Patch Size

Clearcut patch size conformed to the Franklin-Forman model

by remaining constant up to 50 percent cutover, except for a

small anomalous increase in patch-size up to 10 percent cutover. Individual clearcuts had not coalesced into larger clearcuts within the areas sampled: this pattern follows from the

standard procedure of not locating new cuts immediately

adjacent to existing ones. Old-growth and late-successionalforest patch-sizes, however, did not conform to the model:

patch-size decreased linearly instead of remaining constant

within the range of 0 to 30 percent cutover as predicted by

Franklin and Forman.

Franklin and Forman described forest patch-size as remaining

constant during the initial stages of fragmentation, although

patches become “increasingly porous” with progressive clearcutting. This rationale for calculating patch-size ignores the

fact that forest patch-size as measured by area will always

decline with conversion of some part of the patch to clearcut.

An absolute loss of habitat occurs, a reduction in the habitat

“core area” (Temple 1986) devoid of edge effects, and a

decline in the connectivity of remaining forest, however

contiguous the external boundaries of the patch may be.

Landscape-Vertebrate Relationships

Importance of Landscape Scale

Birds, small mammals, and amphibians responded differently

to the three scales of analysis. Birds were influenced by

stand, neighborhood, and whole-landscape variables. Stand

area, primarily a function of fragmentation, was an important

influence on bird abundance, and, to a lesser extent, on richness. Stand elevation and stand age (variables that are not

influenced by fragmentation) also had strong effects on bird

richness and abundance, respectively. Larger-scale factors of

neighborhood and landscape proportions of clearcut or forest

also strongly influenced bird richness and abundance. The

richness and abundance of small mammals and amphibians

in sampled stands was closely associated with few stand or

landscape variables related to fragmentation. Elevation also

was an important determinant of amphibian richness and

abundance. Habitat features are likely to be important determinants of variations in the richness and abundance of birds,

small mammals, and amphibians that are not explained by

landscape features.

The landscape indices alone had little power for predicting

vertebrate richness or abundance. Dominance did show some

significant relation to small mammal abundance and amphibian richness and abundance, and the fractal dimension

was associated with bird and small mammal richness in

old-growth stands. These relations were relatively weak,

however. We believe that these variables alone would have

little utility as indices for management in tracking trends in

richness or abundance. They will have some value, however,

in providing information on the spatial characteristics of the

whole landscape to support the interpretation of other more

specific variables describing habitat composition. The fractal

index, for example, was most strongly correlated with

clearcut area and may be a useful index of logging disturbance, although the correlation was only moderately

strong (r = 0.60).

Response of Species to Fragmentation

Bird richness showed a clear response to fragmentation. Richness in all stands was associated with clearcutting in the

immediate neighborhood of the stand, and also in old-growth

stands with clearcuts in the landscape. These phenomena

were not entirely unexpected: they conform to the conventional edge-effect of richness increasing with disturbance

from the influx of early successional and edge species

(Leopold 1933, Raphael and Barrett 1984, Raphael and others

1988, Rosenberg and Raphael 1986, Verner 1986). Among

the four generalist species well represented in the data set

(American robin, black-throated gray warbler, dark-eyed

junco, and rufous hummingbird), however, only rufous

hummingbird abundance showed a clear correlation with

clearcuts in the surrounding area. Black-throated gray warbler

abundance had a negative relation with late-successional

forest in the buffer, which can be construed as a preference

for young forest as found by Huff and Raley (this volume).

If the edge effect drives bird richness, we would expect

future studies to show bird richness to peak at 50 percent

cutover, when the areas of late-successional forest and

clearcuts are equal and edge length peaks.

The regression model predicted high bird abundance in old,

large, stands surrounded by old growth in the buffer zone,

and in landscapes with a high proportion of clearcut area and

a low proportion of old-growth area. Similar results were

found in old-growth stands where abundance increased with

the reduction of surrounding old-growth and late-successional

forest, and with the addition of more clearcuts in the landscape. We believe this phenomenon is a result of the conventional edge effects described earlier and the “packing”

(Rosenberg and Raphael 1986, Whitcomb and others 1981)

of late-successional-forest birds displaced from adjacent

logged forest into remaining old-growth stands. Abundance

data for red-breasted nuthatches and pine siskins, two common birds normally associated with late-successional forest

habitats (Huff and Raley, this volume), support our packing

hypothesis with greater abundance in stands surrounded by

clearcuts.

