United States

Department

of Agriculture

Forest Service

Rocky Mountain

Research Station

General Technical

Report RMRS-GTR-178

September 2006

Ecology, Biodiversity, 

Management, and Restoration 

of Aspen in the Sierra Nevada

Wayne D. Shepperd

Paul C. Rogers

David Burton

Dale L. Bartos
Rocky Mountain Research Station

Natural Resources Research Center

2150 Centre Avenue, Building A

Fort Collins, CO 80526

Shepperd, Wayne D.; Rogers, Paul C.; Burton, David; Bartos, Dale L. 2006. Ecology, biodiversity, 

management, and restoration of aspen in the Sierra Nevada. Gen. Tech. Rep. RMRS-GTR-178. 

Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research 

Station 122 p.

Abstract—This report was commissioned by the USDA Forest Service Lake Tahoe Basin Management Unit to synthesize existing information on the ecology and management of aspen (Populus 

tremuloides) in the Sierra Nevada of California and surrounding environs. It summarizes available 

information on aspen throughout North America from published literature, internal government 

agency reports, and experienced scientists and managers. The historic distribution, abundance, 

and ecologic role of aspen in the Sierra Nevada are discussed, along with the reproductive physiology of aspen. Issues that affect aspen health and vigor in the Sierra Nevada and elsewhere are 

covered, along with methodology for assessing the condition of aspen and monitoring the effects of management activities to restore and maintain aspen. Descriptions of the types of aspen 

that occur in the Sierra Nevada are presented along with alternative techniques to manage and 

restore aspen that are applicable wherever aspen is found.

Key words: aspen, Populus tremuloides, ecology, management, disturbance, Sierra Nevada, 

Lake Tahoe basin, monitoring, and assessment

The Authors

Wayne D. Shepperd is a Research Silviculturist at the USDA Forest Service Rocky Mountain 

Research Station in Ft. Collins, CO. He holds a B.S. in Outdoor Recreation and M.S. and Ph.D. 

degrees in Silviculture from Colorado State University. He is a recognized authority on aspen 

and has published numerous scientific papers on aspen ecology and management during his 37 

year career with the U.S. Forest Service.

Paul C. Rogers is a Forest Ecology doctoral candidate at Utah State University in Logan, Utah. 

He previously worked as an analyst for the Forest Health Monitoring and Forest Inventory and 

Analysis programs in the Rocky Mountain Research Station. Paul holds a B.S. in Geography from 

Utah State University and an M.S. in Geography from the University of Wisconsin-Madison. His 

research interests include biogeography, aspen, lichenology, forest monitoring, and disturbance 

ecology.

David Burton, a full time Bureau of Land Management and United States Forest Service volunteer 

and recipient of the Chief’s Forest Service Individual Volunteer Award for 2002, is principal coordinator of the Aspen Delineation Project, a collaborative effort of the Pacific Southwest Region 

(R5), California State Office-BLM, and California Department of Fish and Game. The project focuses on how agencies have identified, mapped, treated, monitored, and studied aspen habitats. 

It also explores challenges, issues, and roadblocks surrounding aspen management.

Dale L. Bartos is a Research Ecologist and Project Leader with the Rocky Mountain Research 

Station, and an Adjunct Assistant Professor at the Department of Wildland Resources, Utah State 

University, Logan, Utah.
Ecology, Biodiversity, 

Management, and Restoration 

of Aspen in the Sierra Nevada

Wayne D. Shepperd

Paul C. Rogers

David Burton

Dale L. Bartos

United States Department of Agriculture

Forest Service

Rocky Mountain Research Station

Preface

This publication was undertaken through a cooperative work agreement between the USDA 

Forest Service Lake Tahoe Basin Management Unit. Printing costs were covered by the USDA 

Forest Service Pacific Southwest Region. We thank them for their generous support. Much of the 

information contained in this publication appeared in an internal report submitted to the Lake 

Tahoe Basin Management Unit in January, 2006. It has since been subjected to additional peer 

and policy review and has been revised to include additional sources of information and clarify 

statements made in the earlier report.

The purpose of this effort is to assemble the best information available about the ecology, biodiversity, and management of aspen (Populus tremuloides) within the Sierra Nevada ecosystem 

and nearby environs. However, we feel that much of the information contained in this document is applicable outside the Sierra Nevada proper and will be useful to managers elsewhere. 

To meet our goal, we scoured published literature, internal agency reports, and captured local 

expertise through personal contacts with the intent of finding the best and most pertinent information available about aspen. Because aspen is a minor component of landscapes in the Sierra 

Nevada, the body of published literature is somewhat limited. Therefore, we have interpreted 

results of research published elsewhere in North America and extrapolated that knowledge to 

the biophysical setting of the Sierra Nevada within the limits of our accumulated experience and 

knowledge of aspen ecology. In all instances, we have endeavored to separate fact from opinion, 

and knowledge from assumption. We feel that what we have presented here is the state of the art 

of what is currently known about aspen, but remind readers that local knowledge and experience 

gained from trial and observation through an adaptive approach to management is as valuable 

as anything we have included in this document.

Acknowledgments

We would like to thank the many people who contributed their expertise to this General Technical Report.

Particular thanks are due to Bobette Jones, Ecologist, Eagle Lake RD, Lassen NF; Coye Robbins, 

Wildlife Biologist, Almanor RD, Lassen NF; Paul Schmidt, Wildlife Biologist, Alturas Field Office, BLM; Victor Lyon, Wildlife Biologist, LTBMU; and Beth Brenneman, Ecologist, LTBMU who 

provided us with important aspen assessment data.

We would also like to acknowledge the contributions of George Gruell, Carson City, Nevada; 

David Goin, Reno, Nevada; and Tom Rickman, Wildlife Biologist, Eagle Lake RD, Lassen NF for 

their repeat photography sets that helped capture some extensive changes that have occurred 

to aspen habitats in California.

We are very grateful for the work of Bruce Davidson, GIS Specialist, LTBMU, Jessica Iles, Wildlife Biologist, Pacific RD, Eldorado NF for their many hours in designing and revising the maps 

that are found in this General Technical Report, and Kay Glowes, Penryn CA, for her countless 

hours of editing early drafts of this document.

Finally, we would like to thank Ken Tate, Rangeland Watershed Specialist, UC Davis; Walt Mueggler, Rocky Mountain Research Station (retired); Hugh Safford, Regional Ecologist, USFS, Pacific 

Southwest Region; Shane Romsos, Wildlife Biologist, LTBMU; Mollie Hurt, Wildlife Biologist, 

LTBMU; David Fournier, Vegetation Management Planer, LTBMU; and Linda Joyce, Project 

Leader, RMRS for their diligent review of this synthesis.

iii
Contents

Preface..........................................................................................................iii

Acknowledgments.........................................................................................iii

Chapter 1. Introduction...............................................................................1

Chapter 2. Natural and Historical Setting ...................................................7

Physical Environment........................................................................................7

Defining the Area of Concern ....................................................................7

Ecoregion Sections.....................................................................................7

Aspen Distribution.....................................................................................9

Geology and Glaciation...........................................................................10

Water and Hydrology ..............................................................................12

Climate Effects on Vegetation..........................................................................14

Long-Term Climate Variability..................................................................14

A 1000-Year Record.................................................................................14

Recent Climates and Future Directions ....................................................15

Historical Disturbance Ecology.......................................................................16

Pre-Settlement .........................................................................................17

Euro-American Settlement .......................................................................18

Modern Era..............................................................................................20

Chapter 3. Aspen Ecology..........................................................................23

Physiology of Aspen ........................................................................................23

Aspen’s Ecological Amplitude and Habitat...............................................23

Clonal Habit and Root Systems................................................................24

Damaging Agents Affecting Aspen ..................................................................26

Stem Canker Diseases..............................................................................26

Root Diseases ..........................................................................................28

Boring Insects ..........................................................................................28

Foliage Diseases ......................................................................................28

Defoliating Insects ...................................................................................28

Physical Damage .....................................................................................29

Aspen Genetics ...............................................................................................30

Asexual and Sexual Reproduction............................................................30

Clonal Intermixing ...................................................................................32

Long-Term Heterozygosity .......................................................................33

Frontiers in Aspen Genetics .....................................................................34

Plant Associations ...........................................................................................35

Vegetation Classification in California......................................................35

Aspen Vegetation Types ...........................................................................36

Models for Community Typing in the Sierra Nevada ................................38

Terrestrial Biota...............................................................................................38

Evaluating Diversity in Aspen Systems .....................................................38

Aspen-Related Wildlife ............................................................................40

Plant Diversity in Aspen Forests...............................................................43

Chapter 4. Current Forest Issues Related to Aspen Communities..............45

Threats to Aspen Sustainability .......................................................................45

Advancing Conifer Succession.................................................................45

Limited Aspen Regeneration ....................................................................49

iv
Aspen Habitat and Species of Concern ...........................................................51

Aspen as Prime Habitat............................................................................51

Species of Concern, Lake Tahoe Basin .....................................................51

Invasive Species and Aspen Communities.......................................................53

Invasive Species of the Sierra Nevada ......................................................54

Invasive Plant Survey of Lake Tahoe Basin ...............................................54

Aspen in the Wildland Urban Interface (WUI)................................................58

Development and Natural Process Conflict..............................................58

Lake Tahoe Defense Zones ......................................................................58

Recreation Impacts .........................................................................................60

Water Quality and Quantity............................................................................63

Soils.........................................................................................................63

Overstory and Understory Protection.......................................................64

Shading Quality.......................................................................................64

Evapotranspiration ...................................................................................64

Range Management and Aspen Communities..................................................65

Livestock and Aspen Stands.....................................................................65

Range Management History and Aspen....................................................65

Grazing Effects on Aspen Stand Structure.................................................65

Range Quality..........................................................................................66

Aspen Regeneration.................................................................................67

Conifer Encroachment on Rangelands .....................................................67

Livestock/Wildlife Interactions.................................................................67

Chapter 5. Assessment of Current Aspen Conditions.................................69

Ecological Status .............................................................................................69

Sierra Nevada Section..............................................................................69

Southern Cascade Section........................................................................71

Modoc Plateau Section ............................................................................74

Current Aspen Stand Types in the Sierra Nevada ............................................75

Meadow Fringe (Seral Aspen Community) ...............................................75

Riparian Aspen (Seral Aspen Community)................................................75

Upland Aspen/Conifer (Seral Aspen Community).....................................76

Lithic Aspen (Lava, Bolder, Talus).............................................................77

Snowpocket Aspen (Stable Aspen Community) ........................................77

Upland Pure Aspen (Stable Aspen Community)........................................77

Krümmholz Aspen (Stable Aspen Community).........................................78

Chapter 6. Management Alternatives for Aspen in the 

Sierra Nevada and Tahoe Basin ................................................................79

Characteristics of Aspen in the Sierra Nevada Ecosystem ...............................79

Identifying the Need to Regenerate Aspen......................................................80

Treatment Alternatives to Regenerate Aspen...................................................81

Clearfell-Coppice Harvest........................................................................82

Root Separation .......................................................................................82

Removal of Competing Vegetation...........................................................83

Protection From Browsing........................................................................84

Prescribed Fire.........................................................................................85

Combined Treatment Techniques.............................................................86

Treatment Opportunities Specific to the Sierra Nevada and 

Lake Tahoe Basin .........................................................................................87

Upland Pure Aspen..................................................................................87

Upland Aspen/Conifer .............................................................................87

v
Riparian Aspen ........................................................................................88

Meadow Fringe Aspen .............................................................................89

Snowpocket Aspen ..................................................................................89

Lithic Aspen.............................................................................................89

Krümmholz Aspen ...................................................................................90

Managing Aspen in the Wildland Urban Interface ..........................................90

Managing Aspen on Rangelands......................................................................90

Management Objective: Establish New Stems Above Browse Height.......91

Fencing....................................................................................................91

Salt Blocks and Water Source...................................................................91

Seasonal Utilization.................................................................................92

Class of Animal........................................................................................92

Type of Livestock .....................................................................................92

Cycling of Grazing to Benefit Aspen ........................................................92

Post Fire Recovery ...................................................................................92

Management Strategies Relating to Recreation Impacts..................................92

Management: Water Quality and Quantity .....................................................93

Chapter 7. Assessment and Monitoring Methodology ...............................95

Stand Assessment Methodology ......................................................................96

Developing Qualitative Assessment Protocols..........................................96

Photo Points ............................................................................................97

Quantitative Assessment Protocols...........................................................98

Implementation Monitoring ............................................................................98

Measuring and Monitoring Plant Populations...........................................99

Effectiveness Monitoring ...............................................................................100

Monitoring: Water Quality and Quantity......................................................101

Chapter 8. Summary................................................................................103

Literature Cited..........................................................................................105

Appendix I. Questions and Responses Related to Region 5 

Management Team Concerning Interpretation of Sierra Nevada 

Forest Plan Amendment Decision Notice (2004a).................................. 117

vi
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

Chapter 1.

Introduction

Quaking aspen (Populus tremuloides) is the most 

widespread tree species in North America (Baker 1925; 

Preston 1976; Lieffers and others 2001), and thought to 

be second in worldwide range only to Eurasian aspen 

(Populus tremula) (Jones 1985a). Aspen is found in most 

of eastern Canada and the U.S. (except the Southeast), 

throughout the upper Midwest and Lake States, across 

sub-boreal Canada and Alaska, in the Rocky Mountains 

from Canada through the U.S. and into northern Mexico, 

and in mountain ranges paralleling the west coast from 

Alaska through British Columbia, Washington, Oregon, 

California, and Mexico’s northern Baja California 

(Preston 1976). The species is most abundant in Canada’s 

central provinces and the U.S. states of Colorado and 

Utah (Jones 1985a; Lieffers and others 2001). In much 

of the western U.S., aspen is a mid-elevation shadeintolerant species that is a relatively minor component 

of more widespread conifer forests.

Aspen is an important tree species throughout the 

western United States. One of the few broad-leaved 

hardwood trees in many western forests, it is a valuable 

ecological component of many landscapes, occurring 

in pure forests as well as growing in association with 

many conifer and other hardwood species. While 

aspen provides desirable scenic value, the diversity of 

understory plants that occur in the filtered light under 

aspen trees supply critical wildlife habitat, valuable 

grazing resources, and protection for soil and water. 

Though aspen is a crucial component of many Western 

landscapes, it may be even more valuable in the Sierra 

Nevada, where it is less common or extensive than 

elsewhere.

To that end, this publication presents a broad-based 

synthesis of aspen ecology and management for the 

Sierra Nevada Range of California and Nevada. We use 

the same geographic criteria applied in the Sierra Nevada 

Ecosystem Project (SNEP 1996) and the Forest Service’s 

Sierra Nevada Forest Plan Amendment (SNFPA FSEIS) 

(USDA Forest Service 2004c) to define our area of interest (fig. 1-1). In short, these documents focus primarily 

on the entire Sierra Nevada ecoregion section—hence 

called “ecoregion” (Bailey 1995; Miles and Goudey 

1997)—as well as portions of the Southern Cascade and 

Modoc Plateau ecoregions (fig. 1-2). These ecoregions 

contain aspen on lands administered by the United States 

Forest Service, Bureau of Land Management, National 

Park Service, Nevada and California State Park Systems, 

California Tahoe Conservancy, California Department 

of Fish and Game, and private ownership (fig. 1-3). We 

believe the Lake Tahoe basin can serve as a barometer 

of aspen issues found throughout the Sierra Nevada, and 

we will therefore use the basin and adjacent environs 

for case studies to illustrate issues at finer scales. A base 

map of the Lake Tahoe basin is provided as a reference 

for those less familiar with the area (fig. 1-4).

Within this framework, the primary objectives of 

this publication are to provide resource managers with 

the knowledge and tools to:

Understand contemporary aspen ecology and resource issues.

Develop management direction and goals.

Work toward desired conditions for aspen.

Apply regulatory standards and guidelines to aspen 

situations.

Map out strategies for monitoring that will support 

adaptive management.

To meet these objectives, pertinent information on 

aspen ecology and management was synthesized for the 

Sierra Nevada. Aspen-specific research from the Sierra 

Nevada region is limited. Throughout this publication 

we use research produced elsewhere to address this 

geographic shortcoming in aspen knowledge. Sources 

used include published and unpublished literature, as 

well as administrative records.

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•

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USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

In order to provide a scientific framework for the 

implementation of aspen-related management, we rely 

heavily on the related concepts of “range of natural 

variability” (RNV) (Landres and others 1999) and 

“properly functioning conditions” (PFC) (Campbell 

and Bartos 2001). At their core, these concepts can be 

used to implement management practices based on the 

Figure 1-1. The geographic area covered by this report. It is the same as that in the Sierra 

Nevada Ecosystem Project (1996) and Forest Service Sierra Nevada Forest Plan Amendment (2004).

best available understanding of ecosystem dynamics in 

a context of changing physical and social environments. 

It is proper to discuss aspen ecology and management 

in this framework because it is closely related to 

standards and guidelines of the government agencies 

that administer public lands in the Sierra Nevada. For 

example, in the U.S. Forest Service’s Final Supplemental 
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 

Figure 1-2. The portions of the Sierra Nevada, Southern Cascade, and Modoc Plateau 

ecoregion sections (Miles and Goudey 1997) that are within the geographic area 

covered by this report.

Environmental Impact Statement of the Sierra Nevada 

Forest Plan Amendment (USDA Forest Service 2004c) 

the Record of Decision states:

At either the landscape or project scale, 

determine if the age class, structural diversity, 

composition, and cover of riparian vegetation 

are within the range of natural variability of 

the vegetative community. If outside the range 

of natural variability, consider implementing 

mitigation and/or restoration actions that will 

result in an upward trend. Actions could include 

restoration of aspen or other riparian vegetation 

where conifer encroachment is identified as a 

problem.
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

Figure 1-3. Land ownership within the geographic area covered by this report. The 

“Other” category includes State, County, City, and Tribal Trust administered lands, 

as well as lands under private ownership.

The Bureau of Land Management and the National 

Park Service call for similar attention to properly 

functioning natural conditions in vegetative communities (USDI NPS 1999; USDI BLM 1999a; USDI 

BLM 1999b). Since these directives call for vegetative 

communities to be managed within a range of natural 

variability or in a properly functioning condition, a 

short review of the concepts will provide a foundation 

for much of the discussion that follows.

Landres and others (1999) define natural variability as “the ecological conditions, and the spatial 

and temporal variation in these conditions, that are 

relatively unaffected by people.” The authors illustrate 

how management’s use of natural variability relies on 
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

Figure 1-4. Land ownership and physical features in the Lake Tahoe basin.

two concepts: understanding ecological processes and 

recognizing that those processes are constantly in flux. 

We would like to emphasize the importance of avoiding defining the variability of a plant community in 

too wide a spatial scale or too exact a temporal point. 

Regarding spatial variables, Fule and others (2002) 

point out that changes in site specific characteristics, 

like geography, soils, precipitation, aspect, and slope are 

spatial variations that may cause shifts in natural variability. Likewise, regarding temporal variables, Millar 

(1997) indicates that temporal variation caused by short 

and long term climate changes requires consideration 

of ecosystem relationships and climatic factors rather 

than natural variability tied to one pre-anthropogenic 

reference point. The primary difference between the two 

concepts is that natural variability is used to identify 
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

variation in a vegetative community, and functioning 

condition is used to describe where the current community is in relationship to overall variation.

For our purposes, we consider a vegetative community to be in a properly functioning condition if it is 

within the range of conditions known to have existed 

historically in the area where it is found (Barrett and 

others 1993; USDA Forest Service 1997; Campbell 

and Bartos 2001).

This publication is based on the concept of managing 

aspen within its natural variability. To better understand 

historic variability, we begin in Chapter 2 by looking at 

pertinent spatial and temporal factors found in the Sierra 

Nevada, such as climate and human-caused disturbance, 

which have strongly influenced contemporary aspen 

communities. Next, we examine current knowledge 

of aspen ecology (Chapter 3). This discussion begins 

with aspen physiology and evolves toward a broader 

look at aspen’s role in ecosystems. The relationship of 

aspen to a range of modern land management issues 

is discussed in Chapter 4. Then we review current 

conditions in aspen communities in the Sierra Nevada 

through the use of historical records and existing aspen 

stand assessments (Chapter 5). A review of research 

on aspen management and discussion of applications 

to specific conditions found in the Sierra Nevada follows (Chapter 6). Chapter 7 details the history of stand 

assessment and monitoring techniques for evaluating 

aspen management. We conclude by discussing the use 

of adaptive management techniques to meet aspenoriented objectives. Finally, a brief summary chapter 

will review the important messages presented in this 

document (Chapter 8).
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

Chapter 2.

Natural and Historical Setting

Physical Environment_______

Defining the Area of Concern

The same criteria applied by previous regional examinations published under the title “Sierra Nevada” (SNEP 

1996; USDA Forest Service 2004c), are being used here, 

acknowledging similar ecological and social concerns 

as the previous efforts (fig. 1-1). We will emphasize 

the core Sierra Nevada range, but on occasion will use 

adjacent ecoregions for related discussions.

Three ecoregions (sections) define and describe this 

area: Sierra Nevada, Southern Cascades, and Modoc 

Plateau (fig. 1-2). The Sierra Nevada Foothills section 

has been excluded for the practical reason that there are 

very limited aspen in this region (Potter 1998). Likewise, 

there are other areas adjacent to the Sierra Nevada ecoregion that contain aspen populations. Several features are 

common to these three ecoregions (table 2-1). All have 

warm, dry summers and cool to cold, moist winters. 

Likewise, soils of granitic or volcanic parent material 

that commonly support aspen forests (Potter 1998) are 

abundant in all three sections. Finally, these ecoregions 

occur in higher elevation mountain landscapes (as opposed to interspersed valleys). Regional precipitation 

patterns are presented in figure 2-1. Because temperature 

and precipitation data were only available for certain 

locations, climate in mountainous terrain may deviate 

considerably from the averages presented.

Ecoregion Sections

The Sierra Nevada ecoregion (fig. 1-2) is a blockfault range trending northwest along the eastern edge of 

California. Elevations vary from 1,000 to 14,495 ft (305 

to 4,418 m). The range is tilted to the west meaning there 

are generally much longer, gradual slopes to the west 

than to the east. The west side is generally wetter, with 

the steep east slope lying in a classic rain shadow. Deeply 

incised canyons flow to the west eventually joining the 

central valley, while to the east drainages are relatively 

short, flowing over bedrock into the Great Basin (Miles 

and Goudey 1997). Precipitation generally increases 

from south to north and from low to high elevation in 

the Sierra Nevada (fig. 2-1). The west slope receives 

more precipitation than the east slope and most moisture 

comes in the form of snow. The western foothills get 20 

to 30 inches (50 to 76 cm) of rain annually; the mixed 

conifer forest belt gets between 30 and 60 inches (76 and 

152 cm); and the highest elevations can receive up to 

100 inches (254 cm) (Hornbeck and Kane 1983). Dryer 

east side slopes tend to have similar precipitation zones, 

although they are narrower and occur at slightly higher 

elevations. The Lake Tahoe basin contains elements of 

both moist western slope and rain shadow eastern slope 

precipitation regimes (fig. 2-2).

Local mountains and ridges may alter the general 

precipitation and temperature trends stated above. Boca, 

California is an example from the northern part of the 

range (east side). This town rests at 5,575 ft (1,699 m), 

to the north of Lake Tahoe, and on the lower edge of 

the forested belt. Boca receives an average of 22 inches 

(56 cm) of moisture annually (60 percent as snow), with 

an average temperature of 42° F (5.5° C) (Hornbeck 

and Kane 1983).

The Southern Cascades occupy the northwest portion of our area of interest, extending northwest to a 

point near Mt. Shasta, then due north to Oregon. This 

ecoregion is comprised mainly of volcanic highlands 

dissected by broad valleys. Elevations range from 

2,000 feet (610 m) on the western side of the range to 
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 Table 2-1. Physical and climate features of the three prominent ecoregion sections discussed in this report. Precipitation Temperature Avg. Avg. Avg. Dominant Köppen climate zone annual Percent Avg. annual annual Geo- parent Common Elevation (ft) (Hornbeck p. 24) range snow annual high low morphology material soils Sierra Nevada* 1,000-14,495 Mediterranean 30-60” >60% 420 620 250 westward entisols/ highland tilted block granite inceptisols Southern Cascades** 2,000-14,162 Mediterranean 20-70” not available 42/600 62/720 25/470 volcanic basalt/ inceptisols 	 highland/Mediterrane	 	 	 andesite	 /alfisols an warm summer Modoc Plateau*** 3,700-9,892 Mediterranean 8-30” 30% 480 680 320 block faults basalt aridosols/ mollicsols * Precipitation and temperature data taken from weather station at Boca. ** Precipitation and temperature data taken from composite information of climate zones and weather stations at high/low elevations. *** Precipitatioin and temperature data taken from weather station at Fort Bidwell. Sources:	 Hornbeck, David. California patterns: a geographical and historical atlas. Palo Alto, CA: Mayfield Publishing Company; 1983. 117p. Miles, Scott R. and Goudey, Charles B., compilers. Ecological subregions of California: section and subsection descriptions. San Francisco, CA: USDA Forest Service, 	 Pacific Southwest Region; 1997; R5-EM-TP-005.

14,162 feet (4,316 m) at Mt. Shasta (Miles and Goudey 

1997). This range is deceptive, however, because un-

like the numerous 14,000 foot (4,267 m) peaks in the 

Sierra Nevada, Mt. Shasta towers above much of the 

surrounding Southern Cascades.

The primary watersheds within the Southern Cascades 

proper are the Klamath and Sacramento Rivers on the 

west slope, while lesser waterways drain to the east 

into the Modoc Plateau ecoregion. There is generally 

greater precipitation in the western parts of the Southern 

Cascades. Average annual moisture for the section is 20 

to 70 inches (50 to 178 cm) (Miles and Goudey 1997), 

though some areas near Mt. Shasta receive as much as 

90 inches (228 cm) (Hornbeck and Kane 1983). This 

ecoregion is characterized as being a mix of two climate 

zones in California: the Mediterranean highland and 

Mediterranean warm summer. The upshot of this mix 

is that most of the area follows similar temperature 

and precipitation patterns as the Sierra Nevada section. 

An exception is the southwest portion of this section, 

centered between Mt. Shasta and Lassen Peak, which 

more closely follows the higher annual rainfall and 

warmer winters of the Mediterranean warm summer 

climate zone. This area often receives more moisture 

than the Modoc Plateau to the east (Hornbeck and 

Kane 1983).

The Modoc Plateau ecoregion encompasses the area 

to the north of the Sierra Nevada and much of the 

northeast corner of California. Geomorphologically 

the Modoc Plateau is comprised of blocks and faults 

similar to the Basin and Range formations of the Great 

Basin. These northward trending mountains and valleys 

are infused with volcanic remnants such as craters, 

cones, and lava flows. The elevations range from 

3,700 to 9,892 ft (1,128 to 3,015 m) in the Warner 

Mountains (Miles and Goudey 1997). While some 

rivers flow to the western drainages of the Southern 

Cascades, many drain internally to catchments or to 

the east toward the Great Basin. Average rainfall for 

Fort Bidwell, California (4,500 ft [1,371 m]), in the 

Mediterranean highland climatic zone, is 16.1 inches 

(40.9 cm) (section-wide range of 8 to 30 inches/20 to 

76 cm). Average summer high temperatures are about 

68° F (20° C) (about 7° F/4° C warmer than Boca in 

the Sierra Nevada). The Modoc Plateau receives only 

30 percent of its annual moisture from snow, whereas 

the Sierra Nevada receives at least 60 percent from 

snow (Hornbeck and Kane 1983).
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

Figure 2-1. Precipitation patterns across the 

Sierra Nevada region. Moisture generally 

increases with elevation and latitude. The 

east side of the range is drier than the 

west side.

slope, to 8,800 ft (2,682 m) at the crest of the Sierra 

Range, descending to 5,640 ft (1,719 m) to the east where 

the Sierra Nevada meets the Great Basin. Aspen can 

also be found in abundant numbers in the Carson and 

Monitor Ranges located east and south of Lake Tahoe, 

in the Sweetwater Mountains north of Mono Lake, and 

in the White Mountains east of Bishop.

In the Southern Cascades, aspen is located on both 

sides of the range and, as is the case in the Sierra 

Nevada, is more abundant on the eastern slope of the 

range. In the area of Lassen National Forest and Lassen 

National Park, aspen ranges from 5,500 ft (1,676 m) 

on the western slope to 8,000 ft (2,438 m) in Lassen 

Aspen Distribution

Aspen is located throughout the three ecoregions of 

our area of interest and spills into adjacent provinces 

(fig. 2-3). In the Sierra Nevada, aspen is found in stands 

from the Kern Plateau in the Sequoia National Forest 

in the south to Diamond Peak in the Lassen National 

Forest on the north. Aspen stands are located on both 

east and west sides of the Sierra, though they tend to 

be larger and more abundant on the east slope. Stands 

can be found along a west-to-east transect through the 

Eldorado and Humboldt-Toiyabe National Forests in the 

central Sierra from 5,310 ft (1,618 m) on the western 
10 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

National Park to 6,000 ft (1,828 m) on the eastern 

slope of the range, where the Southern Cascades join 

the Modoc Plateau.

In the Modoc Plateau, aspen is found extensively in the 

Warner Mountains as well as in nearly all the mountains 

at elevations above the broad valley floors. Aspen is 

also located in small isolated sites in the expansive 

Devils Garden plateau, near Alturas, California.

Geology and Glaciation

The modern Sierra Nevada owes its form to geologic 

forces created by the collisions of tectonic plates over 

the last 200 million years (Hill 1975). The Sierra Nevada 

range originated during the Mesozoic Era from 150 to 

200 million years ago. The range is a product of continental shifting and folding, which forced a buckling 

along block fault lines on the present east slope, when 

the North American continental plate collided with the 

Pacific Ocean plate. At the core of the mountain range, 

very large metamorphic granite intrusions of Jurassic 

(135 to 180 million years BP) and Cretaceous (70 to 

135 million years BP) origin were uplifted during this 

mountain forming period (Hornbeck and Kane 1983). 

As mountains rose, a combination of fluvial and glaciofluvial processes began to incise deep canyons along 

Figure 2-2. Precipitation patterns within the 

Lake Tahoe basin.
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 11

prominent waterways, notably those on the western 

slope. Today volcanic rocks that originated during the 

Tertiary Period (26 to 66 million years BP) are common 

in the northern and central Sierra Nevada, although 

small outcrops can be seen throughout the range (Hill 

1975; Clark and others 2005).

Active fault zones dominate the Sierra Nevada 

landscape. Lake Tahoe was formed by uplifting 

along fault lines followed by a series of volcanic 

dams on the upper Truckee River that periodically 

raised the level of the lake. During subsequent glacial 

epochs, additional glacial dams were formed and later 

breached, causing catastrophic flooding downstream 

(Schweickert and others 2000). The present Lake 

Tahoe basin is in an active earthquake zone. The 

Sierra Nevada fault block is currently inching west 

to northwest. A major fault bisects the lake itself, 

forming a boundary between the Basin and Range 

province to the east and the Sierra Nevada proper. 

By geologists reckoning, the same continental shifts 

along fault lines that helped shape the Sierra could 

spawn a magnitude 7.0 earthquake in the Lake Tahoe 

basin today. Evidence of tsunamis—large destructive 

waves generated by earthquakes—is also found in 

Figure 2-3. Subsections of the Ecological 

Units of California (Miles and Goudey 

1997), where aspen is known to occur. 

No inference should be drawn as to 

the number and size of stands in each 

subsection.
12 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

the contemporary geology of Lake Tahoe’s western 

shoreline (Schweickert and others 2000).

Subsequent to mountain formation, the Sierra Nevada 

experienced significant glacial periods that carved out 

prominent canyons, while moving large volumes of 

parent material downslope. Abundant landforms left by 

past glaciers define some of the Sierra Nevada’s most 

prominent scenery, including National Parks, alpine 

peaks, and scenic mountain lakes. Major glaciation 

took place in the Sierra Nevada during the Pleistocene 

Epoch (10,000 to 1.5 million BP). Birman (1964) notes 

at least seven glacial periods during the Quaternary: six 

in the Pleistocene and one in the Holocene. The latter 

occurred specifically during the Little Ice Age (1350 to 

1850 A.D.). Numerous researchers have investigated 

the close link between climate and glacial advances 

and retreats of the Holocene and their subsequent effects on vegetation (see Climate Effects on Vegetation, 

this chapter). In terms of ongoing glaciation, there are 

presently 108 active, mostly small glaciers and 401 

glacierettes (large over-summer snowfields) in the Sierra 

Nevada (Guyton 1998).

In the previous eras of larger glaciers, it is likely that 

aspen was among the first species to colonize recently 

vacated outwash plains. Recently glaciated landscapes 

are free of other vegetation, have deep soil deposits, 

and have a ready source of moisture in glacial melt 

(Muller and Richard 2001). All of these landscapes 

would favor the establishment of aspen seedlings 

(McDonough 1979). It has been hypothesized that new 

genetic varieties of aspen have not been introduced 

to the Sierra Nevada since the last major glaciation 

when climate may have been more conducive to true 

seedling establishment (Strain 1964). However, recent 

documentation of seedling establishment in Yellowstone 

National Park (Romme and others 1997), and research in 

the Sierra Nevada documenting genetic diversity among 

small isolated aspen stands (Hipkins and Kitzmiller 

2004), raises the possibility that more recent seedling 

establishment has occurred here.

Water and Hydrology

The Sierra Nevada contains some of the most intensively used and managed watersheds anywhere on 

earth. Nearly every drop of moisture that hits the range 

is allocated for some downstream human use. Between 

75 and 90 percent of the runoff goes to agricultural uses. 

Every major drainage in the range has been significantly 

altered, either by historic uses or contemporary dams. 

Currently, there are hundreds of large dams in the 

Sierras and thousands of minor dams (Hornbeck and 

Kane 1983; Kattelmann 1996).

In the latter half of the 19th century, mining had 

enormous effects on Sierra Nevada watersheds. Early 

surveys documented seasonal drying up of both major 

and minor rivers in the area from mining activities 

(Sudworth 1900). Though surface erosion is naturally 

low in granitic soils because of high infiltration rates, 

intensive use and redistribution of water during the 

mining era dramatically affected soil and vegetation. 

As a result of large-scale mining, waterways were 

dammed, diverted, polluted, excavated, and filled with 

debris. Hydraulic mining—redirection of water using 

gravity into flumes, down steep slopes, and eventually 

through high pressure nozzles directed at river banks and 

hillsides to remove gold-bearing sediments—was the 

most egregious form of riparian destruction. Kattelmann 

(1996) asserts there were over 400 hydraulic mining 

operations in the Sierra Nevada during peak use, most of 

these being located on the west slope between the North 

Fork of the American and Feather rivers. While much 

of the hydraulic mining took place at lower elevations, 

water was diverted and timber was cut to support these 

activities in areas where aspen occurred.

Clearing of near-stream forest vegetation, including timber cutting for flumes, sluices, mine timbers, 

and construction materials was also related to mining 

operations. This activity led to further sedimentation 

of streams and significant alteration of surrounding 

vegetation. However, clearing of vegetation adjacent 

to streams may have created openings with abundant 

water, allowing established riparian aspen clones to 

expand onto disturbed sites. Such mining activities, 

like other disturbances of this era, likely contributed 

to the creation of riparian aspen stands found on the 

landscape today (Potter 1998). Like mining and logging, 

excessive livestock grazing in the late 19th century was 

also responsible for removing meadow vegetation that 

increased sedimentation and led to downstream gullying 

(Kattelmann 1996).

The 20th century brought new large-scale impacts 

to California’s waterways. In the Sierra Nevada, fire 

suppression was fully instituted (after some debate) by 

the 1920s. In terms of riparian systems, it was thought 

that suppressing fires would reduce erosion. While it is 

true that a pulse of erosion generally occurs after fires, 

periodic fires also contribute important organic matter 

to stream systems (Kattelmann 1996). Suppression of 

frequent surface fires can eventually lead to denser 

forest canopies that burn as larger crown fires which, 

in turn may contribute bigger sediment loads to lakes 

and rivers. Increased logging and development later in 
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 13

the century led to large-scale road building. Improperly 

constructed forest roads can divert streams, increase 

sedimentation, and occasionally lead to slope failure. In 

Kattelmann’s opinion, roads built during the period from 

the 1950s to the 1970s were the greatest disturbance to 

healthy watersheds since the 19th century mining era.

Although many large dams were built at lower elevations during the early- to mid-20th century, numerous 

small dams were also constructed throughout the range 

(Kattelmann 1996). Dams may effectively simulate 

localized drought on downstream riparian vegetation, 

allowing tolerant plants to invade formerly active stream 

channels. This conversion can cause downstream water 

tables to drop, leading to stream incision, reduced 

overbank flooding, and eventual die-off of adjacent 

woody vegetation (Kondolf and others 1996). In one 

documented case around 1940, water was diverted from 

Rush Creek, near Mono Lake, to supply water to Los 

Angeles, drying-up the creek downstream of the diversion and causing die-off of aspen and related riparian 

communities (Stine and others 1984). Water backing up 

behind dams has also flooded aspen habitat at numerous 

reservoirs in the Sierra Nevada. Thin strips of aspen 

remain along reservoir shorelines today where larger 

stands may previously have existed (fig. 2-4).

Cumulative effects on adjacent forest communities 

of major and minor hydrologic alterations throughout 

the region are not well understood. Although we have 

reason to speculate that aspen forests were unintentionally changed by past riparian manipulations, further 

investigation is needed to verify this hypothesis. We do 

Figure 2-4. Aspen growing on the shoreline 

of Caples Lake, formerly known as Twin 

Lakes, in the Eldorado National Forest. 

Aspen habitats, as well as part of the 

Mormon Emigrant Trail, were flooded

when this lake was increased in size as 

part of a hydroelectric development.

know from other regions that forest succession alone 

may have lasting impacts on water balances. Gradual 

succession from seral aspen stands to longer-lived 

conifer forests is evident in the absence of fire and with 

unchecked browsing of aspen regeneration (Mueggler 

1985; Bartos and Campbell 1998; Rogers 2002).

It is thought that forests dominated by conifers 

will use more water than those dominated by aspen 

(Kaufmann 1985), given the greater transpiring surface 

area of conifers and their ability to transpire moisture 

from the soil anytime that temperatures are above 

freezing. In Utah, researchers measured net water 

changes between stands of aspen, subalpine fir (Abies 

lasiocarpa) and Engelmann spruce (Picea engelmannii), 

and combined spruce-fir stands. They found that there 

was a net reduction in runoff in all the conifer stands 

when compared to aspen (Gifford and others 1983a). A 

similar study in Colorado examined seasonal water use 

and available moisture by aspen, spruce, and grassland 

ecosystems (Brown and Thompson 1965). Results from 

this study suggest that aspen use more water during 

the summer season, although more water is available 

from aspen forests (presumably due to retention in deep 

clay/organic soils) than from either spruce or grassland 

types. These authors caution that they did not closely 

examine year-round use and that interception of snow by 

spruce, for example, may alter annual use and moisture 

projections. They go on to speculate that aspen use 

more water during the growing season; conifers likely 

use more water over the entire year. Previous studies of 

forest composition and water retention imply that aspen 
14 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

stands can potentially increase hydrologic flows if they 

are retained on the landscape, but further research is 

needed to confirm these theories.

Climate Effects on 

Vegetation_________________

Recent advances in the field of disturbance ecology—the study of long-term and large-scale catastrophic 

events on ecosystem change—have become more reliant on understanding climate patterns and their effects 

on vegetation (Millar and Woolfenden 1999a; Veblen 

and others 2000; Dale and others 2001; Whitlock and 

Knox 2002; Whitlock 2004; Pierce and others 2004). 

