Discussion 

The historic role of fire in Pacitic Northwest forests is 

critical in understanding how these Douglas-fir forests developed, and to what extent they provide habitat for wildlife. 

The extent to which such panems and processes provide a 

blueprint for future management is less clear. No areas are 

large enough to allow totally free-ranging fire; few areas 

have management objectives that will allow prescribed natural fire (lightning ignitions allowed to bun under certain 

conditions in certain zones). Logging followed by slash buming has sometimes been called a mimic of natural process 

(Agee 1989). Although some aspects (for example, smoke 

production; Fahnestcck and Agee 1983) may be similar, the 

functional result in the past has been quite different: no snags 

are left (shade, woodpeckers) and much more soil distwbawe is present after logging, which may redirect postdistubante plant succession (Scott 1980). 

Historic fire patterns have several implications for remaining 

old-growth forests. Today’s old growth tends to be fragmented-that is, broken into small units-as a result of barvest. Based on historic patterns of fire, increased fragmentation compared to natural conditions should be more significant to the north, which apparently had larger blocks of 

single-aged stands. The increasingly smaller size of oldgrowth stands will create a drier, windier microclimate along 

stand edges, accelerating both windthrow and potential fire 

behavior. Slash bums may increase the risk of fire escape to 

neighboring stands (Agee 1989). Such units will also be 

easier to reach and protect from fire, however, il that is the 

appropriate management sbategy. 

To suggest that fire is an appropriate process in the preservation of old-growth forest seems paradoxical. Yet it is apparent that almost all of our Douglas-fir old-growth forest resource consists of first- or multi-generation forests born of 

fire. Fire is responsible for their destruction; yet it is also 

responsible for their creation and maintenance. Without fire, 

the old-growth forests of the Pacific Northwest would have 

significantly different species composition and structure, and 

would likely function quite differently as wildlife habitat. 

In the short term, management plans to perpetuate oldgrowth forest can ignore fire. So many other disturbances 

are reducing the area of old growth that short-term management should be oriented to a preservation- rather than a 

process-oriented approach: preserve what we have without 

regard to its long-term maintenance. Eventually, we must 

come to grips with the realization that these living systems 

will change, even with complete protection from disturbance. 

The proportion of Douglas-fir will decline, particularly on 

more mesic sites, with attendant changes in wildlife habitat. 

In the long run, we will be forced to recognize a more 

dynamic management strategy, sensitive not only to historic 

fire regimes, but also to the new fire regimes expected with 

global climate change. 111 

33 
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Forest Fragmentation in the Pacific Northwest

and its Potential Effects on Wildlife

John F. Lehmkuhl and Leonard F. Ruggiero

Authors

JOHN F. LEHMKUHL is a research wildlife biologist and

LEONARD F. RUGGIERO1

 is a supervisory wildlife biologist, USDA Forest Service, Pacific Northwest Research

Station, Olympia, Washington 98502.

Abstract

Fragmentation is the creation of a complex mosaic of spatial

and successional habitats from formerly contiguous habitat.

Loss of habitat and the less obvious phenomenon of habitatpatch isolation are aspects ‘of fragmentation that threaten the

viability of wildlife populations, the components of biotic

diversity. Fragmentation of forests on Federal lands in the

Pacific Northwest is the product of staggered-set clearcutting

of late-successional forest. Large-scale block clearcutting on

private lands has minimized fragmentation simply by leaving

few or no fragments. Less than 20 percent of the original

old-growth forest remains, and the fragmentation of remaining old-growth stands may degrade the quality of these areas

for plants and wildlife. Analysis of population vulnerability

suggests that life-history characteristics, along with biotic

and physical environmental factors, influence population

structure and individual fitness. These factors determine the

risk of local population extinction. We compiled a list of 93

species associated with late-successional Douglas-fir forests

in the Northwest and rated the risk for each species of local

extinction from fragmentation on a scale of 1 to 10 based on

the frequency of occurrence, abundance, and variation in

abundance data from the old-growth community studies (this

volume), and on body size, vagility, and migratory status

information from the literature. Over 80 percent of the listed

species had high to moderately high risk scores (7 to 10);

risk for these species was a function of the low frequency

and abundance associated with three combinations of body

size and vagility. Clearcutting in patterns that minimize fragmentation would reduce the impact of isolation and small

patch size for 27 species, likely benefit another 20 species,

but probably have little impact on 46 species.

Introduction

Forest fragmentation and its effects on biotic diversity have

been recognized during the last decade as one of the most

pressing problems faced by conservation biologists. Particular attention has been given to the relatively undeveloped and

unstudied tropical region (Soul) and Wilcox 1980). Recent

planning activities of Federal land-management agencies and

issues related to the conservation of old-growth forest ecosystems, however, have brought forest fragmentation to the

forefront of domestic conservation biology (Harris 1984,

Harris and others 1982, Lehmkuhl 1984, Lumen and Nietro

1980, Meslow and others 1981, Salwasser and Samson 1985).

1 Now supervisory wildlife biologist, Rocky Mountain Forest and Range

Experiment Station, Laramie WY 82070.

35
Forest fragmentation typically is the creation of a complex

spatial and temporal mosaic of forest patches by staggeredset clearcutting, in which small (<20 ha) clearcuts are scattered over the landscape. Aside from the loss of old-growth

forest habitats that results from this practice, the remaining

forest patches become smaller and more isolated as logging

continues. Nowhere is this problem more evident or controversial than in the old-growth forests administered for multiple use by public land-management agencies in the Pacific

Northwest (Franklin and Forman 1987, Meslow and others

1981). The problem is made urgent by the planned harvest of

50 percent of the old-growth remaining on National Forest

lands in western Oregon and Washington over the next 5

years, at a rate of more than 75,000 ha per year (Marcot and

others, this volume; Ohman and others 1988), and by the

planned liquidation of old growth remaining on Bureau of

Land Management (BLM) lands in southern Oregon over the

next 10 to 20 years (Lumen and Nietro 1980).

A substantial literature documents the effects of forest fragmentation on birds in agricultural landscapes of Eastern

North America and Europe (for example, Forman and others

1976, Freemark and Merriam 1986, Galli and others 1976,

Van Dorp and Opdam 1987, Whitcomb and others 1981).

These studies have generally found that the presence or

abundance of forest-interior birds declines with decreasing

size and increasing isolation of forest patches. A few studies

have analyzed changes in the general abundance of forest

birds (Raphael and others 1988), and small mammals and

amphibians (Raphael 1988c) as the result of logging in

Western forests. Little is known, however, about the specific

effects of fragmentation caused by clearcutting formerly

extensive old-growth forests in Western North America. In

contrast to Eastern forests, studies in Western forests

(Lehmkuhl and others, this volume; Raphael 1984; Rosenberg

and Raphael 1986) have found vertebrate richness or abundance only weakly related to stand size and isolation, although some negative effects were suggested for particular

species. Isolation and small-area effects were suggested by

Newmark (1987) as the cause of local extinction for 43 percent of the medium and large mammal species in National

Parks of the Western United States. A comparison of the

western forest situation with that in eastern forests, however,

is weak.

Eastern forests have experienced extensive fragmentation

(Whitcomb and others 1981), providing relatively static and

simple systems with woodlot patches in an agricultural-land

matrix. The relatively static dynamics and high contrast between patch and matrix are ideal for studying the effects of

habitat patch size and isolation. Western forests managed for

timber production, however, are at an early stage of fragmentation with a dynamic and complex pattern that is more difficult to study. Old growth in a managed landscape forms the

matrix, rather than the patch as with woodlots (Harris and

others 1982, Vemer 1986), until a threshold is reached where

cutover forest area exceeds pristine forest area (Franklin and

Forman 1987). Boundaries between old growth and cleat-cuts,

although initially distinct, are dynamic and become increasingly ambiguous with secondary succession of clearcuts

(Rosenberg and Raphael 1986), whereas woodlot boundaries

remain distinct and both boundary and habitat matrix are

relatively static.

The incipient nature of Western forest fragmentation nevertheless allows us the opportunity to study and manage forest

landscapes to prevent or ameliorate the impacts of timber

harvesting on wildlife populations. Our goal in this paper is

to assess the state of old-growth forest fragmentation in the

Pacific Northwest, and the potential effects of that process on

wildlife. Our objectives are to briefly describe the loss and

fragmentation of old-growth forests in the Pacific Northwest,

to review concepts for assessing the effects of forest fragmentation on wildlife population persistence and diversity and

describe the characteristics of species vulnerable to extinction, to compile a list of Pacific Northwest species associated

with old-growth forests and rate their vulnerability to fragmentation, and to examine timber harvest alternatives that

may minimize the effects of fragmentation on wildlife populations. We will primarily discuss the fragmentation of oldgrowth, or late-successional forests (synonymous in view of

problems in defining the old-growth condition; see Marcot

and others, this volume), because they are least likely to be

present in managed forest landscapes. The discussion will be

oriented to public land management, as opposed to private

industrial forest land where old growth has been largely

eliminated (Harris 1984, Ohman and others 1988, Spies and

Franklin 1988) and where wildlife values are incidental

(Hicks 1985) to timber production.

Forest Fragmentation in the Pacific Northwest

Two issues are associated with current harvest patterns of old

growth: quantitative loss of habitat for associated species,

and qualitative loss of habitat resulting from the reduced

capacity of remaining patches to support old-growth conditions and wildlife communities. Assessing the quantitative

loss of old growth is problematic because estimates of the

area of old-growth forest remaining on public lands vary

with the definition of the old-growth condition (Marcot and

others, this volume). We are not prepared to defend any particular definition, but we present estimates from the literature

to provide order-of-magnitude information.

Estimates of old-growth forest remaining in Oregon and

Washington range from 1 to 2.5 million ha, representing 10

to 25 percent of a base 10 million ha of commercial forest

land. Spies and Franklin (1988) estimated that 1 million ha

of old-growth forest (17 percent) remains from the 6 million

ha of commercial forest land that occurred before settlement.

The Pacific Northwest Region of the USDA Forest Service

36
estimates that remaining old-growth on National Forests in

Oregon and Washington are 2.5 million ha (USFS 1988).

More in line with Spies and Franklin, Morrison (1988) reinterpreted Regional data with a stricter definition of oldgrowth and estimated that little more than 1 million ha remains. Northern California appears to have retained a much

larger percentage (50 percent or 400 000 ha) of its original

810 000 ha of old-growth forest (Raphael and others 1988)

from a base of 1.1 million ha of commercial forest land.

The qualitative loss of old-growth habitat by the creation of

many small fragments in formerly contiguous forest is the

second issue associated with forest management. The viability of remnant patches as wildlife habitat is a function of

myriad edge effects that depend on patch size and isolation.

Some research suggests that a fundamental change in microclimate occurs within 160 m of the forest edge, which creates

conditions different from the patch interior (Franklin and

Forman 1987, Harris 1984). Patches 10 ha are effectively all

edge and have lost the essential attributes of the oldgrowth condition. This, and other edge effects, which we

will discuss later in detail, act to reduce the effective size

and functional viability of patches for plant and animal

communities.

Forty years of staggered-set clearcutting on National Forest

(Harris 1984, Spies and Franklin 1988) and BLM lands

(Lumen and Nietro 1980, Monthey 1984) has resulted in

various stages of fragmentation of the remaining forest. Data

on the patch-size distribution of forest fragments are available for only a few areas. Harris (1984) presented data from

the Siuslaw National Forest in the Oregon Coast Range indicating that 3 to 6 percent of the forest area remained in old

growth, depending on the definition used; 34 percent of the

old-growth stands were smaller than 8 ha and 61 percent of

the patches were smaller than 16 ha, accounting for 8 and 21

percent of the total old-growth area. If stands less than 8 ha

are not functional as old growth and are subtracted from the

old-growth area of the Siuslaw National Forest, only 2 percent of the Forest area remains as functional old-growth, even

assuming a total area of 6 percent old growth. The isolation

of forest fragments with distances up to 7 km (Harris and

others 1982) between patches further limits the functioning

of old-growth patches as wildlife habitat.

Monthey (1984) reported a similar patch-size distribution for

BLM lands in Oregon: about 50 percent of the patches were

less than 12 ha, and most were less than 4

(Lehmkuhl and others, this volume) on 48 2025-ha landscapes on National Forest and National Park lands in the

southern Washington Cascade Range indicated that about.

31 percent of the area was old growth; 39 percent of the

patches were 110 ha, but they represented only 4 percent of

the total old-growth area. Excluding study landscapes in

Mount Rainier National Park yielded only a slightly larger

percentage (5 percent), of old-growth stands 10 ha, which

suggests that much of the remaining old-growth in the

southern Washington Cascades may still be in large enough

patches to be considered functional. Those study landscapes,

however, may be biased for relatively high old-growth area

because selection of study landscapes was based on the

requirement that stands be >40 ha for the original stand-scale

studies. The study landscapes may nevertheless represent

the general area as indicated by the similar percentages of

clearcut area (16 percent) for study landscapes and the

Gifford Pinchot National Forest in general (G. Gruelich,

unpubl. data).

Assessing Fragmentation Effects on Wildlife

Species Diversity and Population Persistence

Maintaining species diversity is the ultimate goal of current

conservation efforts. Species diversity as an operational goal

for management, however, is not always desirable (Anderson

1979; Askins and others 1987; Murphy 1989; Raphael, this

volume; Van Home 1983; Vemer 1986). Conventional

methods for measuring species diversity do not distinguish

between species and do not account for their special ecological and management importance. Clearly, we need to be

concerned with individual species, while diversity should be

a conceptual goal for conserving regional biota (Anderson

1979, Askins and others 1987, Murphy 1989, Noss and

Harris 1986, Van Home 1983, Wilcox and Murphy 1985).

To manage for species, we must understand the processes

affecting the persistence and stability of populations of

individual species as components of species richness.

The persistence of a population is primarily a function of its

size, with extinction (local or global) invariably preceded by

reduced population size (Gilpin and Soul) 1986, Goodman

1987a,b, Newmark 1987, Pimm and others 1988, Soul) and

others 1988, Wilcox 1980). The reduction may result from

natural or human-caused disturbance; in our case, smaller

populations are a consequence of a quantitative or qualitative

loss of habitat. In circumstances where population size is

reduced below a threshold number for recovery, the subsequent extinction of the reduced population may be deterministic and unavoidable. More often, extinction depends on the

interplay of stochastic (random) factors with population and

environmental factors (Gilpin and Soul) 1986, Shaffer 1981).

Shaffer (198 1) described four primary stochastic forces of

extinction:

l Stochastic demographic changes in small populations,

such as variation in birth and death rates and sex or age

anomalies randomly affect individuals in a manner that

increases the variance in reproductive rate and increases

the probability of extinction from a chance reproductive

failure (Goodman 1987a, Lande 1988);

37
l Environmental stochasticity, random temporal changes

in environmental quality or interspecific interactions,

affect individuals equally in a manner that increases

variation in abundance and has proportionately large

effects on small populations (Goodman 1987a, Lande

1988);

l Catastrophes may occur at random intervals through

time and can arguably be called intense, punctuated cases

of environmental stochasticity; and

l Genetic stochasticity arises from a reduction in the

effective population size with consequential deleterious

short-term effects of inbreeding on reproductive success,

and long-term losses of genetic variation (heterozygosity)

and adaptability to environmental change from genetic

drift (Frankel and Soule 1981, Schonewald-Cox and

others 1983).

In general, the probability of extinction for small populations

is highest from environmental effects, followed by demographic stochasticity, long-term genetic effects, and lastly catastrophes (Gilpin and Soule 1986, Lande 1988). The importance of stochastic extinction forces is largely determined by

the interaction between the particular characteristics of a

species, the population, and the environment. An appropriate

framework for assessing the strength of this interaction is

population vulnerability analysis.

Population Vulnerability Analysis

Population vulnerability analysis as proposed by Gilpin and

Soule) (1986) has three aspects: the physical and biotic environment of the individual and population; the population

phenotype, or the species life history and habitat requirements; and the expression of the first two aspects in population structure and individual fitness.