The pattern of associations that indicate packing were not

entirely clear for all forest birds, however. The common

winter wren was more abundant in stands surrounded by latesuccessional forest with few clearcuts. Negative correlations

between the abundance of black-throated gray warblers,

hermit-Townsend’s warblers, and western flycatchers and

440
old-growth or late-successional forest surrounding the stand

might be construed to support packing, but are probably a

consequence of these birds’ affinities for younger stands

(Huff and Raley, this volume) rather than a negative response

to clearcutting.

Small mammal richness and abundance, and the abundance

of individual small mammal species, showed little response

to fragmentation. Marsh shrews were more abundant in

stands in disturbed landscapes with relatively more clearcut

than in late-successional forest. Water shrews, however,

showed a strong association with late-successional forest in

the neighborhood of stands.

Amphibian richness and abundance were mainly influenced

by stand-scale variables, primarily elevation, but also showed

an affinity for landscapes with high habitat-dominance. Interpreting the relation to habitat dominance in terms of habitat components is difficult, except to say that dominance was

moderately correlated (r = 0.68) with late-successional forest

in the landscape. That relation is somewhat contradicted,

however, by the negative correlation between amphibian

abundance and old growth (a component of late-successional

forest) in the landscape.

A somewhat weaker packing-effect than seen with birds

could be inferred for amphibian abundance by a negative

association with old growth in the buffer, as indicated by

partial correlation and regression, and the positive correlation

between abundance in old-growth stands and surrounding

clearcut area. The western redback salamander was the only

species with an abundance consistent with that pattern,

showing a positive correlation with stand size and a negative

correlation with old growth in the buffer zone. The negative

association with buffer old growth, however, is consistent

with its greater abundance in young rather than old stands

(Aubry and Hall, this volume). No strong relation were

found for other amphibian species.

Management Implications

The greater richness of birds in stands situated in clearcut

landscapes does not mean that clearcutting is the preferred

practice to maintain biotic diversity on forest lands. The

increase in species diversity is caused by the influx of generalist species adapted to human-disturbed landscapes. Generalist species will likely persist in forest landscapes regardless

of management practices, but late-seral species will decline

with conversion of late-successional forest to early seral

stages. Increasing local diversity by adding generalist species

is less important than maintaining the quality of biotic diversity regionally through retention of specialist or endemic

species (Murphy 1989, Samson and Knopf 1982, Van Horne

1983, Verner 1986).

Why did we not detect strong relations between species richness or abundance and habitat fragmentation? An optimistic

assessment is that we have not yet reached the threshold of

fragmentation where populations begin to decline. Rosenberg

and Raphael (1986) made a similar assessment of the situation in northern California. Models of vertebrate response to

fragmentation suggest that species diversity will begin to

decline when 50 to 75 percent of the landscape is cutover

(Franklin and Forman 1987, McLellan and others 1986)

within a short enough period that late-successional forest will

not have been replaced by succession. Our study areas and

much of the Western public forest lands have not yet reached

that stage of fragmentation. Other interpretations, however,

suggest various processes operating beyond the scope of our

study to mask the potentially negative effects of fragmentation on biotic diversity.

The packing displayed by birds and amphibians suggests that

some animals associated with late-successional forest are at

artificially high populations in some stands. Whitcomb and

others (1981) found that populations of forest birds packed

into remnant patches were later greatly reduced or eliminated

from those areas. This finding has important implications for

interpreting ecological research, monitoring populations, and

assessing habitat relations.

