to discuss ways in which the Technical Assessment Team and 

the Policy-Implementation Team can interact most efficiently. 

5 December 

Presentation on the influence of fire on forest structure and composition 

by John Maupin, Fire Management Officer, Plumas NF, Quincy, California. 

6-10 January 1992-Sacramento 

Team working session. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
20-24 January-Sacramento 

Team working session. 24 

January 

Coordination meeting with Policy-Implementation Team. 

3-14 February-Sacramento 

 4 February 

Discussion of possible approaches to risk assessment. 

11 February 

Presentation on forest conditions to be expected, over the long-term, if. 

the California Forestry Reform Act of 1992, or something like it, is 

enacted by the State Legislature. 

13 February 

Briefing for co-chairs of Steering Committee on current status of the 

Team's discovery process. 

24-28 February-Sacramento 

Team working session. 

2-6 March-Sacramento 

Team working session. 

9 March-Fresno 

Nine of 13 report chapters sent from Fresno to peer reviewers picked by 

presidents of scientific societies: 

American Ornithologists' Union 

Dr. Richard N. Conner, Research Wildlife Biologist, Southern 

Forest Research Station, USDA Forest Service, Nacogdoches, 

Texas. 

The Wildlife Society 

Dr. David Graber, Research Biologist, Sequoia/Kings Canyon 

National Parks, Three Rivers, California. 

The Society for Conservation Biology 

Dr. Stanley A. Temple, Professor, Department of Wildlife Ecology, University of Wisconsin, Madison, Wisconsin. 

Ecological Society of America 

Dr. Ted Case, Professor, Department of Biological Sciences, University of California, La Jolla, California. 

Society of American Foresters 

Dr. Arthur Cooper, Head, Department of Forestry, North Carolina 

State University, Raleigh, North Carolina. 

10 March-Arcata 

Two additional chapters sent to reviewers. 

16-18 March-Sacramento 

Team working session. 

23-31 March-Sacramento 

Team working on last two chapters of the CASPO report. Vacated 

Sacramento offices 31 March. 

1 April-7 May-Fresno and Arcata 

Team finishing report with regard to review suggestions and final revisions. 

8 May-Sacramento 

Final report presented to the Steering Committee. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

Appendix B-Glossary 

Activity center-an area within which an owl or pair of owls finds suitable 

nesting sites and several suitable roost sites, and in which a substantial amount 

of their foraging occurs. An activity center can generally be identified by the 

location of a nest or a primary roost. Other identifiers are recent locations of 

owls, especially of pairs or reproductive pairs. 

Adaptive kernel technique-a method of estimating home-range size in 

which, first, a bivariate probability distribution is estimated using the observed 

locations, and then the area of the contour that contains 95 percent of the 

observed locations is calculated. 

Adaptive management-process of implementing policy decisions as 

scientifically driven management experiments that test predictions and assumptions in management plans. 

Algorithm-mathematical rule for solving a problem. 

Allee effect-a depression in the encounter rate between males and females 

resulting from low population densities; the probability of finding a mate drops 

below that required to maintain the reproductive rates necessary to support the 

population. 

Allowable sale quantity-(ASQ) the maximum quantity of timber that 

may be sold by a given National Forest from land capable, available, and 

suitable for timber production for a time period; usually expressed on an 

average annual basis. 

Basal area-the area of the cross-section of a tree stem near its base, 

generally at breast height and inclusive of bark. 

Biological diversity-the variety of life's forms-that is plants, birds, 

insects, and so on. 

Biomass-the total quantity (at any given time) of living organisms of one or 

more species per unit of space, or of all the species in a biotic community. 

Biomass sale-sale of wood fiber such as logging residue (slash), small 

diameter live trees and cull logs (other than saw logs), for the purpose of 

energy co-generation or chip production. 

Birth-pulse population-a population assumed to produce all of its offspring at an identical, and instantaneous, point during the annual cycle. 

Blowdown-trees felled by high winds. 

Bonferroni confidence interval-an individual confidence interval constructed about each estimated proportion within a multinomial contingency 

table. The width of each confidence interval is adjusted downward to account 

for the estimation of simultaneous intervals. 

Bottleneck-see "population bottleneck." 

Burning period-the anticipated period of greatest fire activity during a 

24-hour period, typically from 1000 until 1800. 

Cambium-a layer of formative cells between the wood and bark in woody 

plants: the cells increase by division and differentiate to form new wood and 

bark. 

Canopy closure-the degree to which the crowns of trees are nearing 

general contact with one another. 

Carrying capacity-the maximum number of animals that can be sustained over the long-term on a specified land area. 

Center of activity-owl's nest site or primary roost area. 

Checkerboard ownership-a land ownership pattern in which every 

other section (square mile) is in Federal ownership as a result of Federal land 

grants to early western railroad companies. 

Cohort-individuals all resulting from the same birth-pulse, and thus all of 

the same age. 