The environment-Fragmentation affects the environment

by reducing the availability and heterogeneity of original

habitats. Forest remaining after logging is a subset of the

original area, and it usually represents a less heterogeneous

array of habitats. The loss of habitat heterogeneity takes two

forms (Goodman 1987a): loss of regional heterogeneity reduces the ability of populations to average effects over their

geographic range; and loss of resource hot spots, or locations of stable resource availability, eliminates refugia in

which populations can persist under periodic severe environmental conditions and from which they can later recolonize

formerly occupied areas.

Fragmentation also alters the context and configuration

of a stand by exposing remnant forest patches to edge

effects. The present connotation of edge effect differs from

conventional usage (Leopold 1933), which suggests that

more species diversity occurs on the ecotone of two habitat

types. Edge effect here means deleterious effects of ecotones

on plant and animal populations (Harris 1989, Laudenslayer

1986, Reese and Ratti 1988, Soule) 1986, Yahner 1989).

Many forest-edge effects have been detailed in the literature,

particularly for eastern U.S. Forests, but each situation has

unique problems (Janzen 1986). Some environmental and

edge effects are described below.

l Competition between interior and edge species

may occur when edge species that colonize the earlysuccessional habitats and forest edges created by logging

(Anderson 1979; Askins and others 1987; Lehmkuhl and

others, this volume; Rosenberg and Raphael 1986) also

use the interior of remaining forest (Kendeigh 1944,

Reese and Ratti 1988, Wilcove and others 1986, Yahner

1989). Competition may ultimately reduce the viability

of interior species’ populations.

l Generalist species occurring in the forest at the time of

fragmentation may benefit from environmental changes

outside the forest, receiving a cross-boundary subsidy

(Janzne 1986). Consequent increases in generalist species’

vigor or populations may alter community interactions,

and result, for example, in greater competition with

other animal species or altered plant-animal interactions

(Janzen 1986). Alverson and others (1988) described

how a cross-boundary subsidy from abundant earlysuccessional habitat in logged areas of Wisconsin maintains a high white-tailed deer population, which has a

major impact on the composition and regeneration of

remnant old forest. Elk likewise receive a subsidy from

early-successional habitat on recently logged sites (Hett

and others 1978, Raedeke and Lehmkuhl 1986) and can

markedly depress understory regeneration of hemlock in

Pacific Northwest forests (Harmon and Franklin 1983).

l Nest predation and parasitism have been shown to increase in eastern forests along edges up to 600 m inside

the patch, which is nearly four times the distance of

microclimatic effects in Pacific Northwest forests (Reese

and Ratti 1988, Wilcove and others 1986, Yahner 1989).

Corvids and small mammalian omnivores are usually

implicated as the most serious nest predators (Reese and

Ratti 1988).

l The patch boundary may be a unidirectional filter

through which animals pass out of the patch and cannot

return (Janzen 1986). Game animals, for example, are

most vulnerable to hunting mortality when they venture

from the cover of forest patches into large open clearcuts.

38
l Secondary extinctions may result from the elimination

of keystone species (Paine 1969, Soule 1986, Wilcove

and others 1986) or altered community processes

(Terborgh 1988). Gilbert (1980b) and Terborgh (1988)

described situations in the tropics where the elimination

of top predators could have cascading effects through the

community by changing the size or structure of prey

populations, which in turn influence plant-community

dynamics through seed dispersal and predation. Soul)

and others (1988) described a meso-predator release

effect on chaparral birds in which the local extinction of

a large predator (coyote) allowed small predators (fox

and domestic cats) to increase, decimating prey species

that formerly persisted with low populations of the small

predators. Maser and Maser (1988) suggested that timber

harvest practices that preclude viable populations of

mycophagous forest-dwelling squirrels may alter fungal

spore dispersal and eliminate a critical link in the symbiosis between hypogeous mycorrhizal fungi and coniferous trees which improves tree productivity.

l Edge creep occurs when extrinsic processes act on patch

edges to progressively decrease patch area or quality

(Soul) 1986). Ground fires along patch edges, for example, may penetrate only a short distance inside the

patch, but they initiate a positive feedback loop of increasing fire effects. The growth of herbaceous vegetation responding to increased light and release from

shrub competition after a fire may provide abundant fuel

for subsequent fires that creep further into the patch and

renew the cycle (Janzen 1986). Other examples are firewood cutters who penetrate progressively deeper into a

patch to cut snags for wood, and chronic windthrow and

subsequent salvage that progressively decreases patch

area.

0 Microclimatic changes along patch edges alter the conditions for interior plant and animal species and usually

result in drier conditions with more available light (Bond

1957, Harris 1984, Ranney and others 1981). The current

estimate suggested for the Pacific Northwest is that

microclimatic effects extend up to two tree lengths (about

160 m) inside a patch (Franklin and Forman 1987, Harris

1984), which is tentatively supported by ongoing research

(T. Spies, pers. comm.). Seral plants become more vigorous or abundant at edges in response to microclimatic

changes, leading to greater abundance of seral plants in

the seed pool and a higher probability of their establishment in patch interiors or colonization of gap disturbances (Janzen 1986, Ranney and others 1981).

The dynamics of forest habitat and environmental change

with fragmentation are poorly understood. Are there thresholds of change in habitat attributes, or are the changes continuous? Franklin and For-man’s (1987) simple checkerboard

model with 10-ha patches suggests several patterns and

thresholds of change: as the forest matrix becomes smaller

with an increasing number of Clearcut patches, the edge

length peaks at 50 percent cutover, but then the relation

reverses and clearcuts become the matrix with forest patches;

total area of forest interior habitat declines linearly to zero at

50 percent cutover, assuming that edge microclimatic effects

occur within 160 m from the edge; and patch size does not

decline until a threshold of 30 percent cutover when forest

patch size drops precipitously until the 50 percent cut level,

after which patch size remains steady.

Research data from Lehmkuhl and others (this volume) contradict the Franklin and Forman patch-size model, however,

indicating that forest patch size declines linearly between 0

and 50 percent cutover. They support Franklin and Forman's

(1987) 50 percent edge threshold when late-successional

forests become the patches in a clearcut matrix. The threshold for more strictly defined old-growth forest, however, was

lower, near 30 percent cutover. Wilcove and others (1986)

suggest that patch numbers increase exponentially with time

under a steady rate of fragmentation. Burgess and Sharpe

(1981) presented data from southern Wisconsin landscapes

that indicated mean distance between patches increased exponentially until about 10 percent of the forest remained.

Population phenotype-The life history characteristics of

species are probably the most important aspect of the population phenotype for population vulnerability analysis

(Margules and others 1982, McCoy 1982, Shaffer and

Samson 1985, Simberloff and Abele 1982). The probability

that a population will persist with habitat loss, which is one

aspect of fragmentation, is largely a function of population

density or abundance: the greater the abundance or density,

the lower the risk of local extinction (Diamond 1984,

Diamond and others 1987, Newmark 1987, Pimm and others

1988, Soul) and others 1988, Terborgh and Winter 1980,

Wilcox 1980). Vulnerability to extinction in isolated habitat

patches may also increase with large body size (Diamond

1984, Diamond and others 1987, Pimm and others 1988);

low fecundity (Pimm and others 1988); specialization on

patchy resources (Karr 1982a, Terborgh and Winter 1980);

behavioral patterns requiring the formation of large groups

and thus concentrated resources, dependence on keystone or

link species, or occurrence at the edge of a species’ range

(Terborgh and Winter 1980); temporal variation in population size (Diamond 1984, Goodman 1987a,b, Karr 1982b);

and metabolic rate (Wilcox 1980).

39
Persistence despite isolation of habitat patches, the second

aspect of fragmentation, is a function of the dispersal abilities

of the species (for example, Harris 1984, Soule and others

1988, Wilcox 1980). Dispersing individuals may rescue populations dwindling to extinction (Brown and Kodric-Brown

1977), or found new populations where they have become

locally extinct. Species with poor dispersal capabilities, especially those with low powers of persistence, have a high risk

of extinction in fragmented landscapes.

Population structure-We have discussed the increased

probability of stochastic extinction from a reduction in population size brought on by the loss of forest habitat. Fragmentation also breaks the population into small subunits, each

with dynamics different from the original contiguous population and each with a greater chance than the whole of local

extinction from stochastic factors. Such fragmented populations are metapopulations, in which the subunits are interconnected through patterns of gene flow, extinction, and recolonization (Gill 1978, Lande and Barrowclough 1987, Levins

1970).

Determining the viability of the subpopulations of a metapopulation in fragmented landscapes is difficult. Long-term study

is necessary because extinctions may be highly stochastic or

observable effects after fragmentation may be delayed until

populations reach an equilibrium with the changed availability of resources (Shaffer 1981). A “source-sink’ effect may

occur, whereby animals dispersing from undisturbed source

areas colonize sink habitat patches that would otherwise not

support populations (Pulliam 1988, Wiens 1976). Animals

may breed in low-quality sink patches but have a negative

reproductive rate, while the appearance of a viable population is maintained by immigration (Pulliam 1988). Then,

interpatch distances are important determinants of population

persistence, and they interact with species mobility and lifehistory characteristics to determine the rate at which immigration is effective in maintaining population size and genetic

diversity.

The dynamics of population response to various amounts of

fragmentation are essentially unknown: is population response gradual or continuous, or is the response nonlinear

with thresholds? Several models predict response thresholds.

Franklin and Forman's (1987) model predicts that forestinterior species richness declines slowly until 30 percent

cutover, then rapidly declines to 50 percent cutover in

response to rapidly decreasing patch sizes. Total species

diversity increases rapidly up to 30 percent forest removal as

edge species colonize in response to increasing edge length,

but then declines slowly with the loss of interior species and

declining edge. Game species, which are mainly edge species, follow a similar trend. A model by McLellan and others

(1986) indicates a threshold of population decline at 50 percent habitat loss for species susceptible to fragmentation (low

vagility, small populations). A population persists longer

when patches of the same habitat are larger and closer. Less

susceptible species begin to decline when 75 percent of the

habitat is removed.

The Pacific Northwest Vertebrate Community

The forests of the Pacific Northwest support a large and

diverse fauna of over 400 species (Brown 1985), of which

58 percent are bird species, 30 percent mammals, 7 percent

amphibians, and 5 percent reptiles (Harris 1984). In a review

of the literature, Harris (1984) concluded that species diversity declines with elevation, increases with moisture, and

increases with the structural diversity associated with increasing stand age, which mainly consists of the vertical structure

afforded by large, living trees; dead, standing trees (snags);

and logs on land and in streams. Recent, extensive, field

studies have shown forest-bird community richness to be

positively associated with increasing temperature, which is a

negative function of elevation and distance from the ocean

(Huff and Raley, this volume). Concurrent studies of small

mammal and amphibian communities indicated richness to

be primarily a function of zoogeographic barriers to dispersal

(mainly the Columbia River and the Willamette Valley) and

localized geographic distributions, especially for amphibians

(Aubry and others, this volume; Bury and others, this volume

a).

Vulnerability Analysis

Assessing the effects of forest fragmentation from timber

harvest on wildlife diversity and population viability raises

several basic questions: “What species are associated with

late-successional forest landscapes?” “What species are at

greatest risk as assessed from an examination of the structural

and life-history characteristics of species’ populations that

influence their persistence in forest landscapes altered by

timber harvest?” and “What are the options for timber management to manipulate forest stand- and landscape-scale

characteristics to minimize the risks of local extinction?”

To answer the first question, we used the list of species associated with late-successional forest from the summary paper

of this symposium (Ruggiero and others, this volume). Other

species were added from the wildlife-habitat relationships

tables in Brown (1985) to include species that use latesuccessional stages (large sawtimber and old growth) of the

temperate coniferous forest and were not sampled by the

old-growth community studies (this volume). The list was

edited to exclude species taken from Brown that are not

found primarily in temperate coniferous forests. We concentrated on species associated with late-successional forest

because this is the stage of forest development least likely to

be represented in managed forest landscapes of the future.

The list of 93 species is not complete (appendix table 2), but

reflects current knowledge of habitat relationships. Earlier

40
lists based on literature reviews are similar in the total number of species listed. Harris (1984) proposed late-successional

forest as primary habitat for 118 species, with 40 species

dependent on old growth. Brown (1985) described 76 species

that use old-growth forest as primary breeding habitat and 65

species that use it as primary feeding habitat.

The vulnerability of listed species was assessed by compiling

research data and general information about the abundance,

life history, and habitat relationships of species, and then

determining the risk of local extinction associated with each

variable. Field data on the frequency of occurrence, abundance, and coefficient of variation in abundance among

stands from the old-growth community studies (Lehmkuhl

and others, this volume) were assembled for 64 of 93 listed

species. The values of those variables for each species were

converted to percentages of the maximum value for species

in the same taxon, and then grouped into three classes: 0 to

33 percent, 34 to 66 percent, and >66 percent of maximum.

Risks associated with the percentile classes were scored from

low (1) to high (3) depending on the variable. Risk scores

declined with increasing frequency of occurrence and abundance, but scores increased with increasing variation in

abundance. Scores were estimated for 29 species (for which

abundance data were not available from the community studies) by examining scores for similar species and consulting

the general literature.

Risks associated with the life-history parameters of body

size, vagility, and migratory status were similarly rated as

low, moderate, or high. Risk scores increased with increasing

body size under the premise that small-bodied species have

smaller home ranges and higher densities than larger species,

and may persist longer in small patches than large species.

Risk decreased with increasing vagility because high vagility

enables dispersing individuals to rescue failing subpopulations in isolated habitat patches, or to recolonize patches with

locally extinct subpopulations. Risk associated with migratory status was scored low (1) for resident species, moderate

(2) for short-distance migrants, and high (3) for long-distance

migrants. Robbins and others (1989) showed that longdistance migrants were more susceptible to stand-area effects

than short-distance migrants or residents. Pimm and others

(1988) found migrants to have a higher risk of extinction

than residents, probably because of greater migration risk

and variability in population size. The intermediate risk we

assigned to short-distance migrants is not clearly supported

by the literature, but was inferred from the arguments of

Pimm and others (1988) concerning migration risk. Ancillary

data on the strength of association with late-successional

forest, and use of structural and special habitats, were compiled to further assess risk and aid in habitat management.

A cumulative risk rating was calculated for each species as

the weighted sum of risk scores for individual variables, by

means of the following formula:

Cumulative risk = 3 * (frequency score + abundance

score) + 2 * (body size score + vagility score)

+ migratory status + variance in abundance.

Weights were assigned to reflect the relative importance of

variables that affect persistence in fragmented landscapes.

Frequency and abundance were weighted by 3 to account for

the primary importance of population size and density. Body

size and vagility were weighted by 2 to reflect the lesser, but

still vital, importance of body size for persistence in isolated

patches, and vagility in the rescue and recolonization of

isolated populations. Variation in abundance and migratory

status scores were unweighted to indicate their relatively

lesser importance for persistence. Cumulative scores were

recalculated according to a scale of 1 to 10 to simplify

ranking of risk.

Eighty percent of the species fell into moderately high (score

7.8) and high-risk categories (scores 9, 10) (fig. 1). The

20 high-risk species have low frequency and abundance,

and one of three body size-vagility combinations (appendix

table 2). Small size and low vagility characterized 12 species

of amphibians and small mammals. Their persistence in

isolated patches of a highly fragmented landscape is problematic because of low frequency and abundance, and poor

probability that subpopulations could be rescued or patches

recolonized because of limited vagility. The limitation of

poor vagility depends on the varying porosity of adjacent

altered habitats to movement of dispersing animals. An additional consideration is the very limited geographic distributions of three amphibians.

These species may persist in relatively smaller patches than

large species, however, because of their small size and relatively higher potential density. Amphibians may have an

additional advantage relative to small mammals because

their lower metabolism and energy requirements presume

smaller ranges and higher densities. Small patches of latesuccessional forest are often regarded as poor habitat because

of microclimatic and other edge effects, but the impact of

edge effects on amphibians and small mammals remains to

be shown. Small patches may be adequate to support viable

populations of amphibians and perhaps small mammals, and

may be important sources for recolonization of adjacent

altered stands made suitable with time by succession. The

elimination of small old-growth patches as poor habitat,

therefore, may be imprudent unless they are imminently

threatened by blowdown.

