Patch Size

Clearcut patch size conformed to the Franklin-Forman modelby remaining constant up to 50 percent cutover, except for asmall anomalous increase in patch-size up to 10 percent cut-over. Individual clearcuts had not coalesced into larger clear-cuts within the areas sampled: this pattern follows from thestandard procedure of not locating new cuts immediatelyadjacent to existing ones. Old-growth and late-successional-forest patch-sizes, however, did not conform to the model:patch-size decreased linearly instead of remaining constantwithin the range of 0 to 30 percent cutover as predicted byFranklin and Forman.

Franklin and Forman described forest patch-size as remainingconstant during the initial stages of fragmentation, althoughpatches become “increasingly porous” with progressive clear-cutting. This rationale for calculating patch-size ignores thefact that forest patch-size as measured by area will alwaysdecline 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 adecline in the connectivity of remaining forest, howevercontiguous the external boundaries of the patch may be.

Landscape-Vertebrate RelationshipsImportance of Landscape Scale

Birds, small mammals, and amphibians responded differentlyto the three scales of analysis. Birds were influenced bystand, neighborhood, and whole-landscape variables. Standarea, primarily a function of fragmentation, was an importantinfluence on bird abundance, and, to a lesser extent, on rich-ness. Stand elevation and stand age (variables that are notinfluenced by fragmentation) also had strong effects on birdrichness and abundance, respectively. Larger-scale factors ofneighborhood and landscape proportions of clearcut or forestalso strongly influenced bird richness and abundance. Therichness and abundance of small mammals and amphibiansin sampled stands was closely associated with few stand orlandscape variables related to fragmentation. Elevation alsowas an important determinant of amphibian richness andabundance. Habitat features are likely to be important deter-minants of variations in the richness and abundance of birds,small mammals, and amphibians that are not explained bylandscape features.

The landscape indices alone had little power for predictingvertebrate richness or abundance. Dominance did show somesignificant relation to small mammal abundance and am-phibian richness and abundance, and the fractal dimensionwas associated with bird and small mammal richness inold-growth stands. These relations were relatively weak,however. We believe that these variables alone would havelittle utility as indices for management in tracking trends inrichness or abundance. They will have some value, however,

in providing information on the spatial characteristics of thewhole landscape to support the interpretation of other morespecific variables describing habitat composition. The fractalindex, for example, was most strongly correlated withclearcut area and may be a useful index of logging dis-turbance, although the correlation was only moderatelystrong (r = 0.60).

Response of Species to Fragmentation

Bird richness showed a clear response to fragmentation. Rich-ness in all stands was associated with clearcutting in theimmediate neighborhood of the stand, and also in old-growthstands with clearcuts in the landscape. These phenomenawere not entirely unexpected: they conform to the conven-tional edge-effect of richness increasing with disturbancefrom the influx of early successional and edge species(Leopold 1933, Raphael and Barrett 1984, Raphael and others1988, Rosenberg and Raphael 1986, Verner 1986). Amongthe four generalist species well represented in the data set(American robin, black-throated gray warbler, dark-eyedjunco, and rufous hummingbird), however, only rufoushummingbird abundance showed a clear correlation withclearcuts in the surrounding area. Black-throated gray warblerabundance had a negative relation with late-successionalforest in the buffer, which can be construed as a preferencefor young forest as found by Huff and Raley (this volume).If the edge effect drives bird richness, we would expectfuture studies to show bird richness to peak at 50 percentcutover, when the areas of late-successional forest andclearcuts 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 anda low proportion of old-growth area. Similar results werefound in old-growth stands where abundance increased withthe reduction of surrounding old-growth and late-successionalforest, and with the addition of more clearcuts in the land-scape. We believe this phenomenon is a result of the conven-tional edge effects described earlier and the “packing”(Rosenberg and Raphael 1986, Whitcomb and others 1981)of late-successional-forest birds displaced from adjacentlogged forest into remaining old-growth stands. Abundancedata for red-breasted nuthatches and pine siskins, two com-mon birds normally associated with late-successional foresthabitats (Huff and Raley, this volume), support our packinghypothesis with greater abundance in stands surrounded byclearcuts.