Estimations of natural variability in ecosystems are 

based on knowledge of disturbance regimes that are 

largely dependent on climatic fluctuations (see Historical 

Disturbance Ecology, this chapter). The overarching 

message from these studies is that climate, and therefore 

natural systems, are not static. The Sierra Nevada is no 

exception to this rule. A full appreciation of the role 

that climate has had on the occurrence and distribution 

of aspen in Sierra Nevada ecosystems requires a brief 

review of climatic variability over long-term, millennial, and recent time scales. This will also allow some 

speculation as to the effect of future climate projections 

on aspen in the region.

Long-Term Climate Variability

On the scale of millions of years, geologic evidence 

of gross change may reflect continental shifting and 

large climate flux. In the realm of centuries—closer 

to our own lifespan—finer-scale disturbance cycles, 

such as flood, fire, insect and disease infestations, and 

human interventions, seem of more importance. Where 

short-term drought events seem important within human 

life spans, they have less standing at millennial scales. 

In general, pre-Holocene climate favored long glacial 

periods over brief regional warming. Thus, glacial 

fluxes affected the spatial distribution of vegetation 

in dramatic ways. For instance, around Owens Lake, 

vegetation alternated between spruce-fir (Picea-Abies

spp.) and juniper (Juniperous spp.) forests from 800,000 

to 650,000 B.P. (Woolfenden 1996). Not only does 

this point to wide elevational fluctuations of forest 

cover, but to changes in spatial distribution as well. For 

example, the closest contemporary stands of spruce 

are in Arizona.

The Holocene (10,000 years to the present) marked 

the end of the last true glacial advance. During the 

Holocene, longer warm periods in the Sierra were 

generally punctuated by a few prominent cooler epochs 

lasting several hundred years. These did not include 

significant glaciation, and occurred around 11,000; 

8,000; and 3,000 B.P. (Woolfenden 1996). Aspen, as 

well as other species, likely resided in different locations 

during these times.

A 1000-Year Record

Multiple authors have documented a warmer climate 

in the Sierra Nevada, known as the Medieval Warm 

Period, occurring from approximately 1000 to 1400 A.D. 

(Swetnam 1993; Scuderi 1993; Woolfenden 1996; Millar 

and Woolfenden 1999a; Millar and Woolfenden 1999b; 

Millar and others 2004). Several lines of evidence, including increases in tree ring growth, fire scars, tree line 

elevations, pollen records for fire-dependent species, and 

decreased Mono lake levels indicate consistent patterns 

of prolonged drought and large-scale disturbance during 

this time. Millar and Woolfenden (1999a) speculate that 

volcanic vents erupted later in this period in the Glass 

Creek watershed and played a role in igniting fires in 

the region. Regardless of ignition source, the warmer 

and dryer conditions prominent during the Medieval 

Warm Period appear to have encouraged more frequent 

fires at low elevations (Swetnam 1993), as well as in 

montane forests (Millar and Woolfenden 1999a). This 

climate and disturbance pattern probably encouraged 

growth of species more resistant to low intensity fire, 

such as Jeffrey pine (Pinus jeffreyii) and Ponderosa 

pine (P. ponderosa).

The Little Ice Age (1400 to 1850 A.D.), as the name 

suggests, was characterized by cool temperatures and 

increased precipitation. Although the later portion of 

this period was moderately dry, the overall cool and 

wet conditions spawned minor glacial advances in the 

Sierras (Stine 1996). In terms of vegetation, much of the 

current “old growth” red fir forest in the Sierra Nevada 

originated during this era when fires were generally 

infrequent, though more intense (fig. 2-5) (Millar and 

Woolfenden 1999a). Swetnam (1993) documented the 

lowest fire return intervals of the past 2000 years during 

the Little Ice Age (except for the 1860 to present fire 

exclusion period). Generally, the longer the fire return 

interval, the easier it is for shade tolerant species such 
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 15

as red fir, white fir (Abies concolor), and mountain hemlock (Tsuga mertensiana) to colonize particular sites. 

However, lodgepole pine (Pinus contorta) and quaking 

aspen can thrive immediately after crown fires at upper 

elevations because of their need for full sunlight and 

ability to regenerate quickly after disturbance. Millar 

and Woolfenden (1999a) describe cycling shifts between 

red fir and lodgepole pine forest as climate changed 

over the past several centuries. The first fir originated 

during the cool/wet early Little Ice Age period, followed 

by a pulse of pine as cool/moderately dry conditions 

prevailed around the turn of the 19th century. A final 

understory layer of fir regeneration is found in present 

stands reflecting a 20th century that was relatively fire 

free and moist (Millar and others 2004). We presume 

that aspen “pulses” parallel those of lodgepole pine, 

given the two species’ similar response to climate and 

disturbance (Skinner and Chang 1996). This would result 

in greater aspen abundance toward the end of the Little 

Ice Age (based on climate conditions) and toward the 

end of the 19th century (resulting from human-caused 

disturbances).

As previously mentioned, the transition from Little 

Ice Age to a warm and moist 20th century was coincident with widespread impacts of human settlement 

(see Historical Disturbance Ecology, this chapter). 

Other authors have noted the difficulty in separating the 

effects of these factors on modern vegetation (Millar 

and Woolfenden 1999a); however, Swetnam (1993) 

does note a clear decline in the fire frequency after 

1860 in lower montane west-side sequoia groves in the 

central Sierras. Conversely, other studies cite frequent 

burning in upper montane sites that was documented 

by early settlers and forest surveyors (Sudworth 

1900; Leiberg 1902; Cermak 1988). Beaty and Taylor 

(2001) attribute frequent fires that proliferated into the 

early 20th century in the northern part of our area 

to a combination of grazing practices (post-season 

burning) and drought conditions. Increased fire starts 

in the upper montane zone during the latter half of 

the 19th century were likely brought on by natural 

climate fluctuations (Millar and others 2004) in conjunction with intentional and unintentional burning 

by settlers. Based on current aspen stand ages (Potter 

1998), we think that these late 19th century climate 

and disturbance patterns favored a widespread pulse 

of aspen regeneration.

Recent Climates and 

Future Directions

Observations of annual branch growth of treeline 

conifers, snowfield invasion, vertical branch growth 

in krümmholz, meadow invasion, and climate records 

have all indicated some decadal climatic generalities 

based on synchrony across methods and sites in the 

range, which characterize the 100-year record for the 

region (Millar and others 2004). Overall, the past century 

was warmer and wetter than several previous centuries; 

relatively warm periods occurred from 1920 to 1940 

and 1976 to 2000. The second half of the century was 

wetter than the first half, and the relatively dry periods 

occurred from 1910 to 1935 and 1945 to 1970.

Many scientists now speculate that we are on the 

brink of another climatic shift fueled primarily by 

human industrialization that will lead to accumulation 

of greenhouse gasses in the atmosphere (Overpeck 

and others 1990; Dettinger and others 2004). Specific 

effects of such a climate shift on aspen are not known, 

but we could expect that aspen growing in ecotones near 

the limits of its growing conditions would be affected. 

There may be far-reaching consequences for aspen 

where exotic species act in tandem with rapid climate 

shifts. A recent study in Utah used conservative (1 

percent/year increase) CO2-based estimates of climate 

warming to model potential impacts of gypsy moth 

(Lymanteria dispar) on quaking aspen in the coming 

century (Logan and others 2006). Their results suggest 

incremental temperature increases in the next century 

will facilitate widespread introductions of gypsy moth 

into previously temperature-limited elevation zones 

containing Gambel oak (Quercus gambelii), bigtooth 

maple (Acer grandidentatum), and aspen. Where 

Figure 2-5. Pollen records ofAbies (white fir and red fir) from

the central Sierra Nevada show an increasing abundance 

since the last major glaciation in the elevation range where 

aspen is present today (Anderson and Smith 1994).
16 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

these hardwoods have no previous exposure to gypsy 

moth, large contiguous stands without natural defense 

mechanisms may be destroyed. Similar potential host 

situations occur throughout western mountain ranges 

where temperature modification will have more pronounced effects by elevation than by latitude.

In the Sierra Nevada, projections are for a warming of about 5.4o F (3o C) during the 21st century 

(Dettinger and others 2004). While these authors are 

less confident in the direction of precipitation change, 

they do expect warming to have significant affects on 

the timing and amount of spring runoff. Where snowmelt occurs earlier in the spring, longer and warmer 

summers will likely result in drying forest fuels, 

facilitating wildfire ignition and expansion. This will 

also result in soils drying out sooner which could lead 

to greater moisture stress in aspen. If this predicted 

climate trend materializes, a practical approach toward 

natural range of variability management in aspen may 

be to emulate patterns and processes favored during 

the Medieval Warm Period, rather than those found 

during the Little Ice Age just prior to Euro-American 

settlement. After all, a warm/dry summer weather 

pattern facilitating increased frequency of fires may 

be one more favorable to aspen expansion, or at least 

more favorable to greater renewal of aspen and regular 

culling of competing conifers. However, unpredictable 

consequences of artificial warming, such as exotic 

pest introductions, cannot be overemphasized. To 

add further caution, most studies cited in this section have stressed a high degree of variability within 

broad climate patterns and especially during periods 

of climatic shift (Millar and others 2004; Dettinger 

and others 2004). The unprecedented nature of human-induced climate change, though not intentional, 

introduces further variability to natural change where 

future vegetation, including aspen, and human health 

are concerned.

Historical Disturbance 

Ecology___________________

We have previously discussed aspen’s great ability 

to adapt to varying environments as evidenced by its 

wide contemporary range (see Physical Environment, 

this chapter). For example, aspen is apparently one of 

the first colonizers following glacial retreat in Canada 

(Muller and Richard 2001). Modern literature suggests 

that some aspen have been in the Sierra Nevada for at 

least the last 8,000 years (Strain 1964), although some 

estimates of clonal persistence for Interior West locales 

date to the Pleistecene (Baker 1925; Grant 1993; Mitton 

and Grant 1996). Mobility of clones over long periods 

is not well understood; however, it is conceivable that 

particular clones could migrate to adapt to changing 

conditions. A study in eastern Oregon found two 

sections of a genetically identical clone over 800 ft apart 

(Personal Comm., Valerie Hipkins, Geneticist, USDA 

Forest Service, National Forest Genetics Laboratory). 

Alternatively, specific climatic conditions, such as those 

following the Yellowstone fires of 1988, may facilitate episodic seedling establishment, thereby greatly 

increasing local aspen range and abundance (Romme 

and others 1997). Potentially, these two strategies work 

in tandem to effect both slow and rapid advancement 

that is dependent on climate pattern.

The role of disturbance in Sierra Nevada forests, 

and specifically in aspen’s place in those forests, is 

largely a study of 19th and 20th century environmental 

history. Region-wide, this 200-year period witnessed 

a human impact on vegetation transformed from 

relatively benign, to very intense, to scientifically 

“managed.” This transformation was based not only on 

great changes in population, but on changes (in orders 

of magnitude) in the scale and intensity of landscape 

modification. Native cultures exploited the mountain 

range at a subsistence level; Euro-Americans extracted 

resources and converted land at an industrial level. 

Cermak (1988) echoes this sentiment in relation to 

magnitude of Native American versus Euro-American 

burning:

It is likely that Indian burning affected this 

natural balance in some places and not others. 

At any rate, Indian burning had little effect upon 

California’s forests compared to the repeated, 

widespread burning of forests and brushlands 

practiced by settlers during the last half of the 

nineteenth century.

There is no doubt that natural disturbance cycles 

have been disrupted over the past 200 years. Our intent here is to discuss the impacts and interactions of 

natural- and human-caused disturbance (or, in the case 

of fire suppression, lack of disturbance) on aspen. In 

addition, parallel and sometimes coincident changes in 

regional climate have strongly influenced disturbance 

and vegetative patterns.

To unravel the effect of these impacts on the limited 

cover of aspen forests in the region we must examine 

pre-settlement, Euro-American settlement, and modern 

era patterns of forest disturbance. Disturbance commonly affects more than a single vegetation community, 

often benefiting one vegetation type to the detriment 
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 17

of another (Rogers 1996). Thus, interplay between 

aspen and its successional associates is an important 

component of our historical assessment.

Pre-Settlement

Prior to Native American migration to the Sierra 

Nevada region, vegetation was largely influenced by 

climate and natural disturbance. During the Holocene, 

California witnessed dramatic climate change that 

resulted in broad elevational shifts in vegetation 

(Woolfenden 1996). From this long-term perspective, 

disturbances—volcanoes, earthquakes, glaciation, 

floods, land slides, snow avalanches, insect and disease 

epidemics, and wildfires—were commonplace. As a 

pioneer species, aspen was likely the first tree species 

on site after most of these disturbances. For example, 

aspen was the most effective colonizer after continental 

glaciation in Canada (Muller and Richard 2001).

Although all of these disturbances have occurred 

in the Sierra Nevada, fire stands out as possibly having the most consistent long-term influence on aspen 

communities. We know more about fire’s influence, 

compared to other disturbances, on aspen from a large 

body of research in this subject area predominantly 

conducted in the Interior West (Gruell 1983; Jones and 

DeByle 1985b; Bartos and others 1994; Floyd and others 

1997; Romme and others 2001; Rogers 2002).

Aspen in the Sierra Nevada is found primarily in the 

upper montane zone (Potter 1998), so we will confine our 

brief review of fire ecology to that vegetation zone. In 

general, aspen regenerate best after stand-replacing fires 

given their root sprouting habit and need for open sunlight unhindered by competing overstory species (Jones 

and DeByle 1985b). However, montane fire regimes 

are not uniform. The most common aspen associates in 

the high Sierra—red fir (Abies magnifica) and white fir 

(A. concolor)—tend to have fire return intervals between 

50 and 150 years (Skinner and Chang 1996; Potter 

1998). In contrast, the slightly lower elevation mixed 

conifer zone tends to have fire return intervals between 

30 and 90 years, and the lowest elevation west slope 

and drier east slope forests have very short fire return 

intervals between 5 and 30 years (Skinner and Chang 

1996; Taylor 2004). Thus, the structure and composition 

of aspen’s vegetative associates can be expected to vary 

considerably under varying fire regimes.

These are broad generalizations. Species composition 

and microhabitat will more directly affect the fire ecology at a given locale (Brown and Simmerman 1986). 

For example, aspen associates such as lodgepole pine, 

Jeffrey pine, sugar pine (P. labertiana), western white 

pine (P. monticola), and whitebark pine (P. albicaulis) 

include widely varying fire characteristics, from frequent 

underburning (Jeffrey pine) to infrequent crown fires 

(lodgepole and whitebark pine). Pure aspen stands in 

Colorado have been described as nearly fire resistant 

because of their high density of moist forbs and higher 

stand humidity (Fechner and Barrows 1976). However, 

under very dry conditions even this dense forb layer 

becomes flammable (Jones and DeByle 1985b). Aspen 

trees also have thin bark that is extremely sensitive 

to fire. Even low intensity fire will often lead to high 

aspen mortality where flammable vegetation is found 

in the understory of aspen stands (Jones and DeByle 

1985b). Where aspen commonly occurs in the Sierra 

Nevada in riparian and other moist areas (Potter 1998), 

we would expect a somewhat longer fire cycle. Though 

little research specific to aspen forests has been done 

here, Skinner and Chang (1996) speculate that “Fire 

return intervals in these locations are likely to be quite 

variable and long.” The natural fire cycle may be 

shortened where conifers are readily invading aspen 

sites. In terms of fire size, most fires in this zone are 

relatively small, often limited by discontinuous fuels 

or natural fire breaks.

Anthropological evidence suggests that Native 

Americans have lived in the Sierra Nevada for the past 

10,000 years (Anderson and Moratto 1996; Parker 2002). 

At least six major tribes and countless smaller bands 

lived in and around this range with a total estimated 

population of about 90,000 to 100,000 prior to European 

settlement (Cook 1978; Parker 2002). Several authors 

have attempted to piece together the environmental 

use and vegetative impacts—primarily intentional 

burning of forests—of Native Americans (Denevan 

1992; Anderson and Moratto 1996; Vale 1998; Parker 

2002; Vale 2002). These assessments run the range from 

Native Americans having widespread regional impacts 

to having very little impact at all on vegetation. An 

exhaustive synthesis of demographics, physical environment, lightning strikes, climate patterns, tree ring 

records, and anthropological land uses conducted by 

Parker (2002) concludes that aboriginal populations did 

modify landscapes intensively near permanent settlements, but effectively left most of the Sierra Nevada 

range to processes of natural disturbance and succession.

Native Americans also modified landscapes in other 

ways, including land clearing for settlement, planting 

for agricultural practices, and harvesting products 

such as nuts, berries, pruned limbs, and other plants 

for food and tools (Anderson and Moratto 1996; Vale 

1998). However, these activities were generally at very 
18 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

local scales, and not common where aspen occurred 

in the upper montane zone. Although modifications 

may have been substantial in places like the Yosemite 

Valley, where large Miwok villages were located, the 

high elevation hinterland was used only on a seasonal 

basis, and mainly for transitory hunting purposes (Vale 

1998). In sum, there was minimal human alteration of 

vegetation during the pre-settlement period at upper 

montane locations where most aspen occurs.

The end of the pre-settlement era (1780 to 1850) 

witnessed minor European exploration from the east 

(American traders and explorers) and west (Spanish 

missionaries and military), and only limited settlement in the area. Euro-American contact had swift and 

devastating impacts on Native American demography. 

From 1800 to 1830, Native American mortality originated primarily from coastal Spanish missions via three 

mechanisms: Native Americans returning from mission 

trading brought disease into their villages; interior 

Spanish military excursions brought disease to foothill 

settlements, which in turn spread epidemics to Sierra 

tribes; and occasional violence upon arrival of military 

treks in Indian Territory (Cook 1978). After 1830, as 

Euro-American contact from the east began in earnest, 

Native mortality increased exponentially resulting in 

an estimated 60 percent of all mortality taking place 

during the period from 1830 to 1848 (Castillo 1978). 

Beesley (1996) estimates that 80 percent of pre-contact 

Sierra Nevada Indian populations died as a result of 

Euro-American contact, mostly from disease and, to a 

lesser degree, violent confrontation. He calculated that 

although some settlers were killed in conflicts, the ratio 

was nearly 50 Native Americans dying for every white 

settler. To the extent that Native Americans impacted 

vegetation prior to contact, whether locally or regionally, the drastically reduced populations of these tribes 

effectively nullified their impacts by 1850. Based on low 

indigenous population densities and a subsistence-scale 

of land use, vegetation impacts, on the whole, were 

driven primarily by natural factors.

A recent pre-settlement fire history near Lake Tahoe 

attributes 90 percent of fire starts to climatic conditions 

that were common during the period from 1650 to 1850. 

Essentially, late summer weather, including dry conditions during La Niña years and periods of peak lightning 

strikes, seems to be sufficient to account for the number 

and seasonality of pre-settlement fire regimes (Taylor 

and Beaty 2005). Van Wagtendonk (2004) pinpointed 

the highest levels of lightning strikes to be east of the 

Sierra crest and between 8,500 and 9,000 ft elevation, 

with the northern half of the range receiving slightly 

more lightning strikes than the southern half. This zone 

favors frequent historical fire ignitions within aspen 

growing elevations (Potter 1998).

Euro-American Settlement

It is well known that the mid-19th century Gold 

Rush brought prospectors and settlers into California’s 

high country in large numbers. Initial settlement was 

followed by successive waves of resource extractiondriven development. For this reason the settlement era in 

the Sierra Nevada is best characterized by intensive use 

and abuse of natural resources, beginning with mining 

and followed by small-scale water diversion, logging, 

and grazing. These efforts were not necessarily mutually exclusive. For example, hydraulic mining diverted 

streams to extract gold-bearing sediments along major 

Sierra tributaries. Palmer (1992) estimates use in northern Sierra watersheds alone as being 36 million gallons 

(136 million liters) of water in a single 24-hour period, 

some three times the use of San Francisco during that 

era. These operations, spread throughout the range, but 

more commonly on the west slope, completely cleared 

adjacent hillsides of vegetation and sent millions of 

tons of sediment downstream to the Sacramento valley. 

Eventually hydraulic mining was stopped, not because 

of its impact on the immediate landscape, but because 

sedimentation severely limited valley farmers’ use of 

water for irrigation. Construction of local sluices not 

only diverted substantial amounts of water from streams 

and increased erosion near prospecting sites, but often 

involved denuding surrounding hillsides for building 

materials (Beesley 1996). Such disturbances could not 

only have favored aspen with the removal of competing 

conifers, but been detrimental to aspen where roots were 

washed away or buried under heavy sediment.

Logging had both local and widespread impacts on 

the central Sierra Nevada. In the Lake Tahoe basin, logging to support the Comstock mining district in Nevada 

nearly denuded the Carson range (east of Lake Tahoe) 

and impacted most forests surrounding the lake to some 

degree (Jackson and others 1982; Strong 1984; Kim 

and Rejmánková 2001; Taylor 2004). Pollen analysis 

of sediment cores at three marsh sites near Lake Tahoe 

show a distinct signature of increased sedimentation and 

decreased pine pollens during the late 19th and early 

20th centuries (Kim and Rejmánková 2001). Historic 

photos from this period show barren hillsides behind 

logging decks stacked high with locally harvested timber 

(Strong 1984) (fig. 2-6). Near Truckee, huge volumes 

of logs were extracted to supply mines, construct giant 

V-shaped flumes for transporting logs, and for use on 
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 19

the Union Pacific rail line (Jackson and others 1982). 

Beesley (1996) estimated that 300 million board feet 

of timber were harvested to construct snow sheds for 

the railroad and an additional 20 million board feet 

per year to maintain the sheds. In Comstock, mining 

efforts consumed an additional 70 million board feet per 

year, for about 10 years, for flume construction, mine 

ties, fuel wood, home building, and construction of a 

narrow-gauge rail line from Virginia City to Spooner 

Summit (and eventually Lake Tahoe itself). Of course, 

this rail line allowed massive exploitation of lumber, 

which was towed by boom from various points across 

the lake (Strong 1984; Beesley 1996). Strong (1984) 

further explains that logging practices at this time often 

involved post-harvest burning, which contributed to 

large fires in 1889, 1898, 1902, and 1903, and resulted 

in expanses of brush fields in the following decades. In 

1900, the government agent George Sudworth made an 

extensive survey of the Stanislaus Forest Reserve on the 

west slope of the Sierra Nevada. He encountered small 

mills in the headwaters of each major and many minor 

west slope drainages and found evidence that mills had 

moved several times after exhausting the entire supply 

of lumber within a 2.5 to 3 mile radius (Sudworth 1900). 

This massive clearing of forests in the Sierra Nevada, 

although not directed at aspen as a species, nevertheless 

had great residual effects by stimulating aspen growth 

in large newly created forest openings.

Even though mining and logging activities were 

widespread during the late 19th century, they probably 

affected less total land than sheep (Ovis spp.) grazing. 

Grazing, especially by sheep, leaves a near-continuous 

impact on the landscape, whereas both mining and 

logging activities tend to leave a “patchy” disturbance 

Figure 2-7. In 1905, Grove Karl Gilbert photographed and noted 

the extensive sheep grazing and browsing on lands near 

Bowman Lake in what is now the Tahoe National Forest. 

Source: U.S. Geological Survey, Denver, Colorado.

footprint. Noting that some of these later “patches” 

could be quite large, their impact in terms of acres could 

not compare to grazing during this period. During the 

“sheep boom” (1870 to 1890) there were no restrictions 

on the number of sheep or the timing and movement of 

herds. Although accurate estimates of sheep use during 

this period are not available, Beesley (1996) says that 

they numbered in “the millions” and Cermak (1988) 

noted 7 million for the state, a substantial portion of 

which likely used this prime high elevation rangeland. 

Although foraging and trampling by sheep can devastate 

meadows (fig. 2-7), both meadow and forest alike were 

affected by the common and widespread practice of 

burning pasturage upon leaving the mountains in the 

fall. In general, aspen persistence thrives in frequent 

Figure 2-6. Intense logging during the 

Comstock era denuded much of 

the Carson range. This photo was 

taken at Spooner Summit in 1876. 

© Reproduced by permission (Goin 

and Blesse 1992).
20 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

fire environments (Brown and DeByle 1989; Bartos and 

others 1991; Skinner and Chang 1996; Rogers 2002). 

Numerous authors detailed these burning practices 

(Sudworth 1900; Leiberg 1902; Jackson and others 

1982; Cermak 1988; Kinney 1996; Beesley 1996) 

typified in a quote from P.Y. Lewis (Cermak 1988): 

“We started setting fires and continued setting them 

until we reached the foothills. We burned everything 

that would burn.”

Burning, sheep grazing, logging, mining, and water 

diversion had major effects on Sierra Nevada vegetation 

during the latter half of the 19th century. Some areas, 

such as the Carson range on Lake Tahoe’s east side, 

were completely transformed (Strong 1984; Taylor 

2004). Potter (1998) aptly relates this human-caused 

disturbance of over a hundred years ago to contemporary 

aspen cover:

In general, the ages of the current aspen component in many stands corresponds relatively well 

with the end of intensive grazing pressures in the 

late 1800s and the institution of fire suppression 

policies in the early 1900s.

Modern Era

In large part, the establishment of Forest Reserves, 

and eventually the Forest Service in the Department of 

Agriculture, were a reaction to the west-wide abuses of 

the land resources during the late 1800s. The modern 

period therefore is characterized by implementation of 

much needed regulation of forest lands, both nationally and in the high Sierra. The age of scientific land 

management began to take hold after documentation of 

natural resource abuses by the likes of Sudworth (1900), 

Leiburg (1902), and preservationists such as John Muir 

(Muir 1982). Grazing was limited and monitored to 

some degree, logging was planned and inspected, and 

mining and water use were closely regulated. In 

hindsight, this approach to implementing emerging 

management practices was fraught with scientific weakness and personnel shortages, but esprit de corps and 

sheer bravado often carried the day in the newfound 

agencies. The Forest Service in California’s newly 

formed Region 5 confronted fire in an all out attack, 

not only on wildfire, but on the burning practices of the 

settlement era and those of Native Americans.

After the big fires of the 1890s and early 1900s, forest 

managers began discussing the idea of fire suppression 

as a means of bringing the former situation under control. Established forest use practices of that time were 

largely ignored in the effort to control wildfire. This 

included Native American use of fire and other resource 

users who intentionally set fires to improve forage for 

livestock, clear brush and logging slash (although this 

often led to more brush), clear land for settlement, and 

improve game forage and hunting visibility. In 1910, 

extreme fire conditions in the northern Rockies (Pyne 

2001) brought the national debate—fire suppression 

versus “light burning”—to the forefront (Hoxie 1910). 

Eventually, Stuart Bevier Show, a Region 5 forester, 

played a national role in advancing the fire suppression 

policy agency-wide (Show and Kotok 1930). This policy 

of suppressing all fire, especially following the heavy 

burning and extraction of the previous era, is likely a 

key factor in the development of contemporary aspen 

forests.

The legacy of early 20th century scientific forestry, 

epitomized by Gifford Pinchot and Henry Graves at 

the national level and S.B. Show in California, was 

the establishment of practices designed to bring both 

nature (even if it was rebounding from large-scale human abuse) and resource extraction into alignment with 

management objectives that conserved and sustained 

forest resource outputs. Hence, fire would be suppressed 

with military fervor; rivers would be controlled with 

dams and diversions; forests would be “managed” for 

highest yields; and game animals would be regulated 

by elimination of large predators and optimization of 

game species’ numbers. The economic engine driving 

forest management in the Sierra Nevada, as in much 

of the nation through this period, was timber harvest. 

More specifically, this meant the intensive management 

of high value, fast growing conifer species. Secondarily, 

forage for livestock and forage for wild ungulates were 

considered forest “products” to be favored in management plans.

Resource extraction did not cease around 1900; it was 

only controlled and managed by the resource agencies 

established at that time. For instance, in the Truckee 

basin, several waves of industrial logging continued 

until about 1940, when “virgin timber” was eventually exhausted (Knowles 1942). In water engineering, 

the scale of resource use may have been larger than 

previously occurred. To some degree, twentieth century 

water diversion on both large- and small-scales probably 

affected riparian uplands where aspen reside through 

the flooding of small meadows and the diversion of 

water from streams. In the Mono Lake drainage, for 

example, diversion of water for Los Angeles led to a 

loss of riparian vegetation, notably large stands of aspen, 

along Rush Creek (Stine and others 1984). Similar 

impacts on aspen communities may have taken place 

throughout the range as a result of politically driven, and 
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 21

scientifically engineered, solutions to economic development. To some extent, riparian-associated vegetation 

like aspen is accustomed to frequent fluvial events that 

accompany wet and dry periods. However, dams and 

diversions effectively mimic continuous downstream 

drought conditions, ultimately leaving deeply incised 

channels and lowering local water tables that support 

moist site trees such as aspen (Mount 1995).

Management philosophy around the middle of the 

20th century began to shift from control of nature to 

understanding, and eventually to working with natural 

processes. This movement was evident in management 

reactions to disturbances, notably wildfire and floods. 

The “multiple use” management approach began with 

the U.S. Forest Service and was eventually adopted 

by many land use agencies. Essentially, multiple use 

meant that agencies were transforming from single use 

missions, such as cutting timber or capturing streams 

with dams, to a variety of “outputs,” often led by the 

same dominant uses.

On the heels of multiple use management came the 

increased environmental and planning requirements 

of the 1970s meant to bolster protection for multiple 

resources. This transformation from a production 

mode, primed by California’s rapid population growth, 

to a stewardship mode, was not easy (Beesley 1996). 

However, the evolution of resource management is ongoing nationally, with greater recognition of disturbance 

processes having become more accepted in today’s 

management decisions (Rogers 1996).

The modern era, especially in response to settlement practices, has had a great impact on the Sierra 

Nevada landscape. The extent of aspen was certainly 

influenced by the combined effects of fire suppression, management favoring conifers (cutting and 

replanting commercial species), and grazing of aspen 

sprouts by livestock and unfettered wild ungulates. To 

a lesser extent, a relatively moist 20th century may 

have helped boost the confidence of managers in their 

quest to suppress most wildfires. The impact on aspen 

communities in this 100-year period, beginning with 

widespread disturbance that favored regeneration, has 

been a general trend toward advancing succession in 

the absence of fire, where many stands have “a few 

remnant aspen below an almost closed canopy of 

conifers” (Potter 1998).

Whether or not the absence of fire is the result of 

successful fire suppression, wetter climate, or combined effects, the end result is a century of relatively 

low fire occurrence on the heels of elevated levels of 

disturbance (in the latter part of the 19th century). 

However, a fire suppression management strategy 

may not be sustainable under warmer and dryer years, 

which could lead to more intense fire activity at the 

start of the current century.

Aspen forests have probably never covered large 

areas of the Sierra Nevada. Additionally, aspen ecosystems (as well as other vegetation) have been grossly 

disrupted by human activities over the past 200 years. 

The challenge for today’s managers, in light of today’s 

intense human development and use of Sierra forests, 

is to return aspen communities to some semblance of 

their natural range of variability.
22 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 23

Chapter 3.

Aspen Ecology

Physiology of Aspen________

Many years of ecological research have revealed a 

great deal about how aspen thrive on the landscape. 

We believe the basic physiological research—cited 

here primarily from the Interior West of the U.S. and 

Canada—is equally valid in Sierra Nevada aspen stands. 

A key prerequisite to managing aspen community 

health is understanding the basics of aspen’s growth 

habit, ecology, and its reproductive physiology that 

emphasizes vegetative rather than sexual reproduction. 

Further discussion related to seed dispersal and sexual 

reproduction can be found in the Aspen Genetics section 

later in this chapter.

Aspen’s Ecological Amplitude 

and Habitat

Although aspen occurs throughout the western United 

States, in California aspen is found in the Sierra Nevada, 

southern Cascades, Modoc Plateau, Transverse and 

Peninsular Ranges, and select highlands of the Basin 

and Range province (Strain 1964; Barry 1971; Thorne 

1977; Di Orio and others 2005). California aspen is 

generally more limited in range, most often confining 

itself to areas of above average soil moisture, such as 

stream banks, meadows, springs, and subsurface water 

sources (Barry and Sachs 1968; Potter 1998; Smith and 

Davidson 2003). However, in the Southern Cascades 

and Modoc Plateau ecoregions, as well as parts of the 

eastern Sierra Nevada, aspen may also be found outside 

of riparian settings.

At the continental scale, aspen has several physiological characteristics that permit it to attain great 

geographic amplitude. Lieffers and others (2001) 

outline the following important adaptive traits of aspen: 1) Among the wide ranging genus Populus spp. 

(cottonwoods, poplars, aspen), aspen seems to have a 

very high stress tolerance. Usually, high stress tolerance 

is associated with slow growing species and those with 

a limited reproduction strategy; 2) Aspen appears to 

rely on vegetative reproduction via root suckering more 

than other Populus spp. These authors assert that the 

passing of extensive root systems between generations 

enhances tolerance to absorb climate stress (DesRochers 

and Lieffers 2001); 3) Aspen also has the ability to adapt 

leaf size to xeric and mesic conditions (that is, smaller 

leaves for drier sites). Its smaller leaf size could keep 

the leaf surface slightly cooler, allowing earlier shut 

down of stomata, thus tempering water stress during 

drought; 4) Aspen seems to tolerate cold temperature 

and short growing seasons better than most hardwoods 

(Pearson and Lawrence 1958); 5) Leaf fluttering may 

be an adaptive advantage in cooling leaf surfaces of 

many Populus; and 6) Aspen appears to have a higher 

photosynthesis capability than other Populus spp., 

which is comparable to that of high yield poplar hybrids. 

Aspen photosynthesizes well in low light (for example, 

competitive situations) and its bark is also capable of 

photosynthesis, which helps to ameliorate respiration 

during periods of high insolation (before spring leaf-out) 

(Pearson and Lawrence 1958). Photosynthesizing bark 

may help aspen recover from injuries and infestations 

(Jones and Schier 1985; Lieffers and others 2001) and 

may allow aspen to photosynthesize at low levels during 

the winter giving the tree a photosynthetic “boost” prior 

to leaf-out (Pearson and Lawrence 1958; Shepperd and 

others 2004). As leaf chlorophyll increases during the 

summer, bark chlorophyll decreases causing bark to 

become whiter (Strain 1964).

Most aspen stands are composed of one to several 

clones that may persist along a continuum of successional stages, from sparsely growing individuals to 
24 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

apparently stable pure or near-pure groves. Although 

clones are often separate and distinct from one another, 

studies have demonstrated spatial intermingling where 

multiple clones are co-located (DeByle 1964; Mitton 

and Grant 1980; Wyman and others 2003; Hipkins and 

Kitzmiller 2004). In eastern North America, clones tend 

to be smaller in size (less than 5 acres [2.02 ha]) and 

tree establishment from seed is not uncommon. In the 

Interior West, clone sizes are generally larger (from 20 

to 100 acres [8.09 to 40.47 ha]) and seedling events 

are rare (Kemperman and Barnes 1976). Compared to 

conifers, aspen ramets—individual stems, or suckers, 

of the same genotype from a parent root system—are 

relatively short lived. This is due to succession (replacement of aspen by more shade tolerant species) 

and/or a typical onslaught of mortality related to stem 

decays and diseases from ages 80 to 100 years (Baker 

1925; Hinds 1985; Potter 1998; Rogers 2002). Aspen 

thrive where somewhat regular and frequent disturbance promotes regeneration (DeByle and Winokur 

1985). Occasionally, aspen stands appear to perpetuate 

themselves with regular low-level regeneration in multilayer, stable stands (Mueggler 1988; Cryer and Murray 

1992). Healthy ramets (trees) can live over 300 years 

(Personal Comm., John Shaw, Forester, USDA Forest 

Service, Rocky Mountain Research Station) and attain 

diameters of at least 38 inches (96.5 cm) diameter at 

breast height (dbh). Aspen in the western U.S. are longer 

lived than elsewhere. Many mature stands in Colorado 

are currently over 120 years of age (Shepperd 1990). 

Tree form varies from shrubby at upper and lower forest margins to over 100 ft (30.5 m in height) in prime 

locations with average heights of 50 to 60 ft (15 to 18 

m) (Baker 1925).

It should be noted that variation in physical traits of 

aspen is often determined by genotype and therefore 

expressed on a clonal basis by all stems within a particular 

clone. For example, in Colorado the timing of spring leaf 

development, color during growing season and during 

senescence, size, bark color, and leaf fall may be used 

to distinguish separate clones (Shepperd 1982). The following characteristics are common to aspen: leaves are 

1.5 to 2 inches (3.8 to 5.1 cm) long, with serrate margins, 

semiorbicular shape, ending in a distinct point (similar 

to the “spade” in a deck of playing cards). New sprouts 

will often have considerably larger leaves allowing 

increased photosynthesis and quick growth. Flattened 

petioles allow leaves to flip or “quake” in a breeze. 

Aspen bark may vary from green-white, yellow-white, 

pure white, to deeply furrowed black on the lower trunk 

of older specimens (Shepperd 1982, 1990). Quaking 

aspen is characteristically dioecious, meaning there are 

separate male and female plants. However, perfect flowers, containing staminate and pistillate flowers, seem to 

occur in 5 to 20 percent of ramets (McDonough 1985). 

Catkins containing the developed pistillate seeds are 

from 2 to 4 inches (5.1 to 10.2 cm) long and can readily 

be seen dangling from aspen twigs in the late spring. 

As the tiny seeds mature, tufts of white cotton protrude 

from catkins. These tufts will hold seeds aloft as they 

are dispersed in the wind (fig. 3-1).

Clonal Habit and Root Systems

Though the topic is touched on elsewhere in this 

publication (see Aspen Genetics, this chapter), here 

we focus more on the physiology of aspen clones and 

their associated root systems. Aspen generally sprout 

profusely (up to 500,000 stems per acre) following 

disturbance, although there is some continuous lowlevel regeneration even in shaded stands. High initial 

numbers of aspen suckers in post-disturbance stands 

typically self-thin following a negative exponential 

decay model, with most losses occurring in the first few 

years (fig. 3-2) (Shepperd 1993). Young trees studied 

in Canada and the U.S. Rocky Mountains grow an 

average of 3 to 6 ft (0.9 to 1.8 m) the first 2 years and 

a total of 9 to 15 ft (2.7 to 4.6 m) in 5 years (Shepperd 

1993; Miller 1996).

Lateral aspen roots are found generally within a 

foot of the soil surface. These roots are mostly 0.25 

to 3 inches (0.6 to 7.6 cm) in diameter and have very 

little taper at distances out to 100 ft (30.5 m) from 

Figure 3-1. Aspen seed dispersing from pods on a branch.
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 25

the parent tree. Fine “sinker roots” may develop anywhere along the lateral roots, with about 30 percent 

originating beneath developing ramets (Baker 1925; 

Jones and DeByle 1985b). The primary role of these 

vertical roots is to tap into soil nutrients and water 

while increasing stem and root system stability (fig. 

3-3). A key ingredient of healthy lateral root systems 

is their ability to store carbohydrates as “fuel” for 

shoot growth in the event of a regeneration pulse. 

Aspen stands in Colorado between the ages of 20 

to 80 years have been shown to have higher rates 

of carbohydrate storage in their roots than older or 

younger stands (Shepperd and Smith 1993). It has been 

demonstrated that carbohydrate levels vary throughout 

the year (Shepperd and others 2004). In addition to 

carbohydrate storage, overall root mass may be related 

to clone vigor. Shepperd and others (2001) found 

greater root masses in aspen stands that were actively 

regenerating versus those that were not.