A lag in resident-species population decline after isolation

caused by fragmentation may further complicate interpretations of abundance. Populations of long-lived species may

not show instantaneous responses to isolation because the

relaxation time may be long (Shaffer 1981). A source-andsink effect suggested by higher small mammal richness and

the abundance of cavity-nesting birds in young stands surrounded by old-growth may also confound evaluation of

habitat use and the effects of patch size and isolation. Our

results suggest that the value of early-successional communities as wildlife habitat may be overestimated where

adjacent habitats are possible sources of immigrants. The

same relation may occur between small sink-patches and

large source-patches of the same habitat. Populations may

appear high in sink-patches, but net reproduction is negative

and the population is maintained only through immigration

from nearby sources (Pulliam 1988, Van Home 1983). A

related process is the “rescue effect” (Brown and KodricBrown 1977), in which immigrants from large, viable habitat

patches rescue declining or extirpated populations in intermittently viable patches and present a false impression of

viability.

441
Finally, extinctions caused by fragmentation are influenced

by stochastic demographic, environmental, and genetic processes (Gilpin and Soule 1986, Shaffer 1981) that may not

have been active during the study period. Long-term studies

are necessary to account for all these unpredictable controls

on population viability.

The prudent manager will realize that fragmentation-that

is, habitat loss and isolation of remaining fragments-will

nearly reduce the populations of species associated with latesuccessional forest and possibly result in their extirpation.

Declining population trends are evident in some areas for

forest birds (Jarvinen and Vaisanen 1978, Raphael and others

1988), and for small mammals and amphibians (Raphael

1988). The ability of populations that have been reduced by

habitat loss to cope with the effects of habitat isolation is

determined by the life history and population structural

characteristics of the species, and by the success of land

managers in implementing low-fragmentation alternatives to

current logging practices and managing the landscape as an

interacting network of habitats. The absence of strong

negative responses to forest fragmentation in our study

optimistically suggests that we have not yet reached the

threshold of decline for most vertebrates, and still have the

management opportunity to ensure the integrity of vertebrate

communities in Pacific Northwest Douglas-fir forests.

Acknowledgments

This paper is contribution 130 of the Wildlife Habitat

Relationships in Western Washington and Oregon Research

Project, Pacific Northwest Research Staiton, USDA Forest

Service. 0

442
Wildlife Habitat Relationships and Viable

Populations

Leonard F. Ruggiero

Author

LEONARD F. RUGGIERO was a supervisory research

wildlife biologist, U.S. Department of Agriculture, Forest

Service, Pacific Northwest Research Station, Olympia,

Washington 98502. He is now at Rocky Mountain Forest

and Range Experiment Station, Laramie, Wyoming 82070.

The issue that has provided the greatest impetus for research

on wildlife habitat relationships on Federal lands has been

concern about preserving biological diversity-that is maintaining viable populations of native plants and animals. Some

animals, like wolves, grizzlies, and spotted owls, have exciting and dramatic natural histories and charismatic appeal

to the general public, so we hear a lot about them. But the

statutes and regulations that guide public land management

clearly direct land managers to maintain viable populations

of all native species as efforts are made to preserve, and

sometimes enhance, biological diversity.

To maintain viable populations, the kinds, amounts, and

arrangements of environments necessary for populations to

survive over long periods-not just for the next few yearsmust be met. The knowledge that these environments are

essential comes from ecological theory and applied research

in plant and animal ecology, including wildlife habitat

studies. The many different terms commonly used in discussions about habitat relationships are important because

they influence the way we think about very complex

ecological interactions.

One frequently used term that has been the focus of attention

ever since the importance of old-growth Douglas-fir forests

to wildlife emerged as an issue is “dependency” (Carey

1984). The term is uncommon in the literature of ecology

and wildlife management, and no definition of dependency

or explicit practical criteria for determining whether a population is dependent on a particular environment had been

published until recently (Ruggiero and others 1988). The

concept of dependency involves an extremely complex and

dynamic array of ecological interactions. Ecology is defined

as “ . ..the scientific study of interactions that determine the

distribution and abundance of organisms” (Krebs 1985).