Colonization-the act or process of establishing a new colony or population. 

Appendices 279 
Commercial forest land-forest land tentatively suitable for the production 

of crops of timber and that has not been withdrawn for other reasons. 

Confidence interval-a region lying above and below a parameter estimate 

(for example, the mean) in which the true parameter value is believed to occur 

with some specified probability. 

Connectivity-a measure of the extent to which intervening habitat truly 

connects habitats for juvenile spotted owls dispersing between them. 

Core area-a defined area that includes the center of activity of a pair, 

including the nest site if known. 

Corridor-defined tract of land, usually linear, through which a species must 

travel to reach habitat suitable for reproduction and other life-sustaining needs. 

Cull-a tree that is not healthy (diseased, broken top, stunted, and so on) and is 

rejected as not being up to standard for regular timber harvest. 

D.b.h.-diameter of a tree at breast height, typicallly measured in inches. 

Demographic rescue-see "rescue." 

Demographic stochasticity-random fluctuations in birth and death 

rates. 

Density-dependent-process, such as fecundity, whose value depends on 

the density of animals in the population. 

Dispersal-the movement, usually one way, and on any time scale, of plants 

(seeds) or animals from their point of origin to another location where they 

subsequently produce offspring. 

Dispersal capability-ability of members of a species to move from their 

area of birth to another suitable location and subsequently to breed. 

Dispersal distance-the straight-line distance traveled by an individual 

from its birth place until it stops dispersing (assumed to be a breeding site) or 

dies. 

Dominant canopy closure (cover)-canopy closure of only the dominant trees in a stand, expressed as a percentage. 

Dry ravel-a form of surface erosion in which dry, unconsolidated material 

moves down slope under the influence of gravity. 

Duff-decaying vegetable matter that forms a layer on the forest floor. 

Eastside pine forest-general name for a habitat type occurring generally 

east of the Sierran crest. It is dominated by ponderosa and/or Jeffrey pine. 

Ecological integrity-the condition in which all key components of an 

ecological system are intact and functioning normally. 

Ecotone-contact zone between two plant communities, where elements of 

each intermingle. 

Edge effects-differences in microclimate, flora, fauna, stand structure, 

habitat values, stand integrity (including resistance to being blown down by 

high winds) that occurs in or as a result of a transition zone where two plant 

communities or successional stages come together. 

Emigration-permanent movement of individuals of a species from a population. 

Environmental stochasticity-random variation in environmental attributes such as temperature, precipitation, and fire frequency. 

Epigeous fungi-above-ground fruiting bodies of fungi, in the form of 

mushrooms; these make up part of the diet of spotted owl prey. 

Even-aged forest-a forest stand composed of trees with less than a 

20-year difference in age between the oldest and youngest. 

Extinction rate-the number of elements (individuals, populations, species) 

lost per unit of time. 

Extinction time-predicted period of time for a population to become 

extinct. 

Fecundity-the number of young per breeding-age female. In model formulations, fecundity usually refers to the number of female young per breedingage female, calculated by assuming a 1:1 sex ratio. 

280 Appendices 

Fire regime-a description of the frequency, severity, and extent of fires that 

occur in an area. 

Floaters-nonbreeding adults and subadults that move and live within a 

breeding population, often replacing breeding adults that die; nonterritorial 

individuals. 

Foothill riparian/hardwood forest-general name for a habitat type 

occurring at low elevations in the Sierran foothills. It includes stands of 

hardwoods immediately adjacent to streams, as well as dense stands of hardwood forests on the adjoining slopes. Tree species along streams include 

cottonwood, California sycamore, interior live oak, California buckeye, Oregon ash, and occasionally white alder. Tree species on the adjoining slopes 

include blue oak, interior live oak, and digger pine. 

Forest landscape-land presently forested or formerly forested and not 

currently developed for nonforest use. 

Fragmentation-process of reducing the size and continuity of patches of 

habitat; specifically in this document, fragmentation is used in reference to 

forests. 

Fuel ladder-dead or living fuels that connect surface fuels to tree or brush 

foliage and promote spread of fire from ground to vegetation crowns. 

Fuel loading-the amount of combustible material present per unit area, 

usually expressed in tons per acre. 

Fuels-combustible materials. 

Gene flow-movement of genetic material between populations. 

Genetic stochasticity-random changes in gene frequencies from such 

factors as inbreeding. 

Genetic variability-the number of different genes possessed by an individual or population. 

Habitat capability-capacity of a habitat to support an estimated number 

of pairs of a species. 

Habitat Conservation Area-(as proposed by the ISC), a contiguous 

block of habitat to be managed and conserved for breeding pairs, connectivity, 

and distribution of owls; application may vary throughout the range according 

to local conditions. 

Habitat fragmentation-see "fragmentation." 

Habitat mosaic-the mix of habitat conditions across a landscape. 

Home range-the area to which the activities of an animal are confined 

during a defined period of time. 