The pattern of associations that indicate packing were notentirely clear for all forest birds, however. The commonwinter wren was more abundant in stands surrounded by late-successional forest with few clearcuts. Negative correlationsbetween the abundance of black-throated gray warblers,hermit-Townsend’s warblers, and western flycatchers and

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old-growth or late-successional forest surrounding the standmight be construed to support packing, but are probably aconsequence of these birds’ affinities for younger stands(Huff and Raley, this volume) rather than a negative responseto clearcutting.

Small mammal richness and abundance, and the abundanceof individual small mammal species, showed little responseto fragmentation. Marsh shrews were more abundant instands in disturbed landscapes with relatively more clearcutthan in late-successional forest. Water shrews, however,showed a strong association with late-successional forest inthe neighborhood of stands.

Amphibian richness and abundance were mainly influencedby stand-scale variables, primarily elevation, but also showedan affinity for landscapes with high habitat-dominance. In-terpreting the relation to habitat dominance in terms of habi-tat components is difficult, except to say that dominance wasmoderately correlated (r = 0.68) with late-successional forestin the landscape. That relation is somewhat contradicted,however, by the negative correlation between amphibianabundance and old growth (a component of late-successionalforest) in the landscape.

A somewhat weaker packing-effect than seen with birdscould be inferred for amphibian abundance by a negativeassociation with old growth in the buffer, as indicated bypartial correlation and regression, and the positive correlationbetween abundance in old-growth stands and surroundingclearcut area. The western redback salamander was the onlyspecies with an abundance consistent with that pattern,showing a positive correlation with stand size and a negativecorrelation with old growth in the buffer zone. The negativeassociation with buffer old growth, however, is consistentwith its greater abundance in young rather than old stands(Aubry and Hall, this volume). No strong relation werefound for other amphibian species.

Management Implications

The greater richness of birds in stands situated in clearcutlandscapes does not mean that clearcutting is the preferredpractice to maintain biotic diversity on forest lands. Theincrease in species diversity is caused by the influx of gen-eralist species adapted to human-disturbed landscapes. Gener-alist species will likely persist in forest landscapes regardlessof management practices, but late-seral species will declinewith conversion of late-successional forest to early seralstages. Increasing local diversity by adding generalist speciesis less important than maintaining the quality of biotic diver-sity regionally through retention of specialist or endemicspecies (Murphy 1989, Samson and Knopf 1982, Van Horne1983, Verner 1986).

Why did we not detect strong relations between species rich-ness or abundance and habitat fragmentation? An optimisticassessment is that we have not yet reached the threshold offragmentation where populations begin to decline. Rosenbergand Raphael (1986) made a similar assessment of the situa-tion in northern California. Models of vertebrate response tofragmentation suggest that species diversity will begin todecline 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 willnot have been replaced by succession. Our study areas andmuch of the Western public forest lands have not yet reachedthat stage of fragmentation. Other interpretations, however,suggest various processes operating beyond the scope of ourstudy to mask the potentially negative effects of fragmenta-tion on biotic diversity.

The packing displayed by birds and amphibians suggests thatsome animals associated with late-successional forest are atartificially high populations in some stands. Whitcomb andothers (1981) found that populations of forest birds packedinto remnant patches were later greatly reduced or eliminatedfrom those areas. This finding has important implications forinterpreting ecological research, monitoring populations, andassessing habitat relations.