41
Nearly 60 percent of the rated species have moderately high 

risk scores (7-8) with only slightly less than high risk of 

local extinction. The large number of species in the group 

mitigates the lower score, making it perhaps the most imnortant uouo for manaeement. In common with high-risk 

25 

&ecies,“tbesk animals hBve low frequency and abundance, 

but with various combinations of body size and vagility. 

z Combinations of body size and vagility range from small 

a 20 Pi” size with low vagility for amphibians and small mammals to 

0 1111 small- and medium-size birds and mammals with high vagilO 15 

~-..............................-..............-., :- 

r I ity. An important group is comprised of medium-& birds 

and mammals, which include the spotted owl, northern 

goshawk, pileated woodpecker, marten, and fisher. The 

vulnerability of the birds primarily is a function of habitat 

of medium body size, and secondvagility. The vulnerability of the marten 

is a function of both habitat loss and 

habitat-population isolation with only 

1 2 3 4 5 6 7 8 910 

Another high-risk group includes three squirrel species of 

small to medium size and moderate vagility. Because of their 

higher mobility, animals with medium size and moderate 

vagility are less at risk than smaller species from the isolation 

of habitat patches and subpopulations. The risk is greater, 

however, from diminishing patch sizes because of the greater 

energy demands of larger body size. Persistence for these 

species is a balance between decreasing patch sizes and 

increasing isolation. 

The third high-risk group is composed of five species of 

birds and mammals with large size and high vagility. High 

vagility usually enables these species to overcome patch 

isolation. Large body size, however, implies greater energy 

weds and larger home ranges than smaller species have, and 

consequently, total available habitat and patch size are most 

important. These species are generalists (elk, mule deer) OI 

tied to special features (for example, bald eagles need tall 

trees or snags near water), and arguably do not specifically 

require extensive late-successional forests. Elk and mule 

deer, however, require some old forest for thermal and hiding 

cover, and they require winter feeding areas during years of 

heavy snow. This need is particularly important for deer in 

areas with snowfall down to sea level, such as southern 

Alaska and British Columbia (see papers in Meehan and 

others 1984). Regardless of specific cases, the risk analysis 

is significant because it indicates the vulnerability of large 

animals. 

Timber Harvest Alternatives 

A foremost consideration in formulating an adequate answer 

to the question of alternative timber-harvest strategies is the 

fact that populations of species closely associated with latesuccessional forest will always experience some detrimental 

impacts from loss of habitat. Minimum-fragmentation alternatives do not reduce the impact of lost habitat, but primarily 

attempt to minimize the additional negative impacts that 

small habitat patch-sties, edge effects, and isolation of remaining forest patches have on the forest environment and 

metapopulation structure. The value of a given timbermanagement alternative therefore depends on how species 

respond to changes in patch size and isolation. Animals 

respond to increasing patch size as a function of their body 

size: large species with large ranges and low density benefit 

from larger patches that allow for larger, more persistent 

populations. Response to declining isolation is a function 

of vagility: animals with low vagility have greater access 

to other habitat patches as isolation decreases. We crosstabulated vagility and body size for the species in our data 

set to examine how the impact of timber harvest may be 

reduced by a minimum fragmentation scheme (table 1). 

Increasing the patch size and decreasing the isolation of habitat patches reduces impacts on at least 27 species (score 4), 

probably helps another 20 species (score 3), and likely has 

little impact on 46 species (table 1). This analysis does not 

consider qualitative habitat loss through edge effects. Those 

species that can be idcntitied as inrerior species may benefit from the reduction of edge through minimum fregmentation practices. Some preliminary evidence from ongoing 

research indicates that the abundance of a few birds (winter 

wren, Swainson’s thrush, and varied thrush) is higher in 

patch interiors (A. Hansen and J. Peterson, pen. comm.). 

42 
Table 1Species groups and number of species associated 

with late-successional forest that are likely to sustain fewer 

impacts from a minimum-fragmentation clearcutting alternative relative to current maximum-fragmentation practices 

Minimum Maximum 

fragmentation fragmentation 

selection 

low ISOLATION high 

large PATCH SIZE small 

LANDSCAPE SCALE 

Other management options may reduce the impacts of timber 

harvest. Alternatives such as minimum fragmentation alter 

the juxtaposition of clearcut “nits at a landscape scale to alter 

the size and isolation of remaining forest patches (fig. 2). 

Management, however, can also act at a stand scale along 

which selection harvest and clearcutting lie at the extremes. 

Current staggered-set clearcutting is in the area of greatest 

impact at the extremes of both stand- and landscape-scale 

modification (top right of fig. 2). Minimum fragmentation 

alternatives move management to the left along the landscape axis, and stand-scale impacts and patch contrast remain 

constant. As harvest changes from clearcut to selection metiads, or as other means of maintaining the stnxtaral and 

functional characteristics of harvested stands are used (Harris 

and others 1982, Nyberg and others 1987), the contrast and 

edge effects between altered and remnant patches and landscape fragmentation decrease concurrently as shown by the 

mows in figure 2. 

Conclusions 

Reducing timber harvest will always increase the probabilities of persistence for vulnerable wildlife species. Viable 

populations may exist with some timber harvest, however, 

especially if harvest practices are modified to minimize the 

effects of logging at landscape and stand scales. Alternative 

plans for arranging clearcut units to minimize landscapescale impacts have been suggested by Franklin and Forma” 

(1987) and are being planned for some National Forests in 

the Pacific Northwest. Alternative stand-scale practices, such 

as retaining green trees and coarse woody debris, are only 

recently being explored. Other beneficial practices include 

the “se of corridors that maintain habitat connectivity across 

the landscape and facilitate wildlife movement from wilderness or between forest patches across managed landscapes 

(Han-is 1984, Ness 1987, Noss and Harris 1986. Simberloff 

and Cox 1987). Population viability will also be enhanced if 

the location of cutting “nits does not remove habitat refugia 

or decrease the habitat heterogeneity across the landscape 

that animal populations depend on during periods of envimnmental stress. Planning for the replacement over time of latesuccessional forest patches or structural attributes like snags 

will ensme the viability of species requiring these key 

habitats. 

We need more research to give managers better information 

and tools. Vemer (1986) gave a good prescription for research into fragmentation effects: emphasis has bee” on birds 

for several reasons, but we need more. research on mammals, 

reptiles, and amphibians; more research is needed on the 

processes affecting wildlife. persistence in fragmented landscapes, rather than patterns related to area alone; we need 

to determine population parameters and model population 

43 
responses to examine the consequences of fragmentation

over space and time, which is not feasible without long-term

research; and more emphasis should be placed on the spatial

dynamics of fragmentation from a landscape perspective.

We suggest several additional research issues: more emphasis is needed on habitat relationships in managed forests, and

the interaction of remnant forest patches and the managed habitat matrix; continued research is needed into the

value of late-successional forests for wildlife to assess longterm trends; examination of habitat relationships from a

landscape-scale perspective is needed to account for processes working above the stand or patch scale: and study is

needed of the ecology of species and species groups that

current research has shown to be clearly associated with

late-successional forests.

We have used facts and speculation to assess the impacts of

timber harvest on the vertebrate community. We can say

definitely that the old axiom good timber management is

good wildlife management does not apply to species associated with late-successional forest. These species will

always sustain some negative impact from logging. Given

that logging will continue and wildlife populations will

continue to be affected, a larger question is “At what point

will forest fragmentation from logging endanger the viability

of wildlife populations on public lands?” Models (Franklin

and Forman 1987; McLellan and others 1986; B. Noon, pers.

comm.) and some field data (Lehmkuhl and others, this

volume; Rosenberg and Raphael 1986) suggest that, for most

species, we have not yet reached a threshold of fragmentation

on most public lands beyond which local population viability

is doubtful and management options are few. This is very

good news. Nevertheless, some species, such as the spotted

owl, will always be exceptionally vulnerable no matter how

viable the majority of species remain. The viability of populations of those vulnerable species will be the touchstone of

our commitment to maintaining biotic diversity on public

forest lands.

Acknowledgments

We thank Martin G. Raphael, Jared Vemer, Andrew B.

Carey, and Stephen D. West for their thoughtful reviews of

early versions of the manuscript.

This paper is contribution 132 of the Wildlife Habitat

Relationships in Western Washington and Oregon Research

Project, Pacific Northwest Research Station, USDA Forest

Service. n

44
Appendix 

Table Z-Risk of local extinction for species associated with late-successional temperate coniferous forest [the species list was 

modified from Rugglero and others (this volume) and Brown (198511 

Species DE ST 

- 

Clouded salamander 2 

Oregon slender salamander 1 

Pacific giant salamander 1 

Larch Mountain salamander 1 

Siskiyou Mountains salamander 1 

Van Dyke’s salamander 1 

Olympic salamander 1 

Bald eagle 1 

Band-tailed Pigeon 2 

Common raven 1 

Red tree vole 1 

Whimfaxed vole 1 

Elk 2 

Northern flying squirrel 1 

Mule deer 2 

Coast mole 2 

western gray squirrel 2 

Mash shrew 2 

Pacific shrew 2 

Douglas’ squirrel 2 

Western redback salama,,der 1 

cascades frog 1 

Red-legged frog 1 

Amexican robin 2 

Bufflehead 1 

Blue grouse 2 

Barrow’s goldeneye 2 

Black-&mated gray warbler 2 

Black-headed grosbeak 2 

common merganser 1 

Cooper’s hawk 1 

Evening grosbeak 1 

Hutton’s virw 2 

Hooded merganser 2 

Northern goshawk 1 

Nashville warbler 2 

Olive-sided flycatcher 1 

Pine siskin 1 

Pileated woodpecker 1 

Red-breasted sapsucker 2 

Swainson’s thrush 1 

Sharp-shinned hawk 1 

Northern spotted owl 1 

Townsend’s warbler 2 

Vaux’s swift 1 

Western tanager 1 

Western screech-owl 2 

Wood duck 2 

White-wingedcrossbill 2 

Yellow-rumped warbler 1 

Fisher 1 

Variables” 

Management score 

SP FR AB BO VA Ml VR RISKb RANK 

1 

1 

1 

1 

1 

1 

1 

2 

z 

2 

1 

3 

3 

3 

1 

2 

1 

1 

2 

1 

1 

1 

3 

2 

2 

2 

2 

2 

2 

2 

2 

2 

2 

2 

2 

2 

3” 

2 

3 

2 

3 

2 

2 

2 

2 

2 

; 

3 

0 

0 

0 

2 

2 

2 

0 

1 

0 

i 

0 

0 

1 

0 

0 

0 

0 

0 

0 

0 

0 

0 

0 

1 

0 

1 

0 

0 

1 

0 

0 

0 

1 

0 

0 

0 

0 

1 

1 

0 

0 

0 

0 

1 

0 

1 

1 

0 

0 

0 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 i 

3 

3 

3 

3 

3 

3 

3 

3 i 

2 

z 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 i 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3’ 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

3 

i 

3 

1 

1 

1 

1 

1 

1 

1 

3 

2 

3 

1 

1 

3 

1 

3 

1 

2 

1 

1 

2 

1 

1 

1 

1 

2 

2 

2 

1 

1 

2 

2 

1 

1 

2 

2 

1 

1 

: 

1 

1 

2 

2 

1 

1 

1 

2 

2 

1 

1 

2 

3 

3 

3 

3 

3 

3 

3 

1 

1 

1 

3 

3 

1 

2 

1 

3 

2 

3 

; 

3 

2 

2 

1 

1 

1 

1 

1 

1 

1 

1 

1 

1 

1 

1 

1 

1 

1 

1 

1 

1 

1 

1 

1 

1 

1 

1 

1 

1 

1 

1 

1 

1 

1 

1 

1 

1 

1 

2 

2 

1 

1 

1 

1 

1 

1 

1 

1 

1 

1 

1 

1 

2 

2 

2 

2 

1 

2 

3 

3 

2 

2 

2 

2 

2 

1 

3 

3 

2 

1 

2 

3 

2 

1 

3 

3 

3 

1 

2 

2 

2 

1 

3 

1 

2 

1 

1 

1 

2 

1 

2 

1 

1 

1 

1 

3 

1 

2 

1 

1 

1 

1 

2 

1 

3 

2 

1 

1 

1 

2 

2 

1 

1 

2 

2 

1 

1 

1 

1 

; 

2 

2 

1 

1 

2 

2 

2 

1 

: 

2 

1 

30 

28 

29 

28 

28 

28 

28 

29 

ii 

28 

28 

28 

28 

28 

29 

28 

28 

28 

28 

26 

27 

26 

26 

27 

26 

21 

21 

27 

21 

21 

26 

26 

21 

26 

26 

26 

26 

27 

26 

21 

27 

26 

21 

27 

21 

26 

27 

ii 

26 

10 

9 

9 

; 

; 

9 

9 

9 

9 

9 

9 

9 

9 

9 

9 

; 

9 

8 

8 

8 

8 

8 

8 

8 

8 

8 

8 

8 

8 

8 

8 

8 

8 

8 

8 

8 

8 

8 

8 

8 

8 

8 

8 

8 

8 

8 

8 

8 

45 
Table 2--continued 

Variables= 

Species DE ST SP FR AB BO VA MI VR RISK” RANK 

Marten 1 3 0 3 3 2 1 1 1 26 8 

ShI.%W-Ill& 1 1 0 2 3 1 3 1 2 26 8 

Forest deer mcu~e 1 1 0 2 3 1 3 1 3 27 8 

water shrew 2 1 0 2 3 1 3 1 3 27 8 

Tailed frog 1 1 0 2 2 1 3 1 3 24 7 

Dunn’s salamander 1 1 0 2 2 1 3 1 3 24 ~7 

Hammond’s flycatcher 1 2 0 2 3 1 1 3 3 25 7 

Marbled mm&t 1 2 0 3 3 1 1 1 1 24 7 

Northern pygmy-owl 1 2 3 1 1 1 1 24 7 

Northern flicker 1 

Noshem saw-whet owl 1 

; 

: z 

3 1 1 1 1 24 7 

1 3 3 1 1 1 1 24 7 

Purple finch 1 2 0 3 3 1 1 1 1 24 7 

Rufous hummingbird 1 2 0 2 3 1 1 3 2 24 7 

Townsend’s solitaire 1 2 0 3 3 1 1 2 1 25 1 

Wilson’s warbler 2 2 0 2 3 1 1 3 2 24 7 

Big brown bat 1 2 1 3 3 1 1 1 2 25 7 

Silver-haired bat 1 2 1 3 3 1 1 1 2 2s 7 

Keen’s myotis 1 2 1 3 3 1 1 1 2 25 7 

Yuma myotis 1 2 1 3 3 1 1 1 2 25 1 

Califomia myotis 1 2 1 3 3 1 1 1 2 25 7 

Long-eared myotis 1 2 1 3 : 1 1 1 2 2s 7 

Lang-legged myotis 1 2 1 3 1 1 1 2 25 7 

Little brown myotis 1 2 1 i i 1 1 1 2 25 7 

Fringed myotis 1 2 1 1 1 1 2 25 7 

Northwestem salamander 1 1 0 2 2 1 2 2 3 23 6 

Rough&in newt 1 1 0 2 2 1 2 2 3 23 6 

@v jay 1 2 0 2 3 1 1 1 2 22 6 

Hairy woodpecker 1 2 1 2 3 1 1 1 2 22 6 

Hermit thrush 1 2 0 2 3 1 1 2 2 23 6 

Red crossbill 1 2 0 3 2 1 1 2 2 23 6 

Steller’s jay 2 3 0 2 3 1 1 1 3 23 6 

Western red-backed vole 1 1 0 1 2 1 3 1 3 21 5 

Brown creeper 1 2 1 1 3 1 1 1 2 19 4 

Red-breasted nuthatch 1 2 1 1 3 1 1 1 2 19 4 

Varied thrush 1 3 0 1 3 1 1 2 2 20 4 

EnSatiM 2 1 0 1 1 1 3 1 2 17 3 

Southern red-backed vole 1 1 0 1 1 1 3 1 3 18 3 

Hermit & Townsend’s warblers 

: ; 

0 1 2 1 1 3 2 18 3 

Western flycatcher 0 1 2 1 1 3 2 18 3 

Golden-crowned kinglet 1 2 0 1 2 1 1 1 2 16 2 

Chestnut-backed chickadee 1 2 1 1 1 1 1 1 2 13 1 

Winter wren 1 3 0 1 1 1 1 2 2 14 1 

o DEpendmcy on late-successional forest: 1 = primary 2 = secmdary. STmctural layer used: 1 = hodzamS 2 = venical (foliage); 3 = blh. special 

managemenL: 0 = none: 1 = snags; 3 = limited distributim. Mlgmmy status scored 8s 1 = resident: 2 = shon-distance migranr; 3 = long-distance migrant. 