Home range of a pair-the sum of the home ranges of each member of a 

pair, minus the area of home-range overlap. 

Home-range overlap-percentages of the home ranges of two individuals 

that are shared between them. 

Hypogeous fungi-below-ground fruiting bodies of fungi, known as true 

and false truffles; these are an important part of the flying squirrel's diet. 

Initial attack-first action taken to suppress a wildfire, via ground or air. 

Interbirth interval-the interval between birth pulses. 

Internal recruitment-addition of new breeding individuals to a local 

population that were born within that same population. 

Lambda-the finite rate of population change (population size in year 2 

divided by the population size in year 1). 

Land Management Plan-a plan written for the management of a National 

Forest unit, as directed by regulations of the National Forest Management Act 

of 1976, in which the integrated management of all major resources has been 

determined through an interdisciplinary team process. 

Lands not suited for timber production-lands incapable of producing 20 cubic feet of wood fiber per acre per year, or lands withdrawn from 

commercial forest harvest for other reasons (see reserved lands). 

Lands suited for timber production-commercial forest land identified as 

appropriate for timber production. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Large sawtimber-forest stands that are characterized by trees that are ≥21 

inches in d.b.h. 

Late seral stage forest-near-final stage in development of a forest from 

grasses and forbs, through shrubs, small trees, and finally to large, old trees. 

Leave strips-generally narrow bands of forest trees that are left along 

streams and rivers to buffer aquatic habitats from upslope forest management 

activities. 

Lefkovich matrix-a two-dimensional array of numbers whose entries 

represent stage-specific estimates of demographic (birth and death) rates. The 

matrix is used to project population stage structures through time. 

Legacy-remnant trees of original forest stands, both alive and dead. 

Leslie matrix-a two-dimensional array of numbers whose entries represent the age-specific estimates of demographic (birth and death) rates. The 

matrix is used to project population age structures through time. 

Life table-mathematical table illustrating the age-specific birth and death 

rates of a population. 

Linear model-a combination of random variables, none of which has 

exponents that differ from 1.0. 

Linear regression model-an equation that explains some amount of the 

variation in a dependent variable with a linear combination of one or more 

independent variables. 

Live oak/bigcone Douglas-fir forest-general name for a habitat type 

that occurs in a narrow band, mostly at mid-elevations, in mountains of all four 

NFs in southern California. Dominant tree species are canyon live oak, coast 

live oak, and bigcone Douglas-fir. 

Locus-the point (for example, along an axis of a graph) at which the shape of 

a mathematical function changes dramatically. 

Long term-here, 50 to 100 years and sometimes beyond. 

M2G, M3G, M3N, M3P, M4G, M4N, M4P, M5G, M5N, M6-see 

timber strata. 

Managed forest-forest land that is harvested on a scheduled basis and 

contributes to an allowable sale quantity. 

Medium sawtimber-forest stands that are characterized by trees that are 

11-20.9 inches in d.b.h. 

Metapopulation-a population comprised of a set of isolated subpopulations 

that are "linked" by the dispersal of individuals, allowing for recolonization of 

unoccupied habitat patches after local extinction events. 

Mesic-moderately moist, in referring to habitats. 

Microenvironment-the sum total of all the external conditions in a small 

or restricted area that may influence organisms. 

Microhabitats-a restricted set of distinctive environmental conditions that 

constitute a small habitat, such as the area under a log. 

Minimum convex polygon technique-a method of estimating home-range 

size in which the smallest possible convex polygon is drawn around the 

outermost locations where an animal was observed; the area within the polygon is then calculated. 

Mixed-conifer forest-general name for similar habitat types in the 

Sierra Nevada and southern California. It is the predominant timber-producing 

forest of the Sierra Nevada, consisting of various mixtures of white fir, 

ponderosa pine (at lower elevations), incense-cedar, sugar pine, black oak, and 

red fir (at higher elevations). Douglas-fir is an important component from 

Yosemite NP northward, and giant sequoia occurs in widely scattered localities. In southern California, this type is best developed at relatively high 

elevations in the San Gabriel and San Bernardino Mountains, and on Mount 

San Jacinto. Species composition is similar to that of Sierran mixed-conifer, 

although Coulter pine occurs, bigcone Douglas-fir occasionally occurs at 

lower elevations; but red fir, Douglas-fir, and giant sequoia are missing. 

Mixed-evergreen forest-a forest community that is dominated by two 

or more species of broad-leaved hardwoods whose foliage persists for several 

years; important western species include madrone, tanoak, chinquapin, canyon 

live oak, and California-laurel. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

Model-an idealized representation of reality developed to describe, analyze, 

or understand the behavior of some aspect of it; a mathematical representation 

of the relationships under study. 

Monitoring-a process of collecting information to evaluate whether or not 

objectives of a management plan are being realized. 

Monitoring program-see "monitoring"; the program used to monitor a 

population and its habitat. 

Natal cluster-a group of adjacent animal territories, in one of which an 

individual was born. 