Because studying dependencies also includes studying the

interactions that determine the distribution and abundance

of organisms, the Krebs’ definition (and common sense)

suggests that the concept of dependency includes virtually

all aspects of ecology. I find this realization rather sobering,

yet in spite of its implications, the terms dependent and

dependency are used freely and rather casually by the media,

in political debate, and in discussions among biologists about

wildlife and old growth. This broad usage has, I believe, led

to some confusion about the nature of wildlife habitat

relationships.

443
Most people define dependency in fairly simple terms-a

habitat or habitat element that is required for a population to

exist. But given the complexity of ecological systems, such

requirements are not always recognizable. Ecological requirements are not necessarily the same across the entire

geographic range of a species. Most important, ecological

requirements may change over time as environmental conditions change. Because requirements can change over time,

the focus of research should not only be on the features of

the environment that are required for a population to exist

under a given set of conditions, but also on the requirements

necessary for the population to persist over time under varying environmental conditions. The profound difference between existence and persistence must be clearly recognized.

Ecological dependency describes the relationship between a

population and the environment or environments required for

its persistence. Populations will persist only if sufficient

kinds, amounts, and patterns of environments are available to

meet the biological needs of individuals within populations

and if these environments provide sufficient resources to

sustain populations over time as environmental, genetic, and

demographic conditions fluctuate (Ruggiero and others 1988).

Population persistence is likely to involve complex and often

subtle interactions that may vary over time (season, year, or

longer) or with chance events that may affect environments

and populations in dramatic ways. So, although all organisms

in some way depend on the environment in which they exist,

dependency is best thought of as an ecological concept rather

than as a precisely or readily measurable state of nature. The

term “ecological dependency” emphasizes the dynamic and

interactive nature of the concept in both space and time. Discussions about wildlife habitat in general and about dependency in particular sometimes fail to consider this perspective.

I am convinced that meaningful scientific and political discussions about maintaining viable wildlife populations require

that participants understand the concept of ecological dependency, and then come to grips with some very real problems with the way people generally think about dependency.

This understanding will mean sharing a common perspective

about ecological requirements.

As a further example, when the ecological requirements of

wildlife are assessed, populations should be the focus rather

than individuals or species. The concepts of dependency and

population viability both focus on the issue of population

persistence (Ruggiero and others 1988, Shaffer 1981). Yet

observations of one or a few individuals are too often generalized to the species or to the population scale of biological

organization. The debate over whether spotted owls depend

on old growth is a good illustration of how focusing on

individuals can confuse the issue and lead to misunderstandings about habitat requirements. Anecdotes about

individual owls observed in unusual environments do not

444

adequately represent the behavior of the population in selecting habitat. Statistical analyses determine when enough individuals from a population have been observed to reliably

estimate the mean and variance associated with a particular

attribute. Until a population has been studied in this way,

how well the behavior of one or more individuals reflects the

ecological requirements of the population to be maintained is

unknown. Thus, questions about dependency must address

population attributes, not the attributes of individuals. A

similar danger exists in overgeneralizing about the ecological

requirements of populations based on broad observations of a

species, which can happen when discrete, ecologically meaningful populations are not identified. It can also result in

confusion about population versus species attributes and

about ecotypic variation within species.

The concept of ecotypes has been around for a long time, but

its importance is sometimes overlooked. Ecotypes are populations adapted to a set of local conditions-in other words,

populations of a particular species that are adapted to some

part of the species’ geographic range (Odum 1971). Defining

populations too broadly can result in generalizations about

ecological requirements that fail to meet the needs of locally

adapted populations. Simply because a given species exists

as a set of ecotypes across a range of environments does not

mean that any given ecotype is equally plastic and can

quickly “adapt” to drastically altered environments. Adaptations exhibited by one ecotype cannot be assumed to be

within the range of genetic potential of another ecotype. The

spotted owl serves once again as a practical example: the

habitat associations and ecological requirements of the

northern spotted owl must not be confused with those of its

southern relatives.

When wildlife habitat requirements are being assessed,

degrees of association should be recognized, rather than two

simple states of naturedependent or not dependent. Some

species and some environments may have no association at

all. Others may have an exclusive association, and the species

will occur in only that one environment. Answering questions

about dependency at these extremes is relatively easy. But at

what point along a gradient of increasing association a population becomes dependent on some habitat is not known.