Most root suckers arise on roots within 6 inches (15 

cm) of the soil surface and on roots from 0.15 to 0.79 

inches (4 to 20 mm) in diameter (Schier and Campbell 

1978). Suckers initially depend on parent roots for 

nutrients and water (Jones and DeByle 1985b) and 

stored assimilates (Tew 1970), but later develop discrete 

root systems (Shepperd and Smith 1993). As a sucker 

grows, the distal parent root enlarges and new branch 

roots arise from the base of the shoot itself (Baker 1925; 

Brown 1935). Dependence on parent root support is 

thought to decrease as suckers develop their own roots 

(Zahner and DeByle 1965) and connected roots die or 

break (Gifford 1966). The parent root system no longer 

offers a competitive advantage to the new suckers after 

a few years (Shepperd 1993), although functional root 

connections between small groups of stems arising 

from the same parent root may exist throughout the 

life of a stand (DeByle 1964; Maini 1968; Tew and 

others 1969).

The mechanism driving the suckering process is the 

ratio of cytokinin to auxin hormones in the roots and 

apical meristem (Schier 1976, 1981; Frey and others 

2003). Auxins translocated from the apical meristem 

are thought to suppress suckering, while cytokinins in 

the root tips are believed to induce new shoot growth 

(Schier 1981). When aspen crowns are receiving 

ample sunlight, auxins are sent to the root system, effectively curbing new shoot development. In the event 

of increased aboveground mortality, such as from fire 

or land clearing, auxin production is sharply reduced 

allowing cytokinins to stimulate bud primordia on roots 

to develop into a proliferation of new suckers. In less 

extreme events, increased soil temperature alone can 

stimulate cytokinins to the point of developing new suckers (Schier 1976; Frey and others 2003). This factor is 

probably instrumental in low-level suckering in partially 

shaded stands. Apparently, soil temperatures are critical 

only where roots are near the surface. Deeper roots are 

less affected by thermal heating of the forest floor. In 

a post-burn study in Arizona, the soil temperature of 

the blackened surface increased down to 15 cm and 

induced greater suckering compared to the unburned 

treatments (Shepperd 2004). These deeper roots seem 

to be capable of producing suckers when auxins are 

Figure 3-2. Self thinning of young aspen sucker populations 

follow a negative exponential decay model. Data from 

a study of aspen regeneration in the Rocky Mountains 

(Shepperd 1993).

Figure 3-3. Lateral roots of aspen form a dense mat just under 

the soil surface. Fine roots descend from the lateral roots 

to access moisture deep in the soil.
26 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

suppressed after stand replacing disturbance (Frey and 

others 2003).

Although aspen ramets are relatively short-lived, 

aspen genotypes may persist for a very long time 

(Schier 1981; Mitton and Grant 1996). Clone longevity, 

especially in very large clones, has been attributed to 

the interconnected nature of their extensive root systems 

(Mitton and Grant 1996; DesRochers and Lieffers 2001). 

This does not necessarily mean that individual roots 

persist for hundreds or thousands of years. Changes in 

root biomass as ramets age suggest that belowground 

biomass rejuvenates each ramet generation as well 

(Shepperd and Smith 1993). Past work noted that young 

ramets became progressively less dependent on their 

parental root system after 25 years of age, eventually 

cutting underground ties completely (Barnes 1966). 

Other researchers have demonstrated that portions of root 

systems appear to remain functionally interconnected 

for more than one generation (DeByle 1964; Jones and 

DeByle 1985b; DesRochers and Lieffers 2001).

Although all stems in a clone may not be connected, 

connections between stems may act as a vital survival 

mechanism where decaying stands must support young 

trees. Desrochers and Lieffers (2001) found that young 

trees in southern Canada connected to a parent root 

system could utilize the pre-existing root network 

when parent stems died. They found that even dead 

parent trees had portions of their root network living. 

Interestingly, these researchers did not find that stem 

decays aboveground were transferred into the still living 

belowground root network. Physically disconnecting 

aspen roots from parent trees can also stimulate the 

suckering process. From studies conducted in Arizona, 

Shepperd (2001, 2004) found that separating lateral roots 

from parent trees resulted in greater sprouting density 

and subsequently demonstrated that root ripping can be 

an effective way of expanding existing aspen clones.

Aspen can grow in a large variety of soils, although it 

prefers relatively deep and nutrient rich soils with ample 

moisture found most often on moderate slope angles. 

Aspen favor deeper soils on flood plains, benches, slope 

bottoms, and concave landforms because they retain 

soil moisture and have fewer subsurface rocks that 

can inhibit lateral root extension (Baker 1925; Jones 

and DeByle 1985c). Exceptions may be found where 

aspen grows in avalanche chutes, near ridge lines, or on 

talus slopes. In much of the West, aspen is commonly 

found on mollisol soil types (Cryer and Murray 1992; 

Bartos and Amacher 1998) containing a well-developed 

organic layer. Soils associated with aspen stands in the 

Sierra Nevada include mostly inceptisols, followed in 

prominence by alfisols and mollisols (Potter 1998). Soil 

richness under aspen has been attributed in part to organic 

matter accumulation from annual leaf fall from aspen. 

Where conifers are invading aspen stands, needle litter 

can alter soil properties over time (Cryer and Murray 

1992). However, it is believed that this change is not 

extreme enough to limit aspen regeneration in the event 

of fire (Bartos and Amacher 1998).

Much of the regeneration “strategy” of aspen 

involves quickly producing suckers following disturbance in order to out-compete rival plants. Elevated 

levels of nitrogen in post-fire soils afford greater 

nutrient availability for developing aspen regeneration 

for 1 to 3 years following fire (Amacher and others 

2001; DesRochers and others 2003). Increased soil 

temperatures work in tandem with elevated nutrient 

levels after fire to give aspen an apparent edge in early 

growth following disturbance (Shepperd 2001, 2004; 

Fraser and others 2002). Shepperd (2004) presented 

data from an experimental burning study in Arizona 

that indicates increased temperature and nutrients 

associated with burned soils led to more numerous 

aspen stems and greater growth in individual stems 

in the early post-fire years. However, DesRochers and 

others (2003) provide a note of caution. They found 

different responses to post-fire nutrient use in Alberta 

among clones in the same area.

Damaging Agents Affecting 

Aspen____________________

A number of insects, diseases, and other damaging 

agents can affect the health and vigor of aspen. Complete 

and extensive discussions of these factors have been 

presented elsewhere (Walters and others 1982; Hinds 

1985; Ostry and others 1988), so we will only briefly 

review them here.

Stem Canker Diseases

Aspen’s living bark makes it susceptible to a number 

of fungal canker diseases that can attack, girdle, and 

ultimately kill the trees by blocking the transport of 

photosynthates to the roots. Some, such as Ceratosystis

spp., or target cankers are slow growing and can take 

years to girdle a tree (fig. 3-4). Others, such as Encoelia 

pruinosa (Cenangium spp.), or sooty-bark canker (fig. 

3-5) can kill a tree in just a few years. Yet others, such 

as Cryptosphaeria canker (fig. 3-6) can kill aspen trees 
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 27

in a single year. All are fungal organisms that infect 

the living bark phloem tissue through wounds, some 

as insignificant as that caused by the staple holding 

the card on the tree in figure 3-6. Once a canker fungi 

spore infects phloem tissue, the fungus grows, feeding 

off nutrients in the living cells and killing them as it 

spreads through the surrounding bark. Although not all 

wounds get infected, any wound is a potential entry site 

for a canker infection. If the fungus is a slow-growing 

species, the tree reacts to the infection by producing 

callous tissue to seal-off the fungus and eventually heal 

the wound by growing new tissue over it, similar to what 

happens when fire scars a tree. Usually, canker organisms spread again the next growing season, resulting in 

an annual infection and callus production that creates 

the characteristic concentric wounds on aspen that will 

eventually girdle and kill the tree.

Trunk rot fungus, Phellinus tremulae (fig. 3-7), affects wood quality if aspen is grown for commercial 

wood products. It also weakens live trees, making 

them susceptible to wind breakage. Trees near houses 

or in developed recreation areas that have hoof-shaped 

Figure 3-4. Ceratosystis stem canker on aspen. Note the 

target-like compact concentric growth rings.

Figure 3-5. Sooty-bark canker (Encoelia pruinosa) on aspen. The large concentric bands indicate annual growth 

progression.

Figure 3-6. Cryptosphaeria stem canker on aspen. This fast 

growing canker can kill a tree in a single year.
28 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

“conks” or fruiting bodies of this fungus should be 

considered for removal to reduce potential hazard and 

spread of disease to adjacent stems.

Root Diseases

Aspen is subject to root diseases such as Armillaria

spp., which can cause significant localized mortality 

within aspen stands, but usually does not kill entire 

stands. Root diseases kill the roots and spread to the 

stems causing them to topple over in light winds. Since 

the disease can spread through the soil, all roots in an 

epicenter usually die, preventing any aspen suckering 

from occurring. While root diseases may contribute 

to clonal decline, they have not been a major cause of 

aspen mortality in our experience.

Boring Insects

Aspen is not susceptible to bark beetles, such as 

Mountain Pine beetle (Dendroctonus ponderosae), or 

spruce beetle (Dendroctonus rufipennis). However, 

wood boring insects and beetles such as Agrilus planipennis (fig. 3-8), do attack aspen, boring directly 

into the wood and affecting wood quality. Although 

wood borers usually do not directly kill the tree, their 

wounds can be entry points for canker infections, which 

can kill trees.

Figure 3-8. Large galleries of Agrilus planipennis wood borer 

under aspen bark.

Figure 3-7. Phellinus tremulae, trunk rot fungus in aspen.

Foliage Diseases

Other fungal diseases, such as ink-spot (Ciborina 

whetzelii) (fig. 3-9), attack aspen leaves, killing cells 

and creating characteristic patterns of leaf mortality 

that can stress trees, but usually don’t directly kill 

them. Such diseases are periodic, not occurring every 

year or in every genotype, but can create distinct color 

patterns in pure aspen forests when viewed from a 

distance.

Defoliating Insects

Insects such as aspen tortrix (Choristoneura conflictana) and western tent caterpillar (Malacosoma 

californicum) (fig. 3-10) can defoliate large areas of 

aspen. Single defoliation events usually do not kill 

many trees, but can trigger suckering events in aspen 

clones. Repeated defoliations for a number of years 

can result in widespread mortality. However, clones 

can naturally regenerate after such events, if growing 

conditions permit.
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 29

Figure 3-9. Foliage disease on aspen caused by Ciborina 

whetzelii, ink-spot fungus.

Figure 3-10. Western tent caterpillar (Malacosoma californicum) can be a serious aspen damage agent.

Figure 3-11. Deep snowpacks can settle and severely deform 

young aspen suckers (note ski pole in foreground).

Physical Damage

A number of physical factors can damage aspen and 

affect stem form and tree longevity. Snow can bend or 

break mature aspen stems and deform young suckers into 

grotesque shapes (fig. 3-11). Aspen’s shallow rooting 

habit makes it susceptible to windthrow, especially if 

protected trees are exposed by partial harvests of trees 

of similar age. Animals that can physically harm aspen 

include large animals such as elk (Cervus elaphus), 

moose (Alces alces), and deer (Odocoileus spp.) (fig. 

3-12) that may feed on bark because of its high food 

value in winter (Shepperd and others 2004). Small 

mammals, such as beaver (Castor spp.) (see Terrestrial 

Biota, this chapter) or voles, also feed on aspen bark 

(fig. 3-13). The degree to which animals harm aspen is 

dependent upon the size of both the animal and aspen 

populations in a given area.

As a disturbance dependent species, aspen has evolved 

to deal with damaging physical agents by absorbing the 

effects through the sheer number of stems in a clone, 

or by rapidly reproducing new stems to replace those 

killed. In properly functioning ecosystems, an uneasy 

balance is struck between aspen and its damaging agents. 

Recognizing when this balance has been disturbed and 
30 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

Figure 3-13. Voles have gnawed bark from the stem of this 

aspen sucker and girdled it.

Figure 3-12. Infected wounds caused by elk on aspen.

taking steps to correct it is a basic tenet of contemporary 

aspen management.

Aspen Genetics____________

In recent years, the subfield of aspen genetics has 

begun to question previous assumptions about aspen life 

histories. For instance, researchers generally considered 

aspen “stands” to be synonymous with individual 

clones, and that clones could be easily discerned based 

on phenotypic characteristics. Also, it has long been 

assumed that rare seedling establishment in the West 

“...is of no importance in the management of aspen 

stands...” (Baker 1925). More recently, researchers are 

pondering the relationship between unusual events in 

time and their impact on species genetic diversity on 

the landscape. Could rare seedling establishment be 

related to aspen’s limited spatial extent in the Sierra 

Nevada? Or, do rare seedling pulses occurring over 

time play a crucial genetic role in maintaining the 

limited extent of aspen on the landscape that is so 

important to vegetation and wildlife diversity in the 

Sierra Nevada? And finally, what role does somatic 

mutation play in maintaining diversity under the 

regime of asexual (clonal) reproduction? Before addressing these topics, a basic understanding is needed 

of reproductive strategies in aspen.

Asexual and Sexual 

Reproduction

It has long been held that quaking aspen in western 

North America reproduce primarily from suckers arising out of a common root system (Baker 1925) (see 

Physiology of Aspen, this chapter). A network of lateral 

roots exists near the soil surface and may produce vertical 

suckers or shoots (fig. 3-3). Mature ramets (stems) produce auxins that suppress development of adventitious 

shoots. When a stem loses apical dominance, auxin 

levels decrease and pre-existing meristems and buds 

can develop into full-fledged shoots (Schier and others 

1985). Over time, clones may expand as regeneration 

takes place on the perimeter of an open grown stand, 

when disturbance eliminates apical dominance of the 

existing clone, or when there is competition from other 

tree species. Depending on light availability and the 

preponderance of disturbance, clones may vegetatively 

expand or contract over time, or physically migrate 

across a landscape.

Could vegetative reproduction alone account for 

the huge range of aspen (Baker 1925; Strain 1964)? 
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 31

Figure 3-14a,b. Micro-photographs of male (a) and female (b) catkins of aspen. The staminate (male) flowers are distinguished

by the four-lobed stamens and the pistillate (female) flowers by their frill-edged style and pear-shaped ovaries. Source:

Andrew Groover, Instiute of Forest Genetics, USDA Forest Service.

a b

Figure 3-15. Profuse aspen seeding along a roadside in 

southern Colorado (note radio for scale). Source: Larry 

Johnson, U.S. Forest Service.

Baker (1925) estimated that lateral roots in Utah could 

expand out 15 to 50 ft (4.6 to 15.2 m) over a 20 year 

period. Other researchers have found maximum root 

lengths to be around 100 ft (30.5 m) (Buell and Buell 

1959; Jones and DeByle 1985b). We have personally 

observed a sucker growing 150 ft (46 m) from the 

edge of an isolated clone in a Utah meadow. Although 

these spread rates are impressive, it seems unlikely that 

they can explain the vast expanse of aspen coverage 

on the continent. Furthermore, physical barriers to 

root expansion, such as bedrock and riparian areas, in 

combination with soil and climatic restrictions, would 

seem to further inhibit purely clonal explanations for 

widespread distribution.

Sexual regeneration results in the production of new 

genets from seed. In the West, recruitment from seed 

is rare in many clonal species and adverse conditions 

may hinder sexual reproduction, such as when plants 

are near their geographic limit or climates are changing 

(Eriksson 1992). Although aspen do produce viable 

seed, many authors have attested to the rarity of seedling 

establishment in comparison to vegetative reproduction 

(Baker 1925; Barnes 1966; Einspahr and Winton 1976; 

Schier and others 1985; Romme and others 1997). Aspen 

is characteristically dioecious, having separate female 

and male plants, which develop in catkins on twigs 

prior to leaf-out in the spring (fig. 3-14a,b). Ratios of 

male to female clones can vary widely. Researchers in 

Colorado, for example, found a decreasing proportion 

of female aspen with increasing elevation (Grant and 

Mitton 1979). Tiny aspen seeds (about 0.04 inches/1 

mm) are dispersed by wind with the aid of a placental 

cotton-like hair that keeps them aloft for up to 200 miles 

(322 km) (Einspahr and Winton 1976) (fig. 3-1). Seed 

production is profuse, with mature trees generating 

millions of seeds in a good seed year (fig. 3-15) (Maini 

1968; Schreiner 1974).

Aspen seedling establishment across regions of 

North America appears limited by a narrow range of 

conditions for germination (Barnes 1966; Barry and 

Sachs 1968; McDonough 1979; Jelinski and Cheliak 

1992). Seeds are only viable between 2 and 4 weeks in 

situ and need exposed mineral soil for bedding (Barry 

and Sachs 1968). Also, temperatures must be above 

freezing and in near-constant moisture to promote 

seed germination and facilitate seedling establishment 

through the first full growing season (McDonough 
32 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

Figure 3-16. In this exceptional situation, an aspen seedling 

takes advantage of moisture availability and lack of competition to establish, if only temporarily, in a street gutter.

1979) (fig. 3-16). Since seedling growth is dependent 

on minimum competition from other vegetation, recently burned surfaces facilitate this requirement. Some 

authors in the western U.S. feel that these requirements 

are rare enough that seedling “events” introducing 

new genes into aspen populations may occur only at 

a given locale on a scale of hundreds (Barnes 1966; 

Romme and others 1997) or even thousands of years 

(Strain 1964). In the rare case where quaking aspen 

seeds do germinate, herbivory from large ungulates 

can severely impact the newly established seedlings 

(Turner and others 2003). While occasional aspen 

seedlings do establish, the events are so rare that we 

cannot depend upon seedling establishment to manage 

aspen in the western U.S.

Clonal Intermixing

It is a common perception that individual aspen 

clones dominate specific sites, and where clones come in 

contact with each other, they can be easily distinguished 

by phenotypic characteristics, such as timing and color 

of spring leaf out and autumn senescence (Baker 1925; 

Jones and DeByle 1985a; Miller 1996). However, 

studies have shown that intermixing of clones on the 

landscape is common (Barnes 1966) and that relying on 

morphological characteristics alone may be misleading 

(Mitton and Grant 1980). In the Sierra Nevada, Hipkins 

and Kitzmiller (2004) used genetic information to 

identify the number of genotypes per stand, assess levels 

of genetic diversity, and detect geographic patterns of 

variation. They found stands in close proximity can 

be genetically quite distinct, with nearly 50 percent of 

individual west slope stands on the Eldorado National 

Forest each containing only a single genotype.

These monoclonal stands in the Sierra Nevada tend 

to be smaller in average size (0.8 acres [0.32 ha]) than 

multiclonal stands (3.1 acres [1.25 ha]) (Hipkins and 

Kitzmiller 2004). All clone sizes in this study were 

more similar to those in the eastern U.S. than in the 

Interior West (Barnes 1966; Miller 1996). For comparison, Interior West clones sizes ranging up to 200 

acres (80.94 ha) (based on phenotypic characteristics) 

were found to be common in south-central Utah. It has 

been hypothesized that clone sizes in semi-arid regions 

are larger because seed germination is rare, thereby 

promoting longer lasting and larger clones bolstered 

by vegetative sprouting following repeated disturbance 

(Kemperman and Barnes 1976). If this hypothesis 

holds, then a possible explanation for smaller Sierra 

Nevada aspen stands may be their general proximity 

to favorable growing conditions. Alternatively, small 

stand size in this region may be due to a lack of recent 

disturbance, or the compromise notion that a paucity of 

disturbance has forced aspen stands to contract toward 

the relative stability provided by increased soil moisture 

availability. Regardless of mechanism, the monoclonal 

nature of stands in conjunction with the spread of some 

genotypes between stands suggests a broader, more 

extensive occurrence of aspen in the Sierra Nevada in 

the past than what we find now.

Hipkins and Kitzmiller (2004) also found a very 

high level of genetic variability throughout their Sierra 

Nevada study area. Overall, they found 82 percent 

genetic variation as measured by polymorphic loci. 

This same high level of genetic variation has been 

confirmed in Canada (Cheliak and Dancik 1982; Jelinski 

and Cheliak 1992) and has led researchers to proclaim 

aspen one of the most genetically diverse plant species 

(Mitton and Grant 1996). Using the same method, 

enzyme electrophoresis, Hipkins (Personal Comm., 

Geneticist, USDA Forest Service, National Forest 

Genetics Laboratory) found 88 percent polymorphic 

loci in a similar unpublished study conducted in eastern 

Oregon. Multiple clones were documented in many 

stands in both the Oregon and California studies (fig. 

3-17). In Oregon, results suggest that aspen stands in 

one drainage were on average small, monoclonal, and 

less diverse—in terms of genetics and male/female 

clone ratios—while the opposite was true in another 

watershed. She hypothesized that greater numbers of 
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 33

Figure 3-17. Often there may be multiple aspen clones within 

a contiguous stand. Here, separate clones can easily be 

distinguished by their early summer leaf development 

and color.

disturbances over long periods in the latter watershed 

created more seedling opportunities and therefore greater 

genetic diversity. Likewise, greater equality in clone sex 

ratios affords more and better chances of viable seed 

crops following disturbance (Personal Comm., Valerie 

Hipkins, Geneticist, USDA Forest Service, National 

Forest Genetics Laboratory). The sexual distribution of 

clones was not established in the Sierra Nevada genetic 

study. However, male and female clones were identified 

in a separate study on the Eldorado National Forest on 

the Western Slope of the Sierra Nevada (Burton 2004b). 

These findings establish sexual duality in aspen on the 

western slope of the Sierra Nevada, contrary to the 

postulations of previous researchers that only male 

clones were present here (Hutchinson and Stebbins 

1986; Johnston 1994).

In Quebec, Wyman and others (2003) used a newer 

technique, microsatellite (loci) analysis, to examine 

clonal intermixing in stands. They discovered that 

suckers from different genotypes are likely to be highly 

intermixed after disturbance with ramets of different 

genotypes located within 6 to 9 ft (2 to 3 m) of a central 

trunk. In their words, “No clear relationship was found 

between the mean distance of the potential ramet to 

the central trunk and whether or not the central trunk 

and ramet were the same genotype.” They conclude 

that it was not possible to distinguish between clones 

based purely on morphological features at their study 

site. However, they also found more genetic variability within stands than between stands, suggesting a 

very different stand make up in eastern forests versus 

western forests (Wyman and others 2003; Hipkins and 

Kitzmiller 2004).

A byproduct of clonal intermixing is a potential for 

natural root grafting. Barnes (1966) found only a single 

root grafted to another ramet in his study of clonal root 

systems in Michigan. Though he did acknowledge 

intergrowth of clones following disturbance, he apparently did not give consideration to the possibility 

of intermixed clones grafting. However, DesRochers 

and Leiffers (2001) found a much higher rate of root 

grafting, most commonly directly under the stems of 

both live and dead mature aspen. Their main conclusions 

focus on the long-term health of clones maintained by 

adoption of established root systems of mature ramets 

within a clone; however, there could be implications 

for genetic fitness in combination with root-associated 

physiological adaptations among intermixed clones 

that graft.

Long-Term Heterozygosity

Genetic diversity, or heterozygosity, can be viewed at 

the broadest scale as a measure of aspen’s reproductive 

health. Though many aspen stands are made up of single 

clones, we have seen that there is considerable genetic 

diversity within local populations and, at least in some 

places, within stands. This high level of heterozygosity seems counter-intuitive in a species so dependent 

on vegetative reproduction from clones. Two known 

mechanisms may account for high levels of genetic 

diversity in aspen: clonal mutation and rare seedling 

establishment.

Somatic mutations occur when random “mistakes” 

happen in the creation of DNA in the process of vegetative reproduction. These chance happenings that 

result in favorable morphological traits are “selected 

for” in the evolutionary process of adaptation to local 

environments. A prerequisite for successful somatic 

mutation in long-lived species like quaking aspen 

is that clones be relatively stable on a landscape for 

many generations—commonly estimated to be several 

thousand years or since the last glacial epoch (Baker 

1925; Strain 1964; Barnes 1966; Jelinski and Cheliak 

1992; Mitton and Grant 1996). Some authors have 

further hypothesized that dry climatic conditions in the 

West are more conducive to clonal permanence (on the 

landscape) and therefore somatic mutation, whereas 

genetic variability in eastern North American forests is 

more reliant on greater frequency of successful seedling 

events (Kemperman and Barnes 1976; Jelinski and 
34 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

Cheliak 1992; Mitton and Grant 1996). Interestingly, 

both asexual and sexual genetic variances are dependent 

on regular disturbance.

As previously established, successful seedling events, 

notably in western locations, are rare due to very specific conditions required for germination (Maini 1968; 

McDonough 1979). We found no reports of seedling 

establishment in the Sierra Nevada. However, recent 

documentation of large-scale seedling establishment 

following the Yellowstone National Park fires of 1988 

questions the notion that sexual reproduction plays a 

limited role in genetic diversity of quaking aspen (Kay 

1993; Hargrove 1993; Tuskan and others 1996; Romme 

and others 1997; Turner and others 2003). In Alberta, 

Jelinski and Cheliak (1992) found genetic variability 

high among aspen and pointed to mutation as the most 

likely contemporary explanation. Even though they 

have no immediate evidence of seedling establishment, Jelinski and Cheliak speculate that uncommon 

“windows of opportunity” are a likely explanation for 

high rates of genetic variability in their populations. 

Yellowstone researchers take this proposition a step 

further with clear evidence of increased genetic variability after the landmark seedling establishments in 

those environments (Tuskan and others 1996; Stevens 

and others 1999; Romme and Turner 2004). However, 

these authors caution that long-term survival of the 

recent seedling crop will be crucial and they have thus 

far been monitoring new gamets for about 15 years. 

Romme and Turner (2004) have reported large-scale 

losses of new seedlings due to elk herbivory, but they and 

Ripple and Larsen (2001) note that “islands” of protected 

seedlings occur when downed wood, predominantly 

falling dead trees from the wildfire, act as natural barriers to ungulates. In addition to increased population 

heterozygosity, Yellowstone studies have demonstrated 

clear range expansions into areas previously unoccupied 

by aspen (Romme and others 1997; Romme and Turner 

2004). These findings could point to a larger regional or 

continental model for long-term genetic variability and 

fitness, as well as a punctuated equilibrium explanation 

for range expansion. We may yet witness “windows of 

opportunity” in aspen genetic variance, depending on 

survivorship in the Yellowstone situation, and possible 

new seedling events elsewhere under a warmer climate 

and increased large-scale fire scenario.

What might we expect with changing climates 

in terms of aspen regeneration generally and sexual 

reproduction (genetic fitness) more specifically? In 

general, we have seen that over the millennia, aspen 

may be one of the most highly adapted tree species 

in North America. This may be shown by its genetic 

variability, the large size and grand age of some regional 

clones, or its wide habitat and climate range across the 

continent (Barnes 1966; Kemperman and Barnes 1976; 

Jones 1985b; Lieffers and others 2001). Westfall and 

Millar (2004) discuss how tree species have adapted to 

climatic shifts in the past. Though they do not discuss 

aspen specifically, they do reflect on survival strategies 

during shifting climatic epochs. Foremost among these 

characteristics is high genetic variability to cope with 

what they see as inevitable climatic shifts. They state 

that “greater genetic variation results in a higher proportion of individuals that are adapted to the changed 

environment.” However, the models they used were 

highly dependent on seed dispersal as a mechanism 

for adaptation. In mountainous landscapes, shifts in a 

species’ range can be elevational as well as latitudinal. 

Of course, with shifts in temperature and precipitation 

there are accompanying changes in disturbance frequencies. For example, cooler and wetter periods accompany 

longer cycles, but more intense fire events (Pierce and 

others 2004). If rare seedling events are brought on by 

epoch climatic shifts accompanied by large disturbance 

(cool/moist scenario), then aspen seems well positioned 

to move over long distances with its small seeds to 

colonize favorable environments, provided adequate 

seed beds and moisture are available (Romme and 

others 1997). We speculate that where more frequent 

disturbance dominates (warm/dry scenario), we might 

expect short-cycle disturbance regimes that are more 

favorable to clonal reproduction, mutation, and creeping 

migration. In either case, models presented by Westfall 

and Millar (2004) emphasize “sufficient plasticity” 

in species in terms of genetic variation and dispersal 

mechanisms as the key to long-term population survival. 

These are the same traits that have been reviewed here 

for quaking aspen.

Frontiers in Aspen Genetics

Investigations in aspen genetics have used a range 

of established and developing techniques. In the past 

10 to 15 years, new molecular techniques have been 

made available that increase the resolution of genetic 

studies (Parker and others 1998). The most commonly 

used method for aspen studies has been enzyme 

electrophoresis (Jelinski and Cheliak 1992; Hipkins 

and Kitzmiller 2004). Advantages of this technique 

include low cost, relative simplicity, and dependability 

in discerning degrees of polymorphism.

New techniques revolve around different ways 

of examining DNA tissue at higher resolution than 
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 35

established enzyme analysis. Though more expensive, 

these approaches give a more detailed “DNA fingerprint” of individual sections of the much larger genetic 

sequence of an organism. For example, Tuskan and others (1996) used a procedure called randomly amplified 

polymorphic DNA (RAPD) to isolate desired sections 

of the aspen genome, thereby eliminating nonheritable 

segments of the genome. In essence, RAPD allows 

researchers to focus more quickly on specific sequences 

with the DNA structure that distinguish clones.

Wyman and others (2003) were most interested 

in the clonal intermixing question addressed above. 

They worked with microsatellite DNA specifically 

because of its strong applications in population studies. Microsatellite genetics is an especially rewarding 

technique where all or most of a particular organism’s 

total genetic make-up, or genome, can be catalogued. 

Fortunately for aspen researchers, the Populus genome was released for public use in September of 

2004 (International Populus Genome Consortium 

2005). Published work has already resulted from this 

breakthrough (Moreau and others 2005) and geneticists will likely gain considerable mileage from the 

Populus genome as it relates specifically to aspen—the 

most widely distributed tree of this genus—using a 

broader variety of DNA sequencing techniques and 

applications.

Recent work using this technique on European aspen 

(Populus tremula) in Finland was able to demonstrate 

that very small clone sizes (average 2.3 ramets per 

clone) appear to be the rule, rather than the exception, in this region (Suvanto and Latva-Karjanmaa 

2005). Microsatellites allow investigators to more 

easily discern up to 30 to 50 alleles at a given DNA 

loci of interest, making positive individual identifications within populations possible and allowing for 

possibilities in tracing clonal heredity (Parker and 

others 1998).

In summary, aspen genetics provide many avenues 

for study and conservation of the species. We have seen 

how a variety of methods have been used to investigate 

population heterozygosity and clonal intermixing. 

Aspen genetics studies may also aid us in understanding 

explanatory mechanisms for the species’ great adaptability to varying environments, as well as its high rate 

of polymorphism. While geneticists and ecologists are 

still unraveling the role of rare seedling events in longterm species genetics, recent developments in DNA 

sequencing techniques have made investigation of finescale population heredity possible. The interdisciplinary 

nature of this final point should not be overlooked; 

genetic knowledge should not be viewed in isolation 

from basic ecology. If addressed jointly, genetics and 

ecology provide a powerful analytical approach to 

understanding and managing aspen.

From a local conservation perspective, further 

research is needed to increase understanding of rangewide and stand-level diversity of aspen. Depending on 

the outcome of further investigation, we may find that 

population isolation in the Sierra Nevada has led to a 

limited genetic resource in aspen. In this case, greater 

effort will need to be placed on a strict conservation 

course of action. However, if we find that genetic diversity is strong, then we can feel more confident in aspen’s 

local adaptability to changing climates. Of course, the 

unknown in these speculations is the amount or probability of future seedling events. Thus far, there has not 

been documentation of recent genotype establishment 

in the Sierra Nevada. Whether reproducing from seed 

or suckering, aspen still must have adequate moisture, 

deep soils, and relatively unhindered disturbance 

regimes. In the absence of these basic requirements, it 

will be difficult to maintain even the small populations 

that currently exist in our area of interest.

Plant Associations__________

Vegetation Classification 

in California

On a statewide scale, California’s vegetation is very 

diverse, from desert to coastal scrub to rainforest to 

alpine. The size of the state and diversity of plant types 

is not conducive to mapping (or even classifying) aspen 

forests at this scale. Aspen is considered a “minor” 

hardwood forest type with about 1 percent of the total 

forest cover in the state (Bolsinger 1988). Previous classification efforts in California illustrate the difficulty of 

elevating an uncommon species to regional prominence 

via systematic land typing at a large scale.

Mapping and habitat classification efforts in the state 

have been piecemeal (regional) or incomplete through 

the mid 20th century (Sawyer and Keeler-Wolf 1995). 

Wieslander (1946) produced a first approximation 

statewide vegetation map based on USDA Forest Service 

ground observations that only covered about half the 

state’s area. A complementary vegetation classification 

system was developed later by Jensen (1947), but was not 

comprehensive for the entire state. In the 1970’s, Barbour 

and Major (1977) developed a statewide compilation of 

physiognomic types centered largely on vegetation types 
36 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

Figure 3-18. The quaking 

aspen/mountain pennyroyal (Monardella 

odoratissima) association is usually found in 

drier or more upland 

locations than the aspen association with 

corn lily (Fig. 3-19). 

These stands will have 

more available subsurface moisture than the 

surrounding general 

forested landscape.

and based somewhat on work done by Barry (1971). 

These authors further divided aspen forests into nine 

distinct “habitats,”which included: sagebrush scrub, 

Jeffrey pine woodland, northern juniper woodlands, 

red fir forest, lodgepole pine forest, subalpine forest, 

mixed conifer forest, ponderosa pine forest, and montane chaparral (Barry 1971; Barbour and Major 1977). 

Unfortunately, this work did not go beyond mention 

of these types, and did not describe physical traits 

separating one type from the next. During this period, 

other regions were moving ahead with habitat typing 

and plant association guides such as those developed by 

Daubenmire and Daubenmire (1968). California later 

developed a systematic manual for forest classification 

(Sawyer and Keeler-Wolf 1995) following numerous 

publications of the previous decade centered on the 

concept of potential natural vegetation (Driscoll and 

others 1984; Allen 1987; Hall 1988; Kauffman 1990; 

USDA Forest Service 1991).

In the past two decades, vegetation mapping and 

classification have been enhanced by remote sensing 

technology. California implemented a Gap Analysis 

Project (GAP) habitat mapping system in the early 

1990s. This study found that 89 percent of all aspen in 

the Sierra Nevada are on National Forest System land, 

with the next largest landowner being “private” with 

6 percent of the aspen stands (Davis and Stoms 1996). 

However, the GAP likely missed many small aspen 

stands (and other infrequent vegetation types) because 

of the coarse resolution (minimum 240 acres [97.1 ha]) 

of this database (Sawyer and Keeler-Wolf 1995). Using 

classification and mapping techniques, Sawyer and 

Keeler-Wolf produced a statewide vegetation manual. 

At this scale, these authors describe a single aspen 

“series” for California that is a co-dominant with red 

fir and/or white fir in the forest canopy. Perhaps, in the 

end, we are left with too small of a vegetation type to 

be adequately represented at a statewide-scale.

Aspen Vegetation Types

Greater progress has been made toward classifying 

aspen as a distinct type, or types, in smaller geographic 

regions, with a more pointed focus on forest vegetation. 

In the Sierra Nevada, the most detailed classification 

of montane forests provides two aspen associations 

or “potential natural communities”—quaking aspen/

mountain pennyroyal (Monardella odoratissima) (fig. 

3-18) and quaking aspen/California corn lily (Veratrum 

Californicum) (fig. 3-19) (Potter 1998). The most 

striking commonalities among aspen associations in 

the montane zone are that they are mostly less than 5 

acres (2.02 ha), seem to have the deepest and richest 

soils of Sierra Nevada forests, and as the stands increase 

in age they are increasingly invaded by shade-tolerant 

conifers. Both described types are found in moderate to 

very moist sites in the red fir belt, with the mountain 

pennyroyal association representing a slightly dryer and 

upland group, and the California corn lily association 
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 37

Figure 3-19. California corn lily (Veratrum californicum) is a 

large understory forb common in wetter aspen stands of 

the Sierra Nevada.

representing a wetter riparian, near-riparian, and wet 

meadow fringe group. The distinction between these 

two associations is slight enough that the California 

corn lily is found on mountain pennyroyal sites, but 

pennyroyal is not found on corn lily sites. Other 

characteristics of these sites are that they are on low 

slope angles (less than 25 percent), have deep water 

retaining soils, usually Inceptisols, and are relatively 

high in plant diversity. Aspen communities in the 

upper montane zone are second in species richness to 

the very diverse western juniper/sagebrush (Juniperus 

occidentalis/Artemisia spp.) association (Potter 1998). 

Interestingly, Potter does not use quaking aspen as a 

plant associate in either the understory or overstory of 

the remaining 24 forest communities described.

Much of the aspen in the Sierra Nevada is commonly 

associated with riparian and meadow communities. 

This seems to be the case for west slope and montane 

forests, but may be less true on the east slope of the 

Sierra Nevada and in the Southern Cascades and 

Modoc Plateau. A recent classification for eastside 

riparian zones features two distinct aspen types in 

a total of 16 “ecological types” (Weixelman and 

others 1999; Kay 2001a). These authors place aspen in 

riparian communities in the higher elevation “eastside 

mixed conifer forest,” the somewhat drier “yellow 

[ponderosa/Jeffrey] pine forest,” and in narrow strips 

along with black cottonwood (Populus trichocarpa) in 

the lowest and driest “Reno floristic section” (although 

they do allude to distinct “hillslope aspen” types that 

join some of their riparian types). The wetter of the 

two types, aspen/mesic graminoid, appears to be different from either of Potter’s (1998) associations in 

terms of the plant community described. This type is 

characterized by coarser soil sediments, moderately 

deep soil profile, an “at field capacity” water table, and 

low slope angles (6 to 7 percent). The highest percent 

constancies on these sites were in graminoids, while 

California corn lily displayed only a moderate presence (20 percent of plots). The second type, aspen/tall 

forb, had deeper soils (also with coarse stream bed 

sediments), a dark/rich surface horizon, and barely 

higher slope angles (8 to 9 percent). Weixelman and 

others (1999) noted slightly drier conditions on these 

types with lower water tables that were adjacent to 

incised stream channels. Aspen/tall forb ecological 

types are more vulnerable to conifer invasion, in their 

estimation, because of their record of past human 

disturbance and suppression of wildfires. In sum, they 

describe two types that are similar to Potter’s (1998) 

plant associations (that is, both Potter indicator species 

are present, but only at moderate levels), but they also 

key in on some distinctions, such as the high presence 

of graminoids on wetter plots and alternate forbs and 

shrubs on tall forb plots.

Both the Modoc Plateau and the Southern Cascades 

aspen forests are less dependent on wetland habitat 

and thus may be considered more similar to aspen 

community types found in the Interior West (Mueggler 

1988). One effort to address ecological communities 

on the Modoc National Forest features an “aspen 

moisture regime” within a Land Type Associations 

(LTA) system that classifies all lands on the National 

Forest (Smith and Davidson 2003). This Forest considers aspen an “emphasized forest type,” meaning its 

managers should be cognizant of, and manage for, 

aspen communities within any LTA where it occurs. 

Though this is not technically an aspen classification 

system, they do distinguish aspen types as being both 

riparian and upland types, with the qualification that 

“aspen most often occurs in azonally moist areas.” 