Giber life histon/ variables Were scored as 1 = low. 2 = moderae. 3 = high risk. 

b Risk = 3’ FRquency + ABundmce) + 2. (Body size + VAgility) + ?vtllgratory status + VaRiance in abundance. Scores for frequency, abundance, and 

variation WCC assessed frm data presented by Lehmkuhl and others @his volume). Total risk wm calculaled 8s the weighed hum. 

46 
Old-Growth Inventories: Status, Definitions, and

Visions for the Future

Bruce G. Marcot, Richard S. Holthausen, John Teply, and W. Dean Carrier

Authors

BRUCE G. MARCOT is a research ecologist, USDA Forest

Service, Pacific Northwest Research Station, Portland,

Oregon 97208. RICHARD S. HOLTHAUSN is a regional

wildlife ecologist, and JOHN TEPLY is regional inventory

biometrician, USDA Forest Service, Pacific Northwest

Region, Portland, Oregon 97208. W. DEAN CARRIER is

Old-Growth Habitat Program Manager, USDA Forest

Service, Shasta-Trinity National Forest, Weaverville Ranger

District, Weaverville, California 96093-l190.

Abstract

Inventories of old-growth forests require a classification system for identifying and arraying forest conditions for a variety of interests. Old growth has value as fish and wildlife

habitat, as a timber stratum, as an economic resource, and as

a component of biological diversity. It is needed for scientific

study, for long-term forest productivity, for recreation, for

aesthetics, for spiritual interests, and for watershed protection.

Several authors have defined old growth. Their definitions

have similar but not identical components, and none suffices

for all forest types, nor do they meet the intent of all interests.

Recent estimates of known and candidate old growth in

Washington, Oregon, and California total to 3.6 million ha.

This sum, however, is based on a variety of definitions, forest types, and recency of data. Estimates based on consistent

definitions and data sources are not available. Differences

between estimates made by the Forest Service and The

Wilderness Society of the amount of old growth on National

Forests in Washington and Oregon can be partially explained

by the stringency of their definitions of old growth and the

methods they used to identify old-growth stands. The amount

and distribution of old growth on National Forests in the

Pacific Northwest in the future will depend on management

allocations in reserved (for example, wilderness) and nonreserved (for example, timber production) status.

Current information needs relative to inventories of oldgrowth forest include descriptions of successional and developmental stages of old-growth forests and criteria for defining

old growth for each forest type in the Pacific Northwest. At

present, the Forest Service is conducting a regional inventory

in the Pacific Northwest, using an attribute-driven approach.

This work will provide the most complete inventory to date

on National Forest lands. Other recommendations for oldgrowth inventories include: taking an attribute-based rather

than a definition-based approach; developing a map-based

rather than just numerical-based product to address the scale,

context, and size of old-growth stands; and integrating any

adopted definition of old-growth as part of a system for

describing the amounts and distributional patterns of older

forest.

47
Introduction-What Constitutes an Old-Growth

Inventory?

Without adequate inventories and without a clear understanding of the amount and distribution of old growth it is difficult

for the decision maker to determine what is practical or

feasible (Ham 1984:69).

Conducting inventories of a natural system across a broad

geographic area is a difficult task. Attempting to inventory

old-growth forests throughout the Western United States and

beyond is challenging because of the large size and great

diversity of the area. An old-growth inventory requires an

explicit and quantitative classification system to identify and

array candidate areas. Potential old-growth stands are first

identified by a set of general criteria, and finally classified

according to a precise set of stand structural attributes.

This paper was prompted by the need to review existing definitions and inventory estimates of old-growth forests in the

Western United States, and to help resolve controversies over

old-growth inventories on National Forests by describing the

bases of the estimates. Specifically, the objectives of this

paper are to describe the proposed definitions that have

guided estimates of the amount and distribution of old-growth

forests, to review estimates of the amount and distribution of

old-growth forests in the Pacific Northwest, to describe the

bases for estimates of the amount of old-growth forests on

selected National Forests in Washington and Oregon, and to

propose methods for improving the reliability of those inventories. This paper is not intended to represent official Forest

Service policy on old growth, nor to present a new inventory

of old growth. The geographic scope of this paper includes

Washington, Oregon, and California for tallying existing

inventories; and Washington and Oregon only (the Pacific

Northwest Region of USDA Forest Service) for describing

definitions of old growth on National Forest land.

Important Considerations for Old-Growth Definitions

and Inventories

In current planning and management activities on National

Forests, old growth has several values (Sirmon 1985). and

one of them is its importance as wildlife habitat (Meehan and

others 1984, Meslow and others 1981, Raphael and Barrett

1984, Thomas and others 1988). Old growth provides optimal

habitat for some management indicator species, including

spotted owl, pileated woodpecker, and marten, and for many

other species of plants, fish, amphibians, reptiles, birds,

and small mammals (Harris and others 1982, Meslow and

others 1981, Raphael 1988c, Raphael and Barrett 1984). It

also provides thermal and hiding cover for ungulates, especially in winter (Schoen and others 1984, Wallmo and Schoen

1980). Old growth, therefore, plays an important role in providing for productive populations of some species of special

ecological and administrative interest. For some of these

species, old growth may be a key factor in providing for

continued population viability.

Old growth also has value as a source of timber. It has been

defined as forested areas that are past mean annual increment

of wood-volume production (DeBell and Franklin 1987).

Silviculturally, the structure and age-classes of old-growth

forests, with large diameter boles, presence of snags, and less

than ideal stocking, represent less than optimal use of growing space for trees. Economically, old-growth stands are

valuable as timber because of the large volume and high

quality of wood they contain (Society of American Foresters

1984).

Additional values of old growth are as natural research areas

for scientific study (Greene 1988, Sheppard and Cook 1988)

and its ecological role in providing long-term forest productivity (Franklin and others 1981, Perry and others 1988).

Other interests in old growth include its recreational,

aesthetic, and spiritual significance (Anderson 1988), its

contribution to watershed protection (Sedell and Swanson

1984), and its importance as a contributor to biological diversity (Harris 1984, Luman and Neitro 1980, Norse and others

1986).

Addressing each of these values requires different concepts

and definitions of old growth. The wide variety of interests

in old growth also requires consideration of several spatial

scales for inventories: stand, watershed, province, and State

or regional scales. Inventories are the basis for describing the

present and future distribution and amount of old growth,

and for evaluating management alternatives and options

available over time.

Review of Old-Growth Definitions

Several definitions of old growth have been proposed or

used for Pacific Northwest forests. Few definitions have

been developed independently. In particular, definitions by

the Old-Growth Definition Task Group (1986) refined earlier

definitions by Franklin and Spies (1984) and Society of

American Foresters (1984), which in turn were based on

Franklin and others (1981). Most of the Forest Service

definitions and those of The Wilderness Society were based

on these definitions in detail or in concept. Definitions are

of two kinds: those based on timber production criteria, and

“ecological definitions” (those based on field observations

of vegetational structure and composition rather than on

economic value, timber age, or stocking-classes).

48
Existing Definitions of Old-Growth Forests in the Pacific

Northwest

Society of American Foresters-The Society of American

Foresters (1984) defined old-growth Douglas-fir forests of

western Oregon and Washington as stands that have two or

(commonly) more tree species with a range in tree sizes and

ages and often including a long-lived dominant (such as

Douglas-fir) and a shade-tolerant associate (such as western

hemlock); a deep, multilayered canopy; some individual live

trees (greater than 25 per ha) either more than 200 years old

or greater than about 100 cm in d.b.h.; significant coarse

woody debris, including more than 25 snags per ha greater

than 6 m tall and more than about 45 metric tons/ha of down

wood, and at least 10 snags and logs per ha over 64 cm in

diameter and 15 m long. SAP did not provide a definition for

other forest types.

Old-Growth Definition Task Group-This task group

provided interim, working definitions of old growth in four

Douglas-fir and mixed-conifer forest types in the Cascade

Range of Washington and Oregon, in northwestern California,

and in the Sierra Nevada of California (Old-Growth Definition Task Group 1986). Their definitions (table 1) included

criteria for live-tree size, age, density, and associates; canopy

structure; snag size and density; and log size and density.

Different criteria were developed for the four forest types to

reflect differences in moisture regimes and expected site

potential Collectively, these definitions are commonly

referred to as ecological definitions.

USDA Forest Service-The Forest Service has published

old-growth definitions for Washington, Oregon, and California in two regional guides to support the planning process for

the management of forest land (table 2). For Washington and

Oregon, the Pacific Northwest Regional Guide defined old

growth with qualitative criteria (table 2) as well as with quantitative, Forest-specific criteria (as in table 3). In addition

to the regional definitions, the draft Forest plans for most

National Forests quantified structural parameters for stands

of specific forest types. Many of these definitions varied by

National Forest.

In the Pacific Northwest Region, a minimum tract size of

4 ha was established as a regional standard, and qualitative

guidelines were presented for optimal tract size to support

functional patches of old growth:

Optimum tract size will be related to the needs of

dependent wildlife species, such as northern spotted

owls or pileated woodpeckers, and the ability to

insulate part of the stand from the edge effects in

created openings.

In California, the Pacific Southwest Forest Service Regional

Guide used qualitative criteria to define old growth (table 2).

The Wilderness Society-The inventory assessment conducted by Morrison (1988) for The Wilderness Society used

the ecological old-growth definition of the Old-Growth

Definition Task Group (1986). Where inventory data were

unavailable, log and snag criteria were not used. Morrison

also defined several additional classes and subclasses of old

growth (Morrison 1988:10). “Classic” old-growth stands

meet all minimum criteria of the Old-Growth Definition

Task Group (1986) and contain at least 20 trees per ha that

exceed 300 years in age or 100 cm in d.b.h. A subclass of

classic old growth is “super” old growth, with trees

exceeding 700 years old or 180 cm in d.b.h. Stands meeting

the definition of the Old-Growth Definition Task Group

(1986), including where inventory data on logs and snags

were not available, were termed “early” old growth, and

contain at least 20 trees per ha over 200 years old or 80 cm

in d.b.h. Such stands may include some older trees but they

are in low numbers. “Mature” stands contain more than 50

trees per ha exceeding 80 years old or 53 cm in d.b.h. Such

stands fail to meet one or more of the minimum old-growth

criteria. Morrison also summarized old-growth tracts more

than 122 m from roads or clearcuts.

Old-Growth Research Studies-Stands classified as mature

and old growth in the studies by the Old-Growth Forest Wildlife Habitat Program were selected to represent the full range

of conditions in Douglas-fir forests more than 200 years old

(T. Spies, pers. comm.). They were selected before the

criteria used by the Old-Growth Definition Task Group were

developed. As a result, not all old-growth stands studied in

this research fully meet the criteria established by the Task

Group.

Comparison of Four Existing Definitions-The three key

definitions outlined above (Old-Growth Definition Task

Group, Pacific Northwest Regional Guide, and Society of

American Foresters (1984)) describe old growth in Douglasfir/western hemlock forests in Washington and Oregon (table

4). Only the Pacific Northwest Regional Guide describes old

growth for true fir/mountain hemlock forests. In the Douglasfir/western hemlock type, all three definitions call for live

trees of two or more species with a variety of sizes and ages.

The Society of American Foresters’ (1984) definition is most

stringent on tree density and size. The Pacific Northwest

Regional Guide does not specify age. All other definitions

allow >200 years old as an alternative to tree diameter.

Criteria for shade-tolerant associates also vary among definitions. All three definitions agree on qualitative canopy

characteristics. Quantitative criteria for snags and logs vary

widely, however, with the Society of American Foresters’

definition calling for the greatest densities and largest

diameters.

49
Table l-Interim minimum standards for old-growth Douglas-fir and mixed-conifer forests in western Washington and

Oregon and in California (from Old-Growth Definition Task Group 1986:4)

Stand

characteristic

Douglas-fir

on western

hemlock sites

(western hemlock,

Pacific silver fii)

Douglas-fir on

mixed conifer sites

(white fir, Douglas-fir)

Douglas-fir

on mixed

evergreen sites

(tanoak, Douglas-fir)

Sierra

mixed-conifer

forests (white fir)

Live trees Two or more species

with wide range of

ages and tree sizes

Douglas-fir 120 per ha

of trees >81 cm in

diameter or > 200

years old

Canopy

Snags

50

Tolerant associates

(western hemlock.

western redcedar,

Pacific silver fir,

grand fii, or bigleaf

maple) 130 per ha

or trees >41 cm in

diameter

Deep, multilayered

==OPY

Conifer snags 110

per ha that are >51

cm in diameter and

>4.6 m tall

Logs 234 metric tons

per ha including 10

pieces per ha 261 cm

in diameter and >15 m

long

Two or more species

with wide age range and

full range of tree sizes

Douglas-fir, ponderosa

pine. or sugar pine

220 per ha of trees

>76 cm in diameter

or 7200 years old

Intermediate and

small size-classes

are typically white

fu, Douglas-fir,

and incense-cedar.

singly or in mixture

Multilayered canopy

Conifer snags 23.7

per ha that are >51 cm

in diameter and >4.6 m

tall

Logs 222 metric tons

per bit including 5 pieces

per ha >61 cm in

diameter and 715 m

long

Douglas-fir and

evergreen hardwood

species (tanoak, Pacific

madrone, and canyon

live oak) associates

(40 to 60% of canopy)

Douglas-fir or sugar pine

215 per ha of trees

78 1 cm in diameter

or >200 years old

Intermediate and

small size-classes

may be evergreen

hardwoods or

include a component

of conifers (e.g.,

Douglas-fir or

white fir)

Douglas-fii emergent

above evergreen

hardwood canopy

Conifer snags Z3.7 per

ha that are >51 cm in

diameter and >4.6 m tall

Logs 222 metric tons

per ha including 5 pieces

per ha >61 cm in

diameter and >15 m

long

Two or more with wide

age range and full

range of tree sizes

Douglas-fir, sugar pine,

or ponderosa pine 220

per ha of trees 781 cm

in diameter or 7200

years old

Intermediate and small

size-classes are typically

white fir with incensecedar or both in some

stands

Multilayered canopy

Conifer snags 27.4 per

ha that are >51 cm in

diameter and 74.6 m

tall

Logs 222 tons per ha

including 5 pieces per

ha 261 cm in diameter

and >15 m long
Table 2-Old-growth definitions from USDA Forest Service

Pacific Northwest and Pacific Southwest Regional Guides

(units converted to metric)

Pacific Northwest Region (Region 6)-Washington and Oregon

For all National Forests in the Pacific Northwest Region, an oldgrowth stand is defined as any stand of trees 4 ha or greater generally with the following characteristics:

(a) Stands contain mature and overmature trees in the overstory

and are well into the mature growth stage

(b) Stands usually contain a multilayered canopy and trees of

several age-classes

(c) Standing dead trees and down material are present; and,

(d) Evidence of human activities may be present but may not

significantly alter the other characteristics and would be a

subordinate factor in a description of such a stand

Pacific Southwest Region (Region 5) -California

Old growth is a stand that is past full maturity and showing signs of

decadence, the last stage in forest succession. The definition of old

growth by tree age, size, height or density varies by timber type.

Among the components of old growth that may be of importance to

wildlife species and that may be affected by land-management practices are large trees, old trees, decadence of standing vegetation,

much dead and down woody material, uneven-aged vegetation, multilayered vegetation, moderate foliar-height diversity, and mesic microhabitats afforded by high canopy closure. High canopy closure does

not always correspond to decadence.

Efficacy of Existing Definitions

Society of American Foresters (1984), the Old-Growth

Definition Task Group (1986)) Morrison (1988), and Forest

Service definitions all describe old-forest conditions as an

“old-growth ecological type” with varying degrees of stringency. Thomas and others (1988:253) recommended that old

growth be ecologically defined on the basis of the composition of plant and animal species, and the vegetative structure (including sizes and densities of live trees, snags, and

down wood, and the number and nature of canopy layers) of

stands. Minimum stand size and its effect on specific ecological functions is another critical characteristic, especially as

related to providing fish and wildlife habitat. The Forest

Service, Pacific Northwest Region, established 4 ha as a

criterion for minimum stand size, and alluded to optimal

sizes for conserving wildlife species and habitats. Morrison

(1988) summarized inventories by two stand size-classes and

by degree of isolation from other old-growth stands. The

Old-Growth Definition Task Group (1986) cautioned that

stands of less than about 32 ha are fully influenced by edge

conditions, and should not be expected to provide interior

forest conditions.