A lag in resident-species population decline after isolationcaused by fragmentation may further complicate interpreta-tions of abundance. Populations of long-lived species maynot show instantaneous responses to isolation because therelaxation time may be long (Shaffer 1981). A source-and-sink effect suggested by higher small mammal richness andthe abundance of cavity-nesting birds in young stands sur-rounded by old-growth may also confound evaluation ofhabitat use and the effects of patch size and isolation. Ourresults suggest that the value of early-successional com-munities as wildlife habitat may be overestimated whereadjacent habitats are possible sources of immigrants. Thesame relation may occur between small sink-patches andlarge source-patches of the same habitat. Populations mayappear high in sink-patches, but net reproduction is negativeand the population is maintained only through immigrationfrom nearby sources (Pulliam 1988, Van Home 1983). Arelated process is the “rescue effect” (Brown and Kodric-Brown 1977), in which immigrants from large, viable habitatpatches rescue declining or extirpated populations in inter-mittently viable patches and present a false impression ofviability.

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Finally, extinctions caused by fragmentation are influencedby stochastic demographic, environmental, and genetic proc-esses (Gilpin and Soule 1986, Shaffer 1981) that may nothave been active during the study period. Long-term studiesare necessary to account for all these unpredictable controlson population viability.

The prudent manager will realize that fragmentation-thatis, habitat loss and isolation of remaining fragments-willnearly reduce the populations of species associated with late-successional forest and possibly result in their extirpation.Declining population trends are evident in some areas forforest birds (Jarvinen and Vaisanen 1978, Raphael and others1988), and for small mammals and amphibians (Raphael1988). The ability of populations that have been reduced byhabitat loss to cope with the effects of habitat isolation isdetermined by the life history and population structural

characteristics of the species, and by the success of landmanagers in implementing low-fragmentation alternatives tocurrent logging practices and managing the landscape as aninteracting network of habitats. The absence of strongnegative responses to forest fragmentation in our studyoptimistically suggests that we have not yet reached thethreshold of decline for most vertebrates, and still have themanagement opportunity to ensure the integrity of vertebratecommunities in Pacific Northwest Douglas-fir forests.

AcknowledgmentsThis paper is contribution 130 of the Wildlife HabitatRelationships in Western Washington and Oregon ResearchProject, Pacific Northwest Research Staiton, USDA ForestService. 0

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Wildlife Habitat Relationships and ViablePopulations

Leonard F. Ruggiero

AuthorLEONARD F. RUGGIERO was a supervisory researchwildlife biologist, U.S. Department of Agriculture, ForestService, Pacific Northwest Research Station, Olympia,Washington 98502. He is now at Rocky Mountain Forestand Range Experiment Station, Laramie, Wyoming 82070.

The issue that has provided the greatest impetus for researchon wildlife habitat relationships on Federal lands has beenconcern about preserving biological diversity-that is main-taining viable populations of native plants and animals. Someanimals, like wolves, grizzlies, and spotted owls, have ex-citing and dramatic natural histories and charismatic appealto the general public, so we hear a lot about them. But thestatutes and regulations that guide public land managementclearly direct land managers to maintain viable populationsof all native species as efforts are made to preserve, andsometimes enhance, biological diversity.

To maintain viable populations, the kinds, amounts, andarrangements of environments necessary for populations tosurvive over long periods-not just for the next few years-must be met. The knowledge that these environments areessential comes from ecological theory and applied researchin plant and animal ecology, including wildlife habitat

studies. The many different terms commonly used in dis-cussions about habitat relationships are important becausethey influence the way we think about very complexecological interactions.

One frequently used term that has been the focus of attentionever since the importance of old-growth Douglas-fir foreststo wildlife emerged as an issue is “dependency” (Carey1984). The term is uncommon in the literature of ecologyand wildlife management, and no definition of dependencyor explicit practical criteria for determining whether a popu-lation is dependent on a particular environment had beenpublished until recently (Ruggiero and others 1988). Theconcept of dependency involves an extremely complex anddynamic array of ecological interactions. Ecology is definedas “ . ..the scientific study of interactions that determine thedistribution and abundance of organisms” (Krebs 1985).Because studying dependencies also includes studying theinteractions that determine the distribution and abundanceof organisms, the Krebs’ definition (and common sense)suggests that the concept of dependency includes virtuallyall aspects of ecology. I find this realization rather sobering,yet in spite of its implications, the terms dependent anddependency are used freely and rather casually by the media,in political debate, and in discussions among biologists aboutwildlife and old growth. This broad usage has, I believe, ledto some confusion about the nature of wildlife habitatrelationships.