When a population is substantially more abundant in a given

habitat than in any other-when it is “closely associated”

with that habitat-it should be assumed to require that

habitat for long-term persistence.

Such close associations can be inferred from patterns of species abundance (for example, when a population is abundant

in old growth but rarely found in younger forests), from

measures of how well a population reproduces in a particular

habitat, and from observations of when the habitat is used

(for example, in instances where a population reaches its

highest reproductive attainment in a particular habitat or
when the habitat is selected during a critical period). But

intensive study of individual species is required to determine

how various amounts and arrangements of the habitat in

question will influence the probability of population persistence. For example, although the loss of old growth would

be expected to result in a significant decline in numbers of a

closely associated population, predictions cannot be more

precise until intensive studies are conducted to better understand why the population exhibits a close association with

old growth.

Nevertheless, a close association with a habitat should be

interpreted to indicate ecological requirements for persistence. This condition must be accepted as indicating dependency unless more intensive research supports a different interpretation. Equivocating and insisting on “absolute proof

of dependency before committing to the appropriate management actions is inappropriate. The scientists who provide

research results and the managers who use those results

must recognize that such absolute knowledge is usually not

attainable.

One of the major accomplishments of the studies reported

in this volume is that they have identified species that are

closely associated with old-growth Douglas-fir forests (see

Ruggiero and others, this volume, for a summary). The

principal limitation in these studies is that without additional

research, recommendations for managing most of these

species cannot be very precise. In addition, several species

were found to be more abundant in old-growth than in young

and mature forests, but still occurred in high numbers in

earlier stages of unmanaged forests. We remain uncertain to

what extent this group would be affected by eliminating old

growth.

Although old-growth forests are unique and potentially

critical components of Western Hemlock Zone ecosystems,

a related but broader issue is emerging. This new issue is

about managed versus natural or unmanaged forest landscapes, and the importance of all developmental stages of

unmanaged forests in meeting the ecological needs of

wildlife.

Under certain conditions, undisturbed Douglas-fir-dominated

forests can occupy a site for over 1000 years and are in oldgrowth condition for 80 percent or more of their lifespan

(Spies and Franklin 1988). But 80 years is a common

rotation age for managed forests in the Pacific Northwest.

How will wildlife respond as the predominant forest type

shifts toward these younger, generally less complex forests?

Will short-rotation landscapes containing islands of dedicated

old growth be better for late-successional wildlife than landscapes managed on a 250-year rotation, but largely devoid of

older forests? Such questions are unlikely to be answered in

our lifetimes. Society should, therefore, be conservative and

recognize the inherent uncertainty in any long-term management strategy. A range of options should be considered for

meeting the needs of late-successional forest wildlife, including retaining the largest remaining old-growth tracts

within some suitable managed forest matrix (Thomas and

others 1988).

The research results presented in this volume demonstrate

the ecological diversity found in all developmental stages of

unmanaged forests, and establish their importance as wildlife

habitat. These results provide a place to start in designing

managed landscapes that will provide maximum benefits to

wildlife species associated with late-successional forests.

Ecology is a young science. Questions related to maintaining

viable populations push this developing science to its limits,

and many problems need to be solved before most of these

questions can be answered. These problems will not be

solved by politicians or lawyers or economists: nor will

solutions be found on the pages of the Harvard Business

Review or by applying corporate models for risk assessment.

Rather, these problems will be solved only by ecologists and

through a significant and sustained commitment of time and

money in the conduct of scientific research. In the meantime,

ecologists should continue to give decision-makers the very

best information they can; they should offer interpretations

and judgments as part of their professional responsibility;

and rather than apologizing for their uncertainty, they should

stress the substantial body of ecological understanding upon

which they base their opinions.

Although the problems are extremely complex, the most

important tools for meeting these challenges are not

multivariate analyses nor sophisticated research methods.