As Should be expected, none of the definitions suffices for

defining, locating, and analyzing old-growth forests for the

full variety of resource interests. Nor do any of the definitions cover all forest types in the Pacific Northwest. None of

Table 3-SpecifIc definitions of old growth as used in the USDA

Forest Service Pacific Northwest Regional Guide for CedarHemlock-Douglas-fir Forest Type (units converted to metric)

At least 60 percent of the overstory canopy is dominated by

large individual trees in some combination of shade-tolerant and

shade-intolerant species.

The stand contains some trees with stem diameters 81 cm or

greater, an average of five snags per ha, and 67 metric tons of

logs per ha.

Trees have mature bark characteristics; for example,

Douglas-fir has to have deeply furrowed bark.

Crown-height growth has slowed, giving the tops a more

rounded shape; tops may be broken.

Limbs are usually heavy and gnarled, often with mosses and

lichens present.

Stands in these forest types will be considered old growth until

fewer than 12 overmature trees remain per ha.

the definitions is part of a classification scheme for describing the full sere of development of old-forest conditions.

Information based on successional development and structural changes will be most useful in developing an understanding of the status of old forests and in making future projections of old forests. Any single definition should only be part

of a whole system for classifying old forests in a scheme that

recognizes the range of young through old-growth and climax

forest conditions.

A Review of Old-Growth Inventories in the

Western United States

Amounts of Old Growth

The following discussion reviews old-growth and old-forest

inventories in Washington, Oregon, and California. Substantial amounts also occur in Alaska and British Columbia

throughout coastal and interior environments, including stands

dominated by Douglas-fir, western hemlock, and western

redcedar. Much of this additional old-forest area, however,

occurs in Sitka spruce and high boreal forest types, including

black spruce and white spruce. These forests are ecologically

much different than the lower elevation and more coastal

forest types dominated by Douglas-fir, Sitka spruce, and

western hemlock in Washington, Oregon, and California.

Inventory amounts from Alaska and British Columbia are

therefore excluded from the summary that follows.

Washington and Oregon-Franklin and Spies (1984) and

the Society of American Foresters (1984) estimated nearly

6 million ha of old growth on commercial forest land in the

51
Table 4-Three old-growth deflnitio&

Stand

characteristic

Old-Growth

Definition

Task Group

Pacific Northwest

Regional Guide

Society of

American Foresters

Douglas-fir on Western Hemlock Sites

Live trees Two or more species;

wide range of age

&Size

Douglas-fir ~20 per ha

of trees d.b.h. >81 cm

or >200 yrs

Tolerant associates

230 Per ha

Canopy

Snags

Deep, multilayered

Conifer snags IlO/ha

>51 cm in diameter and

>4.6-m tall

Logs Logs 234 metric tons/ha

including 10 pieces/ha

26 1 cm in diameter

and >15 m long

Fir-Mountain Hemlock

Live trees Not addressed

Car@PY

snags

Logs

Two or more species of

several age-classes

Gvermature trees 212

per ha with at least

some Douglas-fir with

d.b.h. 281 cm

2 25 trees Per ha with

d.b.h. >I02 cm or

or >200 yrs old

No specific standard No specific standard

Multilayered

s/ha

Logs 267 metric

tons per ha

Two or more species of

several age-classes

Dominant trees

>12/ha

No specific

standard

Multilayered

212fha

245 metric tons/ha

Two or more species;

wide range of age & size

Multilayered

>25/ha >6 m tall and

some >64 cm in diameter

>45 metric tons/ha and

some >64 cm in diameter

and >15 m long ’

Not addressed

’ To develop their estimates, The Wilderness Society (Morrison 1988) defined “early old growth” with the old-growth

criteria of the Old-Growth Definition Task Group (1986). One deviation was that age data in timber-inventory plots were

not quantified precisely enough to use stand age (>200 years) as a criterion, as with the Old-Growth Definition Task

Group (1986) definition. The Society also established definitions for “classic old growth” and “super old growth.”

Douglas-fir region in the mid-1800s, and at present nearly that this estimate may be high. Because of the absence of

4 million ha have been harvested. More recently, Spies and historic field inventories, these figures are approximate, and

Franklin (1988) estimated that about 17 percent of old growth definitions applied to forest conditions are imprecise.

existing in the early 1800s remains in the Douglas-fir region

of the Pacific Northwest. In an independent estimate, Norse According to draft Forest plans, some 2.5 million ha of for-

(1990) suggested that as much as 7.7 million ha of old growth ests classified as old growth remain on National Forests in

may have been present in western Washington and Oregon Washington and Oregon both east and west of the Cascade

before Euro-American settlement, although he acknowledged Crest. This estimate was based on the definition of old growth

52
from the Pacific Northwest Regional Guide (table 2), and

interpreted in the draft Forest plan by each National Forest.

The draft plans were published between 1986 and 1988. The

old-growth figures were generally updated to 1985, but some

were based on earlier inventories that had not been updated.

No information is available on historic amounts of old growth

on these National Forests.

The reported 2.5 million ha represent about 25 percent of the

total area of National Forest land in these two States. An estimated 1.2 million ha of that area occurs on the 10 National

Forests that are located either wholly or predominantly east

of the Cascade Crest, and 1.3 million ha occur on the 9

National Forests’ located either wholly or predominantly

west of the Cascade Crest.

About 0.30 million ha of the 2.5million ha total occur in

areas reserved from timber harvest by decrees of Congress,

such as in wilderness, Research Natural Areas, and other special reserves, and of that 0.30 million ha, about 0.11 million

ha occur in the 10 National Forests east of the crest and 0.19

million ha occur in the 9 forests west of the crest. This leaves

a reported 2.2 million ha of old growth on National Forests

throughout Washington and Oregon that are not in such

reserved lands.

The Wilderness Society (Morrison 1988) also estimated old

growth in Oregon and Washington. This estimate, however,

covered only six of the nine west-side National Forests in

Oregon and Washington, and it used the definition produced

by the Old-Growth Definition Task Group (1986). According

to Morrison (1988), only 45 percent of the area inventoried

as old growth by the Forest Service actually met all criteria

set forth by the Old-Growth Definition Task Group.

Haynes (1986) estimated, based on data from the 1970s, that

about 1.3 million ha of forest stands more than 250 years

old remained on all land (regardless of ownership) in the

Douglas-fir region of western Washington and western

Oregon. Of this remaining old growth, about 0.4 million ha

are protected, mostly in National Parks in Washington.

Haynes’ estimates were substantially lower than Forest Service estimates. Haynes’ definition of old growth included forests older, and thus more scarce, than did the Forest Service

definition, and he estimated old growth from stand-age data

(Morrison 1988). The precision of estimates made by Haynes

and by the Forest Service was not reported.

More recently, Greene (1988) reported that 0.3 1 million ha

of old-growth forest stands more than 200 years old occur in

North Cascades, Mount Rainier, Olympic, and Crater Lake

National Parks. Franklin and others (1988) estimated that

60 percent of the 97 000 ha within Mount Rainier National

Park in the central Washington Cascades are forested, and

most forests are at least 200 years old.

The Bureau of Land Management (BLM) has defined old

growth as stands 4 ha or larger with trees more than 200

years old (R. Metzgar, pers. comm.;2

 1987 BLM Spotted

Owl Environmental Assessment, cited in Greene 1988). In

April 1987, the BLM in western Oregon had 0.19 million ha

of unsold old growth remaining, with a projected 0.17 million

ha remaining by 1990 (Greene 1988). In April 1988, they

had 0.19 million ha of old growth on commercial forest land,

0.06 million ha protected by current plans, and 0.13 million

ha subject to harvest (R. Metzgar, pers. comm.2). The 0.13

million ha were estimated by expanding permanent plot

information, not by mapping stands in place, although BLM

plans to incorporate all old-growth stands in their geographic

information systems during their 1990s inventory. Based on

BLM Timber Management Plans implemented from 1980

through 1984, Ohmann and others (1988) reported 0.17 million ha of old growth 200 years old on BLM land in western

Oregon. BLM land in western Washington adds a negligible

amount, if any, to these old-growth amounts,

Ohmann and others (1988) also reported 2800 ha of old

growth on other public lands and 17 000 ha of old growth on

forest-industry lands in western Oregon. In these estimates,

old growth was defined according to the Old-Growth Definition Task Group (1986).

California--Currently, California has 8.0 million ha of forest

land (U.S. Department of Agriculture, Forest Service 1986);

39 percent is in National Forests, 53 percent in private ownership, and 8 percent in other ownerships. In the mid-1800s,

5.1 million ha of the 8.0 million ha were thought to be old

growth. About half of these occurred in the Sierra Nevada, a

fourth in the North Coast, and the remainder in northeastern

California with less than 3 percent in the mountains of southem California (Laudenslayer 1985).

Based on inventories from an earlier date (dates vary, depending on National Forest), Laudenslayer (1985) estimated that

about 0.9 million ha of forest stands qualified as candidate

old growth on National Forests in California around 1980.

This amount was about 17 percent of estimated historic old

growth. Candidate old growth was defined as all stands that

might meet or do meet structural criteria for old growth as

presented in the Pacific Southwest Regional Guide (table 2).

On National Forests in California, old growth had not been

defined for most forest types and the only data available

were by timber-type strata. Even if old growth was defined

by forest type, existing inventories would not likely be able

1

The nine west-side National Forests are the Olympic, Mount BakerSnoqualmie, Gifford Pinchot, Mount Hood, Willamette, Deschutes,

Umpqua, Siskiyou, and Siuslaw.

2 On file with Bruce Marcot

53
and overmature timber-type-strata and stocking-classes.

Laudenslayer also estimated that, in the future under Forest

Service plans, the 0.9 million ha will decrease to about 0.7

million ha, or about 14 percent of historic amounts. Virtually

all old growth on private lands in California may be harvested in the next 30 to 50 years (F. Samson and others,

unpubl. manuscript).

In California, Resources Planning Act (RPA) Timber and

Wildlife Narratives provide a more recent estimate of candidate old growth. In the Timber Narratives, old growth was

defined under the general regional definition (table 2), and

in the Wildlife Narratives, overmature forest was based

on the definition of old-growth habitat for management

indicator species. In 1987, 0.6 million ha of candidate stands

qualified as old growth on National Forests in California,

with 0.4 million ha on land suitable for timber production

(RPA Timber Narrative, Pacific Southwest Region). These

candidate stands included forests typed by the Forest Service as containing overstory trees with crown diameters of

>7 m and with canopy closure >40 percent (Forest Service

stand classes 4N and 4G). Another Forest Service summary

reported about 0.8 million ha of overmature forest on

National Forests in California in 1985, 0.2 million ha of

which occur in wilderness (RPA Wildlife Narrative, Pacific

Southwest Region).

Summary-The most extensive estimates of old growth

suggest that amounts of old growth and other old forests on

National Forests in Pacific States total about 3.6 million ha

(table 5). This tally uses the Forest Service’s estimate of oldgrowth amounts in Washington and Oregon. If the more

stringent definition and estimate by Morrison (1988) is used,

total amounts would be lower.

This tally of current inventories is very approximate. It combines many forest types, including upper elevation, true fii

forests in the Washington and Oregon Cascade Range;

namely, Pacific silver fir, grand fir, and ponderosa pine

forests in eastern Washington and Oregon; mixed-conifer and

Jeffrey pine forests in northern California and the Sierra

Nevada, and other forest types. The estimate combines data

from different years and is based on a variety of definitions

of old growth. It includes candidate old-growth forests, an

unknown but significant proportion of which would not

qualify as ecological old growth (Old-Growth Definition

Task Group 1986). The estimate, however, does not include

amounts of old growth that may exist on lands of other

Federal, Provincial, State, local, commercial, and private

administration and ownership throughout the geographic area

considered.

Size of Old-Growth Stands

Past clearcutting of old growth has fragmented remaining

stands into smaller patches with greater interpatch distance.

Fragmentation also alters stand shapes. An unknown proportion of existing old-growth forests occur in relatively inaccessible or unproductive areas, such as in canyon bottoms or

along ridges. Such old-growth stands are often linear and

narrow. Most or all of such stands may be influenced by

edge conditions. Effects of small patch-size, increasing isolation, and edge conditions may adversely affect the persistence of some wildlife populations in old-growth areas.

Harris (1984) summarized the sizes of 319 old-growth stands

from 1981 Total Resource Inventory System (TRI) data from

the Siuslaw National Forest in the Oregon Coast Range. He

noted that 61 percent of the stands were less than 16 ha, the

average stand was 28 ha, and the median was 13 ha.

Laudenslayer (1985) noted that on National Forests in

California, candidate old-growth stands of all forest types

averaged about 16 ha, ranging from an average of 7 ha in the

red fir forest type in the north coast to an average of 235 ha

in the Jeffrey pine forest type in the Sierra Nevada. Few data

are available on the distances between old-growth patches,

but old-growth patches are probably becoming increasingly

smaller and isolated over time (Harris 1984).

Old-Growth Inventory and Management

Direction on National Forest Land in Western

Washington and Oregon

Forest Service and The Wilderness Society Estimates

The Wilderness Society estimates of the amount of old

growth remaining in Washington and Oregon (tables 6,7;

Morrison 1988) and estimates by the Forest Service (table 7;

draft Forest plans) differed substantially. Both estimates were

derived from Forest Service timber inventories, but these

inventories were not designed to measure all characteristics

of importance in distinguishing old-growth stands. Both estimates, therefore, required extrapolation from the inventory

values.

The two estimates were produced for different purposes with

different techniques. The Forest Service estimate was derived

from a mapped inventory of old growth for Forest planning,

and therefore relied heavily on photo-interpretation, The

Wilderness Society estimate was intended to be only a numerical estimate. Although Morrison (1988) did use photointerpretation, he depended more heavily on plot data from

the Forest Service timber-inventory process.

54
Table S-Estimated current amounts (in millions of hectares) of old growth and candidate old growth on

selected lands in the Pacific States of the United States

Area Estimate Definition Reference

Washington and Oregon:

Forest Service

National Park Service

Bureau of Land

Management-Oregon

Other public lands and

forest industry, western

Oregon only

California:

Forest Service

Total 3.6

2.5 Old growth variously

(1.3 west- defined in plans and

side only) Regional Guide

0.31

0.19

Forests >2OO yrs old Green 1988

Stands of 4+ ha,

trees >200 yrs old

0.019

0.60

Draft National Forest Plans

R. Metxgar, pers. comm.

Old-Growth Definition Ohmann and others

Task Group 1988

Stands with trees >7.3 m RPA Timber Narrative,

crown diam. and canopy Pacific Southwest

closure >40 Percent Region

Includes all candidate oldgrowth and excludes old

growth on some lands other

than National Forests)

The two estimates were based on different years and on different definitions of old growth. The Forest Service estimates

were generally current for 1985, although several estimates

were based on older data. Morrison’s estimate was based on

data updated to 1988. Morrison adhered to the definition proposed by the Old-Growth Definition Task Group (1986),

except for down wood and snag density criteria. Morrison

also applied the Task Group definition to forest types for

which it was not developed. The National Forests used the

less restrictive definition published in the Regional Guide,

which was modified to reflect conditions specific to each

forest type and each National Forest.

The draft Environmental Impact Statements for the Forest

Plans show that the operational definitions used by National

Forests actually varied from the definition presented in the

Regional Guide. These operational definitions generally

focused on characteristics for which data were more readily

available. Definitions used by the nine west-side National

Forests in Washington and Oregon require fewer large trees,

large snags, or trees of shade-tolerant species than do either

the Old-Growth Task Group or the Regional Guide definitions (see table 4; The Wilderness Society 1988).