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Most people define dependency in fairly simple terms-ahabitat or habitat element that is required for a population toexist. But given the complexity of ecological systems, suchrequirements are not always recognizable. Ecological re-quirements are not necessarily the same across the entiregeographic range of a species. Most important, ecologicalrequirements may change over time as environmental con-ditions change. Because requirements can change over time,the focus of research should not only be on the features ofthe environment that are required for a population to existunder a given set of conditions, but also on the requirementsnecessary for the population to persist over time under vary-ing environmental conditions. The profound difference be-tween existence and persistence must be clearly recognized.

Ecological dependency describes the relationship between apopulation and the environment or environments required forits persistence. Populations will persist only if sufficientkinds, amounts, and patterns of environments are available tomeet the biological needs of individuals within populationsand if these environments provide sufficient resources tosustain populations over time as environmental, genetic, anddemographic conditions fluctuate (Ruggiero and others 1988).

Population persistence is likely to involve complex and oftensubtle interactions that may vary over time (season, year, orlonger) or with chance events that may affect environmentsand populations in dramatic ways. So, although all organismsin some way depend on the environment in which they exist,dependency is best thought of as an ecological concept ratherthan as a precisely or readily measurable state of nature. Theterm “ecological dependency” emphasizes the dynamic andinteractive nature of the concept in both space and time. Dis-cussions about wildlife habitat in general and about depend-ency in particular sometimes fail to consider this perspective.I am convinced that meaningful scientific and political dis-cussions about maintaining viable wildlife populations requirethat participants understand the concept of ecological de-pendency, and then come to grips with some very real prob-lems with the way people generally think about dependency.This understanding will mean sharing a common perspectiveabout ecological requirements.

As a further example, when the ecological requirements ofwildlife are assessed, populations should be the focus ratherthan individuals or species. The concepts of dependency andpopulation viability both focus on the issue of populationpersistence (Ruggiero and others 1988, Shaffer 1981). Yetobservations of one or a few individuals are too often gen-eralized to the species or to the population scale of biologicalorganization. The debate over whether spotted owls dependon old growth is a good illustration of how focusing onindividuals can confuse the issue and lead to misunder-standings about habitat requirements. Anecdotes aboutindividual owls observed in unusual environments do not

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adequately represent the behavior of the population in select-ing habitat. Statistical analyses determine when enough in-dividuals from a population have been observed to reliablyestimate the mean and variance associated with a particularattribute. Until a population has been studied in this way,how well the behavior of one or more individuals reflects theecological requirements of the population to be maintained isunknown. Thus, questions about dependency must addresspopulation attributes, not the attributes of individuals. Asimilar danger exists in overgeneralizing about the ecologicalrequirements of populations based on broad observations of aspecies, which can happen when discrete, ecologically mean-ingful populations are not identified. It can also result inconfusion about population versus species attributes andabout ecotypic variation within species.

The concept of ecotypes has been around for a long time, butits importance is sometimes overlooked. Ecotypes are popu-lations adapted to a set of local conditions-in other words,populations of a particular species that are adapted to somepart of the species’ geographic range (Odum 1971). Definingpopulations too broadly can result in generalizations aboutecological requirements that fail to meet the needs of locallyadapted populations. Simply because a given species existsas a set of ecotypes across a range of environments does notmean that any given ecotype is equally plastic and canquickly “adapt” to drastically altered environments. Adap-tations exhibited by one ecotype cannot be assumed to bewithin the range of genetic potential of another ecotype. Thespotted owl serves once again as a practical example: thehabitat associations and ecological requirements of thenorthern spotted owl must not be confused with those of itssouthern relatives.