The most important tools are the creativity, intuition, and

judgment of our best ecologists, And to the extent that

society demands solutions to ecological problems, these

tools must suffice, and those who possess them must have

a seat at the decision-making table. 0

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Plant and Animal Habitat Associations in

Douglas-Fir Forests of the Pacific Northwest: An

Overview

Leonard F. Ruggiero, Lawrence L.C. Jones, and Keith B. Aubry

I

I

I

-

Authors

Leonard F. Ruggiero was a supervisory research wildlife

biologist, U.S. Department of Agriculture, Forest Service,

Pacific Northwest Research Station, Olympia, Washington

98502 and is now at Rocky Mountain Forest and Range

Experiment Station in Laramie, Wyoming 82070; Lawrence

L.C. Jones is a wildlife biologist and Keith B. Aubry is a

research wildlife biologist, U.S. Department of Agriculture,

Forest Service, Pacific Northwest Research Station, Olympia,

Washington 98502.

Introduction

This book contains the most comprehensive and detailed

research information ever reported for wildlife habitat relationships in the Pacific Northwest, and it is likely that the

scope and intensity of this research program (see Carey and

Spies, this volume) will not soon be duplicated. The results

reported here represent the best information currently available for evaluating the effects of land management decisions

on wildlife and vegetation and for identifying the most productive avenues for new research.

The primary objective of the Old-Growth Forest Wildlife

Habitat Program was to identify wildlife and plant species

that depend upon or find optimal habitat in old-growth

Douglas-fir forests (Carey and Spies, this volume). The

purpose of this paper is to summarize in tabular form the

major patterns of association among forest age-classes for all

wildlife and plant species that were adequately studied in at

least one physiographic province during this research effort.

The information contained in the following matrices was

provided by the scientists responsible for collecting, analyzing, and interpreting the data. These designations result

from statistical analyses, review of pertinent literature, and

the professional judgments of the principal investigators.

Unlike similar compendia of wildlife habitat relationships,

the information in this chapter is derived primarily from

actual field research in the geographic areas listed, and the

interpretation of research findings has been done by the

scientists who conducted the studies. Detailed descriptions of

research methods, data analyses, and interpretive rationales

can all be found elsewhere in this volume (see also Carey

and Ruggiero, in press).

Wildlife habitat relationships are dynamic, especially over

long periods of time, and such relationships are generally

not reducible to “obligatory vs. non-obligatory” associations

(Ruggiero and others 1988). A species’ ecological and biological requirements may change as a function of its reproductive status, age, and environment, and habitat associations

can change accordingly. Moreover, habitat requirements can

be subtle and difficult to detect, and can be based upon or

447
modified by interactions with other species (Simberloff 1983). Further research is urgently needed on at least the following

When available information is limited to general patterns of species or species groups: the Larch Mountain salamander,

species’ abundances derived from extensive community sam- Siskiyou Mountains salamander, Van Dyke’s salamander,

pling, precise descriptions of each species’ habitat require- spotted frog, Keen’s myotis, white-footed vole, carnivores

ments are difficult. Even after intensive autecological study, (especially the marten), accipiter hawks (especially the

habitat relationships may not be precisely definable. goshawk), and numerous nonvascular plant species.

Nature of the Data

Some attributes of the data upon which this summary information is based should be kept in mind. First, the study

stands were preferentially selected to represent a wide range

of site conditions occurring in young, mature, and old-growth

Douglas-fir forests across a broad geographic area (Carey

and Spies, this volume). Consequently, the statistical variation within each age-class makes detecting significant

differences among age-classes difficult. In other words, such

a selection process is expected to result in greater variation

and fewer statistically significant differences in measured

parameters among age-classes than if the stands had been

selected randomly; these results are therefore conservative.

Conversely, ecologically important differences may exist

in the absence of statistical significance (see Connor and

Simberloff 1986). Thus, we recommend careful consideration

of these caveats when interpreting results that fail to demonstrate statistically significant relationships, especially for

those species that have been designated as “associated” with

a particular age-class (see Summary Matrices).