Data published by Morrison (1988) allowed us to estimate

inventory results by using the less restrictive definitions that

the Forest Service used on six National Forests in western

Oregon and Washington (table 6). In addition to areas that

met all the old-growth criteria of the Old-Growth Definition

Task Group (1986), Morrison listed areas that had some

old-growth characteristics but failed to meet individual

criteria (table 6) Most of these areas would likely meet the

less restrictive definition generally used by the Forest

Service. All of these areas have developed beyond a mature

stage marked only by the presence of trees >53 cm d.b.h.,

and all contain at least some trees >80 cm d.b.h. These areas

were summed and compared to Forest Service old-growth

estimates in table 7. This table also shows the sampling error

calculated by Morrison for his old-growth estimates. Sampling error (table 7) for the sum of these four, less restrictive

old-growth classes would actually be greater than the sampling error calculated only for the old-growth class.

Predictably, these adjustments bring the two estimates closer

together. Figures taken from The Wilderness Society report

total to 0.8 million ha or 80 percent of the Forest Service

estimate of 1.0 million ha (table 7). This compares to The

Wilderness Society estimate of 0.4 million ha of old growth

on the six National Forests, which represented 45 percent of

the Forest Service estimate. When sampling error is taken

into account, estimates overlap on two of the six forests. If

the 3 years of harvest between 1985 and 1988 were taken

into account, the two estimates on the Willamette National

Forest would probably also overlap.

55
Table L--The Wilderness Society area-estimates in thousands of hectares of old-growth forest and forest conditions that fail to meet

one or more old-growth criteria (sensu Old-Growth Definition Task Group 1986) as listed in table 4, on 6 National Forests in

western Washington and Oregon

National

Forest

Mount BakerSnoqualmie

Olympic

Gifford Pinch0 t

Mount Hood

Willamette

Siskiyou

(1)

Old growth’

120

43

48

72

121

57

(2) (3)

Fails late-seral Fails snag

tree-c&&on criterion

3 40

1 0

1 23

0 27

29 36

0 18

(4)

Fails

large-tree

criterion

29

21

31

39

35

34

(l-4)

Totalb

192

64

104

138

221

109

(5) (6)

Fails Meets

multilayer- maturecanopy forest

criterion criterion

3 41

1 8

0 80

0 35

0 93

0 7

(l-6)

Total

with

mature

236

74

183

173

314

117

Total 461 829 1097

Source: Morrison (1988).

’ Includes old growth on all land allocations, including wilderness. Other columns do not include wilderness lands established before the date of the

inventory data available from the National Forests.

* Columns 1 to 5 all contain stands with at least some trees SO-cm in d.b.h. Column 6 does not.

Table 7-Estimates of old growth in thousands of hectares by the Forest Service and

The Wilderness Society (Morrison 1988) including areas that fail to meet one or more

old-growth criteria

National

Forest

The

Wilderness Society

old-growth plus other

old-forest stands

(from table 6)

Forest

Servicd

old-growth

estimate Difference

Sampling error

(Morrison 1988)

Mount BakerSnoquahnie

Olympic

Gifford Pinchot

Mount Hood

Willamette

siskiyou

192 270 76 8

64 88 23 2

104 93 10 17

138 140 2 13

221 259 37 25

109 179 70 19

Total 829 1029

’ Derived from draft Forest Plans.

This comparison is not intended to show that both inventories are really the same. Significant differences clearly exist.

The comparison indicates that old-growth estimates are sensitive to the definitions on which they are based. This emphasizes the need for a commonly accepted definition of old

growth, a better understanding of important old-growth characteristics, and better inventories intended to quantify those

characteristics. Inventories must be designed to provide information on individual characteristics rather than to accommodate one fixed definition of old growth. The importance

of such an inventory design is highlighted by the fact that the

stands classified as “old growth” in the Old-Growth Program

research studies (this volume) did not actually meet, nor

were they designed to meet, all criteria from the Old-Growth

Definition Task Group. An inventory designed around this

single definition would fail to inform us about a large

amount of forest land that is important wildlife habitat.

Improved inventories must be completed soon if we are to

make use of information from research (this volume).

56
Current Management of Old Growth and OldGrowth Allocations in National Forest Plans

What will be the fate of the 1.3 million ha of old growth on

the nine National Forests in western Washington and Oregon?

Forest-planning alternatives propose the retention of about

0.6 to 1.2 million ha over time. Preferred alternatives of the

nine draft Forest plans called for a total of 0.8 million ha of

reported old growth to be retained over time, which is about

57 percent of currently inventoried amounts. The proportion

of total historic old growth represented by this amount is

substantially less and is not quantified.

In current planning and management activities on National

Forests, old growth appears as an issue derived from various

interests, including recreation, fish and wildlife habitat, timber supply, long-term forest productivity, scientific research

areas, and aesthetics. Management-area direction established

in Forest plans will determine the future amount and distribution of forest habitat, including some old growth, for wildlife

species, and particularly management indicator species on

National Forests. In addition, management requirements for

standing and fallen dead trees in all areas address a vital

component of old-growth forests that also may be found in

younger stands. Those snags will probably be smaller, however, and may not persist as long as in old-growth stands.

Actual amounts and distributions of old growth for wildlife

species is currently decided by each National Forest through

Forest plans. Amounts and distributions result from applying

regional standards (“minimum requirements”) for maintaining

old-forest habitats for spotted owls, marten, pileated woodpeckers, a variety of other primary excavators, and some

additional key vulnerable species identified in each of

several provinces in the Pacific Northwest Region. Forest

plans establish final allocations, but they must meet or exceed

the regional standards.

Habitat-management areas for old-growth management

indicator species provide one type of allocation where

old growth may occur. Other allocations will also contain

old growth. As an example, the draft Forest plan on the

Willamette National Forest lists the following land allocationclasses that may contain old-growth forest (other National

Forests will retain old growth under similar allocations):

Congressionally withdrawn-no harvest

Wilderness

National Recreation Areas

Experimental Forests

Research Natural Areas

Administratively withdrawn-no harvest

Proposed Research Natural Areas

Old-Growth Groves

Threatened or Endangered Species Habitats (such as for

bald eagle)

Other Special Wildlife Habitats and Habitat Protection

areas

Spotted Owl Habitat Areas

Pileated Woodpecker Habitat Areas

Marten Habitat Areas

Deer and Elk Thermal Cover

Other Special Habitat (Protection and Enhancement)

Semiprimitive (Roadless) Areas

Recreation Sites (Existing and Proposed)

Mining Claims

Lands Technically Unsuited for Timber Production

Partial Harvest Categories

Visual Retention Areas

Semiprimitive (Roadless) Areas

Riparian

Full Harvest Categories

General Forest

Old growth occurring in the full harvest allocation and

several other allocations (such as mining claims) would be

retained only until the time of harvest or management

activity.

Under the Preferred Alternative in the Willamette National

Forest draft plan, 52 percent of old growth will be retained

through time. This totals about 0.18 million ha (table 8). The

remaining 48 percent or 0.17 million ha is planned to be

harvested over the next five decades. According to the draft

Forest plan, old growth on the Willamette National Forest

was defined according to the Old-Growth Definition Task

Group (1986; see table 1) with specifications that vary by

forest type. Like most of the other draft Forest plans in the

region, the Willamette National Forest draft plan quantified

and analyzed the effects of land-allocation decisions on the

distribution of old-growth forest-stands for specific management indicator species (spotted owls, marten, pileated woodpeckers, and primary cavity excavators). Outside of such

management requirements, however, the effects of landallocation decisions on the size and context of old-growth

stands were not analyzed.

Old-Growth Habitat Characteristics of Importance to

Wildlife: A Key to Future Inventories

Some wildlife species may have co-evolved with, and depend

on, specific amounts and conditions of old-growth forests.

Specific kinds, sizes, and patterns of old-growth environments

are, therefore, keys to the long-term survival of these species.

Land allocations affect the distribution of old growth across

57
Table S-Allocation of mature and old-growth forest 

under the preferred alternative in the draft Furest plan, 

Willamette Katiunal Forest 

Categ0ty 

Withdrawn 

Wilderness 

Unsuited for timber 

nroduction 

HtXtareS 

93600 - 

45000 - 

wildlife habitars 

No hanest 18cJJO - 

Subtotal 

Harvestable 

Timber harvest arear 

TOti 

184 : LOO 52 

167200 48 '1 

351400 loo 

the landscape over time and the effectiveness of old growth 

as habitat for wildlife. Resulting spatial patterns of old gmwtb 

influence the viability of many wildlife species that depend 

on the ecological conditions of old forests. Old growth may 

provide population “reservoirs” for species that find early 

successional stages of second-growth conifer stands marginal 

habitat. 

What attributes of old growth affect the perpetuation of these 

old-growth wildlife species? Attributes may be characterized 

at stand and landscape scales. Attributes at each scale may 

be useful for guiding future inventories of old growth as 

wildlife habitat. 

At the stand scale, important vegetational structures include 

the presence of key substrates, food, and resources, especially 

epiphytic mosses and lichens, which are important as winter 

food for ungulates and rodents; and the presence of specific 

thermal and moisture conditions. Other stand characteristics 

important to old-growth-dependent wildlife are related to 

long-term productivity of the forest environment. These 

characteristics include the presence of dead and decaying, 

standing and down wood as habitat for breeding, feeding, 

hiding, and other needs; and the presence and density of 

hypogeous fungi, which are important as food sources for the 

co-evolved mycophagous small mammals that disperse the 

mychonizal fungi needed for conifer growth. 

Landscape attributes affecting the perpetuation of old-growth 

dependent and associated wildlife include the spatial distribution of old growth; the size of stands; the presence of habitat 

corridors between old-growth or old-forest stands; proximity 

to other stands of various successional stages and especially 

for well-developed mature-forest stages and species with 

different seasonal uses of habitats; and the susceptibility of 

the old-growth habitat to catastrophic loss (such as wildfire, 

insects, disease, wind and ice storms, and volcanic eruptions). 

Stand size, in combination with its landscape context (the 

condition, activities, or both on the adjacent landscape that 

affect the stand), is of major significance in perpetuating 

old-growth resources and can have a major effect cm their 

use by wildlife. Wide-ranging species may be able to use 

stands of various structural- , size- , and age-classes. If such 

stands are separated by unsuitable habitat or disruptive activities, however, the remaining old-growth stands become 

smaller in effective (interior) size, more fragmented, and 

possibly not suitable for occupancy 01 for successful reproduction. An old-growth inventory that quantifies such stand 

and landscape attributes is a prerequisite for evaluating possible context and landscape effects on species’ presence. 

The Mature and Overmature (MOM) Forest 

Inventory on National Forest Land 

Rationale for an Old-Forest Inventory 

An old-growth inventory must respond to a set of clearly 

stated goals and objectives (Thomas and others 1988). These 

goals directly determine the classification schemes, parameters to be gauged, spatial scale of resolution for the inventory, and acceptable scales for the reliability of the information. They also help guide how the inventory process and its 

results are integrated into other resource information needs 

(see Davis and Henderson 1977). At present, goals and objectives for conducting an inventory of old-growth forests in the 

Pacific Northwest vary among groups with different interests 

in how the old-growth rescm~ce is managed. 

An old-growth inventory can be category driven or attribute 

driven. With a category-driven inventory, old-forest stands 

are tallied into one or mcxe classes, which are defined a 

priori. By contrast, in an attribute-driven approach, the classifier describes forest stands by a set of structural chamcteristics deemed pertinent to old-forest conditions. These stands 

are then classified a posteriori based on a specified set of 

stand attributes. The utility of both approaches depends on 

the purpose of the inventory. For old-growth? inventories, the 

attribute-driven approach-when used with a classification of 

seral stages of old forests-is more flexible than a categorydriven approach. As Smith (1982x1) observed, It is better to 

invent additional patterns than to warp those observed to fit 

preconceived ones. 

58 
With either approach, we must specify what is being inventoried. Inventories of old-growth forests include the presence

or amount of various stand characteristics, and distributions

of old growth at different scales of resolution and geographic

areas. Amounts should be displayed as totals by State, by

Federal and State agency jurisdiction, by administrative unit

within agencies, by individual and corporate ownership, by

ecophysiographic province, or by other geographic divisions.

Size and context should include contiguous sizes and the

shapes of old-growth stands and their relation to adjacent

landscape conditions and activities, including the proximity

of other old-growth stands. Distribution includes defining the

spatial scale at which an old-growth inventory should be

conducted. This scale greatly affects the kinds of stand

attributes that can be used as inventory criteria and the

resolution at which such attributes should be known.

For example, a stand-by-stand inventory could include a complete census of appropriate characteristics for each vegetative

unit. By contrast, a forest-wide inventory may use a stratified

sampling approach to provide reliable information at the scale

of vegetation types. Then, some of the characteristics of any

given stand are known only as a probability-that is, an estimate of the proportion of all stands in a stratum that have

particular structural characteristics.

An old-growth inventory must be designed with a specified

degree of reliability. The degree of error and confidence in

the statements of amount and distribution should be known,

at least qualitatively. The reliability of an inventory is a function of many factors. These include the correctness and usefulness of the classification scheme used; the quality of the

sampling design by which remote-sensing images are interpreted and vegetation surveys in the field are conducted; the

consistency with which inventory criteria are applied across

various land units, taking into account the need to vary criteria by forest type and land form; the availability and quality

of remotely sensed images: the expense and training involved

in having people interpret the remotely sensed images; the

experience and training of field crews; and the sample sizes

used in field verification testing and from which subsequent

classification strata are derived.

Inventory of Mature and Overmature Forest

A large-scale, attribute-driven inventory of mature and overmature forests has been instituted on National Forests in

Washington and Oregon as part of an inventory of all vegetation stands. The main objective of the inventory of mature

and overmature forest is to provide a set of data to which a

solid, basic set of definitions of the developmental stages of

old forests can be applied. This inventory will provide an

accurate inventory of old-growth stands; allow for the use of

multiple definitions of old-growth forest conditions; and be

accessible on a geographic information system (GE) for analysis of distributional conditions. Plans also call for a periodic

update of the inventory with remote imagery, although this

technology needs further testing.

The inventory of mature and overmature forest was designed

to enhance the existing Vegetative Resource Survey, which

is the standard inventory of all vegetation stands on National

Forests in the Pacific Northwest Region. This enhancement

resulted in a redefinition of mapping criteria and a greater

emphasis on sampling stand attributes that are considered to

be of importance in old forests. These attributes include the

size, shape, and location of stands; the multicanopy structure

of the stands: mixes of tree, shrub, and herbaceous species;

the diameters and heights of live trees; canopy closure within

each canopy layer and for the stand as a whole; the size,

distribution, and decay states of standing and down dead

trees; and heterogeneous characteristics of stands with small

canopy openings.

Information about these attributes will allow interpretations

about the values of stands from several different viewpoints.

One interpretation that could be made would be whether

stands meet a specific definition of old growth. This interpretation, however, would be an outcome of the basic information that was collected, rather than being the driving force

for the design of the inventory.

The Mature and Overmature (MOM) Inventory follows the

two-stage design of the existing Vegetative Resource Survey.

The first stage is done by photo-interpretation. In this step,

each forest stand is delineated on photos and maps. For each

delineated stand, information is recorded on the number of

canopy layers (up to three); canopy closure of the entire

stand; the presence of snags; clumpiness of the canopy; the

presence of large, remnant trees from stands that previously

occupied the site; and volume. In addition, for each of the

canopy layers, information is recorded on tree species, canopy

closure, crown diameter of dominant trees, clumpiness, and

visible snags. All of this information is derived from photointerpretation and is ground verified on about 10 percent of

all stands.

After the photo-interpretation is completed, stands are arrayed

according to their attributes to determine an appropriate distribution for additional, detailed ground plots. The ground plots

are the second stage of the inventory. In other inventories,

the standard design calls for lumping the stands into defined

strata to develop a sampling strategy. With the MOM Inventory, strata are avoided so that plot data can later be combined in several different ways to address various concerns.

Instead of developing strata, the sample plots are distributed

so that the full array of stand conditions are sampled. This

process involves arraying the stands according to species

59
composition, number of canopy layers, tree density or stocking, canopy closure, and crown diameters. The ground plots

follow a IO-point design and include detailed information on

live-tree species including height, density, diameter, and

form by canopy layer; snag density, diameter, height, and

decay state; diameter, length, and decay state of logs; and the

presence of indicator plant species in the understory. This

last category is used to classify the stand by plant association.