As in many dry Interior West forests, where aspen is 

common on non-riparian sites, aspen on the Modoc 

Plateau can be found in relatively moist climatic and 

topographic pockets on lower slope angles.
38 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

Models for Community Typing 

in the Sierra Nevada

A model for aspen forest classification in the Sierra 

Nevada region may be found in Mueggler’s (1988) work 

in the Interior West. Certainly the plateaus and ranges of 

the Modoc Plateau have similarities to Rocky Mountain, 

or Great Basin and Range highlands, where aspen is 

a vital component of forested landscapes. Mueggler’s 

Aspen Community Types of the Intermountain Region

includes eight “cover types” and 56 “community 

types” for Nevada, Utah, southern Idaho, and western 

Wyoming. Fourteen of Mueggler’s community types 

describe two-thirds of all stands sampled—these he 

later calls “major community types.” For those familiar 

with “habitat types” (vegetation communities with an 

assumed successional course toward specific climax 

forest cover), Mueggler is cautious to distinguish 

aspen community types as being more temporary and 

transitory. As a mostly seral species, aspen community 

types are viewed as distinct at any point in time, but 

usually on a path toward some other climax tree cover. 

In his words, “Community types are what the manager 

actually sees in the field.”

At least one of Mueggler’s habitat types, P. tremuloides/Veratrum californicum, resembles a prominent 

type described by Potter (1998) and perhaps Weixelman 

and others (1999), in that the understory species is 

common, along with deep saturated soils in this type. 

Though this habitat type is relatively uncommon for 

the Interior West, it may provide a point of departure, 

where such stands appear common in the Sierra Nevada, 

for an aspen based community typing system in this 

region.

It is likely that further development, with a focus 

on aspen communities rather than all montane forests 

or all riparian ecological types, would yield a more 

refined aspen classification system for this region. The 

earlier work of Barry (1971) may provide a model for 

a detailed aspen classification in the Sierra Nevada. 

He reviewed aspen communities with a cursory listing 

of plant associates for each major cover type along an 

east-west transect of the range. Though the emphasis 

of this work was not a classification system, it does 

provide a framework, with aspen as the focal point, for 

better species and community management.

Vegetation classification systems that look at the 

total forested landscape may be overly focused on 

contemporary aspen coverage. It may be that contemporary aspen forests have retrenched to moister 

physiographic zones as a result of a century of grazing, 

fire suppression, and advancing conifer succession 

(see Chapter 2: Historical Disturbance Ecology). We 

believe that an alternative approach would view any 

occurrence of aspen on the landscape today as being a 

potentially viable aspen community with the addition 

of sucker inducing disturbance (Bartos and Campbell 

1998), provision for a proper growth environment, and 

protection of resulting suckers from browsing animals 

(Shepperd 2001; Rogers 2002; Shepperd 2004). This 

approach, in combination with a “community types” 

or “plant association” classification, would strive to 

delineate both current and potential aspen (those with 

advanced conifer succession) plant communities.

Terrestrial Biota____________

Evaluating Diversity in Aspen Systems

In the early to mid-20th century, forest managers 

were not overtly concerned with vegetation diversity 

and considered wildlife management, a field in its 

infancy at the time, to be primarily the regulation and 

maintenance of game species. A key element in this 

period of forest management history was the principle 

of “scientific management.” That is, having the ability 

to calculate outcomes of management actions in natural 

systems with a high degree of certainty (Zimmerer 

1994; Hirt 1994). An exemplar of scientific management during this era was the same Stuart Bevier 

Show, discussed earlier, who promoted formalization 

of fire suppression in the U.S. Forest Service in the 

early 20th century. Show and colleagues (Show and 

others 1947) addressed aspen as wildlife habitat in 

their 1940s handbook in this way:

Aspen—Though limited in area this type is 

highly productive of food, furnishes good cover or 

is associated with good food and cover types. Can 

easily become a problem with high populations 

of big game animals. Is assigned to the Canadian 

Life Zone although it may follow water courses 

into the Transition [Zone]. Summer water supplies are abundant. Is in the belt of heavy snow. 

Most species using the type are migrant, however 

it is the natural home of the beaver.

Aside from placing beaver firmly in aspen terrain, a 

subject we will return to later, these authors generally 

regard aspen forests as favorable, though transitory to 

most wildlife. Though they are unclear on the “problem” with “high populations of big game animals,” it is 
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 39

Table 3-1. Locally extinct wildlife of the Lake Tahoe basin.

Group	 Common name	 Scientific name

Birds

Canyon wren Catherpes mexicanus

Peregrine falcon Falco peregrinus

Lewis’s woodpecker Melanerpes lewis

Savannah sparrow Passerculus sandwichensis

Mammals

White tailed hare Lepus townsedii

Wolverine Gulo gulo

Heather vole Phenacomys intermedius

Canyon mouse Permyscus crinitus

Mountain sheep Ovis canadensis californiana

Sierra nevada red fox Vulpes vulpes necator

Grizzly bear Ursus artos

Amphibians

Northern leopard frog* Rana pipiens

Fish

Four exotic species have gone locally extinct in the past 

three decades (Lake whitefish, Arctic grayling, Atlantic

salmon, Chinook salmon).

* It is unknown at this time whether this is a true native species.

Source: Schlesinger, Matthew D. and Romsos, Shane J. 2000. Appendix G: Vertebrate 

species of the Lake Tahoe Basin. Murphy, Dennis D. and Knopp, Christopher M., editors. Lake Tahoe Watershed Assessment: Volume II. Appendices. Albany, CA: USDA 

Forest Service, Pacific Southwest Research Station; pp. G1-15.

assumed they are suggesting that limited aspen cover can 

easily be overloaded, thus leading to habitat damage.

The wildlife handbook goes on to promote the 

“working circle” (a period term referring to actively 

managing for all successional stages) as the primary 

tool for increasing productivity of both wildlife and 

domestic livestock. They summarize their advocacy 

thus: “The total capacity of a managed working circle of 

forest land for both wildlife and livestock is far higher 

than for undisturbed virgin forest.” Though the main 

focus of the handbook is on game and fish species, the 

authors do associate aspen with three species: beaver, 

sagehen (also sage-grouse, Centrocerus urophasianus), 

and mink (Mustela vison). We will expand this limited 

treatment of aspen’s terrestrial biota to include both 

plants and animals dependent upon aspen forests (see 

Aspen-related wildlife and Plant diversity in aspen 

forests, below).

Before we discuss aspen related species, it is important to understand the overall biological diversity 

in our area. An example of diversity assessment within 

the region was recently completed for the Lake Tahoe 

basin (Manley and others 2000). This large research 

effort yielded the following category totals for species 

currently in the basin (as opposed to historic sightings of 

transitory individuals): 312 vertebrate species consisting 

of 217 bird, 59 mammal, five amphibian, eight reptile, 

23 fish; 1,308 vascular plants; 115 non-vascular plants 

estimated; 810 invertebrates; and 573 fungi and lichens. 

Perhaps more telling is the assessment of wildlife species 

that have become locally extinct since Euro-American 

settlement (table 3-1). In addition to this short list of 

extirpated wildlife, we may safely presume that some 

native vascular plants, non-vascular plants, invertebrates, and fungi and lichens have been eliminated due 

to both human and natural causes in association with 

ubiquitous 19th century resource extraction and 20th 

century land development in the Sierra Nevada. In this 

same period, numerous species have been introduced 

to this region as well (see Chapter 4: Invasive Species 

and Aspen Communities).

There are certain aspen-related focal species that either directly or indirectly enhance regional biodiversity. 

Alteration of aspen forests may have cascading trophic 

effects on these and (likely) not yet documented plant 

and animal species. In addition to the idea of specific 

species dependence, we will address the concept of 

greater plant diversity in aspen communities. Both of 

these themes should be further considered in light of 

changes in aspen extent at the regional level.
40 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

Figure 3-20. Sapsucker (Sphyrapicus spp.) wounds often 

scar aspen bark in a distinctly horizontal pattern. Excessive 

sapsucker bark penetration may encircle the stem thereby 

girdling and killing a tree.

Aspen-Related Wildlife

For the western U.S., Flack (1976) and DeByle 

(1985c) provide good overviews of bird species 

that specialize in aspen habitats (fig. 3-20). Flack 

conducted a systematic survey of aspen/bird habitat 

in both western Canada and the U.S. Across this vast 

region he looked at pure and near-pure aspen stands 

for patterns associated with forest structure and bird 

species diversity. Fortunately, Flack’s dataset includes 

one plot at Monitor Pass just south of Lake Tahoe. 

The most common birds found at this aspen site were 

warbling vireo (Vireo gilvus), Empidonax flycatcher 

(Empidonax spp.), house wren (Troglodytes troglodytes), and Oregon junco (Junco hyemalis thuberi). In 

fact, the Warbling vireo is a common denominator in 

aspen forests throughout western North American as 

evidenced by Flack (1976) and others (McGraw 1986; 

Turchi and others 1995; Matson 2000; Borgmann and 

Morrison 2004). Heath and Ballard (2003), working in 

the eastern Sierra Nevada, found that warbling vireos 

are highly associated with aspen, which indirectly 

suggests that recent declines in warbling vireo may 

be associated with concurrent reductions in aspen 

extent reported by Bartos and Campbell (1998) and 

Di Orio and others (2005). Matson’s (2000) work in 

Wyoming suggests that warbling vireo, along with 

Orange-crowned warbler (Vermivora celata), which is 

also found in the Sierra Nevada, be used as “indicator 

species” for aspen habitats.

In terms of aspen stand structure and bird diversity, 

Flack’s (1976) broad regional study found bird species 

abundance decreased with increasing tree density or 

decreasing tree diameter. Put another way, bird diversity 

was greatest in older (larger diameter), more open aspen 

stands. The implication is that stand age enhances bird 

diversity as stands naturally tend to thin over time. 

However, this may not be the case where invading 

conifers begin to change the species composition of 

aspen stands.

In Rocky Mountain National Park in Colorado, Turchi 

and others (1995) established a clear positive relationship between number of bird species and aspen cover; 

fewer species were consistently found in surrounding 

conifer stands. They attribute bird species richness 

in aspen to high understory cover and, in particular, 

increased shrub cover.

Similar results have been found in eastern Sierra 

Nevada aspen forests. Bird species richness and 

abundance increased with lower percent conifer cover, 

increased herbaceous cover, and lower shrub-class aspen 

cover (Richardson and Heath 2004). Shrub-class aspen 

are those trees that are either stunted or young, in either 

case often being located on recently disturbed sites. 

While researchers have found decreased bird diversity 

in shrub-class, greater diversity has been noted in mature 

aspen stands (McGraw 1986), especially where conifer 

invasion was minimal (Richardson and Heath 2004). 

These authors and Verner (1988) describe the following 

specific benefits to bird diversity that are provided by 

aspen in eastern Sierra Nevada forest communities: 1) 

thick herbaceous layer for forage and cover; 2) aspen’s 

susceptibility to heart rot as a benefit to both primary 

and secondary cavity nesters; 3) increased abundance 

and diversity of insects; and 4) the ability of aspen sites 

to remain moist provides a ready water source, for birds 

as well as ensuring more insects as a food source. An 

Arizona study of bird populations, also found greater 

diversity in aspen compared to conifer/aspen and pure 

conifer stands (Griffis-Kyle and Beier 2003). Heath 

and Ballard (2003) reported that habitats dominated 

by aspen and black willow trees support “some of the 

most diverse riparian breeding songbird populations in 

the eastern Sierra Nevada.”
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 41

Figure 3-21. Woodpeckers often use aspen for nesting 

because of its thin bark and propensity for decay, thus 

making cavity excavation easier. This red-breasted sapsucker (Sphyrapicus ruber) is shown just below the nest. 

Fledglings still occupy the nest, despite a recent attempted 

predation by an unidentified mammal that peeled bark

away from the entry hole.

A subgroup of birds that nest in both live and dead 

tree cavities seem to favor aspen over other tree species 

in the West (Flack 1976; DeByle 1985c; McGraw 1986; 

Dobkin and others 1995). Common cavity nesters in our 

region include flickers (Colaptes spp.), woodpeckers 

(Picoides spp. and Melanerpes spp.), chickadees (Parus

spp.), and nuthatches (Sitta spp.) (fig. 3-21). Their affinity for aspen may be due to a generally limited hardwood 

resource in many western forests. Additionally, this 

particular “hardwood” actually has fairly soft wood, a 

thin bark, and a propensity toward various heartwood 

decays in older stems (Baker 1925; Hinds 1985; Rogers 

2002) that allow easier cavity excavation.

Decades of western regional research have documented the symbiotic relationship between cavity nesters 

and stem pathogens in aspen (Shigo and Kilham 1968; 

Flack 1976; DeByle 1985c; McGraw 1986; Dobkin and 

others 1995). In southeastern Oregon, Dobkin and others 

(1995) found that 73 percent of trees with cavities had 

some sort of fungus present on the stem. In addition to 

initial cavity builders and nesters, secondary colonizers 

like owls and sparrows also inhabit aspen cavities. They 

found that cavity nesters prefer dead trees over live trees 

and large trees (> 9.4 inches [23.9 cm] dbh) over small 

trees. A similar study comparing Great Basin with Sierra 

nesting habitat concluded that cavity nesters preferred 

mature stands over younger stands (McGraw 1986). 

Concern has been expressed that lack of large live and 

dead trees can lead to declining habitat for cavity nesters, 

especially where grazing is compounding the problem 

by severely limiting regeneration (Dobkin and others 

1995). In sum, invasion by conifers, a limited supply 

of mature trees, and lack of regeneration may lead to 

declining habitat for several bird species dependent on 

aspen forests.

In addition to breeding birds, a few mammals are 

notable for their aspen affinity. In the Sierra, like most 

of the West, beaver were intensively trapped for their 

fur in the early- to mid-19th century. Trappers, and later 

managers, during the late 19th and early 20th century 

began reintroduction programs to improve population 

viability for the fur trade and ecological restoration. It 

was estimated in 1940 that there were about 680 beaver 

across California, most of these near the Sacramento 

River delta (Tappe 1942). Show and others (1947) 

distinguish between two native and one introduced 

subspecies in the Sierra Nevada:

The native Shasta beaver [Castor canadensis 

shastensis] is found in the Pit and Klamath 

drainages and has recently been transplanted to 

the Walker River. The Canadian beaver [Castor 

canadensis canadensis], introduced from Oregon 

and Idaho, is located in the drainages from the 

Feather River to the Tuolumne. Golden beaver 

[Castor canadensis subauratus], a native of the 

Great Valley, has been planted on the Mendocino, 

Los Padres, Stanislaus and Sierra forests.

According to Tappe (1942) the Shasta beaver occurred 

naturally in the Southern Cascades and Modoc Plateau 

areas. This race was nearly wiped out, but subsequent 

reintroductions appear to have revived populations. 

Native beaver populations in the Sierra Nevada proper 

have not been documented, except for a single footnote 

where Tappe interviewed a range rider who attests to 

beaver sign in the upper Carson River drainage during 

the late 19th century. At this point, most beaver populations in the Sierra Nevada have been introduced or 

reintroduced following extensive historic trapping.

Opinions vary widely on how introduced beaver 

have affected and will continue to affect the natural 

aspen community. Periodic introductions and culling 

of exotic beaver populations have had unknown longterm effects on forest ecosystems, including aspen. It is 

known, however, that beaver have a strong preference 

for aspen as a food source and for dam and lodging 
42 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

Figure 3-22. Beaver 

(Castor spp.) damage to a mixed aspen/conifer forest in 

the Sierra Nevada. 

Trees are killed by 

girdling and toppled 

for den material and 

as a beaver food 

source. There is also 

incidental aspen 

mortality from dam 

water inundation.

material (fig. 3-22). Hall (1960) estimates that beaver 

need about 3 pounds of aspen bark per day to sustain 

themselves and they seem to favor eating smaller saplingsized trees—likely because they are also a good size 

for building material—though they do often take large 

diameter stems. Beaver may completely remove a stand’s 

overstory, similar to a logging clear cut or fire, except 

that most downed stems remain on site. Those branches 

and stems that are hauled away by beaver may be used 

for lodging or dams. Aspen felling and dam building 

may temporarily alter forests drastically; a raised water 

table behind the dam will impede further aspen regeneration. However, following eventual dam breaching, a 

longer term outlook may see the area recolonized first 

by aspen, then by beaver. Weixelman and others (1999) 

feel that beaver may be equally as important to riparian 

aspen stand regulation as fire: “Beaver periodically 

renew aspen stands. As long as beaver populations 

are not too dense, aspen sites recover between periods 

of beaver colonization.” Though cyclic interactions 

of the introduced beaver and natural processes—fire, 

flooding, aspen regeneration and growth—are not well 

understood at this time, local studies on these questions 

have resulted in a pessimistic outlook for beaver and 

aspen interactions (Hall 1960; Beier and Barrett 1987). 

Both of these studies concluded that beaver may lead 

to local aspen extinction if population numbers are not 

closely controlled.

The mountain beaver (not a true beaver [Aplodontia 

rufa]) is a candidate for federal endangered status and 

uses aspen forests for habitat and forage. Beier (1989) 

considers mountain beaver habitat to be marginal in 

the relatively dry Sierra Nevada as compared to the 

Pacific Northwest. In the Sierra Nevada, this species 

is somewhat limited to cool and moist regimes with 

deep soils provided by riparian habitat containing aspen 

and non-aspen woody species (Beier 1989; Todd 1992; 

Carraway and Verts 1993). Apparently, mountain beaver 

feed on conifers in the winter when other species are 

not available. In Oregon, researchers found that a high 

percentage of conifer seedlings were eaten by mountain 

beaver (Carraway and Verts 1993). In the Sierra Nevada, 

Steele (1989) documented use of aspen bark for food 

and found clippings in “haystacks,” a mix of vegetation 

used in nest building.

Mule deer (Odocoileus hemionus) are the largest native ungulate currently on the Sierra Nevada landscape. 

Domestic cattle (Bos taurus spp.) and sheep (see Chapter 

2: Historical Disturbance Ecology), though they prefer 

non-woody forage, have been known to trample suckers, denude their foliage, and promote exotic grasses 

and forbs in overgrazing situations (Kay and Bartos 

2000). The combined effects of deer and cattle on aspen 

communities have been examined by researchers in 

the McCormick Creek basin of the Stanislaus National 

Forest (Loft and others 1987, 1991, 1993). One conclusion of this series of studies is that deer and cattle are 

attracted to the same dense forb communities that aspen 

provide, but deer often avoid these sites when cattle 

are present (Loft and others 1993). The aspen/corn lily 

vegetation type (see Plant Associations, this chapter) was 

more highly used by cattle than adjacent willow types 

because of high quality forage (Loft and others 1987). 

They found deer must spend more of their day feeding 

and less time resting, and that there was significantly 

less hiding cover for fawns, when cattle are competing 
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 43

for the same forage in aspen stands (Loft and others 

1993). As summers progress, deer must expand their 

range further to make up for denuded preferred aspen 

habitat that cattle have been grazing (Loft 1989). In sum, 

these authors felt that the limited aspen cover in their 

study area (4 percent of total area) should be devoted to 

mule deer (and other wildlife) habitat rather than cattle 

grazing since the negative effects of competition with 

livestock were so pronounced on deer and livestock 

could use other forage types (Loft and others 1993).

Where native ungulate herbivory is unchecked by 

predation, animal disease, extreme weather, recent 

disturbance, or wildlife management measures, aspen 

regeneration can be severely limited. Though elk exist 

in small numbers in the Southern Cascades and Modoc 

Plateau, they have not become the dominant consumers 

of aspen sprouts as they are in the Rocky Mountains 

(Baker and others 1997; Kay and Bartos 2000; Romme 

and Turner 2004). Further growth of these herds may 

begin to show increased effects on aspen communities 

in this area, particularly in reducing regeneration. In the 

Greater Yellowstone region, research has demonstrated 

that large herbivores relax their natural predator avoidance behavior when top carnivores are absent, linger 

in open aspen stands, and decimate aspen regeneration 

(Ripple and others 2001). Aside from minor predation 

from mountain lions (Felis concolor) in the Sierra 

Nevada, unimpeded deer, or small elk populations 

farther north, may consume prodigious amounts of 

aspen suckers.

Aspen seedlings or sprouts may be partially protected 

from herbivory where recent disturbance, such as fire 

or aspen felling by beaver or windthrow, has resulted 

in large amounts of downed trees. Downed trees can 

impede localized ungulate travel by presenting natural 

barriers. Where downed trees are not dense enough to 

block travel, large logs can physically hide regenerating 

aspen from herbivores (Barry 1971; Ripple and others 

2001). After the 1988 fires in the Greater Yellowstone 

region, a large pulse of aspen seedlings were discovered on burned sites (Romme and others 1997). Many 

of the burned trees, predominantly lodgepole pine, 

subsequently fell to the forest floor providing protection for new growth of various species, including the 

rare crop of aspen seedlings (Ripple and others 2001). 

However, a trade-off exists between the protection 

offered by downed logs and the effect that their shade 

has on soil temperature and sprouting. Shepperd (1996) 

found that in Colorado, heavy slash loadings greatly 

reduced aspen sucker establishment. A final factor, 

the reintroduction of gray wolves (Canus lupus) to 

theYellowstone ecosystem, has provided an additional 

element of protection for aspen seedlings and sprouts 

in that the threat of predation keeps elk on guard and 

moving so that they do not browse excessively in any 

given area (Ripple and others 2001).

The trophic links in Yellowstone may provide a model 

for a healthy aspen terrestrial biota in the Sierra Nevada 

and elsewhere. Burgeoning populations of introduced or 

native herbivores have the potential to severely affect 

aspen stands, which in turn, will have cascading impacts 

on other species, such as rare breeding birds or diverse 

plant communities. Maintenance of ecosystem function 

and structure will likely enhance the broadest species 

composition (Noss 1990), provided that the ecosystem 

is within its “natural range of variability” (Landres and 

others 1999). Our limited discussion of terrestrial biota 

has thus far explored aspen-fauna interactions. Let’s now 

balance our review by discussing the floral diversity of 

aspen communities.

Plant Diversity in Aspen Forests

Just as aspen is associated with many different 

types of animals, aspen is also associated with certain 

plant species. As previously discussed (see Plant 

Associations, this chapter), aspen is usually a minor 

forest type surrounded by, or included within, drier conifer forests in the Sierra Nevada and surrounding areas 

covered in this work. Therefore, we may view aspen 

communities as oases of plant and animal diversity. 

Several authors cite high levels of plant diversity in 

aspen when compared to surrounding vegetation types 

(Mueggler 1985; Potter 1998; Manley and Schlesinger 

2001; Chong and others 2001). Specifically, Manley 

and Schlesinger (2001) observed greater than 60 plant 

species in riparian zones of the Sierra Nevada where 

at least 10 percent of a stream reach was occupied by 

aspen-cottonwood. Additionally, they found increases 

in rare plant species and even mammal observations 

when cover of aspen-cottonwood increased in riparian 

corridors. In the Interior West, however, Mueggler 

(1985) found plant species richness was high in aspen 

types, but did not find a corresponding association 

of aspen endemics. He observed that most plants in 

aspen forests also occurred in surrounding communities, but adjacent types did not bear all of those plants 

in close proximity, hence aspen’s commonly higher 

diversity level. Potter’s (1998) work reflects a more 

detailed study of plant communities, including aspen, 

in the upper montane zone of the Sierra Nevada. As a 

byproduct of his plant association classification work 

he found that aspen/mountain pennyroyal association 
44 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

had higher vegetative diversity than all of its forest 

associates except those in the western juniper/sagebrush (Juniperus occidentalis/Artemisia spp.) series. 

The higher density of plants, many with larger cover 

values, make aspen appear more diverse, though they 

are merely more lush than the western juniper type. 

Potter’s aspen/California corn lily association had a 

slightly lower species richness than the two associations just mentioned.

Little work has been done specifically on aspen 

and lichen communities, though preliminary evidence 

suggests that aspen, often occurring as a sole hardwood 

among conifers, possesses a unique lichen flora. In their 

brief discussion of aspen and macrolichen interactions 

as part of a larger forest monitoring effort in Colorado, 

McCune and others (1998) refer to the “distinctive lichen 

community” found in “mature to old aspen.” They go 

on to suggest that, “Loss of aspen would affect all of 

the species dependent on it, including the characteristic 

lichen communities.” In Colorado forests, as well as in 

European aspen (Populus tremula), the greatest lichen 

diversity was documented where older aspen were 

mixed with conifers; younger and pure aspen stands do 

not appear to provide a diversity of substrates and local 

moisture (humidity) needed to support an increased 

lichen flora (Hedenås and Ericson 2004).
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 45

Figure 4-1. Stem cankers may have profound effects on 

entire aspen stands. As stands age, they become more 

susceptible to serious decay.

Chapter 4.

Current Forest Issues Related to 

Aspen Communities

Thus far we have examined Sierra Nevada natural 

history, human impacts on the forest component, and 

basic ecology of the aspen community. The intent of this 

review was to lay a foundation for discussing current 

aspen-related issues, monitoring activities, and potential 

management actions. We now turn to an evaluation of 

important issues affecting aspen health in the Sierra 

Nevada, often turning to the Lake Tahoe basin for specific 

examples. Although these issues may be interrelated 

and complex, we will attempt to advance an informed 

discussion and recommend appropriate alternatives. A 

clear understanding of ecological benefits, as well as 

potential cautions, of aspen related issues is necessary 

to make informed decisions about managing aspen in 

specific landscapes or to improve aspen community 

vigor at the regional scale.

Threats to Aspen 

Sustainability______________

Advancing Conifer Succession

An overarching theme in this chapter, and one alluded to throughout this publication, is succession of 

vegetation in Sierra Nevada forests over the past century. 

Aspen ramets are relatively short-lived when compared 

to their coniferous counterparts. Numerous authors have 

documented the disturbance dependent nature of aspen 

forests (Sampson 1916; Baker 1925; Mueggler 1985; 

Jones and DeByle 1985b; Bartos and others 1991; Rogers 

2002). Following forest disturbance, such as a wildfire 

or avalanche, aspen suckers sprout from existing root 

stock to take advantage of open sunlight created by 

clearing of the previous forest. As stands age, the new 

aspen cohort will self-thin over time due to competition 

for limited resources. Eventually, in most aspen forests, 

more shade tolerant tree species will colonize stands. 

As these trees begin to overtop the aspen, a decline in 

growth and concordant onslaught of pathogens typically 

causes a rapid reduction in stand vigor (Baker 1925; 

Hinds 1985; Rogers 2002) (fig. 4-1). Remaining aspen 

may persist if they can maintain some open light in the 

canopy of conifers (Ko 2001) (fig. 4-2). These trees may 

act as root-stock refugia that will eventually sucker anew 
46 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

Figure 4-2. Although aspen is often overtopped by conifers, 

it may persist with low levels of regeneration where ample 

gaps allow sunlight to reach the understory. The photo 

shows young aspen reaching for sunlight around this 

small forest opening.

Figure 4-3. Aspen stands in the Lake Tahoe basin. Stands 

indicated in this map are located on land administered by 

the Lake Tahoe Basin Management Unit, California Conservancy, and California and Nevada State Park Systems. 

As of the printing of this map, the Forest Service has not 

inventoried its lands in the northern part of the Basin. 

Verified aspen locations shown at this scale are generally

aligned with canyon bottoms. No data are available for 

aspen stands located on private lands. Each indication on 

this map represents an individual geo-referenced point. 

The size of stands is not reflected on this map.

with a stand replacing disturbance. In the absence of 

stand replacing (or at least stand opening) disturbance, 

all ramets of the previous generation will eventually die 

out. Viable root stock cannot be maintained without 

at least some living ramets to procure photosynthetic 

energy, so complete loss of aspen from a site may be 

the final outcome of vegetation succession from aspen 

to conifers.

As discussed previously, aspen may also regenerate 

from seed, although this appears to be a rarity in the West, 

perhaps occurring on a scale of centuries at any given 

locale (Kay 1993; Romme and others 1997). Although 

we assume that seedling establishment is possible in 

the Sierra Nevada, since both male and female clones 

exist (Burton 2004b), as yet we are unaware of specific 

documentation of true seedlings. Thus, as a short term 

strategy, re-establishment via natural seeding events 

does not appear to be a reliable option for regional 

maintenance of the species.

Aspen is generally associated with canyon bottoms 

and meadow communities in the Lake Tahoe basin. 

Figure 4-3 presents a preliminary census of aspen stands 

in this basin. At this scale, the association between aspen 

and primary streams is evident. Overall, aspen cover 
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 47

Figure 4-4a,b. Dramatic change over a 130-year period at Spooner Summit: (a) an historic photo from 1876 at height of logging/mining boom during the Comstock era. The voracious need for wood for railroads, mining materials, flumes, housing,

and fuel resulted in denuded slopes and transformed landscapes; (b) this 1992 photograph of the same landscape shows 

ample aspen growth in the riparian zone with conifers upslope. Some invasion of aspen by conifers is already underway. 

©Reproduced by permission (Goin and Blesse 1992).

only a small portion of the basin’s landscape—about 

0.5 percent by recent estimations (Manley and others 

2000). Was this always the case, or are there particular 

human-induced circumstances that have led to the current vegetative configuration? One way to examine this 

question is through the historical record of disturbance, 

which we discussed previously (see Chapter 2: Historical 

Disturbance Ecology).

Comparison of historic and recent photographs is 

another method of documenting landscape change over 

time. A few researchers have used historic photos in the 

Interior West to document change in aspen communities (Gruell 1983; Manier and Laven 2002; Kay 2003). 

The photographic record in the Sierra Nevada seems 

to bear out historic and climatic evidence, as reflected 

in vegetative cover, of intense disturbance in the 19th 

century being followed by advancing succession of 

shade-tolerant trees in the 20th century. Figure 4-4 

consists of a photo pair dated 1876 (a) and 1992 (b). 

In the historic photo you can see the immense impact 

that the mining era had on the Carson Range, and specifically at Spooner Summit where forested hillsides 

were denuded to supply logging trains traveling east 

to Virginia City area mines (Strong 1984). Drainage 

bottoms, where aspen exist today (fig. 4-4b) were 

impacted most by the construction of rail lines, wagon 

and horse trails, small houses, and loading platforms. 

All of this activity virtually wiped out the native tree 

cover on the summit. The modern photo shows aspen 

clones occupying moister and deeper soils, with a century 

of conifer growth dominating the higher slopes. Some 

conifers can be seen penetrating through the aspen, 

signifying a gradual succession to conifers. Signs of 

past development (rail tracks, homes, logging flumes) 

are absent from this more recent landscape view. The 

assertion by Potter (1998) that contemporary aspen 

began their development following land clearing by 

post-settlement industrialists is evident in these photos. 

Human activities in the area today, including fence and 

power lines, a picnic area, and adjoining trails, continue 

to shape contemporary forest development, although 

in more subtle ways.

Aspen stands shown near Conway Summit (fig. 

4-5a and b) present a very different view of advancing 

succession. In this instance, near-pure aspen stands in a 

non-forest matrix have proliferated and expanded during 

this 80-year period. Since natural fire is very limited at 

this high elevation, we presume that previous decades 

of intense livestock grazing kept aspen regeneration 

at bay in the 1906 photo. Aspen is not readily visible 

in the 1906 photo that was apparently taken in late 

spring prior to leaf-out, but aspen cover seems to be 

limited in stature and located primarily within drainage 

bottoms. The modern photo of the same area shows a 

more vibrant aspen community, apparently thriving in 

upland conditions (foreground).

Contemporary photos around the Lake Tahoe basin 

depict primarily riparian aspen with conifers invading 

the edges of aspen stands. Figure 4-6 shows 20- to 30-

year old white fir beginning to shade out a much older 

a b
48 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

Figure 4-5a,b. Photo pair taken at Conway Summit depicts a different type of change from our previous example (figure 4-4a,b),

although advancing succession in this instance resulted in greater aspen cover where historic overgrazing and related fires

kept regeneration in check. The historic photo (a), taken in 1906, shows limited aspen cover confined predominantly to the

drainage bottoms, while the modern photo (b), from 1998, illustrates substantial growth of woody plants, mostly aspen, with 

the removal of livestock over recent decades. Source: (1906) W.D. Johnson, Conway Summit-Virginia Creek, Mono County, 

California. Photo #666, Geological Survey Photography Library, Denver, Colorado. ©(1998) George E. Gruell, Carson City, 

Nevada.

a

b
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 49

Figure 4-6. This small aspen 

stand at the edge of a meadow 

in Blackwood Canyon is being 

actively invaded by white fir

(Abies concolor). Ample light 

under mature aspen is encouraging some regeneration of 

aspen, but little regeneration 

is occurring under the heavily shaded fir portion of this

stand.

aspen stand. When taking this photo, we noted that 

regeneration was plentiful under mature aspen, but no 

aspen sprouts were evident where fir was dominant.

Limited Aspen Regeneration

Advancing succession presents only one part of the 

aspen story in the Sierra Nevada. Not only do conifers 

shade out mature aspen trees, but they severely limit 

the possibility of aspen suckering. Overstory clearing, 

whether in small gaps or in large openings, provides 

the needed light for suckers to proliferate. A recent 

study of aspen regeneration in the Lake Tahoe Basin 

suggests that aspen is not declining overall because of 

a high rate of small-scale gap filling by seedlings and 

saplings (Ko 2001). However, this study apparently 

focused on stands where aspen comprised the dominant 

overstory and not on locations where remnant aspen 

trees are now outnumbered by a conifer canopy. Though 

gap replacement may engender small-scale population 

stability, it will not curtail the larger trend of conifer 

succession in stands currently dominated by aspen and 

in stands where conifers have long since overtopped the 

previous aspen forests. Where aspen abut larger forest 

openings, either natural or human-caused, new suckers 

may arise in an “apron” of regeneration where aspen roots 

penetrate into the opening (fig. 4-7). In the past century, 

reduced wildfire, related to relatively moist climate 

patterns (Millar and others 2004) and fire suppression, 

appears to have led to reduced aspen regeneration (fig. 

4-8). Compounding this situation (mostly outside the 

Tahoe basin) is the possibility of losing aspen sprouts 

because of domestic and wild ungulate use (see Chapter 

3: Terrestrial Biota Earlier and Range Management and 

Aspen Communities, below).

Other factors can also affect aspen regeneration 

success. Figure 4-9 shows a conifer removal cut with 

very limited aspen regeneration in the Lake Tahoe 

basin. Since neither livestock nor elk are present, this 

may be due to a limited aspen root system resulting 

from previous conifer dominance and/or persistent 

shading from surrounding uncut trees. This clone may 

require additional disturbance to initiate suckering, so 

additional management actions, such as “root ripping” 

(see Chapter 6: Treatment Alternatives to Regenerate 

Aspen—Root Separation) may be warranted. An example of normal regeneration following disturbance 

is shown in figure 4-10.

In essence, the true threat to aspen sustainability is 

maintaining the status quo: suppressing natural fires and/

or avoiding active management options, such as targeted 

cutting, prescribed burning, and regeneration protection. Alternatives that can promote aspen rejuvenation, 

which is dependent on moderate-to-intense disturbance, 

are limited in the highly developed Lake Tahoe basin. 

The basic choices are: 1) allowing natural disturbances 

to take their course, 2) actively managing for aspen 
50 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

Figure 4-8. Lack of regeneration under mature aspen cover. 

The stand appears to be changing cover types from aspen to tall grass meadow, or possibly pine (Pinus spp.), 

and is showing signs of encroachment near the granite 

outcrop.

Figure 4-9. Three years post-harvest there is very little regeneration in this North Canyon aspen stand.

Figure 4-7. Aspen may continue to 

grow out in “waves” from an established stand where there is a lack of 

competition from other trees, and 

adequate growing conditions (full 

sunlight, moisture, soils, etc.).
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 51

Figure 4-10. Typical flush of aspen regeneration one year

after a fire on the Modoc National Forest. Each alternating

color stripe on the pole equals six inches (13 cm).

rejuvenation, or 3) intelligently crafting geographically 

specific syntheses of both options. For instance, at 

sites farther from human development it may be more 

appropriate to allow natural fire or conduct occasional 

prescribed burns. Management near residential areas 

may require a more precise, less random, course of 

action, such as small patch clearings with precisely 

controlled burning. A more detailed discussion of 

management options for particular situations, including 

highly developed areas, can be found in Chapter 6.

Aspen Habitat and Species 

of Concern________________

Aspen as Prime Habitat

A recent comprehensive watershed assessment of the 

Lake Tahoe basin declares aspen forest types as one of 

nine “Ecologically Significant Areas” (ESAs) (Manley 

and others 2000). The criteria used for this designation 

are minimum human alteration, rarity on the landscape, 

and potential for high biological diversity. Aspen is one 

of two ESAs that is both geographically rare and shows 

high diversity—the other being marshes. Most of the 

nine ESAs were closely related to water proximity. 

This places a premium value on existing aspen stands 

and evaluating options for stands where other forest 

types may be encroaching on the very limited aspen 

cover in the Lake Tahoe basin. Numerous authors have 

broadly discussed the high biotic diversity of aspen 

stands (DeByle 1985c; Chong and others 2001) and we 

have devoted considerable discussion to the topic as it 

applies to the Sierra Nevada (see Chapter 3: Terrestrial 

Biota). But how dependent are identified wildlife species on aspen in the Lake Tahoe basin and how will 

management (or lack thereof) affect those species? If, 

as Manley and others (2000) asserted, aspen is truly 

“ecologically significant,” then important steps should 

be taken to preserve aspen not only for its own sake, 

but for the species that are dependent upon aspen for 

critical habitat.

Species of Concern, Lake Tahoe Basin

Several species within the Lake Tahoe basin are 

formally designated on state threatened or endangered 

lists. In this section we highlight those species of 

concern that are either somewhat or highly dependent 

on aspen ecosystems. Table 4-1 shows threatened 

vertebrates known to use aspen in the Sierra Nevada 

and Lake Tahoe areas. Federally endangered species 

carry the greatest restrictions. California “threatened” 

and Nevada “rare” species include legislative prohibitions on intentional destruction (for example, hunting 

or trapping). The Nevada “watch list” is the least 

restrictive category, designating species of some demographic concern. At this time there are no confirmed 

southwestern willow flycatchers (Empidonax traillii 

extimus), a federal endangered species, in the Lake 

Tahoe basin, although this subspecies is known to 

inhabit the southern Sierra Nevada. Survey data from 

around the basin shows mountain willow flycatcher 

(Empidonax traillii adastus), a threatened species in 

California, either directly or indirectly using aspen 

habitat (Morrison 2005). Preliminary results showed 

an estimated 43 mountain willow flycatcher territories 

in the Lake Tahoe basin (Morrison and others 2002). 

In many cases, willow (Salix spp.) used by willow 

flycatcher is co-located with aspen stands around wet 

meadows or along streams. Where conifers are invading, 

meadows will become drier, decreasing willow/aspen 
52 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

Table 4-1. Listed vertebrates for the Lake Tahoe basin known to use aspen habitat.

Common name Latin name Status notes

Amphibians Mountain yellow-legged frog Rana muscosa Nevada rare species list; federally

endangered in southern California

Birds Northern goshawk Accipiter gentillis Nevada rare species list

Mountain willow flycatcher	 Empidonax traillii adastus	 Califronia threatened species; Nevada

rare species list

MacGillivary’s warbler Oporornis tolmiei Nevada watch list

Flammulated owl* Otus flammelous Nevada rare species list

Red-naped Sapsucker Sphyrapicus nuchalis Nevada watch list

Mammals Sierra (Mono Basin) mountain Aplodontia rufa californica	 Nevada rare species list; see

beaver Terrestrial Biota discussion

Northern flying squirrel	 Glaucomys sabrinus Nevada watch list

North American wolverine Gulo gulo Califronia threatened species; Nevada

watch list

Western red bat Lasiurus blossevillii Nevada rare species list

Sierra Nevada snowshoe hare Lepus americanus tahoensis Nevada watch list

American marten Martes americana Nevada rare species list

American pika Ochotona princeps Nevada watch list

Sierra Nevada red fox Vulpes vulpes necator	 Califronia threatened species; Nevada

watch list

Western jumping mouse Zapus princeps oregonus Nevada watch list

Sources:	 Manley, Patricia N.; Fites-Kaufman, Jo Ann A.; Barbour, Michael G.; Schlesinger, Matthew D., and Rizzo, David M. Biological

integrity. 