When wildlife habitat requirements are being assessed,degrees of association should be recognized, rather than twosimple states of naturedependent or not dependent. Somespecies and some environments may have no association atall. Others may have an exclusive association, and the specieswill occur in only that one environment. Answering questionsabout dependency at these extremes is relatively easy. But atwhat point along a gradient of increasing association a popu-lation becomes dependent on some habitat is not known.When a population is substantially more abundant in a givenhabitat than in any other-when it is “closely associated”with that habitat-it should be assumed to require thathabitat for long-term persistence.

Such close associations can be inferred from patterns of spe-cies abundance (for example, when a population is abundantin old growth but rarely found in younger forests), frommeasures of how well a population reproduces in a particularhabitat, and from observations of when the habitat is used(for example, in instances where a population reaches itshighest reproductive attainment in a particular habitat or

when the habitat is selected during a critical period). Butintensive study of individual species is required to determinehow various amounts and arrangements of the habitat inquestion will influence the probability of population per-sistence. For example, although the loss of old growth wouldbe expected to result in a significant decline in numbers of aclosely associated population, predictions cannot be moreprecise until intensive studies are conducted to better under-stand why the population exhibits a close association withold growth.

Nevertheless, a close association with a habitat should beinterpreted to indicate ecological requirements for persist-ence. This condition must be accepted as indicating depend-ency unless more intensive research supports a different in-terpretation. Equivocating and insisting on “absolute proofof dependency before committing to the appropriate man-agement actions is inappropriate. The scientists who provideresearch results and the managers who use those resultsmust recognize that such absolute knowledge is usually notattainable.

One of the major accomplishments of the studies reportedin this volume is that they have identified species that areclosely associated with old-growth Douglas-fir forests (seeRuggiero and others, this volume, for a summary). Theprincipal limitation in these studies is that without additionalresearch, recommendations for managing most of thesespecies cannot be very precise. In addition, several specieswere found to be more abundant in old-growth than in youngand mature forests, but still occurred in high numbers inearlier stages of unmanaged forests. We remain uncertain towhat extent this group would be affected by eliminating oldgrowth.

Although old-growth forests are unique and potentiallycritical components of Western Hemlock Zone ecosystems,a related but broader issue is emerging. This new issue isabout managed versus natural or unmanaged forest land-scapes, and the importance of all developmental stages ofunmanaged forests in meeting the ecological needs ofwildlife.

Under certain conditions, undisturbed Douglas-fir-dominatedforests can occupy a site for over 1000 years and are in old-growth condition for 80 percent or more of their lifespan(Spies and Franklin 1988). But 80 years is a commonrotation age for managed forests in the Pacific Northwest.

How will wildlife respond as the predominant forest typeshifts toward these younger, generally less complex forests?Will short-rotation landscapes containing islands of dedicatedold growth be better for late-successional wildlife than land-scapes managed on a 250-year rotation, but largely devoid ofolder forests? Such questions are unlikely to be answered inour lifetimes. Society should, therefore, be conservative andrecognize the inherent uncertainty in any long-term manage-ment strategy. A range of options should be considered formeeting the needs of late-successional forest wildlife, in-cluding retaining the largest remaining old-growth tractswithin some suitable managed forest matrix (Thomas andothers 1988).

The research results presented in this volume demonstratethe ecological diversity found in all developmental stages ofunmanaged forests, and establish their importance as wildlifehabitat. These results provide a place to start in designingmanaged landscapes that will provide maximum benefits towildlife species associated with late-successional forests.

Ecology is a young science. Questions related to maintainingviable populations push this developing science to its limits,and many problems need to be solved before most of thesequestions can be answered. These problems will not besolved by politicians or lawyers or economists: nor willsolutions be found on the pages of the Harvard BusinessReview or by applying corporate models for risk assessment.Rather, these problems will be solved only by ecologists andthrough a significant and sustained commitment of time andmoney in the conduct of scientific research. In the meantime,ecologists should continue to give decision-makers the verybest information they can; they should offer interpretationsand judgments as part of their professional responsibility;and rather than apologizing for their uncertainty, they shouldstress the substantial body of ecological understanding uponwhich they base their opinions.