Network-a particular spatial arrangement of entities (blocks or patches of 

owl habitat in this case) that are interconnected in some fashion (by dispersal 

of owls in this case). 

Null hypothesis-a supposition of no difference between test comparisons 

(situation A no different from situation B). 

Old growth-forest stand with moderate to high canopy closure; a multilayered, multispecies canopy dominated by large overstory trees; a high incidence 

of large trees with large, broken tops, and other indications of decadence; 

numerous large snags; and heavy accumulations of logs and other woody 

debris on the ground. 

Owl site-an area of unspecified dimensions where a single owl or a pair of 

owls has been located, usually repeatedly. In demographic and radio-tracking 

study areas, where efforts to locate all owls are more intense than elsewhere, 

most owl sites with single owls have eventually been found to have a pair. All 

owl sites have been mapped and given unique spatial references, so they can 

be tallied. Designation of an owl site makes no assumption about home-range 

or territory boundaries of the owls, although usually a center of activity can be 

identified by the location of a nest or a primary roosting area. The terms "owl 

site" and "site" are general and often used generically to refer to home ranges, 

territories, or to sites designated by agencies for special owl management. 

Owl use area-an irregularly shaped polygon that contains a known or 

predicted activity center and encompasses the amount of nesting and foraging 

habitats typically found within home ranges during the breeding season. 

Pair site-an area of variable dimensions on the landscape assumed to be 

large enough to have an amount of habitat capable of supporting one pair of 

spotted owls; see "owl site." 

Paradigm-an underlying model or representation that characterizes a process. 

Physiographic province-a geographic region in which climate and 

geology have given rise to a distinct array of land forms and habitats. 

Ponderosa pine/hardwood forest-(montane hardwood) general name 

for a habitat type that blends with the upper portion of the foothill riparian/ 

hardwood forest. In the southern Sierra Nevada, ponderosa pine at its lowest 

elevation generally occurs with interior live oak, canyon live oak, and black 

oak, with incense-cedar and white fir coming into stands at slightly higher 

elevations. In the northern Sierra Nevada, tanoak and Pacific madrone commonly contribute to the hardwood component of this type. 

Population-a collection of individuals that share a common gene pool 

through interbreeding. 

Population bottleneck-the phenomenon experienced by a small population that is susceptible to the deleterious effects of demographic and genetic 

stochasticity; also a zone of constriction in the distribution of a population. 

Population density-number of individuals of a species per unit area. 

Population persistence-general term for the capacity of a population to 

maintain sufficient numbers and distribution over time. 

Population viability-probability that a population will persist for a specified period of time across its range, despite normal fluctuations in population 

and environmental conditions. 

Potential habitat-(1) habitat that has been altered (for example, logged 

or burned) and is not presently suitable for owls but is believed to have the 

potential to regenerate into suitable habitat; (2) unsurveyed habitat that ap-. 

pears to be suitable based on comparisons with habitat elsewhere that has 

known owl sites. 

Appendices 211 
Power analysis-a statistical method for estimating the probability of making 

a type-II error (failure to detect a difference or a trend that actually occurs, such 

as a decline in a population of spotted owls). 

R3G, R3N, R3P, R4G, R4N-see timber strata. 

Red fir forest-general name for a habitat type that blends with the higher 

portions of Sierran mixed-conifer forest. It is dominated by red fir, with 

increasing amounts of white fir at lower elevations until it becomes mixed-conifer 

forest. At upper elevations it often includes some lodgepole pine and occasionally quaking aspen. 

Redwood/California-laurel forest-general name for a habitat type 

that is restricted to the central coast range, where coast redwood, Californialaurel, tanoak, Pacific madrone, red and white alder, coast live oak, Santa Lucia 

fir, and bigleaf maple form various mixtures. 

Regulated forest-theoretical managed forest from which the same acreage of trees can be removed annually, in perpetuity. 

Rescue (rescue effect)-periodic immigration of new individuals sufficient to maintain a population that might otherwise decline toward extinction. 

Reserved land-Federal lands unavailable for timber yield or management 

due to being a National Park or classified as a Wilderness Area in a National 

Forest. 

Reserves-tracts of forest temporarily or permanently set aside from logging. 

Restricted harvest-land either withdrawn from logging or where timber 

production is limited to less than clearcutting. 

Riparian/hardwood forest-general name for a habitat type that varies 

considerably in different parts of southern California. In deep canyons in the 

Los Padres NF, for example, it occurs in narrow strips adjacent to permanent or 

near-permanent streams. Common tree species include coast live oak (near 

coast), canyon live oak (interior locations), California sycamore, white alder, 

California-laurel, and cottonwood. In shallower canyons in the Cleveland NF, 

these forests may consist almost exclusively of coast live oak. 

Rotation-the planned number of years between the regeneration of an 

even-aged stand and its final cutting at a specified stage. 