Murphy, Dennis D. and Knopp, Christopher M., editors. Lake Tahoe watershed assessment: Volume I. Albany, CA: USDA, 

Forest Service, Pacific Southwest Research Station; 2000; p.554-557.

* Powers, Leon R.; Dale, Allen; Gaede, Peter A.; Rodes, Chris; Nelson, Lance; Dean, John J., and May, Jared D. Nesting and

Food Habits of the Flammulated Owl (Otus Flammeolus) in Southcentral Idaho. Journal-of-Raptor-Research. 1996; 30(1):15-20.

Nevada Natural Heritage Program: http://heritage.nv.gov/lists/animls04.htm

California Department of Fish & Game: http://www.dfg.ca.gov/whdab/pdfs/TEAnimals.pdf

habitat and increasing predation of mountain willow 

flycatchers (Morrison 2005).

The USDA Forest Service Lake Tahoe Basin 

Management Unit (LTBMU) has recently conducted 

center point bird counts in aspen stands surrounding 

Lake Tahoe. One state listed species, MacGillivray’s 

warbler (Oporornis tolmiei), has been tallied six times 

in these counts along with numerous other birds that are 

not listed as using aspen habitat (data on file, Lake Tahoe 

Basin Management Unit). LTBMU wildlife biologists 

have recently documented northern goshawk (Accipiter 

gentillis) nesting sites associated with aspen within the 

Lake Tahoe watershed (fig. 4-11). A 2004 Lake Tahoe 

basin survey of goshawks detected 23 nest sites and 45 

individuals (29 adult and 16 juveniles). Approximately 

9.5 percent of goshawk nests were located in aspen trees 

in a variety of forest types and over half (61 percent) were 

within 300 ft of a water source (USDA Forest Service 

2004b). Researchers in the Great Basin have documented 

the close relationship of goshawks to aspen communities 

(Younk and Bechard 1994). This study found that goshawks preferred larger and older aspen, with relatively 

open understories, in close proximity to water similar to 

the Lake Tahoe findings described here.

Surveys of intermediate size native carnivores in the 

Sierra Nevada have not confirmed sightings of either 

the North American wolverine (Gulo gulo) or the Sierra 

Nevada red fox (Vulpes vulpes necator) in over 60 years. 

The lack of sightings of these two species may be attributed to their notorious aversion to people (Zielinski 

2004). Zielinski also notes a decline in American marten 

(Martes Americana) throughout the Sierra Nevada, 

especially outside of reserved areas (lands reserved 

from logging and alteration, such as designated wilderness, National Parks, and wildlife preserves). It is 

thought that reductions in marten numbers are due to 

forest fragmentation, but this assertion has not been 

specifically tested in the Sierra Nevada. The LTBMU, in 

cooperation with agency and private enterprise partners, 

is involved in several surveys of mammals with some 

focus on marten. Their interests in marten vary from Off 

Highway Vehicle (OHV) impacts, to general baseline 

biodiversity surveys and urban/development impacts. 

Thus far, they have been monitoring up to 17 marten 

detection sites around the basin. Although Manley and 

others (2000) describe marten as using aspen habitats, 

it is unclear how critical this type is to their survival. 

In the Rocky Mountains, researchers have depicted 
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 53

Figure 4-11. A goshawk (Accipiter gentilis) (inset) and 

its nest located along Taylor Creek in Sugar Pine 

Point State Park on the 

West Shore of Lake Tahoe. 

Source: R. Young. Sierra 

District, California State 

Parks.

marten as an old growth conifer species (Hargis and 

others 1999). Further research is clearly needed on 

habitat types used by American marten and potentially 

specific needs derived from aspen.

In summary, specific surveys are not being conducted 

for most threatened species found in table 4-1 and 

therefore definitive data sets are scarce on these species. 

However, we concur with Manley and others (2000) that 

aspen stands that are apparently succumbing to succession in parts of the Lake Tahoe basin are a potentially 

critical resource for maintaining diverse faunal, as well 

as floral, populations. It would seem reasonable that 

targeting aspen as a keystone species may be beneficial 

to several species of concern. It is clear, however, that 

land managers will be under continued scrutiny to 

monitor threatened aspen communities, as well as to 

document the status of individual species.

Invasive Species and Aspen 

Communities______________

We use the term “invasive” here in the most general 

sense: denoting active spread of non-native plants into 

native landscapes. Aspen communities possess at least 

three characteristics that present fertile ground for 

invasive plants: 1) they have deep, rich, soils; 2) their 

proximity to moist meadows and riparian zones offer a 

ready source of water; and 3) their dependency on disturbance and open light is shared by many invasive species. 

Monitoring for invasives at aspen management sites is 

crucial. Some of the management suggestions advocated 

in this publication may in fact lead to spreading of weedy 

plants if precautions are not taken, such as washing and 

sterilizing machinery to avoid bringing invasive seed into 

aspen stands. The immediacy of human development and 

recreation to forest lands also influences the spread of 

invasives in the Lake Tahoe basin. People are the prime 

transporters of non-native plants. In the Sierra Nevada, 

construction and road building, escaped domestic plants 

from private residences and commercial nurseries, 

importation of fill soil containing foreign seed, and the 

movement of people and domestic animals may also 

carry seed into wildlands (Schwartz and others 1996). 

In addition to these direct modes of introduction, other 

human activities indirectly encourage invasive species, 

which are often generalists that are able to tolerate a wide 

range of conditions. For example, it is well known that 

humans are contributing excessive nitrogen and carbon 

dioxide into the soils and atmosphere from various 

waste products. Excess amounts of these compounds 

often discourage native species growth while favoring 

fast-spreading invasives. “Quite simply, increased human presence means increased risk of plant invasion” 
54 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

(Schwartz and others 1996). This certainly does not bode 

well for the Lake Tahoe basin.

Aspen communities are known for high plant diversity 

(see Chapter 3: Plant Associations), and serve as oases 

of animal habitat even when covering only a small 

proportion of total land area (see Chapter 3: Terrestrial 

Biota). But high diversity does not necessarily equate 

with low plant invasion. Recent research suggests that 

diverse plant communities, such as aspen forests, are 

not necessarily more resistant to invasive plants as is 

commonly believed (Lonsdale 1999; Stohlgren and 

others 2003; Gilbert and Lechowicz 2005). A study of 

several forest types in Rocky Mountain National Park 

found more invasive species present in aspen stands than 

other forest cover. Of 42 total invasive species in the 

study, 90 percent were found in aspen and 39 percent 

of those were exclusive to the aspen type (Chong and 

others 2001). Interestingly, they did not find a large 

cover of invasives, only a large diversity of their presence in aspen stands. Thus, Chong and others caution 

that given appropriate disturbance, these aspen stands 

located in a relatively pristine setting, but with invasive 

seed present, are poised for higher levels of invasion 

that could crowd out native species.

Invasive Species of the 

Sierra Nevada

Many of the highly disruptive activities of the 

settlement era (1850 to 1900, see Chapter 2: Historical 

Disturbance Ecology), in addition to modern road 

building and related development, have provided 

an ecological opening for invasive plants. We might 

imagine that invasive or exotic species colonize wildlands in a rapid or dramatic fashion, but most plant 

invasions have been underway for decades or centuries. 

Often an introduced species will persist at low levels 

for decades before spreading rapidly when favorable 

climatic or disturbance conditions present themselves. 

Many of the exotic species found in California forests today were introduced either intentionally or 

unintentionally by European settlers, beginning in the 

18th century (Bossard and others 2000). Nearly all 

of these species are of Eurasian origin and are noted 

for their ecological amplitude. For example, common 

mullein (Verbascum thapsus) was likely intentionally 

introduced to the eastern United States as a medicinal 

plant in the 1700s from Europe. It was subsequently 

transported by settlers, either intentionally or not, and 

established in California by the 1880s. Jepson (1925) 

acknowledged 292 non-native species in the state. By 

the end of the 20th century the estimate was at 1,045 

invasive plants (Randall and others 1998). Of these 

1,045 non-native species in California, about 10 percent 

are considered serious threats (Bossard and others 

2000). In table 4-2 we list species that constitute the 

most serious threats to Sierra Nevada forests according 

to the California Exotic Pest Plant Council (Bossard 

and others 2000; D’Antonio and others 2004; Calflora 

database [www.calflora.org/index0.html]). Thus far, 

many of these plants have only been documented in 

low quantities above 5,000 ft (1,524 m) in elevation 

where aspen habitat exists. Changing conditions may 

promote further expansion of any one of these plants, 

so it is essential for managers to detect, monitor, and 

eradicate (if appropriate) non-native species at an early 

stage (D’Antonio and others 2004). This strategy is 

recommended for economic efficiency and protection 

of native diversity. Exponential spread of invasive 

species can result in dominance of resources used by 

native plant communities within a short time.

Invasive Plant Survey of 

Lake Tahoe Basin

Though the Lake Tahoe basin has most of the basic 

ingredients for rapid dissemination of invasive weeds, 

one thing working in its favor is its relatively high 

elevation. Generally, the impact of invasive species is 

less at higher elevations. “While there are species (for 

example, Kentucky bluegrass [Poa pratensis]) that 

invade high-elevation Sierra Nevada meadows, the 

number of these is few relative to lower elevations in 

the Sierras” (Schwartz and others 1996). Most invasive 

species found in our list (table 4-2) thrive at elevations 

lower than the basin’s 6,200 foot (1,890 m) base. A 

preliminary survey of invasive species conducted by 

the LTBMU is depicted in figure 4-12. This map shows 

a spotty pattern of non-native intrusions, mostly correlated with heavy development and recreation sites, 

but also with riparian zones where aspen is prevalent. 

Basin-wide geographic patterns of invasive species 

are evident. Those invasive species that are evenly 

distributed throughout the basin and are away from 

major roads, such as bull thistle (Cirsium vulgare), have 

likely been here longer and have efficient dispersal and 

germination mechanisms for this location. Others, such 

as nodding bluegrass (Poa reflexa), display the opposite 

pattern (locally distributed, near roads) suggesting a 

recent introduction or limited dispersal ability in this 
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 55

Table 4-2. Major invasive exotic plants of forested communities above 5,000 feet (1,524 m) elevation in the Siera Nevada.

Common name Latin name Elevation (ft.) Notes

Cheatgrass Bromus tectorum	 up to 7,200	 eastern Sierra Nevada; Modoc Plateau

Hairy whitetop Cardaria pubescens up to 6,600 proliferate in moist riparian uplands

Nodding plumeless thistle Carduus nutans up to 6,500*	 fields and roadsides

White knapweed Centaurea diffusa	 up to 7,500	 northern Sierra; uncommon in Lake Tahoe basin

Spotted knapweed Centaurea maculosa	 up to 6,600	 widespread; aggressive colonizer

Tocalote Centaurea melitensis	 up to 7,200	 woodlands and field, of lesser invasiveness

Yellow starthistle Centaurea solstitialis	 up to 7,500	 dry sites; spreading in Cascades, Sierra, and Modoc

Canada thistle Cirsium arvense	 up to 6,000	 spreading by roots; a robust plant up to 6 ft. in height

Yellowspine thistle Cirsium ochrocentrum up to 10,000 pine, pinyon-juniper, sagebrush communities

Bull thistle Cirsium vulgare up to 7,500	 widespread; colonizes recently disturbed sites

Scotchbroom Cytisus scoparius up to 6,200* found on river banks, road cuts, and forest clearcuts

St. Johnswort Hypericum perforatum	 up to 7,100*	 woodlands and field, of lesser invasiveness

Broadleaved pepperweed Lepidium latifolium up to 7,200 riparian, roadsides, wet meadows

Ox-eye daisy Leucanthemum vulgare up to 7,000 northern Sierra, mountain meadows, riparian forests

Butter and eggs Linaria vulgaris up to 6,300* recently introduced in Lake Tahoe area

Dalmation toad-flax	 Linaria dalmatica up to 7,000* forest openings and grasslands

Purple loosestrife Lythrum salicaria up to 6,000 found in wetlands, riparian, and meadow habitat

Scotch thistle Onopordum acanthium up to 5,200 widespread throughout region

Kentucky bluegrass Poa pratensis up to 8,000 widespread in moist and dry sites

Sulphur cinquefoil Potentilla recta up to 6,400*	 roadside; recently introduced in Lake Tahoe area

Black locust Robinia pseudoacacia up to 6,200 mature groves may shade out native vegetation

Himalayan blackberry Rubus discolor up to 5,200 Bossard et al. (2000), occurs in the Sierra Nevada

Russian thistle Salsola tragus up to 8,900	 Shrublands, Calflora is requesting more information

Common dandelion Taraxacum officinale	 up to 10,800	 widespread; usually in non-wetland areas

Common mullein Verbascum thapsus up to 7,200 broad range in forest cuts, riparian, and meadows

Sources:	 Bossard, Carla C.; Randall, John M., and Hoshovsky, Marc C. 2000. Invasive plants of California’s wildlands. Berkeley, CA:

	 University of California Press; 360 p.

	 D’Antonio, Carla M.; Berlow, Eric L., and Haubensak, Karen L. Invasive exotic plant species in Sierra Nevada ecosystems. In:

Murphy, Dennis D. and Stine, Peter A., editors. Proceedings of the Sierra Nevada science symposium. Albany, CA: USDA Forest 

	 Service, Pacific Southwest Research Station; 2004; PSW-GTR-193. 175-184.

	 Hickman, James C. The Jepson manual: higher plants of California. Berkeley, CA: University of California Press; 1993. 1400 p.

	 Calflora taxon report: www.calflora.org

* Documented at higher elevations than acknowedged elsewhere in Sierra Nevada in Lake Tahoe area by USDA Forest Service, Lake Tahoe 

Basin Management Unit.

area. Not surprisingly, the most invaded areas within 

the Lake Tahoe basin are those with the highest human 

use. A quick glance at figure 4-12 shows moderate invasions at several populated points along major highways 

around the lake, and heavier invasion at high use areas 

on the south shore and in Blackwood Canyon midway 

up the west side of the lake.

A closer look at Blackwood Canyon, a moderately 

infested watershed, reveals patterns and potential risks 

when we compare an invasive species survey with the 

location of delineated aspen stands (fig. 4-13). Of a 

total of 6.05 acres (2.45 ha) of surveyed invasive weeds 

in Blackwood Canyon, 75 percent are St. Johnswort 

(Hypericum perforatum) and 25 percent are bull thistle. 

All introduced plants surveyed here are near roads and 

many are adjacent to aspen stand polygons. Not coincidently, roads play an important role in weed introduction. 

Loosely affixed invasive seeds are often transported by 

cars and deposited at stopping points or while vehicles 

are moving. The road follows the drainage bottom in the 

lower reaches of Blackwood Canyon, which is a riparian 

corridor with deeper soils, common prerequisites for 

aspen stands in the Tahoe basin. Musk thistle (Carduus 

nutans) has been found in aspen treatment areas elsewhere in the Sierra Nevada (figure 4-14). It is classified 

as an “A” rated noxious weed by the California department of Food and Agriculture, and should be targeted 

for eradication or containment when found.

Human disturbance associated with recreation (see 

Recreation Impacts, this chapter) along this corridor 

are also present. Activities such as horseback riding 

or all terrain vehicle use, reduce plant cover, expose 

mineral soil, and help disperse invasive plant seeds. 

Considering that aspen regeneration may require 

additional disturbance and that at least two invasive 

plants are already established adjacent to aspen 
56 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

Figure 4-12. A basin-wide survey of invasive species by the USDA Forest Service Lake Tahoe Basin Management 

Unit depicts the location and diversity of species present. Lake Tahoe basin has a modest level of invasive species compared to lower elevation sites.
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 57

Figure 4-13. Association of field verified aspen stands, and St. Johnswort (Hypericum perforatum) and bull 

thistle (Cirsium vulgare) survey data in Blackwood Canyon, on Lake Tahoe’s west side. This large-scale 

view shows a clear relationship between plant invasions and road and riparian corridors, as well as plant 

invasion in close proximity to aspen stands.
58 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

Figure 4-14. Musk thistle (Carduus nutans) colonizing a recently burned site where the soil was particularly disturbed. 

Young aspen are regenerating in the background.

communities in Blackwood Canyon, preventative measures to eliminate or reduce plant invasions following 

aspen regeneration would be recommended. In addition 

to out-competing native understory species, aggressive 

non-native plants may limit the regeneration success 

of aspen in this area. Careful treatment and long-term 

monitoring will be needed to insure successful aspen 

reestablishment.

Aspen in the Wildland Urban 

Interface (WUI)____________

Development and Natural Process 

Conflict

An aspect of aspen community health that is somewhat unique to the Lake Tahoe basin is its proximity to 

large tracts of human development. Natural disturbance 

processes, as well as forest management actions, often 

conflict with homeowner expectations or values. Many 

residents and visitors to Lake Tahoe view forests as 

beautiful settings for human enjoyment that remain 

unchanged over time. Disturbance ecology tells us that 

there is a certain inevitability, such as “building on 

a flood plain,” that all forests eventually succumb to 

significant change agents (wildfire, landslide, beetle 

infestation, natural pathogens, weather events, etc.), 

whether or not they are populated by people (Rogers 

1996). Aspen are particularly dependent on dynamic 

environments where disturbance spawns stand rejuvenation. Although aspen thrive on disturbance, either 

natural or human engineered, it may be difficult to 

allow stand-replacing disturbance events to occur 

near developed or urban landscapes. Commonly, land 

managers will manage disturbance processes more 

intensively in developed situations near the urban 

interface, while forests farther from settlements are 

allowed greater leeway in the magnitude of disturbances 

that are acceptable.

We must realize that a potential conflict exists 

between encouraging aspen to regenerate and protecting human developments. Patches of forest thinning 

with subsequent prescribed burning may be enough 

to stimulate aspen regeneration in heavily managed 

zones. Although such actions may not be popular, they 

may be necessary to arrest vegetation succession and 

regenerate aspen. Managers and residents alike should 

realize that forests will eventually renew themselves via 

natural disturbance events if preventative actions are not 

periodically taken to reduce forest density. Interference 

in fire regimes, for example by suppression, eventually 

will favor larger fires. To seek acceptable methods for 

managing aspen, understanding basic aspen ecology 

is an essential first step in the dialogue between land 

managers and residents of the area.

Lake Tahoe Defense Zones

The map shown in (fig. 4-15) depicts the designation 

of wildland urban interface zones in the Lake Tahoe 

basin. Defense zones are defined as “the buffer in closest 

proximity to communities, areas with higher densities 

of residences, commercial buildings, and/or administrative sites with facilities” (USDA Forest Service 2004c). 

Threat zones, in most instances, are used to buffer defense 

zones from potentially catastrophic fire situations. Threat 

or defense zones may be adjusted for local conditions 

where fire suppression is projected to be difficult.

Aspen not only thrives on periodic disturbance, but 

pure aspen forests in the Rocky Mountains are considered 

a deterrent to crown fire spread because of the moisture 
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 59

Figure 4-15. A basin-wide view of wildland urban interface defense zones. Defense zones are usually found closest to developed property, while threat zones are meant to buffer defense zones from potential disturbances, 

most commonly fire.
60 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

Figure 4-16. A developed picnic ground in an aspen stand 

at Spooner Pass.

often held in the dense understory and the lack of aspen 

bark flammability (Fechner and Barrows 1976; Jones 

and DeByle 1985b). Thus, silvicultural techniques, 

prescribed burning, or wildfire that serve to restore aspen 

communities may, in turn, provide effective fuel breaks 

in the Lake Tahoe basin and other developed interface 

areas. Of the 388 aspen survey plots in the basin (fig. 

4-3), 63 percent fall within either designated defense 

zones on Forest lands or developed urban private and 

municipal lands. Recall from figure 4-3 that much of 

the north and east portions of the basin had not been 

formally inventoried to date. Nonetheless, 35 percent 

of aspen stands inventoried fall in the wildland urban 

defense zones, providing excellent opportunities for 

fire prevention through bioremediation; in this case, 

promoting aspen health while reducing crown fire 

potential near developed areas.

Nearly all of the inventoried aspen polygons in the 

Blackwood Canyon example fall within the defense 

zone. Including aspen in the defense zone allows maximum flexibility to take preventative actions in an area 

of high use, but only moderate development. Because 

Blackwood Canyon is not a typical highly developed 

defense zone, managers feel no need to designate a surrounding threat zone. Therefore, managers may be able 

to stimulate additional aspen growth using prescribed 

fire or other management techniques in this canyon and 

other places where declining aspen stands occur within 

wildland urban interface (WUI) designated defense 

zones. If aspen regenerate beyond browse height, then 

expanded aspen forests may serve a complementary 

goal of providing a natural fire break near recreation 

and development locations.

Recreation Impacts_________

Recreation is just one of many forest values in our 

area of concern, but recreational use has the potential 

to severely compromise other values (Cole and others 

1987). This section presents an overview of the impacts 

of recreation on aspen habitats and factors that influence the severity of these impacts. In the management 

section (Chapter 6) we will explore ways of reducing 

the conflict between recreational use and conserving 

aspen communities.

From a recreation standpoint, aspen stands are 

aesthetically pleasing locations, and are often used for 

dispersed hiking, hunting, camping, and OHV travel. 

Occasionally, agencies have developed roads or trails, 

or constructed picnic or camping areas in aspen stands 

(fig. 4-16). However, recreational activities at concentrated levels are often detrimental to sensitive aspen 

communities. For example, the thin bark of aspen is 

easily damaged by human activity and subsequently 

penetrated by a variety of diseases (Walters and others 

1982; Hinds 1985). Likewise, human activity may 

impede aspen regeneration through trampling and soil 

compaction.

Many variables determine the impact of recreational 

uses of ecological habitats. The most significant are 

the: 1) amount of use, 2) type of recreational activity, 

3) behavior of recreationists, and 4) the spatial and 

temporal distribution of use. Combinations of these 

factors can have a range of impacts on four ecological 

components of habitats-soil, vegetation, wildlife, and 

water (Cole 1993). In their review of literature on human 

impacts in wilderness areas across America, Leung and 

Marion (2000) summarize a broad range of impacts to 

sensitive areas like aspen stands caused by recreation 

activities (table 4-3).

In a Rocky Mountain study, Hinds (1976) found 

the principal causes of damage to aspen through recreational use are: 1) cutting and carving aspen trees, 2) 
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 61

Table 4-3. Direct and indirect effects of heavy recreation impact on soils, vegetation, wildlife, and water on aspen habitats 

(Leung and Marion 2002).

Ecological Component

Soil Vegetation Wildlife Water

Direct effects Soil compaction Reduced height and Habitat alteration Introduction of exotic

vigor species

Loss of organic litter Loss of ground Loss of habitats Increased turbidity

vegetation cover

Loss of mineral soil Loss of fragile species Introduction of exotic Increased nutrient inputs

species

Loss of trees and shrubs Wildlife harassment Increased levels of

pathogenic bacteria

Tree trunk damage	 Modification of 	 Altered water quality

wildlife behavior

Introduction of exotic Displacement from

species food, water, and 

shelter

Indirect/derivative Reduced soil Composition change Reduced health and Reduced health of aquatic

effects moisture	 fitness	 ecosystems

Reduced soil pore Altered microclimate Reduced reproduction Composition change

space rates

Accelerated soil Accelerated soil erosion Increased mortality Excessive algae growth

erosion

Altered soil microbial Composition change

activities

Figure 4-17. Damaged trees are common in aspen stands 

in developed and dispersed camping areas within aspen 

stands.

trampling community understory, and 3) soil compaction. Human induced wounds to aspen not only damage 

the physical function of the trees, but provide a pathway 

for pathogens to enter individual trees and sometimes 

even entire clones (see Chapter 3: Damaging Agents 

Affecting Aspen). For example, in a Colorado study of 

camp grounds, Hinds (1976) found 83 percent of aspen 

trunks contained mechanical damage from human cutting, carving, and axing of aspen trees (fig. 4-17), whereas 

only two percent of the aspen in an adjacent natural 

setting contained trunk wounds. In these campgrounds, 

47 percent of the living trees had trunk cankers versus 

only 11 percent in the natural setting.

Soil compaction can occur from concentrated human foot travel, horse travel, or use of OHVs (Weaver 

and Dale 1978). Additionally, concentrated camping 

can cause extensive damage to aspen because of soil 

compaction, trampling, and sucker removal (fig. 4-18) 

(DeByle 1985b). Human foot traffic, pack stock, and 

OHV use can significantly alter soil properties. The 

forces exerted upon the soil reduce pore space, particularly macro pore spaces, that provide for air and water 

movement through soils (Cole and others 1987). In a 
62 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

Figure 4-18. Soil compaction 

can result from repeated 

use of dispersed camping 

sites.

study on aspen soils similar to those described for aspen 

in the Serra Nevada (Potter 1998), Shepperd (1993) 

found that near-surface aspen roots were damaged by 

soil compactive forces. This occurred most commonly 

when numerous vehicle passes caused the stripping of 

small moisture-absorbing roots from large lateral roots. 

Soil compaction within stands may increase runoff from 

storm events and lead to erosion and sedimentation of 

waterways. Impacts can differ greatly with the type of 

recreational activity. Weaver and Dale (1978) reported 

that although there are exceptions, the general rule of 

thumb is that motorized uses will usually cause more 

impact than non-motorized uses, and horses will cause 

more impact than hikers. All of these impacts are amplified when soils are saturated. They found that 200 passes 

by a motorcycle removed twice as much vegetation as 

the same number of passes by a horse, and nine times as 

much vegetation as 200 hikers in a controlled experiment 

on a grassland in Montana. Shepperd (1993) found in 

Colorado that compaction and damage of aspen roots 

were affected by the number of vehicle passes and soil 

organic matter content with maximum damage occurring 

after 16 vehicle passes. Compaction effects in this study 

did not diminish for up to 14 years. Cole and Schreiner 

(1981) cite numerous studies in the Sierra Nevada that 

have documented compaction and damage to ground 

vegetation from recreation activities on wetter, more 

developed soils, such as those favored by aspen in this 

region (Potter 1998).

DeByle (1985b) speculated that winter use of aspen 

areas is generally not damaging because of the uniform 

snow cover that normally protects aspen regeneration 

and limits soil compaction. However, there has been a 

great boom in snowmobile use on public lands in the 

winter, and as is the case with concentrated summer 

recreation, attention should be given to concentrated 

uses of these over-snow vehicles for possible habitat 

alteration and increased wildlife harassment. Although 

we found no research data to indicate that snow machines 

harm aspen, their presence in young sprout stands could 

contribute to sucker damage caused by snow compaction 

and settling (See Chapter 3: Damaging Agents Affecting 

Aspen and fig. 3-11).

Extensive concentration of people in aspen stands 

may also have a negative effect on bird and mammal 

behavior patterns in aspen. Cole and Landres (1995) 

found that vegetation in newly established campsites 

can change within a year as sites are trampled and soil 

becomes compacted. Cover begins to decline, especially 

in ground and shrub layers. Hinds (1976) reported that 

dead and downed wood was quickly scavenged for fires 

or removed for safety in Colorado campgrounds in aspen 

stands. Plant species diversity, as well as horizontal and 

vertical structural diversity may decline. Moreover, bird 

communities change in response to habitat alteration. 

Habitat changes will generally cause the greatest reduction in bird species that rely on shrub and ground cover 

(juncos, thrushes, warblers, sparrows, vireos, and wrens) 
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 63

and those that depend on standing dead and downed 

woody debris (woodpeckers and secondary cavity nesters). Additionally, Marzluff (1997), reviewing studies on 

the effect of recreation in a range of habitat types, found 

that disturbance during certain times of the year may 

have an impact on bird behavior. For example, repeated 

intrusions during the nesting season may cause birds to 

minimize or stop singing, decrease defensive behavior 

at nests, and possibly cause birds to abandon nest sites 

leading to an overall decline in nesting productivity. 

Most significantly, Marzluff (1997) reported that the 

potential influence of human disturbance increases with 

the frequency and intensity of disturbance.

From a recreation standpoint, pronounced aspen 

mortality and site degradation lower the quality of the 

recreation experience. In his study of Rocky Mountains 

campgrounds in aspen stands, Hinds (1976) found that 

many of the aspen camping sites were no longer able to 

provide significant shade to campers due to mortality 

of the mature canopy. He reported average campsite 

tree loss of 44 percent in 8 years in one campground, 

68 percent in 12 years in another, and 23 per cent in 

8 years in yet another one. The heaviest loss he found 

was a 90 percent tree loss after 14 years.

In our area of interest, aspen make up a small percentage of the landscape and are disproportionately 

important in affecting ecological diversity. Therefore, 

it would be prudent to avoid developing recreation 

sites in aspen stands, but it might be helpful from the 

recreational quality perspective to place sites within 

view of aspen landscapes or provide trails that pass 

near aspen forests.

Water Quality and Quantity_

Since riparian ecosystems are often found in or 

adjacent to aspen communities, there is an increasing 

interest in relationships between these unique habitats. 

Understanding those relationships is critical to their 

management as will be detailed in the management 

section of this synthesis. Four important variables within 

these aspen habitats contribute to water quality and 

quantity in riparian systems: 1) the nature of the soils 

found under the aspen, 2) the structure and composition 

of the vegetative community found within the habitat, 

3) the interaction of rain and snow with specific soils 

and vegetation cover, and 4) the evapotranspiration that 

takes place in the ecosystem.

Soils

Soils formed in aspen communities play an important 

role in the water quality and quantity of adjacent riparian 

ecosystems because of the aspen soil’s water-holding 

capacity. The most significant factor in the formation 

of aspen soils is the presence of a nutrient-rich litter 

created from the annual leaf fall of aspen. Results from 

a study in the Intermountain west (Bartos and DeByle 

1981) indicate this litter decays rapidly, forming a thin 

surface organic horizon that is typically underlain by 

a thick mollic horizon, high in organic matter content 

and available nutrients. Breakdown of organic matter 

from the understory contributes additional litter to the 

formation and maintenance of the mollic horizon.

In another Intermountain study, Tew (1968) reported 

that the surface 6 inches (15.2 cm) of soil under aspen 

had 4 percent more organic matter, a higher water holding capacity, a slightly higher pH, and more available 

phosphorus than adjacent stands of shrubs and herbaceous vegetation. This is consistent with Potter (1998), 

who reported that the Populus Tremuloides/Veratrum 

Californicum association in the upper montane area of 

the central Sierra Nevada had the highest Available Water 

Holding Capacity (AWC) of any forested associations. 

He defined AWC as the capacity of soils to hold water 

available for use by most plants. He believed that this 

was a reflection of the deep soils, finer soil textures, 

and low levels of coarse fragments throughout the soil 

profile. This association was often found in or adjacent 

to riparian corridors in the upper montane.

Cryer and Murray (1992) suggested that soil types 

in thriving aspen stands in western Colorado are 

significantly different than those of adjacent aspen 

forests that have recently converted to conifer types, 

although other evidence challenges this assertion 

(Bartos and Amacher 1998). The logic behind Cryer 

and Murray’s (1992) assertion is that decomposition 

occurs much faster in aspen than in conifer forests, 

thus making nutrients more readily available for 

soil organisms and plant roots in aspen communities. Soil therefore becomes reflective of what type 

of vegetation has occupied the site for the longest 

time. They feel that in as short as one generation, 

conifer encroachment in western Colorado aspen can 

increase water percolation through the soil profile and 

lead to the formation of an albic (leached) horizon. 

Although Bartos and Amacher (1998) found similar 

morphological changes occurring in soil profiles as 

conifers invaded aspen in a Utah study, they did not 

find any evidence of corresponding changes in chemical 
64 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

properties. Nevertheless, it seems reasonable that the 

cyclic relationship between soil nutrient availability 

with aspen and its accompanying understory and the 

breakdown of organic matter from those species (thus 

returning the nutrients to the soil), is the foundation 

for development of a stable water holding capacity of 

typical aspen soils, wherever they occur.

Overstory and Understory Protection

Overland flow leading to erosion is a major factor in 

lowering water quality in riparian ecosystems. However, 

DeByle (1985b) hypothesized that a well stocked 

aspen stand provides excellent watershed protection. 

The trees, shrubs, and herbaceous layer can provide 

virtually 100 percent soil cover. This is particularly 

true in Potter’s (1998) aspen/California corn lily association in the Sierra Nevada. He reported that the 

mean percent cover of all species was 94 percent with 

a range of 82 to 99 percent cover in that association. 

The aspen/mountain pennyroyal association, found 

in relatively drier conditions, contained a mean total 

vegetative cover of 94 percent, but had a slightly 

wider range from 70 to 100 percent. This mixture of 

high cover of herbaceous and woody stemmed root 

systems, which penetrates and anchors the soil (fig. 

3-3), is likely to reduce the probability of overland 

flow in these stands (DeByle 1985b). In an earlier 

Utah study, Marston (1952) reported that adequate 

infiltration occurred when the combination of aspen, 

shrub, herbaceous, litter, and rock cover was over 

65 percent with only small amounts of bare cover. 

However, Marston did find that erosion can occur if 

cover is lower than 65 percent due to intense or heavy 

ungulate use. This study further demonstrated that 

even storms with 5-minute intensities approaching 

6 inches per hour were able to infiltrate the porous, 

humus-rich soil in Utah. Snow melt, which accounts 

for the greater percentage of precipitation in California 

aspen stands, is rarely this intense.

Shading Quality

A study in Oregon of aspen habitats adjacent to 

riparian corridors found some interesting results relating to the quantity and extent of shading in streams 

(McNamara 2003). The vegetation communities 

surveyed for shading were sedge/grass, willow/shrub, 

alder, lodgepole pine, ponderosa pine, white fir, and 

cottonwood/aspen. The categories included numerous 

species and were derived from the dominant species 

types present. Using randomly selected shade survey 

sites, McNamara (2003) found that the average percent 

of shade, as well as the maximum and minimum percent of shade in aspen/cottonwood, was greater than 

that provided by many conifer species. For example, 

the study found that in streams with a wetted width of 

less than 5 feet, aspen/cottonwood shading averaged 

greater than lodgepole pine, ponderosa pine, or white 

fir. In streams with a wetted width between 5 and 15 

feet, aspen/cottonwood still provided more shading on 

average than lodgepole pine or ponderosa pine. This 

study calls attention to the value of aspen in providing 

shade in a riparian ecosystem. In the monitoring chapter 

(Chapter 7), we review a current study being conducted 

in the Sierra Nevada that should help evaluate the role 

of shade in aspen habitats along the short reaches where 

they occur.

Evapotranspiration

Water quantity is greatly influenced by evapotranspiration rates of the vegetation occupying the landscape. 

There is greater evapotranspiration in conifer dominated 

forests due to the conifer demands in spring and fall 

before aspen leaf bud and after aspen leaf fall. One Utah 

study reported a 5 percent decrease in water yield when 

conifers replace aspen (Harper and others 1981), while 

a second Utah study modeled water yield decreases of 3 

to 7 inches (7.6 to 17.8 cm) under similar circumstances 

(Gifford and others 1983b). This loss of water means 

that it is not available to produce understory vegetation, 

recharge soil profiles, or contribute to streamflow (Bartos 

2001) (see Chapter 2: Physical Environment—Water 

and hydrology). An early study that completely cut a 

mixed aspen/conifer watershed at Wagon Wheel Gap, 

Colorado increased measurable streamflow for 7 years 

until aspen reclaimed the site (Bates and Henry 1928). 

DeByle (1985b) reported an additional increased yield 

after fire because of removal of understory vegetation. 

However, increases in stream flow are proportional, 

not only to the amount of conifer or aspen removal, but 

also to other factors including the shape of watershed, 

drainage patterns and soil characteristics, and proximity of cuts to stream sides. For example, Tew (1967) 

found that aspect, elevation, and the age of vegetation 

affected the amount of soil moisture during the growing 

season in Utah. Significantly, Johnston (1984) found that 

suspended and bedload sediments during post treatment 

indicated good quality water and generally low erosion 

rates in treated aspen stands in Utah. While he did find 
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 65

significant difference in pH, Ca, Mg, and NO3, he also 

found significant difference in the control, indicating 

that treatment was not a factor. Apparently, this was a 

result of other environmental fluctuations not associated 

with low levels of canopy removal. In Utah, where aspen 

distribution is more extensive than California, Johnston 

reported that removal of less than 20 percent of the 

forested canopy from an aspen dominated watershed 

did not cause a detectable change in stream flow.

Range Management and 

Aspen Communities________

Livestock and Aspen Stands

Like wild ungulates and many other terrestrial species, 

domestic livestock, whether cattle, sheep, or horses, are 

drawn to aspen communities because of their association 

with water and the quality of forage that is consistently 

found under aspen (Potter 1998; Mueggler 1988). In 

California, for example, we know that plant diversity 

in two recognized aspen types was among the highest 

of all the plant communities of the upper Montane 

zone of the Central and Southern Sierra Nevada (Potter 

1998). This included not only total species, but also the 

number of species providing hiding cover (see Chapter 

2: Plant Associations).

Not only are aspen stands more diverse in understory 

plants, but they appear to have greater quantities of 

herbage as well. Woods and others (1982) found that 

Rocky Mountain aspen stands have significantly more 

herbage in their understory than other forest types. 

Reynolds (1969) showed in an Arizona study that 

aspen may have 10 times more herbaceous vegetation than that found in conifer understories. Mueggler 

(1988) reported that productive aspen communities in 

Wyoming can produce as much as 2,900 lb/acre (3,200 

kg/ha) of air-dry undergrowth material. However, he 

notes that forage production in aspen habitats is site 

specific and that aspen communities are generally 

less productive in their northern and southern ranges. 

Richardson and Heath (2004) working in the Sierra 

Nevada, and Dobkin and others (1995) working in the 

Great Basin, found that vegetative diversity plays an 

important role in distribution of wildlife species found 

in aspen communities.

While we would expect regional variation in the biota 

associated with ungrazed aspen, domestic livestock 

have altered natural patterns over time. For example, 

Loft and others (1987) noted that the removal of hiding 

cover by intense cattle grazing and browsing adversely 

affected wildlife in Sierra Nevada aspen communities. 

Kie and others (1991) showed that the natural range of 

deer in the Sierra Nevada was affected by cattle grazing. 

Mueggler (1988) noted extensive alteration of plant 

communities related to a history of excessive grazing 

and browsing in the Intermountain West. In addition, 

DeByle (1985a) reminds us that “any ground-nesting 

bird can be adversely affected by heavy grazing during 

the nesting season.”

Range Management History 

and Aspen

In their study of 20th Century management of 

rangelands in the Sierra Nevada prepared for the Sierra 

Nevada Ecosystem Project, Menke and others (1996) 

found that throughout most of the central part of the 

last century (1930 to 1970), use and changes of range 

intensity and range management improvements in the 

Sierra Nevada were most often driven by socioeconomic 

and forage production reasons rather than decisions 

based on ecological condition. There was little focus on 

the potential for natural vegetation to develop on site. 

Most ranges were stocked above carrying capacity until 

very recently (1979 to the present) (Menke and others 

1996). Alterations in plant communities have resulted 

from changes in intensity, frequency, and seasonality 

of livestock use. These factors, in combination with fire 

suppression, have likely had significant impacts on the 

remaining forage for both livestock and wildlife.

In the second half of the century, Menke and others 

reported that general range conditions in the Sierra 

Nevada may have slightly improved or at least remained 

static. However, they found this was not the case for 

drainages, meadows, watering places, and other natural 

concentration areas that generally continued to decline. 