Although the problems are extremely complex, the mostimportant tools for meeting these challenges are notmultivariate analyses nor sophisticated research methods.The most important tools are the creativity, intuition, andjudgment of our best ecologists, And to the extent thatsociety demands solutions to ecological problems, thesetools must suffice, and those who possess them must havea seat at the decision-making table. 0

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Plant and Animal Habitat Associations inDouglas-Fir Forests of the Pacific Northwest: AnOverview

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

II

I-

AuthorsLeonard F. Ruggiero was a supervisory research wildlifebiologist, U.S. Department of Agriculture, Forest Service,Pacific Northwest Research Station, Olympia, Washington98502 and is now at Rocky Mountain Forest and RangeExperiment Station in Laramie, Wyoming 82070; LawrenceL.C. Jones is a wildlife biologist and Keith B. Aubry is aresearch wildlife biologist, U.S. Department of Agriculture,Forest Service, Pacific Northwest Research Station, Olympia,Washington 98502.

IntroductionThis book contains the most comprehensive and detailedresearch information ever reported for wildlife habitat rela-tionships in the Pacific Northwest, and it is likely that thescope and intensity of this research program (see Carey andSpies, this volume) will not soon be duplicated. The resultsreported here represent the best information currently avail-able for evaluating the effects of land management decisionson wildlife and vegetation and for identifying the most pro-ductive avenues for new research.

The primary objective of the Old-Growth Forest WildlifeHabitat Program was to identify wildlife and plant speciesthat depend upon or find optimal habitat in old-growthDouglas-fir forests (Carey and Spies, this volume). The

purpose of this paper is to summarize in tabular form themajor patterns of association among forest age-classes for allwildlife and plant species that were adequately studied in atleast one physiographic province during this research effort.The information contained in the following matrices wasprovided by the scientists responsible for collecting, ana-lyzing, and interpreting the data. These designations resultfrom statistical analyses, review of pertinent literature, andthe professional judgments of the principal investigators.Unlike similar compendia of wildlife habitat relationships,the information in this chapter is derived primarily fromactual field research in the geographic areas listed, and theinterpretation of research findings has been done by thescientists who conducted the studies. Detailed descriptions ofresearch methods, data analyses, and interpretive rationalescan all be found elsewhere in this volume (see also Careyand Ruggiero, in press).

Wildlife habitat relationships are dynamic, especially overlong periods of time, and such relationships are generallynot reducible to “obligatory vs. non-obligatory” associations(Ruggiero and others 1988). A species’ ecological and bio-logical requirements may change as a function of its repro-ductive status, age, and environment, and habitat associationscan change accordingly. Moreover, habitat requirements canbe subtle and difficult to detect, and can be based upon or

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modified by interactions with other species (Simberloff 1983). Further research is urgently needed on at least the followingWhen 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, carnivoresments are difficult. Even after intensive autecological study, (especially the marten), accipiter hawks (especially thehabitat relationships may not be precisely definable. goshawk), and numerous nonvascular plant species.

Nature of the DataSome attributes of the data upon which this summary in-formation is based should be kept in mind. First, the studystands were preferentially selected to represent a wide rangeof site conditions occurring in young, mature, and old-growthDouglas-fir forests across a broad geographic area (Careyand Spies, this volume). Consequently, the statistical vari-ation within each age-class makes detecting significantdifferences among age-classes difficult. In other words, sucha selection process is expected to result in greater variationand fewer statistically significant differences in measuredparameters among age-classes than if the stands had beenselected randomly; these results are therefore conservative.Conversely, ecologically important differences may existin the absence of statistical significance (see Connor andSimberloff 1986). Thus, we recommend careful considerationof these caveats when interpreting results that fail to demon-strate statistically significant relationships, especially forthose species that have been designated as “associated” witha particular age-class (see Summary Matrices).