Saw kerf-the cut or channel made by a saw. 

Scansorial-adapted for climbing. 

Search capability-the ability of a dispersing juvenile or adult owl to 

locate suitable habitat. 

Search efficiency-proportion of dispersing juveniles or adults that locate 

minimally suitable habitat before they die. 

Search time-number of days required for an average dispersing individual 

to locate suitable or better habitat. 

Seed-tree cut-an even-aged regeneration cutting in which only a few 

seed trees per acre (fewer than for a shelterwood cut) are retained until after 

new tree seedlings are established. 

Senescence-state of being old; characterized by having attributes associated with old age. 

Sensitivity coefficient-term that measures relative degree of change in 

outcome of a mathematical expression or equation after a specified change in 

an individual component. 

Shelterwood cut-an even-aged regeneration cutting in which new tree 

seedlings are established under the partial shelter of seed trees. 

Short term-here, 1 to 5O years. 

Sink-population whose average reproductive rate is less than its average rate 

of mortality; area that attracts immigrants not expected to contribute significantly to future populations (see "source"). 

Site-an area considered from the standpoint of its use for some specified 

purpose (for example, habitat studies, owl locations-see "owl site," and 

logging operation). 

Small sawtimber-forest stands that are characterized by trees that are 

<11 inches in d.b.h. 

282 Appendices 

Snag-standing dead tree. 

Source-an actively breeding population that has an average birth rate that 

exceeds its average death rate; produces an excess number of juveniles that 

may disperse to other areas. 

Standards and guidelines-directions generated and followed in management plans. 

Stochastic-random, uncertain; involving a random variable. 

Stochastic fecundity-random fluctuation in a population's rate of producing offspring. 

Subpopulation-a well-defined set of interacting individuals that comprise 

a proportion of a larger, interbreeding population. 

Suitable habitat-here, an area of forest vegetation with the age-class, 

species of trees, structure, sufficient area, and adequate food source to meet 

some or all of the life needs of a California spotted owl. 

Sustained yield or production-the amount of timber that a forest can produce 

continuously from a given intensity of management; implies continuous production; a primary goal is to achieve a balance between incremental 

growth and cutting. 

Synoptic weather pattern-large-scale weather pattern. 

Territory-the area that an animal defends, usually during the breeding 

season, against intruders of its own species. 

Threshold phenomenon-a pattern or trend, as in population growth rate, 

that exhibits relatively long periods of slow change followed by precipitous 

increase or decrease in response to a slight change in an environmental 

gradient. 

Timber strata-M = mixed conifer, R = red fir, P = ponderosa pine; 2 = 

trees <l2 inches in d.b.h., 3 = trees 12-23.9 inches in d.b.h., 4 = trees ≥24 

inches in d.b.h.; G = good canopy cover (70+ percent), N = normal canopy 

cover (40-69 percent), P = poor canopy cover (0-39 percent). Hence, an M4G 

stand is mixed-conifer with trees ≥24 inches in d.b.h., and canopy cover ≥70 

percent; an R2P stand is red fir with trees <l2 inches in d.b.h., and 0-39 percent 

canopy cover. 

Torpid-having lost temporarily all or part of the power of sensation or 

motion, as a hybernating animal. 

Total canopy closure (cover)-canopy cover by all vegetation 7 feet or 

higher above the ground, expressed as a percentage. 

Truffles-the below-ground fruiting bodies of hypogeous fungi, which are a 

major food source for flying squirrels and other small mammals. 

Turnover-a term in population analysis that indicates the rate or number of 

identifiable adults that die and are replaced during a specified period. 

Type conversion-conversion of an area from one habitat type to another, 

such as oak woodland to annual grassland. 

Type-I error-statistical term for the error made when a null hypothesis 

that is true is rejected; for example, concluding that a difference exists between 

two populations that are identical. 

Type-II error-statistical term for the error that is made when a null 

hypothesis that is false is not rejected; that is, concluding that no difference 

exists in a comparison between two populations when a difference does exist. 

Variance-a statistical tern that indicates a measure of variability within a 

finite population of a sample; the total of the squared deviations of each 

observation from the arithmetical mean divided by one less than the total 

number of observations. 

Viability-ability of a population to maintain sufficient size so that it persists 

over time, in spite of normal fluctuations in numbers; usually expressed as a 

probability of maintaining a specific population for a specified period. 

Vital rates-collective term for the age-specific birth and death rates of a 

population. 

Windthrow-a tree or group of trees uprooted by the wind. 

Xeric-dry, in referring to habitats. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Appendix C-Species List 

Common Name Scientific name 

Lower Plants 

Algae ................................. Division Chlorophyta (various species) 

Fungi, epigeous ................. Divisions Ascomycota and Basidiomycota. 

Common genera in the Sierra Nevada 

 include Boletus, Amanita, Cortinarius, 

and Armillaria 

Fungi, hypogeous .............. Divisions Ascomycota and Basidiomycota. 