Second, the influence of stand size and surrounding habitats

(stand context) on species’ abundances within old-growth

study stands was largely uncontrolled. Old-growth stands

varied considerably in size. Although the old-growth study

stands were relatively large-78 percent were larger than

100 ha-they were often irregularly shaped and located in

various contexts within landscapes containing on average

only about 25 percent old growth (J. Lehmkuhl, pers. comm.).

Lastly, the survey techniques used were designed to sample

the broadest possible assemblage of species within a community. As a result, relatively few data were collected on

rare or highly vagile species. Thus, for certain species of

special interest or concern to forest managers, such as the red

tree vole and pileated woodpecker, the ability to evaluate

observed patterns of abundance or occurrence was limited by

small sample sizes. More intensive research will be necessary to generate adequate information on the habitat relationships of these species. Other species were either not

effectively sampled with our techniques (for example, highly

mobile avian and mammalian predators) or were so locally

distributed (for example, Larch Mountain salamanders) that

insufficient data were collected for any conclusions about

their habitat associations to be made. Lack of information,

however, should not be equated with a lack of concern about

the potential effects of forest management on these species.

Structural Diversity in Naturally Regenerated

Forests

Virtually all of the study stands were naturally regenerated

after wildfires. Consequently, these results do not apply to

intensively managed forest stands. Because some structural

or vegetative components of stands, such as large snags,

logs, and live trees, generally survive even catastrophic

wildfires, such components often carry over from the preceding mature or old-growth stand to the new, young stand

(Spies and others 1988, Spies and Franklin 1988). Intensively

managed forest stands, which generally lack such components

(Spies and Cline 1988), may not provide suitable habitat for

wildlife associated with these features.

Because of these carryover components and variation in

establishment patterns, naturally regenerated Douglas-fir

stands can be structurally diverse, even in the younger ageclasses (Spies and Franklin, this volume). Moreover, the

regional pattern of natural disturbances typically created

forest mosaics where stands of different ages resulting from

a varied history of disturbance were interspersed with relatively old, undisturbed forest (Agee, this volume; Spies and

Franklin 1988). This process created widespread habitat conditions that consisted either of late-successional forests or

younger forests containing structural or vegetative characteristics of late-successional forests. The structural diversity of

naturally regenerated young stands and an interspersed pattern of disturbance probably interacted to maintain high

ecological diversity in natural landscapes (Hansen and

others, in press).

Given that presettlement landscapes in this region were

composed of a wide variety of vegetative conditions, including large expanses of suitable habitat for species adapted

to late stages of forest development, some species would

logically have become adapted to structurally diverse forests

(Thomas and others 1988). Current forest management

practices have resulted in extensive simplification of stand

structures (Spies and Cline 1988) and fragmentation of

natural forests (Lehmkuhl and Ruggiero, this volume;

Morrison 1988). Although young plantations that have

already been established for timber production can be

modified to improve their habitat capability (Spies and

others, in press), the ecological values of new approaches to

harvesting remaining late-successional forests (Franklin

1989) have not yet been evaluated through field research.

448
Alternative silvicultural strategies may be needed to maintain

populations of species adapted to structurally diverse stands

or to unfragmented forest conditions. Perhaps such species

can be maintained by aggregating disturbances at the landscape scale and leaving various components of harvested

stands to add complexity and important functional elements

to managed stands and landscapes (Franklin 1989). The

results presented here suggest that such a scenario imitates

natural disturbance regimes more closely than traditional

silvicultural practices and is more compatible with the goal

of maintaining biological diversity. The extent to which such

alternative management strategies could functionally replace

extensive blocks of relatively undisturbed late-successional

habitat is unknown, however.