Because this plot information can be collected only for a

sample of stands, it will not be available for all the stands

that are photo-interpreted. Data from the MOM Inventory

can be analyzed in two different ways to draw inferences

about stands that were not sampled. First, a collection of

stands can be defined based on any combination of the photointerpreted characteristics. Plot data can then be summarized

for all stands in this collection that were ground sampled.

Because these collections of stands are not being defined as

a priori strata, the values of attributes for such a summary

could vary widely.

The second analysis opportunity is to develop correlations

between characteristics that were photo-interpreted and characteristics that were measured in plots. For each stand, this

comparison would allow us to identify photo-interpreted

characteristics with some degree of certainty, and to identify

ground-sampled characteristics with some known degree of

reliability. Currently, the ability to correlate ground-sampled

characteristics with photo-interpreted characteristics is the

largest unknown in the inventory design, and the reliability

of inferences about these ground-sampled characteristics is

also unknown.

The combined MOM and Vegetative Resource Inventories

will result in five products:

l A stand map for each forest stored on a GIS.

l A data card for each stand containing the photo-interpreted data

for that stand. This information will be stored in the attribute file

of the GIS.

l A database containing all plot data.

l Summarized plot data for collections of stands defined according

to any combination of the photo-interpreted characteristics.

l Statistical correlations between photo-interpreted characteristics

and plot-sampled characteristics.

The MOM-Inventory process is currently being implemented

on the Siuslaw and Willamette National Forests as part of the

Vegetative Resource Inventory. It is expected to be completed

on all nine of the west-side forests in Oregon and Washington

by 1994.

Recommendations for Old-Growth Inventories

We offer the following recommendations for developing a

multipurpose, old-growth inventory. First, an old-growth

inventory should be attribute-based rather than definitionbased. An example is the inventory of mature and overmature forests currently being conducted by the Pacific Northwest Region of the Forest Service.

Second, an old-growth inventory should be map-based to

address landscape issues, including that of scale. Map-unit

sizes should be small enough to identify old growth where

edge effects eliminate interior habitat. A map-based approach

would also benefit from further exploring the use of Landsat

as a tool for updating inventories over time. In addition, a

map-based inventory would benefit from the application of

a geographic information system for evaluating the data,

especially the landscape and spatial conditions of stand size,

shape, and context. The MOM Inventory is currently being

installed on GIS to facilitate the evaluation of spatial

components.

Third, any adopted definition of old growth should be part of

a system of describing the amount and distribution of various

stages of forests, especially older ones. The full classification

system would include descriptions of the total vegetation inventory from seedling to old-growth successional stages, including noncommercial forest lands and nonforest vegetation.

Specifically, the Old-Growth Task Group definitions should

be the basis for ecological definitions used in a broader

framework. In this manner, a consistent definition can be

applied to the attribute-based data and also allow analyses

of stand conditions that meet various other definitions.

Acknowledgments

We thank Jerry Franklin, William Laudenslayer, Jr., Peter

Morrison, Janet Ohmann and Tom Spies for thorough and

helpful reviews of the manuscript. We also wish to acknowledge the policy reviews and helpful comments from Hugh

Black, Wendell Jones, and John Lowe. Franklin Burch

reviewed the inventory figures for the Pacific Southwest

Region of the Forest Service.

60
Ecological Definitions of Old-Growth

Douglas-Fir Forests

Jerry F. Franklin and Thomas A. Spies

Authors

JERRY F. FRANKLIN is a plant ecologist, USDA Forest

Service, Pacific Northwest Research Station, and Bloedel

Professor of Ecosystem Analysis, College of Forest

Resources, University of Washington, Seattle 98195.

THOMAS A. SPIES is a research forester, USDA Forest

Service, Pacific Northwest Research Station, Corvallis,

Oregon 97331.

Abstract

We examined the potential for definitional and indexing

approaches to identifying old-growth Douglas-fir stands.

An interim definition created by the Old-Growth Definition

Task Group uses multiple structural characteristics of forest

stands-specific levels of large or old live trees, snags, logs,

and foliage canopy layers. This definition and numerous

variants with different parameters and parameter values were

tested for their ability to discriminate old growth from young

and mature natural forests with an independently collected

data set. The criteria used in the definitions appear appropriate but variants of the definition differ significantly in their

ability to distinguish old-growth in particular geographic

areas. Definitions that used density of all large (>lOO cm in

diameter at breast height) trees, rather than just Douglas-fir,

performed best. Further refinement of definitions should probably be site specific. Developing approaches that recognize

the continuous variability in old-growth stands are recommended for the whole region. These approaches could be

simple indices based on multiple structural characteristics or

using discriminant analysis; examples are provided to demonstrate the merits of both concepts. Maintaining a holistic

perspective on old-growth forest ecosystems is critical in

these and other current efforts to characterize old growth by

individual attributes.

Introduction

Old-growth forest is a biological or ecological concept that

presumes ecosystems systematically change as they persist

over long periods. An ecosystem has, in effect, a series of

linked life stages, progressing from origin (birth) to old age

(senescence), which vary in composition, function, and structure. Such progressions can take a very long time in forests

because the dominant organisms, trees, typically live very

long.

Characterizing old-growth forests is possible based on these

concepts. Obviously, a series of ecological attributes must be

considered because of the many relevant compositional, functional, and structural features. For practical reasons, however,

a working definition-one for everyday use in gathering

stand data-emphasizes structural and compositional rather

than the conceptually important functional features that are

difficult to measure.

61
A generic definition of old-growth forests, applicable to most

temperate and subalpine forests, is possible. Old-growth forests are later stages in forest development that are often compositionally and always structurally distinct from earlier

successional stages. For example, forests at low to moderate

elevations in western Washington and Oregon often shift

from strong dominance by Douglas-fir in early stages of succession to mixed stands with large amounts of western hemlock and other tolerant species at mid to late stages in succession (Franklin and Hemstrom 1981). Even where both earlyand late-successional forests are composed of the same species, such as many sites with ponderosa or lodgepole pine,

the old stands differ from young stands in structure; for

example, old stands have a much larger range in tree sixes.

The age at which forests become old growth varies widely

with forest type or species, site conditions, and stand history.

Structurally, old-growth stands are characterized by a wide

within-stand range of tree sizes and spacing and include trees

that are large for the particular species and site combination.

Decadence is often evident in larger and older trees. Multiple

canopy layers are generally present. Total organic matter

accumulations are high relative to other developmental stages.

Functionally, old-growth forests are characterized by slow

growth of the dominant trees and stable biomass accumulations that are constant over long periods. Respiration reduces

net annual additions of living organic matter to low amounts

relative to earlier successional stages, despite high gross primary productivity in old-growth stands. All climax forests”

forests that appear to have stable composition and structure

in the absence of a major disturbance”qualify as old growth,

although most old-growth forests are not climax; that is, they

still contain seral species and are undergoing slow directional

changes in composition and structure. Not all virgin or primeval forests are old growth and not all old-growth stands are

totally free of human disturbance. Old-growth forests could

theoretically originate after either human or natural disturbance because they are defined by composition, structure, and

function and not by origin or naturalness.

General characterizations of old-growth forest are conceptually important, but specific definitions are essential to forestland management. Inventories to identify the quantity and

location of old-growth forests require such specificity. Forest

policy, including the allocation of forest lands to various

uses, are partially based on these inventories. Old-growth

forests need to be characterized so that important attributes

can be retained and attempts made to create similar forests.

Characterizations are also needed by scientific researchers.

In this paper, we will consider the state and future of old

growth in the Pacific Northwest. Our primary focus is on the

mesic temperate forests dominated by Douglas-fir and located

west of the crest of the Cascade Range in Washington,

Oregon, and northern California. We review the history and

adequacy of the existing old-growth definition and explore

directions for future development of such characterizations

including local refinement and extension to other forest

types, developing indices with continuous scales of variation,

and using statistical methods, such as discriminant analysis.

Old-growth forests in our study area vary widely in their age

and ecological state (for example, in composition and structure), which reflects a similar wide variability in their history

and physical environments. Old-growth Douglas-fir forests

are from about 200 to over 1000 years old, they undergo

gradual but significant autogenic change during those centuries of existence and may also be subjected to varying

numbers and intensities of disturbance events, such as windstorms. As a consequence, old-growth Douglas-fir forests can

differ substantially in their degree of “oldgrowthness”-that

is, in the degree to which they express the various structural

and functional features associated with these forests; this

variability must be considered in efforts to define and manage

old growth.

Current Definition of Old-Growth Douglas-Fir

Forests

Historical Development of the Interim Definition

The history of the interim definition of old-growth Douglas

fir forests (Old-Growth Definition Task Group 1986) is discussed in detail by Marcot and others (this volume). It began

with National Science Foundation funding of the Coniferous

Forest Biome in 1969, as part of the International Biological

Program (Edmonds 1982), and of subsequent ecosystem studies, particularly at the H. J. Andrews Experimental Forest.

The USDA Forest Service also contributed to this research.

These studies provided a wealth of data and concepts on the

nature of old-growth forests and associated streams (see Blinn

and others 1988, McKee and others 1987).

In 1976 the research group associated with this work received

a request from a National Forest planner for assistance in

characterizing old-growth forests; as he put it, “We know

that old-growth forests are more than just some big old trees,

but we don’t know how else to describe them. Could your

group provide us with a more complete characterization?” A

workshop, convened at Wind River, Washington, in 1977,

produced the synthesis entitled, “Ecological Characteristics

of Old-Growth Douglas-Fir Forests” (Franklin and others

1981).

All current attempts at ecological definitions of old-growth

Douglas-fir are based on the 1977 synthesis and subsequent

research, much of it under the auspices of the Forest Service’s

Old-Growth Forest Wildlife Habitat Program. These studies

clarify the distinctiveness of old-growth forests in composition, structure, and function and, perhaps even more

important, the complexity of most naturally regenerated

62
forests developing after wildfire, windstorm, or other

catastrophic events in the Pacific Northwest, regardless of

successional stage. For example, the structural complexity of

young natural forests is high because of the carryover of

large snags,, logs, and even large live trees from the predisturbance stand; conditions in these natural stands obviously

contrast with the simplified structure of most young forests

managed for timber production. Past old-growth studies also

illustrate the need to use multiple attributes to characterize

old-growth forests and the value and validity of emphasizing

structure in working definitions. Among the first iterations of

old-growth definitions based on these ecosystem concepts are

those by the Society of American Foresters (1984) and

Franklin and Spies (1984).

In 1985, an interagency group-the Old-Growth Wildlife

Habitat Research and Development Program Steering Committee-created the Old-Growth Definition Task Group to

develop an interim definition of old-growth Douglas-fir forests. The group was composed of technical experts from the

USDI Bureau of Land Management, Oregon State University,

and National Forest and Research branches of the Forest

Service. The draft reports they wrote were extensively reviewed by management groups and scientists and revised

before the interim definitions were published (Old-Growth

Definition Task Group 1986).

The Interim Definition and Its Adequacy

The interim definition consist of separate definitions for oldgrowth Douglas-fir forests under three broad environmental

conditions indexed by plant series (table 1). The three definitions are based on multiple structural criteria: density of live

trees of large or old Douglas-firs and of associated mediumto large shade-tolerant tree species; multiple canopy layers;

standing dead tree (snag) densities, including larger sizes;

and logs, including total weight and a minimum number of

large pieces. The rationale for selection of specific characteristics is discussed in the original publication (Old-Growth

Definition Task Group 1986).

Although the definitions have been cussed and discussed

since their publication, no data have been available to test

their adequacy in discriminating old-growth from young and

mature forests. Forest-inventory procedures usually do not

collect data on all of the characteristics used in the definitions; however, Morrison (1989) was able to effectively use

USDA Forest Service inventory plot data by adapting the

definitions to the more limited set of parameters that were

included in the inventories.

Spies and his associates have provided the first complete and

independent data set (Spies and others 1988; Spies and others,

in press; Spies and Franklin, this volume) appropriate for testing the performance of one of the interim definitions-the

definition developed for stands on western hemlock and

mid-elevation Pacific silver fir sites (table 1). This interim

definition is the only one subsequently addressed in this

paper. The data are from 196 young, mature, and old-growth

stands in the Oregon and southern Washington Cascade

Range and Oregon Coast Ranges (Spies and others 1988;

Spies and others, in press; Spies and Franklin, this volume).

Stands were selected that contained Douglas-fir and appeared

to have originated after a high-intensity or stand-regenerating

wildfire. None of the stands were selected according to any

preconceived set of old-growth criteria. Five systematically

spaced plots were used to characterize each of the selected

stands. Stands were classified as young (~80 years old),

mature (80-199 years old), and old (>200 years old), according to the ages of the dominant Douglas-fir trees. The

85 age-defined old-growth stands from this data set are the

standard against which the performance of the various

definitions and other discriminating procedures are tested.

Table l-Interim minimum standards for oldpowth Douglasfir forests in western Washington and Oregon

Douglas-fin Douglas-fir Douglas-fir

on western on mixed- on mixedhemlock sites conifer sites evergreen

Stand (western hemlock, (white fir, sites (tanoak,

characteristic Pacific silver fir) Douglas-fir Douglas-fir)

Live trees Two or more Two or more Douglas-fir

species with species with and evergreen

wide range of wide age range hardwood

ages and tree and full range (tanoak, Pacific

SiZSS of tree sizes madrone, and

canyon live oak

associates (40

to 60 percent

of canopy)

Douglas-fir Douglas-fir, Douglas-fir or

E20perha ponderosa sugar pine

of trees > 81 cm pine, or sugar 2 15perhaof

diameter or pine >- 20 per trees > 81 cm

> 200 years old ha of trees diameter or

>76 cm dia- > 200 years old

meter or >200

years old

Tolerant Intermediate Intermediate

associates and small and small size-

(western hemlock, size-classes classes may

western redcedar, are typically be evergreen

Pacific silver white fir, hardwoods or

fir, grand fii, Douglas-fir, include a

or bigleaf maple) and incense- component of

2 30 per ha of cedar, singly conifers (e.g.,

trees >41 cm or in mixture Douglas-fii

diameter or white fir)

See foolnote on nexl page.

6 3
Table l-continued”

Douglas-fir

on western

hemlock sites

Stand (western hemlock,

characteristic Pacific silver fii)

Deep,

multilayered

Canopy

Snags Conifer snags

2 10 per ha

which are

>51 cm

diameter and

>4.6 m tall

Logs >34 metric Logs 222 Logs 2 22

tons Per acre metric tons per metric tons per

including 10 acre including acre including

pieces Per 5 pieces per 5 pieces per

ha >61 cm acre >61 cm acre > 61 cm

diameter and diameter and diameter and

>15 m long >15 m long >15 m long

Douglas-fir Douglas-fir

on mixed- on mixedconifer sites evergreen

(white fir, sites (tanoak,

Douglas-fir Douglas-fir)

Multilayered

Canopy

Douglas-fir

emergent above

evergreen

hardwood

Canopy

Conifer snags

2 3.7 Per

ha that

are >51 cm

diameter and

>4.6 m tall

Conifer snags

2 3.7 per

acre that

are>51 cm

diameter and

>4.6 m tall

’ From Old-Growth Definition Task Group (1986). Plant series are shown

in parentheses.

The selection of attributes used in the interim definition generally appear appropriate; that is, they are important features

of old-growth forest (Spies and others 1986, Spies and others

1989) and several discriminate old-growth from younger forest stages (Spies and Franklin, this volume). Analyses (Spies,

this volume; Spies and Franklin, this volume) have identified

other features that distinguish old-growth forests, such as

coverage of western yew, coverage of herb and deciduous

shrub species, and measures of crown decadence in overstory

trees.

The parameter values chosen in the interim definition were

intended as “minimum standards,” and they do fall well

below average or characteristic values for measured stands.

The values were, in fact, selected as a “lowest common

denominator,” with the intent that most old-growth stands

would meet the criteria and most young or mature forests

would not. As an example, the 85 old-growth stands sampled

by Spies and his associates average 66 tons per ha of logs

(Spies and others 1988), and the minimum standards call for

only 35 tons per ha on western hemlock sites (Old-Growth

Definition Task Group 1986). Similarly, large snags averaged about 12 per ha in the old-growth samples versus the

task group requirement for 10 per ha.