 Common genera in the Sierra Nevada 

 include Rhizopogon, Gautieria, Geopora, 

 Melanogaster, and Hymenogaster 

Lichens .............................. Division Ascomycota (various species) 

 Chartreuse ....................... Letharia vulpina 

 Hair .................................. Bryoria fremontii 

 Staghorn .......................... Usnea ceratina 

Moss .................................. Division Bryophyta, Class Muscopsida, 

(various species) 

Grasses/Forbs 

Bitterroot, meadow ............ Lewisia nevadensis 

Brodiaea, golden ................ Brodiaea lutea 

Buckwheat, California ....... Eriogonum fasciculatum 

Chinese nests .....................Collinsia concolor 

Grass . ................................ Gramineae spp.

Lupine, broad-leaf ............. Lupinus latifolius 

Trefoil (bird's foot) ............ Lotus spp.

Shrubs 

Blackberry, California ....... Rubus ursinus 

Buckbrush .........................Ceanothus cuneatus 

Buckeye, California ........... Aesculus californica 

Ceanothus ..........................Ceanothus spp.

Chamise ............................. Adenostoma fasciculatum 

Cherry, hollyleaf ................ Prunus ilicifolia 

Chinquapin, golden ...........Castanopsis sempervirens 

Chokecherry ...................... Prunus virginiana 

Coffeeberry, California . .... Rhamnus californica 

Currants .............................Ribes spp.

Deerbrush ..........................Ceanothus integerrimus 

Elderberry .......................... Sambucus spp.

Gooseberry, rock ............... Ribes quercetorum 

Hazel, California ...............Corylus cornuta var. californica 

(Hazelnut) 

Jim brush ...........................Ceanothus sorediatus 

Juniper, mountain .............. Juniperus communis var. saxatilis 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

Common Name Scientific name 

Shrubs 

Manzanita . ..............................Arctostaphlyos spp.

Mountain mahogany, ..............Cercocarpus ledifolius 

curlleaf 

Poison oak ...............................Rhus diversiloba 

Rose, California wild . .............Rosa californica 

Sage, coastal . ..........................Artemisia californica 

Sage, White .............................Salvia apiana 

Sagebrush, big . .......................Artemisia tridentata 

Sagebrush, California . ............Artemisia californica 

Serviceberry, western ..............Amelanchier alnifolia 

Sumac, lemonade ....................Rhus integrifolia 

Toyon ......................................Heteromeles arbutifolia 

Whitethom, mountain . ............Ceanothus cordulatus 

Willows ...................................Salix spp. 

Yerba-Santa . ...........................Eriodictyon spp. 

Yucca ......................................Yucca whipplei 

Other 

Mistletoes, dwarf . ...................Arceuthobium spp. 

Trees 

Alder, red . ...............................Alnus rubra 

Alder, white .............................Alnus rhombifolia 

Ash, Oregon ............................Fraxinus latifolia 

Aspen, quaking . ......................Populus tremuloides 

Boxelder, California . ..............Acer negundo 

Buckeye, California . ...............Aesculus californica 

California-laurel (bay) ............Umbellularia californica 

Cedar, incense - .......................Libocedrus decurrens 

Chinquapin, giant ....................Castanopsis chrysophylla 

Cottonwood .............................Populus spp. 

Dogwood, Pacific ....................Cornus nuttallii 

Douglas-fir ..............................Pseudotsuga menziesii

Douglas-fir, bigcone ...............Pseudotsuga macrocarpa

Fir, red (Shasta) . .....................Abies magnifica 

Fir, Santa Lucia .......................Abies bracteata 

Fir, white .................................Abies concolor

Hazelnut, California ................Corylus cornuta var. californica

 (hazel) 

Hemlock, mountain .................Tsuga mertensiana

 (black) 

Incense-cedar . .........................Libocedrus decurrens

Juniper, California . .................Juniperus californica

Juniper, western ......................Juniperus occidentalis, J. australis

Laurel, California- (bay) .........Umbellularia californica

Madrone, Pacific .....................Arbutus menziesii 

Appendices 283 
Common Name Scientific name 

Trees 

Maple, bigleaf ........................Acer macrophyllum 

Oak, black . .............................Quercus kelloggii 

Oak, blue ................................Quercus douglasii 

Oak, California scrub .............Quercus dumosa 

Oak, canyon live .....................Quercus chrysolepis 

Oak, coast live ........................Quercus agrifolia 

Oak, interior live ....................Quercus wislizenii 

Oak, shrub live .......................Quercus turbinella 

Oak, valley ..............................Quercus lobata 

Pine, Coulter ...........................Pinus coulteri 

Pine, digger .............................Pinus sabiniana 

Pine, foxtail ............................Pinus balfouriana 

Pine, Jeffrey ............................Pinus jeffreyi 

Pine, lodgepole . ......................Pinus contorta 

Pine, pinyon ............................Pinus monophylla 

Pine, ponderosa ......................Pinus ponderosa 

(yellow) 

Pine, sugar . .............................Pinus lambertiana 

Pine, western white .................Pinus monticola 

Redwood (coast) .....................Sequoia sempervirens 

Sequoia, giant .........................Sequoiadendron giganteum 

Sycamore, California ..............Platanus racemosa 

Tanoak ....................................Lithocarpusdensijlorus 

Willows ..................................Salix spp. 