It was not until the 1970s and 1980s that resource 

protection became a greater emphasis relative to forage 

resource production.

Grazing Effects on Aspen Stand 

Structure

While there is not extensive historical evidence on 

the effects of grazing in the Sierra Nevada on aspen 
66 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

Figure 4-19. Excessive ungulate 

grazing can lead to the introduction and multiplication of invasive 

weeds.

communities, Potter (1998), in his study of upper 

montane vegetative communities in the Central and 

Southern Sierra Nevada, noted that the age of the current 

aspen component in many stands corresponds well with 

the end of the intensive grazing pressures of the late 

1800s and the institution of fire suppression policies in 

the early 1900s. This interpretation is consistent with 

other views in the West. Mueggler (1988) speculated 

that over a century of frequently intense grazing in the 

Intermountain West (sometimes by multiple classes of 

livestock in a season in addition to wildlife) left both 

pronounced and subtle alterations of species composition. From a contemporary standpoint, Potter (1998) 

found that many of the stands he sampled in the Sierra 

Nevada were found adjacent to meadows and other 

moist areas where livestock congregate in the summer 

season for shade, forage, and water. He observed that 

livestock would often graze the understory heavily 

and use aspen as a primary browse species. Kie and 

Boroski (1996) and Loft and others (1991) quantified 

this observation in Sierra Nevada studies, finding that 

cattle had a strong summer-long preference for riparian 

habitats. This is consistent with Menke and others (1996), 

who reviewed range management records of Sierra 

Nevada national forests. They reported that potentially 

productive habitats located in drainages, meadows, 

watering places, and other areas where livestock concentrate were in the poorest shape.

Range Quality

In the Interior West, research has focused on the 

effects of intensive grazing and browsing in aspen 

communities. We believe these findings are applicable 

to California aspen communities as well, since the intensity of livestock use appears to be similar. As a rule 

of thumb, livestock grazing tends to shift plant species 

composition in the understory to those of lower palatability (Houston 1954). Intense grazing can also lead to 

increases of annuals, the introduction of invasive weeds 

(fig. 4-19), and a lowering of ground cover (DeByle 

1985a). For example, Mueggler (1988) found in the 

Intermountain West that in many of the stands that were 

severely overgrazed for extended periods of time, the 

amount of perennial forbs was generally reduced and the 

proportion of annuals or graminoids increased. As palatable species were eliminated by repeated use, a change 

in dominance to lower growing, more drought resistant, 

and less palatable species occurred. Mueggler (1988) 

found severely depleted ranges to be dominated by annuals, ruderals, and unpalatable perennials. Dominance 

by a single species or very few species can be a sign of 

overgrazing (Houston 1954). Another potential effect 

that may be critical to shallow-rooted aspen is that litter 

cover is lessened as it is pulverized and trampled, and 

soil may be compacted with a potential increase in soil 

erosion (DeByle 1985a).
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 67

Aspen Regeneration

It seems logical to assume that light grazing while the 

herbaceous understory is lush and succulent is less likely 

to damage aspen regeneration than grazing late in the 

season after the herbaceous plants cure and become less 

palatable (DeByle 1985a). We acknowledge, however, 

that early season grazing may cause more intense soil 

compaction, thereby limiting growth in many species. 

It has been shown that domestic livestock consume 

aspen with increasing pressure through summer and 

early fall as preferred forage decreases in volume and 

nutritional quality (DeByle 1985a; Fitzgerald and others 

1986). This browsing can be very severe, especially 

on young succulent sprouts, and can be site specific 

by all ungulates. This increased utilization of aspen by 

domestic livestock as the season progressed is parallel 

to a similar pattern found with deer browse in Utah 

(Julander 1952) and observed in California (Personal 

Comm., Chuck Lofland, Wildlife Biologist, USDA 

Forest Service). Julander also noted that deer also utilize 

aspen leaves that have fallen on the ground after leaf 

fall, which does not harm the aspen. However, repeated 

heavy browsing will lead to bushy, multi-stemmed aspen 

shrubs, leaving them susceptible to browsing year after 

year until use ceases, or the aspen eventually disappear 

(Keigley and Frisina 1998).

Conifer Encroachment 

on Rangelands

Much of the aspen rangeland in our area of interest 

is found in the fire-adapted ecosystems of the Sierra 

Nevada, Southern Cascades, and Modoc Plateau (Taylor 

1998, 2000). Fire regimes began to change radically 

with the advent of intense grazing during the late 1800s 

and early 1900s. Grazing reduces “fine fuels” that 

would carry frequent low intensity fires through the 

understory of forests and shrubland ecosystems. The 

advent of fire suppression resulted in an initial increase 

in shrub components followed by conifer invasion that 

eventually led to the development of dense conifer 

canopy covers, which burn as high-intensity crown 

fires. While we are not aware of specific studies of the 

effect of conifer encroachment into aspen in the Sierra 

Nevada, Bartos (2001) documented loss of vegetative 

diversity in aspen forests in Utah and attributed it to fire 

suppression and subsequent conversion to conifer 

dominated systems. Conifer encroachment shades 

out understory plants, lowering plant cover, forage 

productivity, and species diversity. Mueggler (1988), 

in his study of aspen communities of the Intermountain 

West, found that production of understory herbaceous 

and shrubby cover decreased as conifer overstory 

increased. This effect becomes apparent when as 

little as 15 percent of the basal area is made up of 

conifers. Furthermore, Reynolds (1969) reported that 

an Arizona study found aspen habitats contained 10 

times more forage than adjacent ponderosa pine forests. 

Succession of aspen to conifers therefore results in a 

considerable lowering of the grazing capacity and a 

preference by both wild and domestic ungulates for 

aspen communities over conifer communities. This 

issue is significant because with no changes in stocking 

intensity, livestock and wildlife are forced to compete 

for diminishing resources.

Livestock/Wildlife Interactions

Removal of forage, as well as disruption of nesting 

site cover, can have negative effects on both large and 

small mammals (Kie and others 1991; Loft and others 

1987 ; Dobkin and others 1995). Kie and others (1991) 

reported that deer in the Sierra Nevada of California 

increased their home range as cattle grazing increased. 

Loft and others (1987) found that deer and fawn hiding 

cover decreased with increases in browsing intensity 

by domestic livestock in the Sierra Nevada.

Livestock grazing is limited to stringer meadows, 

riparian areas, brush fields, and transitional areas in 

mixed-conifer forest at mid-elevations on the western 

slope of the Sierra Nevada (for example, areas created 

after wildfires, or plantations installed after clear cutting 

or small group selection harvests). Many of the aspen 

on the Western slope of the Sierra Nevada are found in 

stringer meadows and riparian areas. Loft and others 

(1993) found that as the grazing season progressed, 

cattle were attracted to the patchy meadow-riparian 

and aspen habitats where herbaceous forage was most 

available. Therefore, we feel confident in stating that 

grazing management is a critical issue in aspen type 

in the Sierra Nevada and that timing and intensity of 

livestock use is critical as it relates to aspen regeneration 

and wildlife values.

A final consideration is that elk herds have recently 

become established in the Southern Cascades and Warner 

Mountains (Personal Comm., Mary Flores and Tom 

Rickman, Wildlife Biologists, USDA Forest Service, 

Modoc NF and Lassen NF). Elk can put added pressure 

on aspen habitats, especially when aspen are found on 
68 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

winter ranges (DeByle 1985c; Baker and others 1997; 

Suzuki and others 1999; Barnett and Stohlgren 2001). 

We conclude it is essential that management of both 

domestic and wild animals be closely linked to forage 

carrying capacity to avoid adverse effects on the aspen 

resource.
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 69

Chapter 5.

Assessment of Current Aspen 

Conditions

Ecological Status___________

To understand the significance of the management 

discussion in Chapter 6, we feel it is best to first examine 

the current ecological condition of aspen communities 

in our area of interest. Individual aspen inventories and 

assessments have been conducted throughout the three 

ecoregions in this area (fig. 2-3) by the U.S. Forest 

Service, Bureau of Land Management, California 

Department of Fish and Game, California Tahoe 

Conservancy, California and Nevada state park systems, 

and National Park Service. While these inventories and 

assessments have yet to be unified for statewide analysis, 

in the areas where they have occurred they individually 

increase the body of knowledge about: 1) aspen spatial 

distribution, 2) current ecological condition of the stands 

assessed and, 3) further management implications at a 

variety of scales. They illustrate, among other things, 

the importance of focusing on site-specific evaluation 

of information when looking at such issues as range of 

natural variability and how current ecological conditions may have been affected by anthropogenic factors 

such as fire suppression and browsing intensity. We 

examined three large data sets, one from each region, 

to gain understanding of the ecological condition of 

the aspen habitats over the broad range of our area. We 

believe that these data sets are representative of the other 

assessment efforts that we have examined.

Sierra Nevada Section

The first data set is from the Forest Service’s Lake 

Tahoe Basin Management Unit (LTBMU). There are 

four state or federal agencies in the Lake Tahoe basin 

that have been conducting aspen assessments since 2002. 

While this data set does not represent the entire potential 

inventory of the LTBMU, it is large enough to help 

understand the distribution and condition of aspen across 

the Lake Tahoe basin landscape. The LTBMU conducted 

assessments between 2002 and 2005 (fig. 4-3) using a 

protocol developed by the Aspen Delineation Project 

(Aspen Delineation Project 2002, Data on Record at the 

LTBMU). Detailed information about this protocol can 

be found in Assessment and Monitoring Methodology 

(Chapter 7). The protocol examines three subjects: 1) 

spatial descriptors—where the stand is located and its 

extent; 2) stand condition data—ecological measures to 

assess risk of decline and/or imminent loss of an aspen 

stand; and 3) management information—what unique 

conditions exist that may affect management options.

We focus here on 542 stand assessments from the 

2002 to 2005 survey of the Lake Tahoe basin. Aspen 

stands are located from lake level at 6,226 ft (1,897 m) 

to over 8,800 ft (2,682 m) with 40 percent of the stands 

within a 1 to 300 ft (0.3 to 91 m) height gradient above 

lake level (fig. 5-1). This propensity for low elevations 

is not necessarily related to their close proximity to 

Lake Tahoe, but rather to the broad canyon bottoms 

of the Lake Tahoe basin (fig. 5-2). Stand sizes range 

from less than a quarter acre (0.1 ha) to 97 acres (39.2 

ha) (fig. 5-3), with 51 percent of the stands being half 

an acre (0.2 ha) or smaller. The LTBMU assessments 

also note stand associations with specific geographic or 

habitat relationships (fig. 5-4). For example, 57 percent 

of the stands are associated with riparian corridors or 

springs. Probably the most significant result of this 

effort was the assessment of the ecological condition 

of each aspen stand. Each stand was assigned a “stand 
70 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

Figure 5-1. Elevation range 

of the 542 aspen assessments conducted 

between 2002 and 2005 

by the Lake Tahoe Basin 

Management Unit.

Figure 5-2. Aspen stands in Blackwood 

and Ward Creek Canyons. The location 

of aspen appears to be associated with 

certain soils and landforms in the Lake 

Tahoe basin.
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 71

Figure 5-3. Distribution of stand sizes for 542 aspen assessments conducted between 2002 and 2005 by the Lake 

Tahoe Basin Management Unit.

Figure 5-4. Distribution of 542 aspen stands by landform. 

From an assessment conducted between 2002 and 2005 

by the Lake Tahoe Basin Management Unit.

Figure 5-5. Distribution of 542 aspen stands assessed 

between 2002 and 2005 by the Lake Tahoe Basin 

Management Unit, grouped as to relative risk of loss.

risk” rating based on a classification developed by 

Bartos and Campbell (1998). The classification includes 

five risk categories: highest, high, moderate, low, and 

none. The highest and high categories apply to aspen 

stands with extensive risk factors that may either cause 

them to disappear from the landscape entirely in the 

near future, or at the very least, diminish in extent and 

ecological significance. Moderate risk stands have risk 

factors associated with growing threats to the stand’s 

vitality. Low and no risk stands are basically healthy and 

viable stands that require no management intervention 

at the present time. The LTBMU assessments found 

that 70 percent of the stands sampled in the basin were 

at moderate to highest risk related to their ecological 

viability (fig. 5-5). The map in figure 5-6 illustrates how 

the assessment of ecological risk is distributed across 

aspen stands in two canyons in the Lake Tahoe basin, 

Blackwood Canyon, and Ward Creek.

Southern Cascade Section

To examine the condition of aspen in a representative location in the Southern Cascades, we reviewed 

an assessment from the Eagle Lake Ranger District, 

Lassen National Forest. This area is located in eastside 

ponderosa pine forests bordering Lassen National Park 

on the west and Eagle Lake, which is part of the Modoc 

Plateau Region, on the east. District personnel mapped 

and assessed aspen stands from 2000 to 2004 using 

RIPARIAN

48%

SPRING

9%

MEADOW

18%

POND

3%

OUTCROP

13%

TALUS

9%
72 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

Figure 5-6. Aspen in Blackwood Canyon and Ward Creek (map developed by Lake Tahoe Basin Management 

Unit from 2002 and 2003 data). Assessment data are grouped by ecological factors indicating stands at 

risk of diminishing in ecological significance or being completely lost from the landscape.
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 73

Figure 5-7. Distribution of factors indicating risk of stands 

diminishing in ecological significance or being lost completely from the landscape. Data are from 522 aspen stands 

assessed in the Southern Cascades in 2002 and 2003 by 

the Eagle Lake Ranger District, Lassen National Forest, 

using the Bartos and Campbell (1998) protocol.

Figure 5-8a, b. Repeat photos taken in (a) 1915 along the Susan River in the Lassen Nation Forest reveals an extensive aspen 

stand. A repeat photograph (b) taken in 2003 reveals increased conifer densities with no aspen present. Arrows have been 

added for orientation, noting a bend in the river that can be seen in both photos. Ground surveys confirmed that there are

no longer aspen on this site.

a b

the Bartos and Campbell (1998) risk factor protocol 

described above (Personal Comm., Bobette Jones 

and Tom Rickman, Wildlife Biologists, USDA Forest 

Service, Lassen NF, and data on file at the Eagle Lake 

RD). The assessment, which covered 522 stands on 

3,157 acres (1,277 ha), found that 430 (82 percent) 

of the stands were at high to highest risk of loss (fig. 

5-7). Seventy-three percent of the inventoried acreage 

fell into these risk categories, meaning that most of the 

aspen stands on the Eagle Lake District may disappear 

in the near future without intervention.

A significant element of the Eagle Lake Ranger 

District protocol was the identification of 37 stands where 

there are no longer any living aspen stems present. The 

Eagle Lake assessment revealed that 491 aspen stands 

(94 percent) were in need of conifer removal, and 321 

aspen stands (61 percent) needed control of browsing to 

allow aspen regeneration to establish. All told, conifer 

removal was recommended for 3,122 acres (1,263 ha) 

of aspen, or 99 percent of the District’s aspen area. 

Control of browsing was an added management action 

recommended for 1,534 acres (621 ha) or 48 percent 

of the District’s aspen.

It is interesting to note that the Eagle Lake Ranger 

District also conducted an extensive search of historic 

records that have helped establish a better interpretation 

of the spatial and ecological condition of aspen in their 

area. For example, only 24 percent of the stands in the 

entire inventory has been affected by wildfire since 1910. 

This is in a locality where historic mean and median 

fire interval rates of 5.0 and 7.2 years respectively have 

been reported for sites adjacent to meadows, along with 

mean fire intervals in upland pines sites of 14.4 years 

and 25.6 years in mixed conifer sites (Taylor 1998, 

2000). Additionally, the Ranger District has been able 

to historically document the loss of at least one specific 

aspen stand. Fig. 5-8a is a 1915 photograph of the Susan 

River showing an extensive aspen stand. The stand is no 

longer visible in figure 5-8b, taken from the same photo 
74 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

Figure 5-9. Percentages of 199 aspen stands assessed in 

the Modoc Plateau in 2003 and 2004 by the Alturas Field 

Office, Bureau of Land Management, that identified risk

to stands diminishing in ecological significance or being

lost completely from the landscape. The Aspen Delineation protocol (2003) described in Chapter 7 was used in 

this assessment.

Figure 5-10. Distribution of Modoc Plateau aspen stands in 

figure 5-9 by land form.

Hightest

22%

High

32%

Moderate

28%

Low

16%

None

2%

point in 2003. An intensive on-the-ground assessment of 

the area conducted by District staff found no aspen.

Modoc Plateau Section

Two assessments of aspen habitat have occurred in 

the Warner Mountains in the Modoc Plateau. One effort 

was a 2-year (2002 to 2003) assessment conducted in the 

BLM’s Alturas Resource Area (data on file at the Alturas 

Field Office). The other was an assessment conducted 

by the California Department of Fish and Game in the 

Modoc National Forest (Di Orio and others 2005).

The BLM effort included 199 aspen stands. Sixtyfour percent of the stands were found to be half an acre 

in size or smaller. Overall, the ecological condition of 

these aspen stands was at-risk. The assessment found 

54 percent of the stands were in the high or highest risk 

category, with 28 percent at moderate risk of loss (fig. 

5-9). Of interest geographically, 62 percent of the stands 

in the Modoc assessment were found to be associated 

with talus, lava flows, or rocky outcrops, whereas only 

38 percent of the stands were associated with areas 

more closely linked to surface moisture sources such 

as riparian zones, meadows, springs, and ponds (fig. 

5-10). This is consistent with Smith and Davidson’s 

(2003) Modoc National Forest’s Terrestrial Ecological 

Unit Inventory (TEUI). The User’s Manual for the 

TEUI states that sources of azonal moisture (above 

average moisture compared to the adjacent area) can 

be riparian springs and seeps, moist meadow, or stream 

sides, but can just as likely be related to topographic 

orientations, positions, and site protections such as 

found in toe slopes, north slopes, talus slopes, and areas 

that accumulate and retain snow because of topography 

and/or weather patterns.

The second assessment in the Warner Mountains 

was undertaken by the California Department of Fish 

and Game (Di Orio and others 2005). The authors 

compared, scanned, and orthorectified photographs 

from 1946 that were repeated in 1994. From this, 

they calculated that there was a 24 percent decline 

in aspen coverage during the 48-year period between 

the photos. Total aspen acreage in the study area went 

from 9,689 acres (3,921 ha) in 1946 to 7,495 acres 

(3,033 ha) in 1994, a loss of 23 percent. However, 

there were actually 8 percent more aspen polygons 

identified in 1996, due to increased fragmentation of 

the stands over time.
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 75

Figure 5-11. Meadow Fringe aspen. Note absence of aspen 

in the meadow.

Current Aspen Stand Types in 

the Sierra Nevada__________

The above assessments illustrate that aspen can be 

found in communities located in a range of elevations, 

geographic settings, and biotic relationships with other 

species. These communities can be adjacent to moist 

meadow complexes or on steep talus slopes. They can 

exist in an ever-changing seral relationship with conifer 

species or exist in pure “stable” communities (Mueggler 

1988). The dynamics of these aspen communities have 

been shaped by climate, soil development, topographic 

settings, moisture regimes, natural disturbance, and 

human impacts over an extensive spatial and temporal 

scale.

After examining many of the existing inventory 

and assessments data sets, reviewing the ecological 

classification of Potter (1998) and Smith and Davidson 

(2003), as well as calling upon our own knowledge 

of aspen in California, we have noted that a number 

of characteristics are common to aspen within the 

defined area of interest. We feel it would be valuable to 

describe these aspen types and use them as a basis for 

our discussion of management alternatives in Chapter 

6. We offer the following descriptions of seven common aspen community types that can be found in the 

Sierra Nevada. This approach is not meant to replace the 

value of a more detailed ecological classification, but 

will provide a framework for discussing possible aspen 

management alternatives. Aspen management is best 

approached on a site-specific basis since that is the level 

at which management activities will occur. Site-specific 

characteristics will determine which management options are necessary and, in some cases, even possible. 

Actions that work well on one site may not be suitable 

on another. We hope discussing ecological variability 

found within the following seven classifications will 

help the reader in developing site-specific management 

alternatives for aspen.

Meadow Fringe (Seral Aspen 

Community)

This classification includes aspen communities 

located on the fringe of meadows or within meadow 

complexes (fig. 5-11). In these cases, the meadows 

themselves are often found in sites of very high soil 

moisture. For proper root function, aspen require that 

soils are unsaturated at least seasonally, hence their 

restriction to the drier meadow fringe. These stands 

generally have a rich diverse herbaceous component 

within the stand except where moderate to heavy 

conifer encroachment has occurred. Soils are deep and 

seasonally very damp. These communities, which may 

be composed only of pure aspen in the overstory, are 

still considered seral because of the close proximity 

of conifer seed sources. With conifer seed sources 

nearby the aspen can provide a “nursery” location 

(that is, an ideal location for seeding establishment) 

for shade tolerant species such as white fir (Abies 

concolor). Many of the stands in this category fall into 

the Quaking Aspen/California Corn Lily association 

described in Potter (1998).

Riparian Aspen (Seral Aspen 

Community)

Riparian aspen communities are located along permanent or seasonal watercourses or adjacent to fens, springs, 

or seeps (fig. 5-12). The stands may be: 1) alongside the 

watercourse’s high water mark, 2) on alluvial material 

near water courses, or 3) along watercourses where 

soil buildup has occurred from water flow slowing or 

flooding. Stands may contain thick herbaceous material 

or may contain little herbaceous material depending on 

moisture presence and conifer encroachment. If a riparian corridor is wide, aspen communities can be large. If 

narrow or steep, the communities can take on the form 

of narrow stringers located between the stream and the 
76 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

Figure 5-12. Riparian aspen. These stands are restricted 

to areas along watercourses where sufficient moisture

is available for aspen to grow. They may occur as either 

pure aspen, or mixed with conifers.

Figure 5-13. Upland aspen/conifer. These stands occur 

outside of riparian zones and can be conifer forest with 

a minor aspen component, or aspen stands containing a 

minor conifer component. All can be expected to become 

pure conifer forests in the absence of disturbance.

upland forest landscape. This category also includes 

aspen communities that are located along alder groves 

or with other deciduous species like willows. In many 

cases, the aspen community exists in a narrow line 

between the alders or willows and the forest canopy. 

Depending on soil moisture content, alders or willows 

may or may not be intermixed with the aspen.

Upland Aspen/Conifer (Seral Aspen 

Community)

In California, aspen is a facultative wetland species 

that is equally as likely to occur in wetland or nonwetland locations (USDI FWS 1997). These aspen 

communities are located away from obvious surface 

moisture regimes (fig. 5-13) on flats and hillsides in upland locations and generally have drier soil regimes than 

those located around meadows, seeps, springs, and fens. 

These stands have an early successional relationship with 

the particular conifer communities they are associated 

with, meaning that we can expect conifers to replace 

the aspen in the absence of disturbance. Associated 

conifer species can be Jeffrey pine (Pinus jeffreyi), 

white fir (Abies concolor), red fir (Abies magnifica), 

western juniper (Juniperus occidentalis), and lodgepole 

pine (Pinus contorta). Because of fire suppression, 

climate change, and browsing over the past 100 years, 

these successional relationships may have changed 

significantly. Openings in these aspen communities 

may contain very thick herbaceous material or little or 

no herb or forb component depending on subsurface 

moisture presence and conifer encroachment. Smith 

and Davidson (2003) described these sites as subhygric 

or slightly moist in relation to zonal sites. They often 

found these associations: 1) at the base of steep forest 

hillsides or moraines, 2) at the base of talus slopes, or 

3) in forest zones where no riparian watercourses are 

present. In the Sierra Nevada, many of these sites fit 

into the quaking aspen/mountain pennyroyal association described by Potter (1998). We have personally 

observed that these communities are often upslope, but 

still adjacent to the damper soil of the meadow fringe 

aspen communities. Potter also found these stands on 
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 77

Figure 5-14. Lithic aspen. These stands are associated 

with glacial moraines, talus-colluvium, rock falls, or lava 

flows.

Figure 5-15. Snowpocket aspen. These stands are associated with snow accumulation areas in the Great Basin and 

eastern Sierra Nevada.

benches and high on slopes where subsurface water 

appears to be plentiful.

Lithic Aspen (Lava, Bolder, Talus)

Lithic aspen communities are located on lateral or 

terminal moraine boulder material, talus-colluvium, 

rock falls, or lava flows (fig. 5-14). It has been said 

that such sites act as refugia for aspen (Jones and 

others 2005b). There are four significant relationships that support the concept that lithic sites act 

as refugia for aspen: 1) the damp bare mineral soils 

found in these rocky sites may provide an ideal site 

for one or more aspen seeding events that could account for the clone or clones currently on-site; 2) the 

rocky locations may be subhydric—that is, extremely 

moist in relationship to the zonal site that surrounds 

the community (Smith and Davidson 2003), again 

making them suitable for establishment of aspen 

clones; 3) over time, these sites may have provided 

a safe haven for aspen regeneration from herbivory 

by both native and domestic animals; or 4) the sites 

may limit conifer establishment. Additionally, lithic 

stands of aspen are naturally resistant to wildfire due 

to limited surface fuels. This is somewhat perplexing 

given the fact that we believe aspen to be a generally 

fire-dependent forest type.

Snowpocket Aspen (Stable Aspen 

Community)

Snowpocket aspen communities occur in topographic 

positions where snow accumulates (fig. 5-15), mostly 

at higher north facing elevations. While these aspen 

communities are much more common in the Great 

Basin, they occur in our area of interest, mostly east of 

the Sierra Nevada proper. Smith and Davidson (2003) 

describe these sites as subhygric—that is, slightly moist 

in relation to the surrounding vegetation types. We 

have observed such communities in the Sweetwater 

Mountains north of Bridgeport and on Glass Mountain 

southeast of Mono Lake. The aspen trees in these 

communities are often short and stunted and rarely 

reach true tree stature because of a short growing 

season and harsh winter conditions. Snowpocket aspen 

stands tend to be pure and “stable” aspen stands. Even 

though conifers may be within seeding distances, they 

are unable to become established within the “snow 

pocket” because of the short growing season available 

to the species.

Upland Pure Aspen (Stable Aspen 

Community)

Upland pure aspen stands are rare, except in locations 

in the Southern Warner Mountains of the Modoc Plateau 

ecosystem, to the south of Lake Tahoe on the eastern 

side of the Sierra Crest, or on ranges that branch off 
78 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

Figure 5-16. Upland pure aspen. 

These stands occur outside of 

riparian zones, do not contain 

conifers, and can be expected 

to persist on the landscape as 

aspen.

the main Sierra fault block such as the Monitor Range 

and the Sweetwater Mountains and Glass Mountain 

south and east of Mono Lake. Probably the largest 

established pure aspen communities are located west 

of Highway 395 in the Conway Summit area north of 

Mono Lake (fig. 5-16). Mueggler (1985) recognized 

two conditions that help clarify what is meant by the 

term stable or pure aspen. First, he conceded that 

over extended time periods—sometimes as much as 

1,000 years—stable aspen could become successional 

to conifers. He also felt that the presence of some 

conifers in a pure aspen stand doesn’t necessarily 

drop the community from the stable classification. 

Seral classification in Mueggler’s (1985) view implies 

“incipient or actual prominence of conifers, which 

suggests active replacement of the aspen overstory 

by more shade tolerant trees. Conifers, however, must 

be prominent, not merely present. Occasional conifers 

can be found in a basically stable aspen community 

because of highly unusual and temporary conditions 

that favor their establishment.”

Krümmholz Aspen (Stable Aspen 

Community)

Krümmholz aspen communities are pure aspen 

stands located in a range of habitats including some 

lithic situations. Their distinguishing characteristic is 

that the aspen component of the community is always 

found growing in a distinct shrub or krümmholz stature 

(fig. 5-17) with little stem height development and a 

highly deformed appearance. They can be found on 

ridgelines or other windswept locations, but are most 

often found on the upper elevation limits of other aspen 

communities described in our classifications.

Figure 5-17. Krümmholz aspen. These pure, 

stable aspen communities are limited to 

ridgelines, avalanche tracks, and windswept locations that restrict stem development.
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 79

Figure 6-1. Large aspen tree in Blackwood Canyon, Lake 

Tahoe basin.

Chapter 6.

Management Alternatives for 

Aspen in the Sierra Nevada and 

Tahoe Basin

Characteristics of Aspen in 

the Sierra Nevada Ecosystem_

As discussed earlier, aspen in the Sierra Nevada is 

the same tree species (Populus tremuloides) that grows 

throughout North America. However, it does not occur 

as extensively, or grow in large stands as elsewhere, 

but rather is restricted to sites where it has been able 

to establish in the past and successfully compete with 

other vegetation within the Sierra Nevada ecosystem. 

Although aspen is a minor component of Sierra Nevada 

landscapes, particularly on the west side of the Sierra 

(Potter 1998), we nonetheless can expect it to have the 

same ecologic and adaptive characteristics as aspen 

found elsewhere. First of all, it is intolerant of shade, 

needing full sunlight to establish, grow, and prosper. 

It grows best on deep heavy soils as elsewhere, but 

can exist on a variety of soil types, including rocky 

soils, glacial till, volcanic ash, and alluvial deposits. 

Aspen’s growth rate is directly dependent upon available soil moisture and the microclimate of the site. 

The size and growth rates of individual aspen trees 

in some Sierra Nevada locations rival those of aspen 

anywhere (fig. 6-1).

Limited information exists on the ages of aspen in 

the Sierra Nevada. Aspen is notoriously difficult to age 

properly and requires special techniques to prepare wood 

samples to obtain accurate ages (Asherin and Mata 

2001). However, given the recovery of forests since 

settlement-era logging and fires in the Sierra Nevada, 
80 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

we can assume that much of the aspen growing in association with post-settlement conifers is of similar age.

Earlier, we discussed how aspen can occur either in 

pure, stable stands that do not succeed to other vegetation 

types, or in mixed species stands associated with conifers 

(Mueggler 1988). Although mixed aspen/conifer stands 

predominate in the Sierra Nevada, there are instances 

where pure, stable aspen stands can be found. These 

include isolated “snowpocket aspen” described in the 

previous chapter that occur in the eastern Sierras, in the 

Modoc Range, and near Mono Lake. Since no conifers 

exist within these stands or within seeding range, they 

can be considered to be stable. That does not mean, 

however, that individual stems in stable stands live to 

long ages in the absence of disturbance. Disturbances 

that are active in these stands are more subtle than 

the fires that periodically reset succession in mixed 

aspen/conifer stands. Such factors as drought, insect 

defoliation, frost, disease, etc. can kill enough trees 

to allow suckering to periodically introduce new age 

classes of aspen into the forest.

Although crown fire is often thought to be the disturbance of renewal in aspen/conifer forests, it does 

not necessarily have to occur at a landscape scale to be 

effective. Mixed severity fire regimes, where surface 

and crown fire are both active at sub-landscape scales, 

can create a mosaic of age-class patches across a given 

landscape. It is likely that the active fire regimes of the 

past resulted in a healthier and more diverse distribution 

of aspen age, stem sizes, and successional stages than 

exist in much of the Sierra Nevada today. The key to 

successful aspen management in the Sierra Nevada is 

to seek ways to sustain and increase the diversity of 

aspen by emulating natural disturbance regimes (Rogers 

1996; Franklin and others 2002).

Identifying the Need to 

Regenerate Aspen__________

From an ecological standpoint, before we can “fix” 

aspen we must be able to identify what is “broken.” 

This requires identifying the chronologic, pathologic, or 

successional stage of development for particular aspen 

clones or stands. By that we mean determining whether 

or not aspen trees that currently make up a clone are 

all of one age or contain multiple age classes; whether 

they are young, mature, in decline from competition and 

disease, or being actively replaced by conifers or other 

vegetation. A dichotomous key showing how managers 

might use some of these features to identify aspen stands 

in need of management intervention will be presented 

later in Chapter 7: Assessment and Monitoring. Our 

intent here is to discuss the physiologic and ecologic 

factors behind those classifications, from the perspective 

of the aspen, so readers might understand the reasons 

for their development.

First, aspen has evolved to be a disturbance dependent 

species. Given the proper growth environment, it can 

establish from seed on bare mineral soil left after fire, 

flood, or other major disturbance. However, if a clonal 

root system exists on a site, disturbances that remove 

competition from other vegetation and kill existing 

aspen trees can stimulate the root system to initiate a 

suckering response. Suckers arise from pre-existing 

roots and will appear only where roots from the previous aspen exist, normally no more than a tree-height 

from the edge of the previous aspen stand (although 

lateral roots have been documented to spread farther 

[see Chapter 3: Aspen Genetics).

Because new roots establish with each ramet generation (Shepperd and Smith 1993), periodic disturbances 

with replacement of some or all of the stems in a clone 

will result in the expansion of the clonal footprint 

through time. Although very large and old clones have 

been documented in Colorado (Grant 1993) and Utah 

(Mitton and Grant 1996), and hypothesized in the Sierra 

Nevada (Potter 1998), the spatial extent of an aspen 

clone’s lateral root system depends as much upon the 

existence of suitable resources for the aspen to grow 

and survive as the length of time a clone has occupied 

a site. Aspen may be limited to where sufficient water 

exists for growth, such as riparian areas, or along the 

lee side of ridges where snow accumulates, or aspen 

may be limited by competition from other trees. Aspen 

has been documented to persist in mixed aspen/conifer 

stands in Colorado for multiple generations (McKenzie 

2001), where periodic small-scale disturbances provided 

for the establishment of new cohorts and maintained a 

viable root system. Conversely, aspen/conifer forests 

may become pure aspen given the right disturbance 

(for example, a high intensity crown fire that kills all 

conifers in a landscape). Some landscapes in Colorado 

are currently occupied by aspen that sprouted after large 

extensive fires in the 19th century and may cover more 

area than before the fires (Kulakowski and others 2004). 

But again, those aspen forests would not exist today if 

root systems from a previous generation of aspen had 

not been present to initiate new suckers.

Similarly, the number of suckers that arise following a disturbance is dependent upon the number and 

density of roots that exist from the previous generation 
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 81

(Shepperd and others 2001). A vigorous, dense clone 

will produce more suckers if burned or cut than a poorly 

stocked clone because of the proportionality of aboveto below-ground biomass in an aspen clone (Shepperd 

and Smith 1993; Shepperd and others 2001). Therefore, 

it may be easier to regenerate a healthy, dense young 

clone than an old, sparsely-stocked one.

The presence of suckers or younger established 

stems in a clone indicates that the clone is healthy, has 

a vigorous root system, and is genetically predisposed 

to regenerate with minimal disturbance. Conversely, 

a poorly stocked clone with few remaining live stems 

is not likely to have a vigorous and extensive root 

system (Shepperd and others 2001). Aspen roots are 

living tissue and have respiration demands that require 

replacement of stored carbohydrate reserves through 

photosynthesis to remain alive. Most roots in a mature 

clone are physically connected to only a few stems 

(DeByle 1964; DesRochers and Lieffers 2001) and will 

therefore die if separated from the parent tree when 

conditions do not allow the successful establishment 

of new suckers.

The belief that an extensive aspen root system exists underground where an aspen grove once stood is 

a fallacy, as is the notion that thousands of suckers 

will spring up if a clone containing only a few live 

trees is cut or disturbed (Ohms 2003). A better rule of 

thumb would be “as goes the above-ground portion of 

an aspen clone, so goes the below-ground portion.” 

Consideration of this rule is essential when estimating 

the need for protecting aspen sprouts from browsing 

after a regeneration treatment.

Large, dense, vigorous aspen clones are likely to 

produce many more sprouts than small, poorly-stocked 

clones. However, reintroduction of repeated disturbance 

in poorly-stocked clones may eventually allow significant regeneration and clone spread over a number of 

disturbance cycles as the clonal root system develops 

over time. Although all of the above examples are drawn 

from studies conducted outside of the Sierra Nevada, it 

seems reasonable to assume that similar dynamics are 

occurring in Sierra Nevada ecosystems.

Treatment Alternatives to 

Regenerate Aspen__________

As discussed earlier, laboratory studies of the general physiology of the species indicate that vegetative 

regeneration of aspen requires the interruption of auxin 

flowing from shoots to roots to stimulate root buds to 

begin growing (Schier and others 1985). This can result 

from disturbances that kill the parent trees outright, such 

as a fire, disease, or timber harvest, or from disturbances 

that only temporarily defoliate the parent tree, such as 

a late frost, defoliating insect attack, or light herbicide 

application. Severing lateral roots from parent trees 

can also cut off auxin flow and initiate suckering. This 

might occur when fire, burrowing animals, or human 

factors (for example, road building) kill portions of a 

lateral root or when roots are mechanically separated 

from parent trees (Shepperd 2004). This sucker-initiating 

process has been referred to as interruption of apical 

dominance (Schier and others 1985).

In any case, the initiation of shoot bud growth must 

also be accompanied by sufficient sunlight and warmer 

soil temperatures to allow the new suckers to thrive 

(Doucet 1989; Navratil 1991). Optimal aspen sucker 

growth occurs when soil temperatures are 59o F (15° 

C) or above. Full sunlight to the forest floor best meets 

these requirements. However, young aspen suckers 

are susceptible to competition from other understory 

plants and herbivory from browsing ungulates, even if 

abundant suckers are present.

The interaction and co-dependency of factors that 

affect aspen sucker initiation, growth, and survival 

can be expressed as a triangle model similar to the fire 

behavior triangle used by firefighters (Shepperd 2001) 

(fig. 6-2). Successful aspen suckering depends upon 

three key interacting components: hormonal stimulation, 

growth environment, and protection of the resulting 

suckers. One or more of the silvical characteristics 

of aspen discussed above is involved in each of these 

factors. Any manipulation of aspen must satisfy all 

Figure 6-2. The aspen regeneration triangle management 

decision model (Shepperd 2001, 2004).
82 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

three of these requirements to successfully regenerate 

the species.

The three elements of the aspen regeneration triangle 

may not always need to be actively provided by managers when trying to regenerate aspen. One or more 

of the elements could already exist in any particular 

aspen stand, so identifying which factors are lacking 

is crucial.

Techniques that can be used to initiate aspen suckering 

and provide a favorable growth environment include 

removal of existing trees through harvest, separation 

of roots from parent trees, removal of competing vegetation, and prescribed burning. Protection of suckers 

from browsing can be provided by satiating the demand, 

constructing physical barriers to browsing animals, or 

controlling animal movement.

Clearfell-Coppice Harvest

Complete removal of all aspen trees has been the 

traditional method of regeneration where commercial 

markets exist for aspen. The correct silviculture term 

for this activity is clearfell-coppice (Ford-Robertson 

1971) rather than clearcutting, since the forest will be 

regenerated by root suckering and not by seeding or 

planting. Clearfell-coppice regeneration fully stimulates the roots to produce new suckers by completely 

removing all parent trees. It also provides an optimal 

growth environment by allowing full sunlight to 

reach the forest floor. Because commercial quality 

aspen stands are generally quite large, harvest blocks 

can be large enough to add an element of protection 

from browsing animals and diseases through the sheer 

numbers of suckers that are produced.

Clearfell-coppice harvest probably has limited application in the Sierra Nevada. In addition to requiring 

large aspen stands, commercial markets for the aspen 

trees that are removed are needed in order for projects 

to be economically viable. Clearfelling does not work 

well in areas where aspen stands are small, unless 

cut units are fenced from browsing animals (where 

they are a problem) following treatment. Although 

clearfell-coppice harvest can introduce new age classes 

of aspen into landscapes, old trees, which provide 

many ecologic characteristics that are desirable for 

aspen forests, are eliminated within cutting units. Soil 

compaction and death of lateral roots from which 

suckers arise has occurred during harvest operations 

in Colorado (Shepperd 1993) and may also occur on 

similar clay soils (described by Potter [1998]) in the 

Sierra Nevada.