Table 2-Performance of the interim definition for Western

Hemloc kSeries

Province

Southern

Oregon Oregon Washington

Parameter and coast Cascade Cascade

minimum standard Ranges Range Range All

Douglas-fir > 80 cm in

d.b.h. (> 20 Per ha) 96 94 77 89

Shade associates > 40 cm

in d.b.h. (2 30 Per ha) 72 76 97 83

Snags > 50 cm in d.b.h.

>5 m tall(>10 per ha) 68 74 92 79

Log biomass (34 tons

per ha) 92 88 97 92

All criteria 52 56 70 60

’ Performance is the percentage of old-growth stands fulfilling individual

and collective minimal standards based on stand means + standard

deviation. 15 percent of mature stands also fulfilled all criteria.

’ From Old-Growth Definition Task Group (1986).

The interim definition does not perform as well as expected

in distinguishing this set of old-growth stands, however,

even though parameter values are well below mean measured

values for old-growth stands (table 2). For example, only

70 percent of the 24 old-growth (>200-year-old) stands

sampled by Spies and his associates in the Washington Cascade Range met all minimum criteria, even when 1 standard

deviation around the mean stand value is allowed. Percentages were even lower for the Oregon Coast (52 percent) and

Cascade (56 percent) Ranges (table 2). Furthermore, the

interim definition did not completely exclude mature and

young stands; discrimination was poorest in the Coast Range,

where 30 percent of the mature (80- to 199-year-old) stands

also met the minimum standards for old growth.

The mediocre performance of the interim definition results

from at least two factors. First, the statistical distribution of

measured stand attributes, such as number of large snags, are

skewed so that the median values lie well below the means.

Hence, many stands fail to fulfill minimum values even

though average values for old-growth stands indicate that a

higher percentage should meet them. Second, a higher percentage of stands fulfill individual criteria than can fulfill all

of the criteria (table 2). For example, old-growth stands measured in the southern Washington Cascade Range meet the

four criteria 77, 92, 97, and 97 percent of the time, but only

70 percent of the stands meet all four criteria simultaneously.

Revising the Interim Definition

Several approaches are possible for modifying the interim

definition: adjust the values in the interim definition so that it

does a better job of either characterizing or distinguishing

64
old-growth Douglas-fir stands, developing more site-specific

versions of the definition, such as for individual habitat types

or restricted geographic regions; or a combination of the two

approaches.

Fifteen iterations of a revised interim definition for oldgrowth Douglas-fir forests on western hemlock sites were

developed by using different variables and parameter values

as minimum standards. The selection of variables and range

of values were based on conditions encountered in the old

(>200-year-old) stands sampled by Spies and his associates.

Four different large-tree variables were tried: Douglas-firs

>80 cm in d.b.h. and >100 cm in d.b.h. and density of all

trees >80 cm in d.b.h. and >100 cm in d.b.h. The range of

values used in the 15 definitions are

Douglas-fir >80 cm in d.b.h. 10-20 per ha

Douglas-fir >lOO cm in d.b.h. l10-20 per ha

All trees >80 cm in d.b.h. 20 per ha

All trees >lOO cm in d.b.h. 10-15 per ha

Shade-tolerant trees >40 cm in d.b.h. 10-30 per ha

Snags >50 cm in d.b.h. and >5 m tall 4-10 per ha

Log biomass 30-34 tons per ha

The definitions were tested by determining the proportion of

stands in the old-growth data set that they correctly identified

based on having values within 1 standard deviation of the b

mean stand value. Generally, definitions were judged by their

ability to identify old-growth stands (minimize Type I error),

although their ability to exclude mature and young stands

(minimize Type II error) were also considered.

The best revised definition correctly identifies 82 percent of

the old-growth (>200-year-old) stands sampled by Spies and

his associates (table 3). This definition uses density of large

trees of all species rather than just Douglas-fir. It also uses

much lower parameter values than those used in the original

definition: all trees >100 cm in d.b.h. 10 per ha; shadetolerant trees >40 cm in d.b.h., 10 per ha; large snags, 4 per

ha; and log biomass, 30 tons per ha. These substantially

lower values relax the criteria to the point where 24 percent

of the mature stands also meet the old-growth criteria. A

definition that uses a value of 15 per ha for all trees >lOO cm

in d.b.h. performs almost as well in identifying old growth

but does much better at discriminating against mature stands.

Old-growth stands in the Oregon Coast Ranges were the most

difficult to distinguish correctly under any of the definitions,

including the original.

Among individual attributes, density of large trees best differentiates old growth from young and mature stands of natural

origin; coarse woody debris (snags and logs) does poorest.

This is true of all versions of the definition and reflects the

substantial similarities between natural stands of all ageclasses. It is also consistent with the finding of Spies and

Table 3-Performancea of the “best-performing” revision of the

interim definition of old growth for the Western Hemlock Series

Province

Southern 

Oregon Oregon Washington

Parameter and Coast Cascade Cascade 

minimum standard Ranges Range Range All

All species > 100 cm in

d.b.h. (> 10 per ha) 96 96 92 95

Shade associates > 40 cm in

d.b.h. (> 10 per ha) 88 88 100 92

Snags > 50 cm in d.b.h. 84 94 100 94’

>5 m tall (>4 per ha)

Log biomass (30 tons per ha) 92 92 100 95

All criteria 76 80 92 82’

’ Performance is the percentage of old-growth stands fulfilling individual

and collective minimal standards based on stand means +1 standard deviation. 24 percent of mature stands also fulfilled all criteria.

* From Old-Growth Definition Task Group (1986).

Franklin (this volume) that the best classification success in

discriminant analyses is achieved using an equation based on

overstory attributes.

The revised definition continues to focus on minimum standards, with the objective of improving its ability to distinguish

old from young and mature stands. An alternative approach

would be to use mean values and associated variances for

characterizing old-growth stands. By replacing the minimum

standards or values with mean or median values, the definition would provide a much better perspective on what an

average old-growth stand is like. Obviously, such a definition

would do much more poorly than one based on minimum

standards at distinguishing old-growth stands from other

stages in forest succession. It would decrease the probability

of “poor quality” stands-stands with few old-growth structural features-being included in an old-growth pool.

Substantial efforts to further refine one general definition of

old-growth forests for the Western Hemlock Series is probably not warranted at this time. First, appropriate data sets

are not available for old-growth stands over large geographic

areas-for example, old-growth Douglas-fir forests on

Western Hemlock Series sites in the central and northern

Washington Cascade Range, Olympic Mountains, British

Columbia, and the northern Oregon Coast Ranges. Data sets

are very limited for old-growth Douglas-fir on other series or

habitat types in southwestern Oregon or in eastern Washington and Oregon. Even within geographic areas that have

been sampled, sampling has avoided old-growth stands of

species other than Douglas-fir, such as western redcedar or

western hemlock. Further development of general definitions

seem pointless until such data are available.

65
A second and perhaps more important reason for suspending

efforts at further refinement of a general definition is the

large variability in old-growth stands apparent from the existing data sets. We infer from this high variability that regional

variations in environment and disturbances (stand ages and

disturbance histories) are very important (Spies and Franklin,

this volume). As data are added from additional geographic

areas, the range of variability known for old-growth forests

can only be expected to increase. A definition that attempts

to encompass these increasing amounts of variability becomes less and less valuable as a tool for identifying and

characterizing old-growth stands in specific locales.

Further development of old-growth definitions should probably be directed toward developing more site-specific definitions, such as for specific habitat types, geographic locales,

or both.

We believe that all classificatory work, whether as discrete

types or along gradients, should be locally fine-tuned for

maximum value. An example might be to define old-growth

on the Western Hemlock/Swordfern and closely related habitat types in the northern Oregon Cascade Range. The Area

Ecology program of the Forest Service’s Pacific Northwest

Region (6) provides an excellent opportunity to carry out

locale-specific efforts of this type-that is, quantifying more

specifically the characteristics of old growth with much more

intensive sampling.

The interim Douglas-fir definition can be effectively extended to other forest types. Because the definition was. by

direction, for old-growth Douglas-fir and not for old-growth

conifer forests in general, Douglas-fir had to be a component

of all of the stands, but most other temperate and subalpine

old-growth stands in the Douglas-fir region follow a similar

pattern in stand structural attributes. As suggested in the interim definition, The Douglas-fir old-growth criteria in table 1

can be modified for use with other species or types. The

major change is replacement of Douglas-fir as the dominant

species...Sitka spruce or western redcedar can be substituted...

[and] In old-growth western hemlock or Pacific silver fir

forests, these species would dominate all size classes including the [larger trees]. (Old-Growth Definition Task Group

1986). The revised definition, which uses density of large

trees of all species rather than just Douglas-fir, is already

better adapted for use in these other forest types than was the

original interim definition. Obviously, data would have to

be gathered to provide parameter values for these forest types

because very little currently exists; such stands were excluded

from the Old-Growth Forest Wildlife Habitat Program.

Characterizations by Using Continuous Scales

The problem with these categorical approaches to old-growth

characterization is that they attempt to pigeon-hole oldgrowth forests, which exhibit continuous temporal and

spatial variability. Systems of characterization based on continuous scales of variability avoid this problem and allow us

to recognize varying degrees of old-growthness. They also

mesh well with the approach to old-growth inventory recently

suggested by a multi-interest consensus group facilitated by

the Pacific Northwest Region of the Forest Service; this

group agreed that stands should be characterized in terms of

structural attributes rather than simply identified as falling in

or out of a specific old-growth definition (Marcot and others,

this volume). Alternative approaches to old-growth identification include developing indices based on multiple characteristics and discriminant analysis. Our explorations of indices follow; results of discriminant analysis are mainly

reported elsewhere (Spies and Franklin, this volume).

Indices to successional stage are probably best comprised of

characteristics that are ecologically important, such as providing wildlife habitat, and functional features of the ecosystem as well as those strongly related to forest age. Many

stand-structure attributes exhibit one of two typical patterns

of change (table 4, fig. 1): a U-shaped curve, such as is typical of the amount of coarse woody debris with highest

amounts early and late in succession; and an S-shaped curve

rising to an asymptote from low values early in succession,

which is typical of average tree size. As discussed by Spies

and Franklin (1988), these two general patterns of development are useful in constructing an old-growth index by

simply summing absolute or transformed values of characteristics, so that the highest index numbers occur in the later

stages of stand development (fig. 2). We illustrated this

scheme with three stands (Spies and Franklin 1988): a stand

with high densities of large trees and large amounts of coarse

woody debris has an index value of more than 0.75; a young

stand with high amounts of woody debris inherited from the

previous old stand is indexed at 0.5; and a young stand lacking both woody debris and large trees has an index value

below 0.25.

We constructed and tested four versions of a more complex

structural index by using both pooled and province-specific

data sets. Attributes used in this index are amount of crown

decadence of Douglas-fir (broken tops, dead tops, broken

tops with upturned leaders, and multiple tops); density of

large trees defined in the four versions as Douglas-firs >80

cm in d.b.h., Douglas-firs >100 cm in d.b.h., all trees >80 cm

in d.b.h., and all trees > 100 cm in d.b.h. density of shadetolerant associates either >25 cm or >40 cm in d.b.h.; density

of large snags; and log biomass, using 60 tons per ha as the

base value. Two versions of each index were constructed and

tested for each province: one based on mean values for that

province and another using means for the pooled (multiple

province) data set.

66
Table 4-ClaSification” of ecosystem characteristics according 

to their expected pattern of change during succession in 

Douglas-fir forests 

Characteristics following Characteristics following an 

a U-shaped curve (curve 1) S-shaped curve (curve 2) 

Amount of coarse woody debris 

Number of large snags 

Coarse woody debris as percentage 

of total ecosystem biomass 

Hetemgeneity of understory 

Plant species diversity 

Mammal diversity 

o From Spies and Franklin 1988 

Average tree size 

Diversity of tree sizes 

Incidence of broken top 

Forest floor depth 

Surface ana of boles and 

branches 

Wood biomass 

Stand age 

This set of five structural attributes works well at indexing 

forest stands along an age gradient in most of its forms (see, 

for example, figs. 3,4). One version performs particularly 

well with the complete set of stands, as well as the stands in 

both the Oregon and southern Washington Cascade Range 

(figs. 3, 4). This version uses density of all trees >80 cm in 

d.b.h. and of shade-tolerant trees >40 cm in d.b.h., along 

with the measures of Douglas-fir crown decadence, largesnag density, and log biomass. Other versions also work well, 

however-particularly in specific provinces. We infer from 

the variability in performance among versions that fine-tuning 

indices for more limited geographic areas is needed, just as 

with the definition. The high degree of overlap between 

natural young, mature, and old stands using these indices 

(fig. 3,4), is important to note, however. 
The best approach to accurately identifying old-growth 

stands and assigning them to a successional stage appears to 

be discriminant analysis (Spies and Franklin, this volume). 

Although this procedure is not actually indexing, it is based 

on multiple structural attributes (as we have applied it) and 

deals statistically with the wide variability in old-growth 

forests. Numerous equations have been developed and tested 

by using various combinations of attributes as repotted 

elsewhere (Spies and Franklin, this volume). The equation 

that has the best classification success (94 percent of the 

old-growth stands accurately identified) uses an overstory 

attribute set: standard deviation d.b.h., mean tree diameter, 

density of Douglas-firs >lOO cm in d.b.h., and total tree 

density. 

Conclusions 

The interim definition has worked well in identifying the 

essential characteristics of old-growth Douglas-fir forests. It 

has provided a basis for considering issues of inventory and 

allocation. Perhaps our current understanding of old-growth 

forests is now sufficient to allow interested parties to discuss 

the nature of this highly variable phenomenon objectively. 

The merit and utility of using multiple ecological attributes 

and of the specific variables selected in the interim definition 

has been demonstrated. A version with different parameters 

of the definition is proposed based on existing data sets, but 

generic definitions will always be less useful than those 

developed for a more limited geographic area. Hence, further 

improvements in the definition should be its refinement for 

specific habitat types and geographic locales and extension 

to other forest types. 

Indices and diwiminant analyses are superior to general 

definitions in distinguishing old-growth Douglas-fir forests 

where that is the objective. Such approaches more easily 

incorporate the wide variability in old-growth forests in time 

and space and fit well with current concepts of forest inventory based on ecological attributes. Indexing allows recognition of various degrees of “old-growthness,” as well as consideration of variability in individual attributes, such as snags 

or large trees, that may be important in management. 

All three approachesdeflmition, indexing, and discriminant 

analysishave merit and specific applications in considering 

policy, management, and research issues associated with 

old-growth Douglas-fir forests. 

We hypothesize, based on the high structural variability in 

old-growth Douglas-fx forests, multiple developmental-or 

successional routes to old-growth-like forests. If this hypothesis proves correct, it could have important management implications. Specifically, stands appear to differ widely in density 

and other structural features as a consequence of varied histories. Hence, developing multi-aged and multi-sized stands 

through partial cutting may be one alternative to long rotations in re-creating stands that resemble existing old-growth 

forests functionally and sbuchxally. These stands could be 

created by retaining selected green trees, as well as large 

snags and logs, at the end of each harvest cycle. 

Much additional research on old-growth forests is urgently 

needed. Although millions of research dollars have been 

directed to northern spotted owls and other vertebrates (as 

demonstrated by this volume), almost no funds are being 

expended on studies of old-growth forests as ecosystems. 

This lack is true for Douglas-fir forests and even more so for 

other coniferous forests on both sides and at the summit of 

the Cascade Range. 

68 
Our failure to study old-growth forests as ecosystems is increasingly serious in considerations of old-growth issues.

Without adequate basic knowledge of the ecosystem, we

risk losing track of its totality in our preoccupation with

individual attributes or species. Definitional approaches to

old growth based on attributes, including those that we have

presented here, predispose us to such myopia. The values

and services represented by old-growth ecosystems will he

placed at ever greater risk if we perpetuate our current ignorance about these ecosystems. It will also increase doubts

about our ability to manage for either old-growth ecosystems

or individual attributes (for example, species and structures)

associated with old growth. We must increase ecosystem

understanding and management emphasis on holistic perspectives as we plan for replacement of old-growth forests. How

can we presume to maintain or re-create what we do not

understand? Some may presume that ignorance (on ecological

values of old growth) is bliss, but this attitude creates high

risk that we will continue to be blindsided by subsequent

discoveries. 

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