Invertebrates 

Beetle, bark .............................Dendroctonus spp., Ips spp., Scolytus spp.

Beetle, fir engraver .................Scolytis ventralis 

Beetle, June ............................Ploeocoma hoppingi 

Cricket ....................................Gryllus spp.

Fly, hippoboscid ............. .......lcosta americana, Ornithomya anchineuria 

Worn, flat ....................... .......Cestoda 

Worm, round .................. .......Nematoda 

Worm, spiny-headed ...... .......Acanthocephala 

Birds 

Goshawk ......................... .......Accipitergentilis 

Hawk, Cooper's . ............. .......Accipiter cooperi 

Hawk, red-tailed . ............ .......Buteo jamaicensis 

Hawk, sharp-shinned ...... .......Accipiter striatus 

Mallard . .......................... .......Anas platyrynchos 

Owl, barn (common) ...... .......Tyto alba 

Owl, barred ..................... .......Strix varia 

Owl, boreal (Tengmalm's) ......Aegolius funereus 

284 Appendices 

Common Name Scientific name 

Birds 

Owl, California spotted ............. Strix occidentalis occidentalis 

Owl, eagle .................................. Bubo bubo 

Owl, great gray .......................... Strix nebulosa 

Owl, great horned ...................... Bubo virginianus 

Owl, long-eared ......................... Asio otus 

Owl, Mexican spotted ............... Strix occidentalis lucida 

Owl, northern hawk ................... Surnia ulula 

Owl, northern spotted ................ Strix occidentalis caurina 

Owl, pygmy (Eurasian) ............. Glaucidium passerinum 

Owl, short-eared ........................ Asio flammeus 

Owl, snowy ................................ Nyctea scandiaca 

Owl, tawny ................................ Strix aluco 

Owl, ural .................................... Strix uralensis 

Partridge, gray ........................... Perdix perdix 

Raven ......................................... Corvus corax 

Sparrowhawk (Eurasian) ........... Accipiter nisus 

Woodpecker, acorn .................... Melanerpes formicivorus 

Woodpecker, red-cockaded . ...... Picoides borealis 

Mammals 

Bats ............................................ Chimptera 

Chipmunk .................................. Eutamias spp.

Gopher, Great Basin pocket ...... Thomomys talpoides 

Gopher, mountain pocket .......... Thomomys monticola 

Gopher, southwestern pocket .... Thomomys bottae 

Mole ........................................... Scapanus spp.

Mouse, brush ............................. Peromyscus boylii 

Mouse, cactus ............................ Peromyscus eremicus 

Mouse, California ...................... Peromyscus californicus 

Mouse, California pocket .......... Perognathus californicus 

Mouse, deer . .............................. Peromyscus maniculatus

Mouse, house ............................. Mus musculus

Mouse, pinyon ........................... Peromyscus truei

Mouse, western harvest ............. Reithrodontomys megalotis

Mouse, western jumping . .......... Zapus princeps

Mouse, white-footed .................. Peromyscus spp. 

Pika ............................................ Ochotona princeps

Rabbit ........................................ Sylvilagus spp. 

Rabbit, brush ............................. Sylvilagus bachmani

Rabbit, cottontail ....................... Sylvilagus audubonii

Shrew ......................................... Sorex spp. 

Squirrel, ground ......................... Spermophilus spp. 

 California ............................... Spermophilus beecheyi

 Golden-mantled ..................... Spermophilus lateralis

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 
Common Name Scientific name 

Mammals 

Squirrel, northern flying . ...............Glaucomys sabrinus

Squirrel, southern flying ................Glaucomys volans

Squirrel, tree . ................................. Sciurus spp. 

Vole.................................................Microtus spp. 

Vole, California . ............................Microtus californicus 

Vole, long-tailed . ........................... Microtus longicaudus 

Woodrat . ........................................ Neotoma spp. 