Root Separation

Mechanically severing lateral roots at some distance 

from parent trees is one means of regenerating aspen 

while retaining an older tree component in the aspen 

forest. This technique relies on the wide-spreading root 

habit of aspen to establish suckers in locations where 

they have a more favorable growth environment than 

that found under dense large aspen. Severing lateral roots 

blocks the flow of auxin from parent trees and provides 

the hormonal stimulation to allow pre-existing buds to 

produce suckers, provided a good growth environment 

exists and suckers are protected from excessive animal 

browsing.

This particular treatment technique was developed 

after a study in Central Colorado (Shepperd 1996) 

found that bulldozed areas produced more suckers than 

cut areas and that more suckers established in fenced 

areas than in those left unfenced. However, leaving all 

cut or bulldozed aspen trees on site clearly inhibited 

aspen sucker establishment. The stimulation effect of 

the bulldozed treatments was attributed to the complete 

severing of the stems from the roots. Apparently the 

stumps from cut trees retained some auxin, which had 

an inhibitory effect on subsequent suckering when the 

stumps were left attached to the roots.

Results from this initial study prompted the establishment of two additional studies in Arizona to investigate 

alternative mechanical treatments that might be used to 

stimulate aspen suckering (Shepperd 2004). In the first, 

a crawler tractor with a ripper attachment was used to 

sever lateral roots in an open mature aspen stand that 

had been partially harvested 15 years earlier (fig. 6-3). 

This treatment stimulated about 486 suckers per acre 

(1,200 suckers / ha) while an unripped, but fenced portion 

produced only half that amount. Although fencing the 

stand clearly influenced increased sucker survival, the 

extra hormonal stimulation provided by tractor ripping 

doubled the number of suckers without any mortality 

to overstory trees.

A second study on the Coconino National Forest in 

Arizona (Shepperd 2001) ripped along the edge of a 

small isolated aspen clone that was growing beside a 

meadow. Using a single tractor pass cutting to a depth 

of 7.9 inches (20 cm) roots extending into the meadow 

away from the existing trees were severed. This simple 

treatment resulted in the establishment of the equivalent 

of over 10,500 stems per acre (26,000 stems/ha) up to 

45.9 ft (14 m) away from existing trees into the meadow. 

In this case, lateral roots produced suckers about 1 to 

11⁄2 tree heights away from existing mature trees. No 

suckers were noted between the ripped zone and existing 
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 83

Figure 6-3. Severing lateral aspen 

roots to stimulate suckering 

using a tractor-mounted ripper. 

Coconino NF, Arizona (Shepperd 2001, 2004).

Figure 6-4. Map of aspen suckers in a 6 x 45 foot (2 x 14 m) transect extending into a meadow adjoining 

an edge-ripped aspen clone, one year after treatment. Coconino NF, Arizona. Each circle represents 

one aspen sprout (Shepperd 2001, 2004).

trees (fig. 6-4), indicating that auxin from the parent 

trees inhibited suckering in the portion of roots in that 

zone. As in the previous study, no existing trees were 

killed by the ripping treatment.

These results are consistent with natural suckering 

events that we have observed in isolated Sierra Nevada 

aspen clones surrounded by meadows or shrublands. 

Therefore, we feel that ripping offers the potential for 

expanding the size of some existing Sierra Nevada aspen 

clones, or introducing new aspen age classes into others without sacrificing existing aspen trees. However, 

if clones are small and browsing animals are present, 

protection of sprouts may be necessary. A single pass 

of the ripper along the edge of existing trees should 

be sufficient to isolate roots and stimulate suckering. 

Multiple passes may excessively injure roots and result 

in diminished suckering. Care should be taken if root 

diseases are present as ripping will provide entry ways 

for disease.

Removal of Competing Vegetation

Changing the growth environment may be all that 

is needed to successfully regenerate aspen, if hormonal stimulation already exists and protection for the 
84 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

Figure 6-5. Removing competing conifers and fencing this 

two-stem aspen clone allowed new suckers to establish. 

Kaibab NF, Arizona (Shepperd 2001, 2004).

Figure 6-6a-d. Photo series showing aspen regeneration 

response to removal of competing overstory conifers. 

Photo (a) was taken in 2000, prior to treatment. Photo 

(b) was taken in 2001, photo (c) in 2003, and photo (d) 

in 2004. Eagle Lake Ranger District, Lassen NF.

a

b

c

d

suckers will be provided. If older aspen trees are 

stressed, they may already be trying to sucker, so the 

hormonal stimulation to regenerate already exists. This 

is often the case in late successional aspen/conifer stands 

where aspen is a minor component of the stocking. 

Removing the conifers will allow sunlight to reach the 

forest floor, raising soil temperatures, and providing 

the proper growth environment for aspen suckers to 

thrive. Managers can “punch holes” in the conifer forest 

surrounding isolated pockets of residual aspen and this 

will often cause the area to be restocked with aspen. As 

with other treatments, care should be exercised when 

browsing animals are a problem.

Removal of competing conifers will enhance any 

natural sucker production already occurring in declining 

clones and can retain any remaining old aspen trees 

for aesthetic and wildlife purposes. This technique has 

been successfully used in the Sierra Nevada (Jones and 

others 2005b). However, new aspen suckers in the clones 

in advanced stages of decline may require protection 

in order for them to successfully establish.

The effects of removing competing vegetation can 

be quite dramatic. Shepperd (2004) described a case in 

Arizona where removing conifers and fencing the area 

surrounding a clone consisting of two mature aspen 

trees resulted in over a hundred established aspen trees 

after 5 years (fig. 6-5). Removing competing vegetation 

produced similar results in (Jones and others 2005b) 

Sierra Nevada study (fig. 6-6).

Protection From Browsing

If an aspen clone is attempting to sucker, and if 

suckers are heavily browsed and shrubby in appearance 

with no central growth axis, protection from browsing 

may be all that is needed to successfully re-establish 
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 85

the stand. Obviously, the hormonal stimulation to 

sucker exists if few overstory conifers are present 

and if a favorable growth environment also exists. 

The heavily browsed suckers would indicate that 

they need to be protected in order to grow above the 

reach of animals.

Direct protection of aspen reproduction will likely 

be expensive because of the cost of constructing fences 

(Rolf 2001; Kees 2004). However, it may be the only 

way to successfully reestablish aspen in the many areas 

in the Sierra Nevada where aspen is a minor component 

of forested landscapes and browsing animals are present. 

Manipulating logging slash (Rumble and others 1996) 

and “hinging,” or partially felling aspen or conifer 

trees around the perimeter of clones (Kota 2005), have 

been used in South Dakota to keep browsing animals at 

bay and may also be an option in some Sierra Nevada 

situations.

Whatever the method used, the goal is to prevent 

browsing of the terminal leader of the young aspen suckers, which can lead to “hedging,” or a shrubby growth 

form that will never develop into a tree. Efficient fence 

designs have been developed (Rolf 2001; Kees 2004), but 

require regular maintenance to be effective. A perimeter 

clearance of one tree length of forest on both sides of 

the fence is suggested to minimize damage from trees 

falling on the fencing. Leaving too much slash on the 

ground can inhibit suckering (Rumble and others 1996; 

Shepperd 1996). Hinging requires skilled sawyers and 

a sufficiently dense aspen stand to create a perimeter 

barrier around the clone (Kota 2005). Chemical browse 

repellents were found to be effective at high dosages in 

a Colorado study using penned elk (Baker and others 

1999), but are likely too expensive for most wildland 

applications.

The length of time that suckers need protection 

depends upon whether the browsers are domestic 

livestock or elk. Domestic livestock will usually not 

bother suckers over 6 ft (2 m) in height, but elk can 

break off and consume aspen saplings up to 1.5 inches 

(4 cm) in diameter at breast height (4.5 ft or 1.4 m). 

In most cases 8 to 10 years of normal growth are 

necessary for suckers to attain these sizes (Shepperd 

2004).

Prescribed Fire

Because aspen is a fire-adapted species, prescribed 

fire can be used very effectively to regenerate aspen. 

Fire provides two of the three essential elements of 

the aspen regeneration triangle. Killing overstory 

stems and injuring lateral roots provides hormonal 

stimulation to initiate sucker production. Removal of 

competing vegetation and blackening the soil surface 

(allowing it to be warmed by the sun) creates ideal 

growing conditions for suckers. Burning also releases 

nutrients that contribute to the growth of suckers. 

However, fire may not provide protection for the 

new sprouts, unless large enough areas of aspen have 

been burned to satiate browsing animals’ appetites for 

aspen sprouts.

Pure aspen forests are somewhat difficult to burn 

because fuel loadings are generally light and the lush 

understory vegetation usually has high moisture content 

and does not contain sufficient biomass to burn effectively (Fechner and Barrows 1976). Effective burning 

to regenerate aspen in these stands requires timing 

the fire when fuels are dry, or using alternative fuels 

to carry the fire into the aspen (for example, burning 

adjacent shrublands) (Shepperd 2004). Fire will usually burn into the aspen far enough to stimulate new 

aspen suckering along the edges of clones, even if the 

overstory aspen trees are not killed outright. This can 

create a diverse landscape in which some clones are 

completely replaced by new suckers, while others have 

some surviving overstory stems, but with new suckers 

beneath them and extending out from the periphery 

of the surviving trees. In both cases, the footprint or 

the area occupied by aspen in these landscapes will 

be increased to the area occupied by lateral roots surrounding the existing clone. This has been reported to 

be about 11⁄2 to 2 times tree height away from existing 

aspen stems from studies in South Dakota (Keyser 

and others 2005), Arizona (Shepperd 2004), and Idaho 

(data on file, Rocky Mountain Research Station, Ft. 

Collins, CO). Similar results could be expected in the 

Sierra Nevada.

Prescribed crown fire has been used successfully in 

southern Utah in mid- to late successional aspen/conifer 

forests where conifer crown bulk density is sufficient to 

carry a crown fire (Shepperd 2004). It should also work 

in similarly structured mixed aspen/conifer forests in 

the Sierra Nevada. This technique requires natural fuel 

breaks to keep the fire from spreading outside desired 

treatment areas. Such burns should be planned when 

soil moisture is high to avoid excessive damage to the 

shallow aspen roots. Although risky, prescribed crown 

fire provides all elements of the aspen regeneration 

triangle and can reintroduce large areas of pure aspen 

into mixed species landscapes. Although it also carries 

the social stigma of resembling a wildfire (killing all 

existing aspen as well as conifers), there are many 

positive benefits to this approach. Prescribed crown 
86 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

Figure 6-7. Sucker densities (with standard deviation bars) 

before (1994) and 5 years after a spring prescribed burn 

in light logging slash. Coconino NF, Arizona (Shepperd 

2004).

Figure 6-8. Average dominant sucker height (with standard 

deviation bars) before (1994) and 5 years after a spring 

prescribed burn in light logging slash, Coconino NF, Arizona 

(Shepperd 2004).

Figure 6-9.Average soil temperatures during the first growing

season after treatments for burned and unburned aspen 

sites, with standard error bars. Coconino NF, Arizona 

(Shepperd 2004).

fire will not only rejuvenate aspen and reset vegetation succession, but it can also increase understory 

vegetation diversity, forage production, and water 

yields, as well as improve habitat for many wildlife 

species (Bartos and Campbell 1998).

A major disadvantage to using prescribed crown fire 

is safety. It can be used in isolated aspen/conifer stands 

surrounded by non-forest vegetation or where mixed 

crown and surface fire can be tolerated across large 

landscapes. Although there is a certain degree of risk 

that an unintended wildfire might result, this type of 

fire is probably what maintained many Sierra Nevada 

aspen/conifer forests in pre-settlement times.

Combined Treatment Techniques

Many times a combination of mechanical treatment and prescribed fire is the best course of action 

to regenerate aspen in mixed aspen-conifer stands. A 

combined treatment can provide a means of emulating 

natural fire regimes by providing maximum hormonal 

stimulation and optimal growth environments for aspen 

suckers as well as eliminating or reducing competing 

conifers. Shepperd (2001, 2004) combined commercial 

harvest, prescribed burning, and fencing to successfully 

regenerate aspen in a study in northern Arizona. All 

ponderosa pine were removed within and surrounding 

isolated aspen clones using a commercial timber sale, 

and the entire area was fenced with an elk-proof wire 

fence. Logging slash was then scattered throughout 

the area and a prescribed burn applied to half of the 

area the next spring following snowmelt when soils 

were wet. The prescribed burn had a striking effect 

on both the numbers of suckers that were produced 

and survived over a 5-year period (fig. 6-7) and on the 

sucker height growth (fig. 6-8). Part of this effect was 

undoubtedly due to nutrients introduced into the soil 

by the fire, but the solar warming of the soil during 

the first few growing seasons following the fire (figure 

6-9) likely contributed as well.

Burning heavy logging slash in harvested areas can 

be detrimental to aspen suckering, especially when 

conditions are dry (Shepperd 2004). Intense heat 

penetration into the soil from the burn can kill aspen 

roots beneath heavy fuel concentrations (fig. 6-10), but 

studies in Colorado have demonstrated that adequate 

suckering can be maintained if soil conditions are wet 

when burning heavy slash (Shepperd 2004).
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 87

Figure 6-10. An excavated aspen root segment ends at the 

edge of soil scorched by burning a small pile of slash. 

Roots under the pile were consumed by the fire.

Treatment Opportunities 

Specific to the Sierra Nevada 

and Lake Tahoe Basin_______

Aspen in the Sierra Nevada and Lake Tahoe basin 

are typically characterized by small stands that are 

intermixed with conifers, or isolated pure stands that 

occur in riparian areas, snow accumulation zones, or 

other topographic zones where moisture conditions are 

favorable for aspen. Although aspen in the Sierra Nevada 

is the same species and occurs in association with the 

same vegetation as elsewhere in the west, it does not 

grow in extensive landscape-wide forests. Therefore, 

management opportunities and techniques are more 

limited in the Sierra Nevada than those available to 

managers elsewhere.

A major difference between aspen in the Sierra 

Nevada and other places in the West is that commercial 

markets do not exist for aspen here. Consequently, the 

need to optimize growth of defect-free trees is not a 

primary goal of management. Instead, the principal 

objective of aspen management in the Sierra Nevada is 

to retain aspen on the landscape and restore declining 

or disappearing clones to a more healthy condition.

The seven Sierra Nevada aspen types described in 

Chapter 5 can serve as a framework on which to base 

our discussion of factors to consider in developing 

management alternatives. These classifications are 

not defined by associated vegetation, or limited to 

individual genotypes or clones, but are based solely 

on our observations of aspen’s occurrence in the Sierra 

Nevada across physiographic positions, soil conditions, 

and associations with conifers.

Upland Pure Aspen

These stands are typically found along the east side 

of the Sierra Nevada or Modoc Plateau in association with grass or shrublands, where conifers are not 

present (fig. 5-16). Although site productivity is not 

high, some of these stands are several hundred acres 

in size. They may appear single-aged, but such stands 

usually contain cohorts of different ages and likely 

are made up of multiple aspen genotypes, or clones. 

Since these stands are similar to those found on the 

eastern edge of the Great Basin and in the Rocky 

Mountains, we recommend using the key by Campbell 

and Bartos (2001) (see Chapter 7) to identify the need 

for management intervention. Basically, intervention 

is needed if clones are in decline and no successful 

aspen suckering has occurred.

Management alternatives could include any of those 

discussed earlier, including clearfelling to introduce 

new aspen age classes into a landscape. These large 

stands are likely the only type of aspen occurring in 

the Sierra Nevada where clearfelling might be a viable option to stimulate sufficient suckering to satiate 

browsing animals.

Upland Aspen/Conifer

Aspen is most commonly associated with conifers 

on upland (nonriparian) physiographic locations in the 

Sierra Nevada (fig. 5-13). These associations can occur 

with most, if not all, conifer forest types in the Sierra 

Nevada and can range from aspen forests containing a 

conifer understory to a few isolated individual aspen 

stems surviving in an otherwise pure conifer forest. 

The common feature of aspen in these situations is 

that the current ramet generation will most likely be 

replaced by conifers without a stand replacing fire or 

other disturbance to open the canopy and allow aspen 

to regenerate.

Clearly the stands most in need of immediate 

management intervention are those that contain the 

smallest component of aspen—the isolated mature stems 

described above. Identification of stands in need of treatment can be done by using the assessment techniques 

described in Chapter 7 developed by Campbell and 

Bartos (2001). To have successful regeneration, aspen in 
88 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

this situation will require the most intensive management 

actions. A large number of mature conifers may have to 

be removed to allow enough light to reach the forest floor 

for successful suckering. Even so, sucker densities are 

likely to be low because aspen lateral root systems are 

likely to be sparse in these situations. Fencing will likely 

be required if browsing animals are present. Prescribed 

burning is not likely useful if overstory conifers are large 

fire-resistant species. Prescribed surface fire may be 

useful in stimulating suckering, provided activity fuels 

are not too heavy. Burning in spring when soils are wet 

will avoid excessive damage to aspen roots.

Mixed aspen-conifer forests that contain a sizable 

component of aspen are most likely to benefit from 

conifer removal. They still have sufficient stocking 

and root density to sucker well, and at high densities. 

Complete conifer removal should be implemented to 

maximize the longevity of the treatment and introduce 

the attributes of a pure aspen forest into the landscape. 

However, an alternative treatment would be to “punch 

holes” in the conifers around the aspen (see discussion 

above in this section) to allow suckering to occur from 

roots extending into the conifers. Removal of some 

aspen may be necessary to break apical dominance if 

clones are vigorous and healthy, but complete removal 

of all aspen is unnecessary.

Care should be taken in the logging process to 

avoid damaging aspen trees that are intended to be 

left. Track-mounted mechanical feller-bunchers are 

the most efficient means of accomplishing conifer 

removal without damaging aspen stems or roots, but 

careful directional hand felling can work, too. Conifer 

logging slash should be removed to allow sunlight to 

reach the forest floor, unless a prescribed fire is planned 

to stimulate additional suckering. In the latter case, only 

scattered branches and tops should be left. Broadcast 

burning of heavy loadings of 1000-hour fuels will likely 

kill too many shallow aspen roots and result in poor 

suckering. In any case, a prescribed fire burning through 

logging slash will likely kill any remaining overstory 

aspen stems. Similarly, any piling and burning of slash 

should be done outside of the aspen lateral root system 

footprint that extends at least one tree height away from 

existing stems. Burning even small hand-piles can kill 

aspen roots (fig. 6-10).

Two additional issues should be discussed with 

regard to upland aspen/conifer forests. We remind the 

reader that goshawk nesting sites are off limits to any 

cutting under the record of decision for the FSEIS 

SNFPA guidelines (Appendix I). Therefore, any aspen 

occurring in these areas cannot be actively managed. 

Second, conifer trees larger than 30 inches can be cut 

outside goshawk PACs, if the action is properly justified 

(Appendix I). We believe that the removal of large conifer 

trees is justified (and in fact ecologically necessary) to 

restore aspen, if the aspen is overtopped by large trees. 

Leaving large conifers may not allow enough light to the 

forest floor to stimulate adequate aspen suckering and 

will certainly provide a ready source of conifer seed to 

quickly re-establish a dense conifer understory.

Riparian Aspen

These aspen stands occur along perennial and intermittent watercourses throughout the Sierra Nevada and 

are especially common within the Lake Tahoe basin 

(fig. 5-12). Although individual clones may extend 

upland beyond the riparian zone, our concern here is 

for those aspen stands where growth and development 

is influenced by the deeper soils and moist growing 

conditions associated with the streamside riparian plane. 

Because these aspen stands are located in riparian zones 

defined in the Sierra Nevada Forest Plan Amendment 

(USDA Forest Service 2004c), management activities 

and options are more restricted here than elsewhere. 

Some riparian clones appear to be in good shape and 

contain multiple age classes of healthy trees, while 

others are rapidly being replaced by conifers. Still others 

have aspen regeneration that is being over-utilized by 

browsing animals.

Providing the proper aspen growth environment 

and protecting aspen regeneration from browsing will 

probably be essential to rehabilitating these clones. 

Prescribed burning and hand-felling of small conifer 

trees may help alleviate conifer competition in some 

cases, but removal of large mature conifers will often be 

required to provide the proper growth environment for 

aspen. Such activities are not specifically prohibited in 

the Record of Decision for the FSEIS SNFPA (USDA 

Forest Service 2004c) (Appendix I), but will require 

additional planning, coordination, and innovative techniques to avoid adverse effects to the riparian zone.

Soil disturbance and compaction is a major concern 

with mechanical treatment activities in the riparian zone. 

Compaction in aspen soils increased with the number of 

times equipment passed over the site in a Colorado study 

(Shepperd 1993), so innovative harvest techniques may 

be needed to remove large trees. Mechanical harvesting 

by a tracked feller-buncher, which can drive into the 

riparian area, harvest a tree, and carry it directly back 

out over the same track without turning, will create 

very little soil disturbance and minimize compaction. 

Similarly, harvesting in winter when soils are frozen, 
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 89

dry, or snow covered minimizes soil disturbance and 

compaction. Regardless of the harvest technique used, 

whole-tree harvesting will remove all slash and allow 

maximum light to reach the forest floor and stimulate 

new aspen sprouting.

It is extremely important that all overstory conifers 

be removed to allow light to reach aspen roots. This 

should include cutting conifers that likely shade aspen 

roots that extend away from existing aspen stems. A 

good rule of thumb would be to remove conifers from 

a large enough area to allow sunlight to reach an area 

11⁄2 tree heights away from existing aspen stems as 

described earlier (see Chapter 6: Treatment Alternatives 

to Regenerate Aspen).

Although prescribed burning will remove competing conifer seedlings and maximize aspen sprouting, it 

may not always be feasible to use fire in riparian zones 

where soot and ash might reach streams and potentially 

affect water quality.

Meadow Fringe Aspen

This type of aspen is similar to riparian aspen, but 

instead of occurring along stream corridors, the aspen 

is situated along the edges of meadows, juxtaposed 

between the mesic grassland vegetation and the drier 

upland forest in a narrow band where conditions are 

ideal for aspen (fig. 5-11). Often, the presence of 

residual aspen stems and downed logs in the conifer 

forest behind the aspen indicates that these stands may 

have been larger under the frequent fire regimes of the 

past. In some instances, the presence of younger aspen 

stems near the meadow indicates that aspen is continuing to invade the meadow. However, in some marshy 

meadows, further aspen invasion appears to be limited 

by saturated soils.

Opportunities exist to expand many meadow fringe 

aspen by removing conifers behind the aspen, away 

from the meadow, and allowing the aspen to re-colonize 

the area where aspen lateral roots still exist. Careful 

inspection of the conifer forest behind the aspen can 

locate ephemeral aspen sprouts in the understory that 

will reveal the extent of the aspen root system. Conifers 

should be removed for a sufficient distance (at least 1 

to 1 1⁄2 tree heights) beyond the aspen roots to allow 

full sunlight to reach the forest floor and stimulate 

sprouting. If logging slash remains, it should either 

be removed and piled outside the area occupied by 

aspen roots, or burned when soil conditions are wet to 

minimize damage to aspen roots. Fencing may also be 

necessary if browsing animals are present since aspen 

sprouting may be sparse due to low root density under 

the conifer forest.

Snowpocket Aspen

These types of aspen stands are common along the 

eastern fringe of the Sierra Nevada and into the Great 

Basin where snow accumulates along the lee side of 

ridges and isolated mountains. The extra moisture that 

accumulates in these topographic locations is sufficient 

to support aspen within and just beneath the zone of 

maximum snow accumulation (fig. 5-15). These aspen 

stands are often pure, or contain few conifers, and are 

characterized by small misshapen stems that have been 

damaged and contorted by the drifting snow. The spatial 

extent of these stands is often limited by the topographic 

conditions that allow extra snowpack to accumulate. 

Heavy snow years damage aspen trees and provide 

sufficient mortality to stimulate the periodic production 

of new aspen suckers.

Snowpocket aspen is more resilient than other types 

of aspen in the Sierra Nevada because these stands are 

often pure aspen (not being invaded by conifers) and 

contain multiple age cohorts as a result of periodic 

snowpack disturbance. Active management intervention 

may not be needed if all of these features are present, 

even though the stand may not fit our ideal vision of 

what an aspen stand should look like. However, if a 

snowpocket aspen stand is in obvious decline without 

any new recruitment, or the aspen have been largely 

replaced by conifers, then active management may be 

needed. Alternatives may include fencing to exclude 

browsing animals or removal of conifers by mechanical 

means or by prescribed crown fire. The latter option 

may be viable because snowpocket aspen is often 

topographically isolated and surrounded by grasslands 

or shrublands that provide natural fire breaks. If snowpocket aspen occur on active sheep allotments, simply 

requiring herders to avoid them may be sufficient to 

allow the aspen to successfully regenerate.

Lithic Aspen

We use this term to describe aspen that is growing on 

talus slopes, basalt flows, and other rocky situations that 

would seem to be the antithesis of what thriving aspen 

require (fig. 5-14). In spite of the apparent contradiction, such rocky conditions serve as refugia for aspen 

and allow it to persist in some landscapes. Wide-ranging aspen roots can penetrate into the spaces between 
90 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

rocks to access pockets of soil. The rocks essentially 

act as mulch, limiting water evaporation and preventing 

buildup of fuels that would allow fire to kill aspen. Aspen 

suckers can rely on root reserves to grow quickly above 

the rocks, where they enjoy full sunlight and may be 

protected somewhat from browsing animals.

Some lithic aspen may require removal of competing 

conifers and fencing to protect sprouts from browsing 

animals, but other management options may be limited. 

Before embarking on further actions, we recommend 

monitoring to ensure that the aspen is persisting and 

not in danger of disappearing.

Krümmholz Aspen

These are rare, but ecologically unique aspen stands 

that occur near upper treelines in the Sierra Nevada 

(fig. 5-17). Aspen often do not achieve tree form in 

these stands, but persist as misshapen shrubs that 

barely cling to life near the upper reaches of forest 

vegetation. Any attempt at stimulating additional 

sprouting by use of mechanical treatment or prescribed 

fire in Krümmholz aspen stands is likely to upset the 

delicate balance under which they exist and may do 

more harm than good. Management intervention for 

these aspen stands should probably consist of careful 

monitoring to ensure that browsing animals are not 

adversely affecting the aspen, and fencing clones that 

are being over-browsed.

Managing Aspen in the 

Wildland Urban Interface___

We’ve included this section to discuss alternatives 

that might be used to enhance aspen growing in areas 

that would not normally be managed for aspen or where 

other management objectives overshadow management 

for aspen. With some additional thought, actions can 

be undertaken in many of these circumstances that will 

fulfill the primary management objective and also benefit 

aspen that happens to occur in these areas.

Currently, many fuels treatment activities are underway within the Wildland Urban Interface (WUI). 

These treatments include thinning, mechanical mastication, and prescribed burning to remove understory 

conifers and accumulated ground fuels. Such activities 

will inherently benefit any aspen in these forests by 

allowing light to reach the forest floor to stimulate 

new suckering. However, aspen could benefit further 

by the removal of all conifers over an aspen clone’s 

root system footprint. Such removals would add spatial 

diversity to the forest and create pockets of pure aspen 

that may also act to alter the behavior of fire burning 

through the area. Including aspen-benefiting activities in WUI fuel treatments is certainly justified and 

beneficial to the forest ecosystem in the long run. We 

should note that diameter limits apply to the removal 

of conifers in fuel treatment projects (Appendix I). If 

large conifers need to be removed to benefit aspen, the 

activity needs to be planned and funded as an aspen 

restoration project so that diameter limits would not 

apply (Appendix I).

Similarly, private landowners whose property adjoins 

and is interspersed within the WUI (fig. 6-11) can also 

benefit from activities that retain and regenerate aspen. 

Because aspen will not burn with the intensity of conifers 

it can be planted or retained closer to structures than 

conifer trees. Creating aspen glades on even a small 

property will alter fire behavior (Fechner and Barrows 

1976) while retaining a forested appearance. We encourage counseling private property owners about the 

benefits of retaining aspen wherever possible.

Managing Aspen on 

Rangelands________________

Adaptive management of aspen communities on 

range allotments may be one of the most valuable 

tools in preserving regional aspen communities. As 

was described in Chapter 4: Range Management and 

Aspen Communities, Potter (1998) found that many 

of the stands he sampled in the Sierra Nevada were 

located adjacent to meadows and other moist areas 

where livestock congregate in the summer season for 

shade, forage, and access to water. During his study, he 

found that livestock would often graze heavily in these 

areas and use aspen as a primary browse species. This is 

consistent with Menke and others (1996) who found the 

most degraded range habitat to be located in drainages, 

meadows, watering places, and other natural livestock 

concentration areas. Loomis and others (1991), Kie 

and Boroski (1996), and Loft and others (1991) also 

noted similar intensive use of these types of habitats 

in the Sierra Nevada, as did Julander (1955) in Utah. 

Therefore, active management (restricting livestock) 

on rangeland will likely be necessary to successfully 

establish aspen suckers and retain biodiversity when 

aspen stands occur in these locations.
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 91

Figure 6-11. Aspen regenerating 

after fuels treatment activities 

on a private lot.

While we focus this part of our discussion on browsing 

of aspen by domestic livestock, we want to accentuate 

that long term management of aspen communities in 

rangelands needs to address the cumulative impacts 

of both wild and domestic ungulates. For example, by 

using three way enclosures in Utah, Kay and Bartos 

(2000) were not only able to document when domestic 

livestock were preventing aspen from regenerating, 

but they also found that fluctuations in deer herd size 

affected aspen’s ability to regenerate. The effects that 

livestock have on aspen ecosystems depends upon the 

type and class of livestock, animal density, and the 

seasonal timing, intensity, and frequency of use (Roath 

and Krueger 1982) (See Chapter 7 for techniques to 

monitor these variables).

Management Objective: Establish New 

Stems Above Browse Height

A principle objective of regenerating aspen on 

rangelands is to establish new suckers above the height 

that browsing animals can seriously damage them. The 

size at which aspen sprouts are vulnerable to browsing depends upon the size of the animal eating them. 

Smith and others (1972) and Sampson (1919) found 

that sheep will browse up to 45 inches (1.14m) and 

cattle and deer up to 5 ft (1.5m). These heights have 

been generally accepted as being adequate to establish 

new aspen. However, in areas of recent elk introduction 

in the Southern Cascades and Modoc Plateau, aspen 

may need to reach at least 12 to 15 ft (4 to 5 m) and 

at least 1.5 inches (4 cm) dbh to avoid damage similar 

to that reported in other areas (Shepperd 2004). There 

are several site specific management techniques that 

are appropriate for modifying livestock distribution, 

alleviating concentration problems, and minimizing real 

or potential conflicts with aspen resource values.

Fencing

Fencing is an obvious tool for ungulate management. It is important to identify whether intense 

browsing by wild ungulate, domestic livestock, or a 

combination of both is keeping a stand or group of 

stands from successfully regenerating. Fence designs 

differ depending on which animals are browsing the 

aspen. Steel, plastic, and pole fencing have been used 

successfully for aspen protection (Rolf 2001; Kees 

2004). Additionally, using brush piling or conifer trees 

that have been hinged when felled to create livestock 

barriers has been successful (Kota 2005). All of these 

techniques require close monitoring during those times 

that regeneration is vulnerable to browsing to ensure 

that barriers remain intact.

Salt Blocks and Water Source

Location of water and salt blocks are magnets for 

cattle grazing, browsing, trampling, and bedding (Roath 

and Krueger 1982). Keeping salt and water sources 

away from regenerating aspen communities will help 

disperse cattle and relieve grazing pressures.
92 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

Seasonal Utilization

Seasonal herbivory on aspen regeneration generally 

increases when the herbaceous vegetation in and around 

an aspen community has cured and lost much of its value 

(for example, after the first killing frost). Aspen utilization 

will also occur when the amount of herbaceous vegetation 

available within or near aspen communities can no longer 

provide the carrying capacity of the range. Proper timing 

of allotment use can greatly benefit aspen. For example, 

successful regeneration of aspen has been demonstrated 

on the Sierraville Ranger District, Tahoe National Forest 

by moving cattle away from aspen as forage preferences 

change from herbaceous vegetation to aspen (Personal 

Comm., Fred Kent, Range Management Specialist, 

USDA Forest Service, Tahoe National Forest).

Wild ungulates may exhibit similar seasonal preferences. For example, it was observed on the Amador 

Ranger District, Eldorado National Forest that deer with 

fawns began bedding down in an aspen/meadow community and browsing intensely on aspen regeneration 

every year in mid-July (Personal Comm., Chuck Lofland, 

Wildlife Biologist, USDA Forest Service, Eldorado 

National Forest). Evidence of multi-stemmed and bush 

shaped aspen suckers were evidence that this process 

had occurred regularly over many years. Constructing 

a temporary woven fence around the area prior to July 

15 each year allowed those suckers to release and grow 

in subsequent years.

Class of Animal

While we were unable to find research to document 

this type of event, we feel that changing livestock from 

cow-calf to dry cow may increase the movement of the 

cattle enough to successfully allow aspen suckers to get 

above browse height. Successful aspen regeneration 

occurred in a number of aspen stands on the Sierraville 

Ranger District of the Tahoe National Forest within three 

seasons of moving from cow-calf to dry cows (Personal 

Comm., Fred Kent, Range Management Specialist, 

USDA Forest Service, Tahoe National Forest). It has 

also been noted that yearlings do less damage to aspen 

than cow-calf pairs.

Type of Livestock

Sheep have been reported to brows aspen more than 

cattle, which tend to prefer grazing over browsing. Sheep 

are often moved into steeper topography that cattle seem 

to avoid. This may place increased pressure on upland 

aspen communities. Good range management objectives 

should include herding sheep away from regenerating 

aspen, especially if the aspen is intended to be used as 

bedding grounds.

Cycling of Grazing to Benefit Aspen

To promote regeneration, DeByle (1985a) recommended moderate grazing until an aspen overstory begins 

to decline, then heavy grazing for a couple of years to 

eliminate or weaken much understory competition, and 

then removing all grazing pressures for 3 to 5 years. This 

process could be repeated every 20 to 30 years to establish 

uneven aged stands. We believe that this approach needs 

to be modified to remove grazing pressures when aspen 

is “ready to regenerate” (show signs of unsuccessful 

sprouting). Waiting for the end of the life cycle of a mature 

age cohort to allow the production of a new age cohort 

may be ill-advised. A stand that is becoming decadent 

may not have enough root structure left to provide proper 

stocking (Shepperd and others 2001; Ohms 2003). Thus, 

we recommend close monitoring of aspen stands and 

providing protection to aspen clones when they naturally 

begin to regenerate a new age cohort.

Post Fire Recovery

Some National Forests in the Pacific Southwest 

Region (for example, the Modoc NF) currently practice 

a minimum of 2 years of allotment rest after wildfires to 

meet vegetation recovery objectives. This would benefit 

aspen clones when they are most likely to produce a 

new age cohort. Aspen clones are more susceptible to 

elimination by repeated browsing if all older aspen trees 

were killed during the fire. Aspen regeneration should 

be closely monitored during initial years following the 

rest period to ensure successful establishment of a new 

aspen stand. Because aspen suckers can be suppressed 

by chronic post-fire herbivory (Bartos and others 1994), 

poor range management or excessive wild ungulate use 

could hasten the death of a clone.

Management Strategies 

Relating to Recreation 

Impacts___________________

As we discussed in Chapter 4, establishing developed 

recreation areas in aspen stands is not a good idea. 
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 93

Although aspen stands are highly desirable for their 

aesthetic appeal, their susceptibility to damage from concentrated human activities make them poor candidates 

for picnic areas and camp grounds. Root compaction 

from vehicles and intense foot traffic can kill aspen roots 

and stress trees. Any injury to the living bark of aspen 

is a potential entryway for canker infections that can 

kill the tree (Walters and others 1982) and aspen trees 

in developed recreation areas are commonly damaged 

by irresponsible users.

Research has looked extensively at recreation impacts 

on wildlands. Cole (1993) and Cole and others (1987) 

identified the importance of recognizing problems 

and carefully evaluating all potential solutions. As we 

discussed in our section on impacts of recreational use 

of aspen habitats (Chapter 4), the principal problem 

is from concentrated recreational uses in aspen. Any 

single recreational activity done in excess can set in 

motion events that can cause impacts to the ecology 

of the landscape. Most recreation activity conflicts that 

we have observed elsewhere in aspen were associated 

with developed and dispersed camping locations, along 

developed road and trail systems, in areas of high OHV 

use, and in or adjacent to pack stock corral or holding 

areas.

There are a number of options available to the manager 

to alleviate or prevent damage to the ecological values of 

aspen in these situations. First, the location of use within 

problem areas can be modified. Although it may not 

be possible to move existing developed campgrounds, 

planning of new facilities should keep the protection of 

aspen in mind. Steps can be taken in existing recreation 

facilities that have been established in aspen stands to 

prevent further injury and loss of aspen. Raised trails 

covered with wood chips or mulch can protect roots, 

and aesthetically pleasing fencing can be constructed to 

keep people away from aspen stems. Interpretive signs 

explaining that aspen bark is living, just like a person’s 

skin, can be used to discourage carving.

If shading is an important campground element, 

more resilient tree species can be established in existing campgrounds. Access to dispersed camping sites 

in highly impacted aspen areas can be discouraged or 

prohibited. Special use facilities such as corrals and 

horse stables can be located away from aspen. Pack 

stock use concentrations in aspen can be discouraged 

through education or prohibited through action.

If camping sites cannot be closed, educational programs can be developed to inform potential visitors of 

the disadvantages of damaging the critical aspen component of campgrounds. One element of the educational 

program could be the development of interpretive signs 

explaining the ecological value of aspen habitats.

Generally speaking, we are not overly concerned 

about the impacts of dispersed recreation use of aspen 

communities. Activities like walking, picnicking, nature 

study, photography, and other permitted consumptive 

uses such as fishing and hunting each have slightly different impacts on aspen, but in moderation their effects 

are usually light. Where concentrated uses of aspen 

habitat occurs, cumulative recreational impacts can 

cause ecological damages equal to those in developed 

or dispersed camping areas. This is especially true along 

trails, roads, or in OHV use areas, where more intense 

recreation management practices would be needed.

Management: Water Quality 

and Quantity______________

Several of the aspen types discussed earlier are 

associated with moist physiographic positions where 

possible effects of management actions on water quality 

and quantity could be an issue. Management of aspen 

within the Forest Service’s Riparian Conservation 

Areas (RCAs) classification is within the parameters of the Sierra Nevada Forest Plan Amendment 

(SNFPA) (USDA Forest Service 2004c). In fact, the 

SNFPA singles out aspen as a species that can be 

managed in RCAs. This interpretation of the SNFPA 

has been confirmed by the Regional Office (Appendix 

I). Likewise, the Lahonton Regional Water Quality 

Control Board, which has regulatory control of the 

Stream Environment Zones (SEZ) on lands on the 

Eastern Slope of the Sierra Nevada (including the 

Lake Tahoe basin), has allowed activities to occur 

near streams, but under very close scrutiny. Agencies 

must work closely to develop management methods 

that will pass regulatory review.

Implementation of management objectives for 

restoring aspen communities adjacent to, and within 

riparian ecosystems, should have the principal goals of 

minimizing soil erosion, preserving water quality, and 

improving stream flow. This may occur by implementing treatments that minimize soil disturbance and allow 

aspen communities the opportunity to return quickly 

to a state where it is contributing to the quality of the 

riparian ecosystem. Conifer removal and prescribed fire 

treatments should be designed to reach these goals, and 

range management practices should be adjusted to limit 
94 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

habitat degradation and improve water quality within 

riparian zones.