Second, the influence of stand size and surrounding habitats(stand context) on species’ abundances within old-growthstudy stands was largely uncontrolled. Old-growth standsvaried considerably in size. Although the old-growth studystands were relatively large-78 percent were larger than100 ha-they were often irregularly shaped and located invarious contexts within landscapes containing on averageonly about 25 percent old growth (J. Lehmkuhl, pers. comm.).

Lastly, the survey techniques used were designed to samplethe broadest possible assemblage of species within a com-munity. As a result, relatively few data were collected onrare or highly vagile species. Thus, for certain species ofspecial interest or concern to forest managers, such as the redtree vole and pileated woodpecker, the ability to evaluateobserved patterns of abundance or occurrence was limited bysmall sample sizes. More intensive research will be neces-sary to generate adequate information on the habitat rela-tionships of these species. Other species were either noteffectively sampled with our techniques (for example, highlymobile avian and mammalian predators) or were so locallydistributed (for example, Larch Mountain salamanders) thatinsufficient data were collected for any conclusions abouttheir habitat associations to be made. Lack of information,however, should not be equated with a lack of concern aboutthe potential effects of forest management on these species.

Structural Diversity in Naturally RegeneratedForestsVirtually all of the study stands were naturally regeneratedafter wildfires. Consequently, these results do not apply tointensively managed forest stands. Because some structuralor vegetative components of stands, such as large snags,logs, and live trees, generally survive even catastrophicwildfires, such components often carry over from the pre-ceding mature or old-growth stand to the new, young stand(Spies and others 1988, Spies and Franklin 1988). Intensivelymanaged forest stands, which generally lack such components(Spies and Cline 1988), may not provide suitable habitat forwildlife associated with these features.

Because of these carryover components and variation inestablishment patterns, naturally regenerated Douglas-firstands can be structurally diverse, even in the younger age-classes (Spies and Franklin, this volume). Moreover, theregional pattern of natural disturbances typically createdforest mosaics where stands of different ages resulting froma varied history of disturbance were interspersed with rela-tively old, undisturbed forest (Agee, this volume; Spies andFranklin 1988). This process created widespread habitat con-ditions that consisted either of late-successional forests oryounger forests containing structural or vegetative character-istics of late-successional forests. The structural diversity ofnaturally regenerated young stands and an interspersed pat-tern of disturbance probably interacted to maintain highecological diversity in natural landscapes (Hansen andothers, in press).

Given that presettlement landscapes in this region werecomposed of a wide variety of vegetative conditions, in-cluding large expanses of suitable habitat for species adaptedto late stages of forest development, some species wouldlogically have become adapted to structurally diverse forests(Thomas and others 1988). Current forest managementpractices have resulted in extensive simplification of standstructures (Spies and Cline 1988) and fragmentation ofnatural forests (Lehmkuhl and Ruggiero, this volume;Morrison 1988). Although young plantations that havealready been established for timber production can bemodified to improve their habitat capability (Spies andothers, in press), the ecological values of new approaches toharvesting remaining late-successional forests (Franklin1989) have not yet been evaluated through field research.

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Alternative silvicultural strategies may be needed to maintainpopulations of species adapted to structurally diverse standsor to unfragmented forest conditions. Perhaps such speciescan be maintained by aggregating disturbances at the land-scape scale and leaving various components of harvestedstands to add complexity and important functional elementsto managed stands and landscapes (Franklin 1989). Theresults presented here suggest that such a scenario imitatesnatural disturbance regimes more closely than traditionalsilvicultural practices and is more compatible with the goalof maintaining biological diversity. The extent to which suchalternative management strategies could functionally replaceextensive blocks of relatively undisturbed late-successionalhabitat is unknown, however.