Woodrat, bushy-tailed . .................. Neotoma cinerea 

Woodrat, desert . ............................ Neotoma lepida 

Woodrat, dusky-footed .................. Neotoma fuscipes 

Appendix D-Authors and 

Their Chapters 

Thomas W. Beck is a Forest Wildlife Biologist on the Stanislaus 

 National Forest in Sonora, California. He was a member of 

 the Core Group . .................. ...............................................................1, 3 

Bruce B. Bingham is a Plant Ecologist for the Pacific Southwest 

 Research Station, USDA Forest Service, at the Redwood 

 Sciences Laboratory in Arcata, California . ............................................5 

Douglas R. Call is a Wildlife Research Technician at Humboldt State 

 University in Arcata, California . ............................................................5 

Gary P. Eberlein is a Statistical Technician for the Pacific Southwest 

 Research Station, USDA Forest Service, at the Forestry Sciences 

 Laboratory in Fresno, California . ...........................................................6 

Gordon 1. Gould, Jr., is a Nongame Wildlife Biologist for the 

 California Department of Fish and Game in Sacramento, 

 California. He was a member of the Core Group . ..........................1, 3, 4 

R. J. Gutiérrez is a Professor of Wildlife, Humboldt State 

 University, Arcata, California. He was a member of the 

 Core Group . ................................................................................1, 4, 5, 8 

Susan J. Husari is a Regional Fuels Management Specialist 

 for the Pacific Southwest Region, USDA Forest Service, 

 in San Francisco, California . ................................................................12 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-133. 1992. 

James D. Johnston is a Forest Archaeologist on the Lassen 

 National Forest in Susanville, California ................. ........................7, 11 

William S. LaHaye is a Wildlife Biologist and Project Leader 

 of the San Bernardino Spotted Owl Study for Humboldt 

 State University in Arcata, California ..................... ...........................5, 8 

Daryl W. Lutz is a Biologist for the Wyoming Department of 

........Game and Fish in Laramie, Wyoming . ................... ...............................8 

Kevin S. McKelvey is a Wildlife Biologist for the Pacific 

 Southwest Research Station, USDA Forest Service, at the 

 Redwood Sciences Laboratory in Arcata, California. He 

 was a member of the Core Group ............................ ..............1, 5, 6, 7, 8, 

 9, 11, 13 

Christine A. Moen is a master's degree candidate at Humboldt 

State University in Arcata, California . .................... ...............................8 

Barry R. Noon is a Research Wildlife Biologist and Project 

 Leader for the Pacific Southwest Research Station, USDA 

 Forest Service, at the Redwood Sciences Laboratory in 

 Arcata, California. He was a member of the Core Group .......1, 5, 6, 8, 9 

Howard F. Sakai is a Wildlife Biologist for the Pacific South 

 west Research Station, USDA Forest Service, at the 

 Redwood Sciences Laboratory in Arcata, California ...........................10 

John S. Senser is an Archaeologist on the Stanislaus National 

 Forest in Mi-Wuk Village, California. He was the Technical 

 Team's photographer on all field trips ........................................ 5, Cover 

George N. Steger is a Biological Technician (Wildlife) for the 

 Pacific Southwest Research Station, USDA Forest Service, 

 at the Forestry Sciences Laboratory in Fresno, California ..................5, 6 

Robert J. Taylor is Director of Wildlife Ecology for the 

 California Forestry Association in Sacramento, California ....................2 

Jan W. van Wagtendonk is a Research Scientist for the USDI 

 National Park Service at Yosemite National Park in California ...........12 

Jared Verner is a Research Wildlife Biologist and Project 

 Leader for the Pacific Southwest Research Station, USDA 

 Forest Service, at the Forestry Sciences Laboratory in Fresno, 

 California. He was Leader of the Technical Assessment Team. ...1, 2, 4, 

 5, 6, 10 

Jeffrey R. Waters is a Wildlife Biologist for the Pacific 

 Southwest Research Station, USDA Forest Service, at 

 the Redwood Sciences Laboratory in Arcata, California . ...................10 

C. Phillip Weatherspoon is a Supervisory Research Forester 

 and Project Leader for the Pacific Southwest Research 

 Station, USDA Forest Service, at the Silviculture Laboratory 

 in Redding, California . ..................................................................12, 13 

Daniel F. Williams is a Professor of Biology at California 

 State University-Stanislaus, in Turlock, California . ............................10 

Cynthia J. Zabel is a Research Biologist for the Pacific South 

 west Research Station, USDA Forest Service, at the 

 Redwood Sciences Laboratory in Arcata, California .........................6, 7 

Appendices 285 
Persons of any race, color, national origin, sex, age, religion, or 

with any handicapping conditions are welcome to use and enjoy 

all facilities, programs, and services of the U.S. Department of 

Agriculture. Discrimination in any form is strictly against agency 

policy, and should be reported to the Secretary of Agriculture, 

Washington, DC 20250. 

The Forest Service, U.S. Department of Agriculture, is responsible for Federal leadership in forestry. 

It carries out this role through four main activities: 

• Protection and management of resources on 191 million acres of National Forest System lands 

• Cooperation with State and local governments, forest industries, and private landowners to help 

protect and manage non-Federal forest and associated range and watershed lands 

• Participation with other agencies in human resource and community assistance programs to 

• improve living conditions in rural areas 

• Research on all aspects of forestry, rangeland management, and forest resources utilization. 

The Pacific Southwest Research Station 

• Represents the research branch of the Forest Service in California, Hawaii, American Samoa 

and the western Pacific.