In concluding this chapter, we remind readers 

that successful aspen management simply involves 

understanding aspen’s unique growth habits and applying treatments that favor the species. Regardless 

of the type of aspen they encounter, the descriptions 

discussed earlier can help managers identify the need 

for intervention. The three critical elements of the Aspen 

Regeneration Triangle (fig. 6-2) describe what must 

be provided in order for any attempt at regenerating 

aspen to succeed.
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 95

Chapter 7.

Assessment and Monitoring 

Methodology

As described in the introduction to this synthesis, 

directives and supporting standards and guidelines from 

multiple governmental agencies call for the ecological 

assessment of vegetative communities such as aspen, 

and for management actions that will move plant communities toward their range of natural variability. This 

chapter will focus on the actual methodologies that have 

been developed for: 1) deriving ecological assessments 

of aspen communities, 2) monitoring herbivory of aspen 

regeneration, and 3) examining whether management 

objectives to change the ecological status of aspen 

are being met. Our discussions will be framed with 

the realization that time and monetary constraints are 

always factors in the development and implementation 

of ecological assessments and monitoring protocols. 

To accommodate these constraints, we will tier our 

discussion—moving from qualitative to quantitative 

methods in all three cases. We recognize that no single 

method or protocol fits all management situations and 

that whatever methodology is used should be designed 

to measure whether management objectives are being 

met (Elzinga and others 1998).

The Record of Decision of the Sierra Nevada 

Forest Plan Amendment’s Final Environmental Impact 

Statement (USDA Forest Service 2004c) calls for identifying whether a vegetative community is outside the 

range of natural variability or moving in that direction. 

Therefore, resource managers need to identify indicators 

that can reflect the ecological status of the ecosystem. 

Many biotic relationships can act as indicators for 

the natural variability of the ecosystem (Richardson 

and Heath 2004; Martin and others 2004). Bartos and 

Campbell (1998) presented a case for using aspen 

itself as an indicator. They believe: 1) loss of aspen 

will cause a parallel loss of biotic diversity within the 

ecosystem; 2) aspen regeneration is closely linked to 

the natural disturbance factors in the ecosystem; and 

3) aspen clones provide a guide as to the spatial extent 

of aspen communities through time (see Chapter 3 for 

additional discussion of these topics).

The loss of aspen itself may be the best variable to 

indicate whether a stand has moved outside its range of 

natural variability. The complete loss of a clone marks, 

for all practical purposes, an ecologically irreversible 

event (Campbell and Bartos 2001). Planting aspen does 

not appear to be a viable method of replacing lost aspen 

clones (Shepperd and Mata 2005).

Because the loss of aspen is irreversible if the root 

system dies, development of assessment protocols capable of determining whether aspen is at risk of being 

lost is crucial to establishing the ecological condition of 

the ecosystem. Also essential are monitoring protocols 

that measure whether aspen will remain in a properly 

functioning condition (PFC) as part of these unique 

ecological systems.

To understand how aspen can be lost from the landscape, we must first look at: 1) the relationship of aspen 

to the rest of the habitat’s biota; 2) how site variables 

and natural disturbances affect aspen in space and over 

time; and 3) how historic and current human use of the 

landscape may be affecting the ecological viability 

of aspen. Some researchers and resource managers 

(Romme and others 2001) believe that human induced 

factors can increase intensity of environmental stressors 

on aspen communities and result in aspen loss from the 

habitat. For example, aspen’s apparent dependence on 

vegetative reproduction makes excessive browsing by 

wild or domestic ungulates a significant stressor to the 

viability of aspen regeneration and a potential factor 

in the loss of aspen. The remainder of this chapter will 
96 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

illustrate how the relationship of aspen to stressors like 

these can be used to: 1) assess the risk of aspen being lost 

from the habitat; 2) measure the amount of herbivory; 

and 3) monitor changes to the ecological condition of 

an aspen habitat under a given management practice.

Stand Assessment 

Methodology______________

Developing Qualitative Assessment 

Protocols

Assessment protocols examining the ecological 

relationships between aspen and stressors on the habitat 

have become common techniques for identifying the 

ecological condition of an aspen habitat and for identifying the intensity of risk factors to aspen’s survival 

in that habitat. Elzinga and others (1998) illustrate the 

value of using ecological models in assessing a plant 

community, designing management objectives, and 

developing sampling designs for monitoring.

Many assessments have as their framework the 

materials presented in this synthesis. They capture 

the asexual reproductive characteristics of aspen, the 

interaction of aspen with a range of disturbance factors 

creating its range of natural variability, and aspen’s 

relationship to human induced variables such as fire 

suppression and livestock browsing. For example, 

Mueggler (1989) introduced an assessment model for 

identifying the risk of aspen loss from the ecosystem 

(fig. 7-1). Mueggler’s model used variables such as the 

ecological relationship between aspen and conifers, 

stand age, and browsing intensity to assess aspen condition. Bartos and Campbell (1998) further developed 

Mueggler’s basic model by quantifying risk factors for 

aspen stands. They surmised that aspen stands were 

at risk when: 1) conifer cover in the understory and 

overstory was > 25 percent; 2) aspen canopy cover 

was < 40 percent; 3) dominant aspen trees were > 100 

years old; 4) aspen regeneration < 500 stems per acre; 

or 5) sagebrush cover was > 10 percent.

Later, Campbell and Bartos (2001) developed a key 

for prioritizing risk factors for landscapes with aspen 

(fig. 7-2). The resulting model has been used extensively by resource managers for rapid assessment of 

the ecological condition of aspen stands (Brown 2001; 

Burton 2004a; Jones and others 2005b). An example 

of the use of the Campbell and Bartos model was illustrated in Chapter 5.

A cooperative effort of the Forest Service and the 

Bureau of Land Management (Aspen Delineation 

Project 2002) used a qualitative approach to assessing 

the ecological condition of aspen habitats that was also 

based on the Campbell and Bartos model. This effort 

incorporated an additional level of assessment to assist resource managers in evaluating the management 

implications of existing aspen stand conditions. Burton 

(2004a) explains that stand structure is the key variable 

used in this protocol to analyze a stand’s ecological 

condition. The definition of “stand,” adopted from the 

CNPS (California Native Plant Society 2004), calls for 

separating aspen ecosystems into units that have both 

compositional and structural similarity. By using this 

definition of stand, the resource manager can establish 

not only the presence and condition of aspen age cohorts, 

but the relationship of those age cohorts to any conifer 

encroachment, as well as the effects of browsing by wild 

and domestic ungulates. This delineation presents the 

resource manager with a clear picture of the ecological 

condition of the stand and provides an indication of 

possible restoration efforts.

Other assessment protocols developed in the West 

have used similar approaches. Jones and others (2005a) 

used the Bartos and Campbell (1998) risk factors 

in the Eagle Lake Ranger District, Lassen National 

Forest (we reviewed these Eagle Lake findings in 

Chapter 5). A protocol developed for the Bureau of 

Land Management in Oregon by Otting and Lytjen 

(2003) used aspen age/size classes similar to those in 

the Aspen Delineation Project protocol—overstory, 

recruitment (mid-aged cohorts free from browsing) and 

understory (vulnerable to browsing). Additionally, the 

Oregon assessment quantified a protocol for establishing 

overstory senescence as the percent of overstory trees 

that are dead or showing signs of decline—that is, trees 

including many dead limbs, conks, or weeping cankers. 

In Utah, a protocol developed for the Fishlake National 

Forest (Brown 2001) assessed stand structure, conifer 

and aspen dominance or co-dominance, and the presence or absence of regeneration. This protocol looked at 

the presence of “successful aspen regeneration” rather 

than the presence or absence of aspen regeneration still 

within the browse zone. The Fishlake National Forest 

protocol also used the Campbell and Bartos (2001) 

prioritized key discussed earlier in this chapter. The 

common denominator in all these protocols is that they 

are qualitative assessments.
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 97

Figure 7-1. Assessment model 

for maintaining aspen stands 

in the Intermountain Region 

developed by Walt Mueggler 

(1989).

Photo Points

Any discussion of qualitative assessments of aspen 

should include the value of photo points. Whether used 

with a field conducted assessment or as a stand-alone 

process, photo points can be particularly effective in 

documenting seral stage relationships between aspen 

and conifers in an aspen ecological assessment (Kay 

2001b, 2003). In addition to documenting the ecological condition of the stand, photo points can assess the 

need for implementing treatments to move stands back 

to an earlier seral stage. For example, figure 6-6a-d 

illustrates how conifer harvest and biomass removal 

re-established an aspen-dominated forest. Mechanical 

harvest was chosen over prescribed fire as a treatment 

option because only an intense crown fire, difficult to 

apply, could accomplish the same ecologic effect.

Photo points have an additional value. Repeat 

photographs taken from specific photo points can be a 

valuable tool in capturing the changes of aspen condition from the perspective of history or for illustrating 

whether management objectives are being met (Gruell 

and Loope 1974; Kay 2001b, 2003). Photo points are 

also valuable in monitoring aspen conditions into the 

future (see Implementation monitoring, later in this 

chapter). However, it is important to geo-reference all 

photo points when using repeat photographs. Agency 

publications (Hall 2002) have demonstrated the significance of the repeated photos in general, and published 

studies using repeated photo points emphasize their 
98 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

Figure 7-2. A key developed by Campbell and Bartos (2001) used to prioritize aspen areas for restoration and 

conservation. The authors assume in this key that aspen are present with a density of at least 20 mature 

trees per acre. Note: Option 1 refers to relative cover; options 2 through 5 refer to absolute cover.

value as it relates to aspen (Gruell and Loope 1974; 

Kay 2001b).

Quantitative Assessment Protocols

While resource managers may not find the collection 

of quantitative data necessary or even economically 

feasible, such data are particularly valuable in providing baseline data for monitoring trends or effectiveness 

treatments. For example, if the management objective 

is to establish a new aspen age cohort above browse 

height, a quantitative assessment of the current condition 

of aspen suckers within the browse zone could act as 

the baseline data for management planning (Jones and 

others 2005b). However, if a manager wants to determine 

if a new practice is successful in establishing a new 

aspen age cohort, then a quantitative assessment of the 

current condition of aspen suckers within the browse 

zone would act as the baseline data in the effectiveness 

monitoring process (Kilpatrick and Abendroth 2001). 

Similar data taken in plots where management practices 

have been changed, and in plots where the practice has 

not been changed, can provide the additional element 

of a control treatment. Since quantitative assessments 

are so closely linked to effectiveness monitoring, they 

will be discussed in detail later in this chapter.

Implementation Monitoring__

Federal agencies managing lands in the Sierra Nevada 

have standards and guidelines limiting browsing to 

specific percentages of the annual leader growth of 

mature riparian woody vegetation (USDI BLM 1999a; 

USDI BLM 1999b; USDA Forest Service 2004c). 

Resource managers may find it necessary to develop 

monitoring protocols for determining whether agency 

standards and guidelines are being achieved. Likewise, 

directives such as the Forest Service’s SNFPA (USDA 

Forest Service 2004c), which calls for removal of 

livestock when browsing of woody stemmed vegetation 

is evident, are helpful when managers or permitees 

need ways to impartially and repeatedly evaluate this 

impact.
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 99

Measuring and Monitoring 

Plant Populations

Elzinga and others (1998) state that implementation 

monitoring, or assessing whether management practices 

are carried out as designed, is the appropriate technique 

for monitoring browse intensity. Previously existing 

protocols were developed to measure woody stemmed 

shrub vegetation (Keigley and Frisina 1998; USDA 

Forest Service 2004c). However, it is important to use 

a protocol that recognizes that aspen is a tree and not 

a shrub, as terminal leaders of young aspen must be 

protected so they can grow into trees.

The USDA Forest Service Pacific Southwest Region 

protocol (USDA Forest Service 2004a) focuses on assessing browse damage to the terminal leaders of young 

aspen stems. Terminal leader damage is used as the key 

indicator of browse because the terminal leader needs 

to remain intact while within the browse zone in order 

for an aspen tree to have uninterrupted growth (Keigley 

and Frisina 1998). This protocol also considers that the 

browse zone height may vary, depending on the species 

of the animal involved. If the terminal bud is damaged 

or removed, it takes up to two years for a new primary 

stem to establish. Jones and others (2005b) use 5 ft (1.5 

m) as the level at which the terminal leaders of aspen 

sprouts are above the reach of sheep, cattle, and deer, 

while (Shepperd 2004) recommends 7.5 ft (2.3 m) with 

a dbh of 1.5 inches (4 cm) as an appropriate height and 

size to avoid elk damage.

Monitoring locations for the Pacific Southwest 

Region protocol are chosen based on key or “critical 

areas” used in the interagency technical report titled 

Sampling Vegetation Attributes (USDI BLM 1996). A 

critical area is defined as an area that must be treated with 

special consideration because of inherent site factors, 

size, location, condition, values, or significant potential 

conflicts among users (Society of Range Management 

1998). Because aspen habitats have a unique biodiversity 

(Dobkin and others 1995; Bartos and Campbell 1998), 

the protocol adopted by the Pacific Southwest Region 

recognizes that individual aspen stands shall be referenced as critical areas, or identified as key areas if they 

reference what is happening in a larger area as a result 

of on-the-ground management actions. A key area is 

therefore a representative sample of a larger stratum, 

which can be defined as a collection of aspen stands, 

a livestock pasture, a watershed, or an entire grazing 

allotment. Therefore, management decisions based on 

implementation monitoring can be applied to the entire 

stratum that the key area represents.

The Pacific Southwest Region protocol recommends 

making monitoring sites permanent. This practice is 

valuable because the power to detect change is often 

much greater with permanent sampling units, and spatial 

variability associated with repeated sampling using 

different plots is removed from analysis (Elzinga and 

others 1998). The reliability of permanent sampling units 

depends on the degree of correlation between sampling 

years. Elzinga and others suggested that the increase 

in power afforded by using permanent sampling points 

outweighs the initial increased costs of establishing 

them on the monitoring site.

The objective of the Pacific Southwest Region protocol is to determine whether a current management 

practice exceeds the Regional utilization standard of 20 

percent for aspen regeneration. The same basic process 

may be applied to any level of utilization (Personal 

comm., John Willoughby, California State Botanist, 

USDI Bureau of Land Management). For this protocol, 

the objective is to obtain a 95 percent confidence level 

around a mean value of 20 percent use. It was reported 

(Burton 2004a) that a sample size of 90 met these objectives. To obtain this number, Burton assumed that the 

20 percent use was achieved by sampling aspen suckers 

up to 5 ft (1.5 m) tall and recording whether or not the 

terminal leader was browsed. These binomial data (yes/

no) allow for calculation of confidence intervals based 

only on the initial estimate (in this case, 20 percent) 

and the sample size (Zar 1999). Table 7-1 shows upper 

and lower 95 percent confidence limits for 20 percent 

use with different sample sizes. Using a sample size of 

90, the lower confidence limit is 0.123 and the upper 

confidence is 0.298. Thus, lower and upper confidence 

limits are within 10 percent of the utilization standard 

(20 percent).

It is important to remember that this protocol will 

quantify browsing intensity of a particular management 

practice at a specific time. While this protocol will not 

indicate to a resource manager whether the current 

management practice is moving the aspen habitat toward 

a desired condition (management objective), the utilization measurement, together with range management 

information (the type and number of animals, length 

of grazing season, when animals were brought on and 

removed), may help in the analysis of effectiveness 

monitoring data.

Keigley and Frisina (1998) developed another method 

to monitor browse utilization of aspen for the Montana 

Fish Wildlife and Parks. Similar to Pacific Southwest 

Region protocol, this method stresses the importance 

of focusing on utilization on the terminal leader of the 

primary stem. In addition to measuring the presence or 
100 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

Table 7-1. Upper and lower 95 percent confidence limits for different sample sizes to

estimate 20 percent browse utilization of aspen sprouts (Burton 2005).

	Number of sampling units 	 Lower confidence 	 Upper confidence

(aspens < 1.5 m) limit limit

 30 0.077 0.386

40 0.09 0.357

50 0.1 0.338

60 0.107 0.324

70 0.113 0.313

80 0.118 0.305

90 0.123 0.298

100 0.126 0.292

150 0.139 0.274

200 0.146 0.263

500 0.165 0.238

1000 0.175 0.227

absence of browse, the intensity of browse and historic 

browsing trend of a stem are noted. The Keigley and 

Frisina technique can be implemented either as a qualitative rapid assessment or as a quantitative assessment. 

In either case, it is based on the identification of four 

architectural structures for young aspen stems and their 

relationship to browsing history. The first growth form, 

described as “uninterrupted-growth-type,” is the treelike structure produced under light or moderate levels 

of browsing when the terminal leader is vulnerable 

to being killed. The second “arrested-type” form is a 

bush-like structure produced when a plant experiences 

an intense level of browsing throughout its life. The 

third, “retrogressed-type,” is a bush-like structure and 

the base of a dead tree structure, produced by a change 

from a light-to-moderate to an intense level of browsing. 

The fourth, “released-type” growth form, identifies a 

tree-like structure growing out of a bush structure and 

is produced by a change from intense browsing that 

caused arrested or retrogressed structures, to a light-tomoderate level of browsing.

The quantitative version of the protocol developed by 

Keigley and Frisina (1998) is also based on these same 

four growth forms. It is a comprehensive-level survey 

that assesses browsing history by actually identifying 

when browsing occurred. Since this protocol can be used 

to establish current browsing intensity, it is still a utilization protocol, but since it examines stems over time, it 

can be used for evaluating the history of the stand.

Effectiveness Monitoring____

As reported earlier, multiple agencies have identified the importance of identifying whether current 

management practices are keeping vegetative communities like aspen within a range of natural variability. 

Directives such as the SNFPA FSEIS (USDA Forest 

Service 2004c) call for “an active and focused adaptive 

management and monitoring strategy” to establish if 

vegetative community goals are being met. Elzinga and 

others (1998) diagram an adaptive management cycle. 

They explain that the effectiveness of any corresponding monitoring lies in the development of management 

objectives that “set a specific goal for attaining some 

ecological condition or change value,” and the development of sampling objectives to measure the condition 

or change in value. In other words, know what your 

objective is before designing a scheme and selecting 

indicators to monitor it.

Successful regeneration of aspen suckers has often 

been the management objective of resource managers 

(Kilpatrick and Abendroth 2001; Jones and others 

2005b). Reliable and repeatable methods for measuring regeneration success are needed. For example, a 

four-year study on the Eagle Lake Ranger District on 

the Lassen National Forest by Jones and others (2005a) 

examined the effect of removing conifers from a mixed 

aspen/conifer stand that was not subject to heavy browsing pressures. They tallied aspen sucker counts into four 

size classes to measure changes in aspen height in the 

plots. The size classes used were adapted from those used 

in research studies reported by Shepperd (2004):

Size Class I = less than or equal to 1.5 ft (0.46 

m)—meant to capture new recruitment of suckers;

Size Class II = greater than 1.5 ft (0.46 m) to 5 ft 

(1.5 m)—meant to capture older suckers that are 

vulnerable to browsing of the terminal leader;

Size Class III = greater than 5 ft (1.5 m) and up to 

1.0 inch (2.5 cm) dbh—meant to capture young 

•

•

•
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 101

aspen that have grown past the browsing threat to 

the terminal leader; and

Size Class IV = greater than 1.0 inch (2.5 cm) 

dbh—meant to capture information of the size classes 

of the remaining cohort in the plot.

Using these size classes on permanent plots makes 

it possible to monitor the growth and establishment 

of aspen suckers through time as they shift to larger 

classes, or to identify the need for remedial protection of 

suckers if they do not grow into larger size classes.

Using these protocols, Jones and others (Aspen 

Delineation Project 2002) developed a protocol titled 

Effectiveness Monitoring of Aspen Regeneration 

on Managed Rangelands for application on Forest 

Service rangelands in the Pacific Southwest Region. 

The authors stress the significance of monitoring site 

selection for aspen. Relative abundance and distribution 

must be carefully considered during establishment of 

monitoring plots. This protocol calls for plot sites that 

are randomly chosen based on the spatial distribution 

of suckers. It calls for establishing stratified random 

sampling using existing protocols (USDI BLM 1999a; 

USDI BLM 1999b) for small or large aspen stands, 

that have suckers sparsely distributed and/or in small 

clumps, and for establishing restricted random sampling 

transects for small or large aspen stands with uniformly 

distributed suckers.

Another protocol was developed for the Wyoming 

Game and Fish Department to monitor the effects 

of prescribed fire and mechanical treatment in aspen 

stands near elk winter feeding grounds (Kilpatrick 

and Abendroth 2001). This protocol specified tallying suckers by 1-foot (0.3 m) to 10-foot (3 m) 

height classes in randomly-placed circular plots. It 

recommends choosing a plot size based on pretreatment tree density estimates (for example, 1/50th to 

1/100th acre (200 to 40 m2) plots for stands with 150 

to 250 trees per acre (370 to 617 trees per ha), and 

1/100th to 1/500th acre (40 to 8 m2) plots for 4,000 to 

15,000 trees per acre (9,880 to 37,050 trees per ha). 

The study also recommends increasing plot size with 

increased stand/clone heterogeneity. Larger sample 

plots will likely be needed to assess regeneration from 

sparsely-stocked stands because root density and 

subsequent suckering will be lower in these situations 

(Shepperd and others 2001).

•

Monitoring: Water Quality 

and Quantity______________

While there have been monitoring and research studies examining the positive effect of aspen communities 

on riparian ecosystems (Johnston 1984; Bartos and 

Campbell 1998), the intense scrutiny by regional regulatory agencies of water quality relating to management 

activities in riparian ecosystems warrants continued 

monitoring. Such monitoring can provide managers 

with the information necessary to adjust management 

practices in order to obtain desired conditions and 

address regulatory concerns. Monitoring key stream 

indicators such as stream flow, stream canopy, stream 

and air temperature, aquatic macroinvertebrates, pH, 

dissolved oxygen, electrical conductivity, turbidity, total 

suspended solids, total N, total P, nitrate, ammonium, 

phosphate, and potassium will help evaluate the effect 

of treatments on water resources. Monitoring stations 

established above, within, and below treatment and 

control stands can clarify how these variables are affected by management actions. With reliable monitoring 

data, interested parties will become increasingly more 

confident in presenting and reviewing future management actions. Currently, an ongoing research study is 

examining how these variables are affected by conifer 

removal treatments in aspen stands along short reaches 

of streams (Tate 2003). Results from this study should 

provide a foundation for decisions related to aspen 

management along riparian corridors as well as develop 

efficient monitoring protocols for successful adaptive 

management.

Our goal in this chapter was to present an overview 

of aspen stand assessment and monitoring methodologies that have enabled resource managers to effectively 

develop, implement, and evaluate aspen management 

activities. The consistent threads through all of the 

protocols we have reviewed are attention to measuring 

the ecological condition of aspen communities, capture 

of factors changing that ecological condition, and assessment of the degree of change. We believe that these 

three focuses will be helpful to managers as they design 

and adjust management activities to benefit aspen in 

the Sierra Nevada and surrounding areas covered in 

this report.
102 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 103

Chapter 8.

Summary

Our discussion of aspen in this document has centered 

on its place in the landscapes it occupies. Aspen is considered to be the most widely distributed deciduous tree 

in North America (Preston 1976). In western landscapes, 

this tree grows over wide ecological amplitude and is an 

important component of many ecosystems. Appreciation 

and concern for aspen in the western U.S. is held by 

managers and lay people alike. Many believe that 

aspen is a true keystone species in forested landscapes 

of the West. This means that the condition and health 

of aspen is a reflection of ecosystem composition and 

processes at large.

This effort is the first time since the publication 

of General Technical Report RM 119 (DeByle and 

Winokur, 1985) that a comprehensive synthesis of the 

existing knowledge and literature pertaining to western 

aspen has been compiled in one document. Because 

aspen-specific research from the Sierra Nevada is 

limited, we often used studies produced elsewhere to 

address this shortcoming. We view this approach as 

being informative to the Sierra Nevada region, as well 

as to the greater aspen literature. Although we have 

emphasized the ecology and management of aspen 

in the Lake Tahoe Basin, much of what is discussed 

here can be applied throughout the Sierra Nevada and 

potentially to other parts of the West.

Our discussion began with a presentation of the 

physical and natural environment in which aspen is 

found. We then discussed the setting for the existence 

of aspen in the Sierra Nevada as well as the climatic 

influences associated with aspen in this region. The 

impacts of humans (recent as well as historical) on the 

aspen system were reviewed in detail, as was the ecology of aspen. We not only included the physiology and 

genetics of aspen, but discussed its role as a component 

of the greater terrestrial biota of the Sierra Nevada.

A number of threats and issues related to the health 

and continued existence of aspen in the Sierra Nevada 

are discussed. Advancing conifer succession is a major 

threat to aspen’s sustainability in the Sierra Nevada, as is 

the restricted ability of aspen to vegetatively reproduce 

via root suckering. Both of these factors are related 

to the limited presence of aspen in the Sierra Nevada 

under the modified natural disturbance regimes that 

exist today. The threat of introducing invasive species 

into Sierra Nevada ecosystems also must be considered 

when managing a disturbance-dependent species like 

aspen. Social values associated with development in 

the Wildland Urban Interface influence management 

options available for aspen in these areas, as do the 

incompatibilities between heavy recreation use and 

aspen. Consumption of young aspen sprouts by domestic 

livestock and wild ungulates must also be dealt with to 

ensure successful establishment of new aspen age classes 

in the Sierra Nevada and surrounding environs. Water 

quality and disturbance in riparian zones is another 

issue that must be considered when planning aspen 

management activities in these areas.

We describe seven distinctive aspen stand types that 

occur in the Sierra Nevada and discuss their current 

ecological status. These types include aspen types that 

we can expect to succeed to conifer forests within one 

generation in the absence of disturbance. These would 

include meadow fringe aspen, riparian aspen, and

upland aspen/conifer types. Other aspen types that lack 

a conifer component are likely to remain as pure aspen, 

as existing trees die and are replaced by new suckers. 

These types include lithic aspen, snowpocket aspen, 

upland pure aspen, and Krümmholz aspen.

Many management techniques exist that can be used 

to treat or restore aspen. These include, but are not 

limited to, cutting, burning, protection from ungulates, 

severing lateral roots, removal of competing conifers, 

or various combinations of these techniques. Selection 

of a treatment alternative depends upon the health 

and vigor of an aspen stand, its successional status, 
104 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

its susceptibility to browsing, and management objectives for the stand in its particular location. Treatment 

alternatives that are appropriate for the seven aspen 

stand types found in the Sierra Nevada are reviewed 

and discussed. A decision tree was presented that can 

be used to identify aspen stands in need of management 

intervention, along with the aspen triangle model 

identifying key factors affecting aspen establishment. 

Because aspen is a critical component of rangelands 

in the Sierra Nevada, a section of the document is 

devoted to managing aspen on rangelands. Special 

management considerations related to urban interface 

areas, recreation activities, and water quality issues 

are also discussed.

Accurate assessment of the current ecologic condition 

of aspen in the Sierra Nevada is essential to making 

sound management decisions. Several protocols that 

have been used to delineate and assess aspen in the 

Sierra Nevada and surrounding areas are reviewed. 

Likewise, future changes in aspen condition and the 

effectiveness of any aspen management activities 

need to be monitored. Any treatment (including the 

no-treatment alternative) should be documented and 

tracked to determine its relative success and to guide the 

adaptive management process. Monitoring techniques 

that range from the most simplistic (photo points), to 

the more complicated, are given. In the recent past, 

considerable effort has been made to monitor aspen 

in the Sierra Nevada and elsewhere in the West. The 

knowledge we have gained from these efforts is shared 

with the reader.

A wealth of knowledge and experience about aspen 

has been presented in this document. This information 

should be beneficial to those individuals anywhere 

who are concerned about aspen, or are responsible for 

planning, implementing, and monitoring aspen management activities. While we focused this document on the 

Sierra Nevada and surrounding areas, the information 

synthesized here should be applicable to other areas of 

the western United States as well.
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 105

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Appendix I.

Questions and Responses Related to Region 5 Management 

Team Concerning Interpretation of Sierra Nevada Forest Plan 

Amendment Decision Notice (2004a)

Wayne D

Shepperd/RMRS/USD

To

Michael Landram/R5/USDAFS@FSNOTES

03/22/2005 04:37 PM

Subject

Aspen Questions

Mike:

It was good to visit with you this afternoon. The memo with our SNFP aspen questions is attached.

Wayne D. Shepperd, Ph.D.

Research Silviculturist

Rocky Mountain Research Station

970-498-1259

email: wshepperd@fs.fed.us

(See attached file: AspenQuestions-Ltr.doc)

(Attached file: AspenQuestions-Ltr.doc)
118 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

United States Forest Rocky Mountain 240 West Prospect

Department of Service Research Station Fort Collins, CO

Agriculture 80526-2098

File Code: 4000 Date: March 22, 2005

Route To:

Subject: Aspen Questions Pertaining to Sierra Nevada Forest Plan Amendment Record of Decision

To: Mike Landram, R5

As you know, The Rocky Mountain Research Station has been funded by the Tahoe Basin Management Unit to 

produce a synthesis publication entitled “Ecology and Management of Aspen in Sierra Nevada with Emphasis 

on the Lake Tahoe Basin.” We have identified several topic areas where we would like to request clarification on 

management policy. Specifically, we would like interpretations of some guidelines covered in the January, 2004 

Sierra Nevada Forest Plan Amendment Record of Decision. so we might formulate aspen management alternatives that are consistent with the Record of Decision. If possible, we would like to receive written responses to 

the following five questions in the form of a signed memo or other document that we might cite in our publication.

The Sierra Nevada Forest Plan Amendment, Final Supplemental Environmental Impact Statement, Appendix 

A, p. 361 (A. Management of Uses Other Than Fire Hazard Reduction) states: “Standards and guidelines for 

crown closure and tree diameter apply only to thinning and regeneration harvest. Exceptions to the vegetation 

management standards and guidelines include responding to past infestation outbreaks and restoration activities, 

such as aspen regeneration, hardwood regeneration, sugar pine management, sequoia regeneration.” Does this 

statement still apply, and if so, under what conditions outlined in the Record of Decision? Are we correct in 

interpreting this to mean that the standard of retaining all live conifers 30” dbh or larger in mechanical thinning 

treatments (Record of Decision, Appendix A. p. 50, #6) does not apply when restoration of aspen is ecologically 

justified?

Guideline #105 for RCO#2 (ROD, Appendix A, p. 64) specifies: “If conditions are outside the range of natural 

variability, consider implementing mitigation and/or restoration actions that will result in an upward trend. Actions 

could include restoration of aspen or other riparian vegetation where other conifer encroachment is identified as 

a problem.” Does the exception to the general forest requirements (SNFPA, FSEIS Appendix A, p. 361) apply 

in aspen restoration in riparian corridors? Or, do diameter limits on conifer removals still apply? Similarly, may 

prescribed burning techniques other than backing fires be used to restore aspen in riparian corridors (SNFPA, 

FSEIS Appendix A, p. 344)? Our concern is that a backing fire might have a longer residency time than a head 

fire and may damage aspen lateral roots under heavier fuel loadings.

Can mechanical activities to restore aspen occur within the 500 ft. radius buffer surrounding Spotted Owl and 

Goshawk PAC’s specified in Guideline #73 (ROD Appendix A, p. 60) provided such activities maintain desired 

conditions (p. 38) within the overall buffer? Aspen stands are generally small and rare in many Sierra Nevada 

landscapes and would normally comprise only a small portion of a PAC or buffer area. Retaining viable aspen 

within these zones may provide a valuable component of the habitat. However, doing so may require removal 

of competing conifers that could not be killed by prescribed burning.

1.

2.

3.
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 119

SNFPA, FSEIS Appendix A, p. 345 states: “Strategies should recognize the role of fire in ecosystem function 

and identify those instances where fire suppression or fuel management actions could be damaging to habitat or 

long-term function of the riparian community.” Is it therefore proper to advise that (1) wildfires be allowed to 

burn through aspen stands located in riparian corridors and (2) prescribed burning can be used to ecologically 

restore aspen communities in riparian corridors?

Since the goals of aspen (a hardwood) restoration are synonymous with the desired goals for Lower Westside 

Hardwood Ecosystems (ROD Appendix A, p. 35-36), could the Standards and Guidelines for Hardwood 

Management (ROD, Appendix A, p. 53, #18, 19, 22, 24, 25, and 26) apply also to aspen?

Please let me know if you would like additional clarification, or would like to discuss these questions further. I 

look forward to your response.

 /s/

WAYNE D. SHEPPERD, Ph.D.

Research Silviculturist

Rocky Mountain Research Station

Michael Landram/R5/USDAFS 

To

Tom Efird/R5/USDAFS@FSNOTES

03/22/2005 05:27 PM

cc

Stephen Bishop/R5/USDAFS@FSNOTES

Subject

Fw: Aspen Questions

Tom—This is formal request for interpretation of SNFP from The Rocky Mountain Research Station, funded 

by the Tahoe Basin Management Unit to produce a synthesis publication entitled “Ecology and Management of 

Aspen in Sierra Nevada with Emphasis on the Lake Tahoe Basin.”

They need a written response which will affect the kinds of alternative treatments considered and described in 

their work.

Mike Landram

mlandram@fs.fed.us

ph. (707) 562-8688

fx. (707) 562-9034

4.

5.
120 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

----- Forwarded by Michael Landram/R5/USDAFS on 03/22/2005 05:01 PM -----

Tom Efird <tefird@fs.fed.us>

04/13/2005 01:52 PMTo

wshepperd@fs.fed.us

cc

Michael Landram <mlandram@fs.fed.us>, Kathy Clement <kclement@fs.fed.us>

bcc

Subject

Re: Fw: Aspen Questions

Wayne, here are the responses to your questions. Please call if my response is not clear or you need further 

explanation.

(See attached file: Response to Wayne Shepperd.doc)

Thomas C. Efird

Implementation Team Leader

Sierra Nevada Framework

Pacific Southwest Region

1323 Club Drive Vallejo, CA 94592

Voice: (707) 562-8976

Cell: (707) 688-3941

FAX: (707) 562-9049

(file: Response to Wayne Shepperd.doc)

Questions related to Sierra Nevada Framework and Aspen Management

1. The Sierra Nevada Forest Plan Amendment (SNFPA), Final Supplemental Environmental Impact 

Statement (FSEIS), Appendix A, p. 361 (A. Management of uses other than fire hazard reduction) states: 

“Standards and guidelines for crown closure and tree diameter apply only to thinning and regeneration 

harvest. Exceptions to the vegetation management standards and guidelines include responding to pest 

infestation outbreaks and restoration activities, such as aspen regeneration, hardwood regeneration, sugar 

pine management, Sequoia regeneration.” Does this statement still apply, and if so, under what conditions 

outlined in the Record of Decision (ROD)? Are we correct in interpreting this to mean that the standard of 

retaining all live conifers 30” or larger in mechanical thinning treatments (Record of Decision, Appendix 

A. p. 50, #6) does not apply when restoration of aspen is ecologically justified?
USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006 121

Response:

Yes, the statement still applies. The Record of Decision, Appendix A. page 51, #9 states 

“Standards and guidelines #6, 7, and 9 above apply only to mechanical thinning harvests 

specifically designed to meet objectives for treating fuels and/or controlling stand densities.” 

This statement was intended to provide clear direction that activities, such as aspen 

management, are not subject to harvested tree size, basal area retention and/or residual canopy 

closure limitations that apply to fuel and/or density reduction treatments.

2. Guideline #105 for RCO #2 (ROD, Appendix A, p. 64) specifies: “If conditions are outside the range 

of natural variability, consider implementing mitigation and/or restoration actions that will result in an 

upward trend. Actions could include restoration of aspen or other riparian vegetation where other conifer 

encroachment is identified as a problem.” Does the exception to the general forest requirements (SNFPA, 

FSEIS Appendix A, p. 361) apply in aspen restoration in riparian corridors? Or, do diameter limits on 

conifer removals still apply? Similarly, may prescribed burning techniques other than backing fires be used 

to restore aspen in riparian corridors (SNFPA, FSEIS Appendix A, p. 344)? Our concern is that a backing 

fire may damage aspen lateral roots under heavier fuel loadings.

Response:

In the ROD, page 3, it states, “All of the management direction for this decision is included 

in this document (Appendix A). The SEIS represents an analysis and planning document and 

does not provide management direction.” So yes, as described above, Appendix A. page 51, #9, 

provides the direction to exclude non-fuel and/or density reduction management actions, such as 

aspen restoration, from harvested tree size, basal area retention and/or residual canopy closure 

limitations. Standards and Guidelines for Riparian Conservation Areas (RCAs) and Critical 

Aquatic Refuges (CARs) # 91 thru #124 apply to all projects within these designated areas. 

With regard to prescribed fire techniques, Appendix A does not restrict choices. A site-specific 

analysis and biological evaluation would be required to assess the consequences of proposed 

restoration treatments within RCAs and CARs.

3. Can mechanical activities to restore aspen occur within the 500 ft. radius buffer surrounding Spotted Owl 

and Goshawk PACs specified in Guideline #73 (ROD Appendix A, p. 60) provided such activities maintain 

desired conditions (p. 38) within the overall buffer? Aspen stands are generally small and rare in many 

Sierra Nevada landscapes and would normally comprise only a small portion of a PAC or buffer area. 

Retaining viable aspen within these zones may provide a valuable component of the habitat. However, 

doing so may require removal of competing conifers that could not be killed by prescribed burning.

Response:

No. Mechanical treatments (Including: pre-commercial thinning, biomass thinning, commercial 

thinning, salvage harvesting, group selection, piling, crushing, and mastication) are prohibited 

in California spotted owl and Northern Goshawk activity centers (500 foot buffer around the 

nest site) within the approximately 300 acre Protected Activity Center (PAC). ROD Standard 

and Guideline # 73. California spotted owl and Northern Goshawk PAC land allocations have 

the objective to “avoid vegetation and fuels management activities within PACs to the greatest 

extent possible.” Table 1 ROD page 45. Mechanical treatments for project objectives other than 

hazardous fuels reduction within the PAC are not addressed in the ROD. A site-specific analysis 

and biological evaluation would be required to assess the consequences of proposed restoration 

treatments within PACs. Based on the site-specific analysis and biological evaluation a nonsignificant forest plan amendment may be prepared.
122 USDA Forest Service, Gen. Tech. Rep. RMRS-GTR-178. 2006

4. SNFPA, FSEIS Appendix A, p. 345 states: “Strategies should recognize the role of fire in ecosystem 

function and identify those instances where fire suppression or fuel management actions could be 

damaging to habitat or long-term function of the riparian community.” Is it therefore proper to advise 

that (1) wildfires be allowed to burn through aspen stands located in riparian corridors and (2) prescribed 

burning can be used to ecologically restore aspen communities in riparian corridors?

Response:

Referring to the associated objective statement, which calls for enhancing or maintaining the 

physical and biological characteristics with riparian-dependant species, both advice statements 

would be appropriate considerations during wildland fire decision-making.

5. Since the goals of aspen (a hardwood) restoration are synonymous with the desired goals for Lower 

Westside Hardwood Ecosystems (ROD Appendix A, p. 35-36), could the Standards and Guidelines for 

Hardwood Management (ROD, Appendix A, p. 53, #18, 19, 22, 24, 25, and 26) apply also to aspen?

Response:

The ROD does not include aspen communities within the definition of the Lower Westside 

Hardwood Ecosystem, which is comprised of the montane hardwood forest and blue oak 

woodland vegetation communities. FEIS Volume 1, Chapter 2—page 17. However, site-specific 

analysis of aspen restoration projects can consider including these, or other, standards and 

guidelines.
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