Summary Matrices

The designations listed in the following matrices areintended to summarize the results of the studies reported inthis book. These designations therefore reflect comparisonsof indices to abundance among old-growth (200-730 yr),mature (80-195), and young (35-79) age-classes of unman-aged, naturally regenerated, closed-canopy forests in theDouglas-Fir/ Western Hemlock Zone of western Oregon andWashington and northwestern California (Franklin andDyrness 1973). Although a large percentage of the specieslisted are primarily forest-dwelling species that occur mostcommonly in the forest zone we studied, many also occur inother forest types or in precanopy seral stages of Douglas-fir/western hemlock forest. Consequently, because some speciesmay have closer associations with other habitats not con-sidered here, these designations should be viewed withcaution-they only reflect relative differences in patterns ofabundance among the three seral stages of unmanaged forestwe studied. The following designations represent the oper-ational definitions used by the principal investigators toassign degrees of association among age-classes for eachspecies.

* Closely Associated. A species was designated as “closelyassociated” with a seral stage if it was found to be sig-nificantly more abundant (based on statistical signifi-cance levels set by each investigator) in that seral stagecompared to the other seral stages, or if it is known tooccur almost exclusively in that seral stage.

+ Associated. A species was designated as “associated”with a seral stage if it was found to be numericallymore abundant in that stage compared to the other seralstages. [Note: some species were also designated asassociated with old growth if they were determined torequire one or more habitat features that are charac-teristic of that seral stage, such as very large diameterlogs, snags, or broken-top live trees.]

P Present. A species was designated as “present” in a seralstage if it occurred in that seral stage in numbers lowerthan 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 thespecies was not studied or the data collected wereinsufficient for any designations to be made.

/ A species was designated with a diagonal slash across allseral stages if it does not occur in the geographic areasampled.

For species designated as closely associated with old growth,we predict that a significant reduction in old growth wouldresult in a marked decline in the population numbers of thosespecies. The extent to which such a decline would threatenthe persistence of each population will depend on variousenvironmental, demographic, and stochastic factors. Theconsequences of a significant loss of old-growth habitat onthe population viability of species designated as associatedwith old growth, are unknown.

The principal investigators that provided the designationsshown here are as follows: diurnal forest birds: Andrew B.Carey (Oregon Coast Range), Frederick F. Gilbert (OregonCascade Range), David A. Manuwal (Southern WashingtonCascade Range), Mark H. Huff (Regional Analyses forOregon and Washington; Southern Washington CascadeRange-Winter), Richard W. Lundquist and Jina M. Mariani(Southern Washington Cascade Range-Snag-DependentBirds), C. John Ralph (northern California and southernOregon); small mammals: Paul Stephen Corn and R. BruceBury (Oregon Coast Range), Frederick F. Gilbert (OregonCascade Range), Stephen D. West (Southern WashingtonCascade Range); Keith B. Aubry (Regional Analyses forOregon and Washington), C. John Ralph (northern Californiaand southern Oregon); bats: Donald W. Thomas (OregonCoast Range; Southern Washington Cascade Range);amphibians: R. Bruce Bury and Paul Stephen Corn(Oregon Coast Range; Regional Analyses for Oregon andWashington), Frederick F. Gilbert (Oregon Cascade Range),Keith B. Aubry (Southern Washington Cascade Range),Hartwell H. Welsh, Jr., and Amy J. Lind (northern Californiaand southern Oregon); vascular plants: Thomas A. Spies(Oregon and Washington) and Bruce B. Bingham (northernCalifornia and southern Oregon); hypogeous fungi: DanielL. Luoma (Oregon Cascade Range).

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For birds, mammals, amphibians, and reptiles, all species forwhich data collected were sufficient for analyses in at least

Acknowledgments

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

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

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

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

for hypogeous fungi was not as intensive nor extensive as forother taxa, the likelihood of not detecting the presence of aspecies within a particular age-class was high. For thisreason, dashes were used in the matrix for hypogeous fungito indicate that the species was not detected in forests of thatage-class.

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