Summary Matrices

The designations listed in the following matrices are

intended to summarize the results of the studies reported in

this book. These designations therefore reflect comparisons

of indices to abundance among old-growth (200-730 yr),

mature (80-195), and young (35-79) age-classes of unmanaged, naturally regenerated, closed-canopy forests in the

Douglas-Fir/ Western Hemlock Zone of western Oregon and

Washington and northwestern California (Franklin and

Dyrness 1973). Although a large percentage of the species

listed are primarily forest-dwelling species that occur most

commonly in the forest zone we studied, many also occur in

other forest types or in precanopy seral stages of Douglas-fir/

western hemlock forest. Consequently, because some species

may have closer associations with other habitats not considered here, these designations should be viewed with

caution-they only reflect relative differences in patterns of

abundance among the three seral stages of unmanaged forest

we studied. The following designations represent the operational definitions used by the principal investigators to

assign degrees of association among age-classes for each

species.

* Closely Associated. A species was designated as “closely

associated” with a seral stage if it was found to be significantly more abundant (based on statistical significance levels set by each investigator) in that seral stage

compared to the other seral stages, or if it is known to

occur almost exclusively in that seral stage.

+ Associated. A species was designated as “associated”

with a seral stage if it was found to be numerically

more abundant in that stage compared to the other seral

stages. [Note: some species were also designated as

associated with old growth if they were determined to

require one or more habitat features that are characteristic of that seral stage, such as very large diameter

logs, snags, or broken-top live trees.]

P Present. A species was designated as “present” in a seral

stage if it occurred in that seral stage in numbers lower

than or equal to those found in the other seral stages.

? Unknown. A species was designated as “unknown”

across all seral stages in a given geographic area if the

species was not studied or the data collected were

insufficient for any designations to be made.

/ A species was designated with a diagonal slash across all

seral stages if it does not occur in the geographic area

sampled.

For species designated as closely associated with old growth,

we predict that a significant reduction in old growth would

result in a marked decline in the population numbers of those

species. The extent to which such a decline would threaten

the persistence of each population will depend on various

environmental, demographic, and stochastic factors. The

consequences of a significant loss of old-growth habitat on

the population viability of species designated as associated

with old growth, are unknown.

The principal investigators that provided the designations

shown here are as follows: diurnal forest birds: Andrew B.

Carey (Oregon Coast Range), Frederick F. Gilbert (Oregon

Cascade Range), David A. Manuwal (Southern Washington

Cascade Range), Mark H. Huff (Regional Analyses for

Oregon and Washington; Southern Washington Cascade

Range-Winter), Richard W. Lundquist and Jina M. Mariani

(Southern Washington Cascade Range-Snag-Dependent

Birds), C. John Ralph (northern California and southern

Oregon); small mammals: Paul Stephen Corn and R. Bruce

Bury (Oregon Coast Range), Frederick F. Gilbert (Oregon

Cascade Range), Stephen D. West (Southern Washington

Cascade Range); Keith B. Aubry (Regional Analyses for

Oregon and Washington), C. John Ralph (northern California

and southern Oregon); bats: Donald W. Thomas (Oregon

Coast Range; Southern Washington Cascade Range);

amphibians: R. Bruce Bury and Paul Stephen Corn

(Oregon Coast Range; Regional Analyses for Oregon and

Washington), Frederick F. Gilbert (Oregon Cascade Range),

Keith B. Aubry (Southern Washington Cascade Range),

Hartwell H. Welsh, Jr., and Amy J. Lind (northern California

and southern Oregon); vascular plants: Thomas A. Spies

(Oregon and Washington) and Bruce B. Bingham (northern

California and southern Oregon); hypogeous fungi: Daniel

L. Luoma (Oregon Cascade Range).

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For birds, mammals, amphibians, and reptiles, all species for

which data collected were sufficient for analyses in at least

Acknowledgments

one province are included. For vascular plants, only those R.S. Holthausen, M.H. Huff, and T.A. Spies made important

species showing an association with a particular age-class in contributions to both the structure and content of this paper

at least one province are included. Hypogeous fungi were through their helpful comments on the manuscript and in

only studied in the Oregon Cascade Range. Because sampling many stimulating and insightful discussions. 0

for hypogeous fungi was not as intensive nor extensive as for

other taxa, the likelihood of not detecting the presence of a

species within a particular age-class was high. For this

reason, dashes were used in the matrix for hypogeous fungi

to indicate that the species was not detected in forests of that

age-class.

450