Table l-continued

Common name 1984 1985 Total

Rodentia

Forest deer mouseb

539 375 914

Unidentified deer mouse 96 60 156

Southern red-backed vole 737 775 1512

Long-tailed vole 0 4

Creeping vole 2; 30 59

Water vole 1 6 7

Unidentified vole

Pacific jumping mouse 3;

7 8

51 90

Carnivora

Ermine 5 13 18

All species totals 3339 3412 6751

l Captured more frequently in 1985 than in 1984.

Captured more frequently in 1984 than in 1985.

Table 2Small mammals captured in snap traps

during 1984 on 8 clearcut areas in the southern

Washington Cascade Range

Common name Total

fnsectivora:

Vagrant shrew 2

Montane shrew 34

Marsh shrew 1

Trowbridge’s shrew 7

Unidentified shrew 3

Shrew-mole 2

Rodentia:

Yellow-pine chipmunk 19

Townsend’s chipmunk 15

Cascade golden-mantled ground squirrel 2

Northern pocket gopher 4

Deer mouse 61

Forest deer mouse 36

Unidentified deer mouse 64

Southern red-backed vole 6

Long-tailed vole 3

Heather vole 6

Creeping vole 12

Unidentified vole 1

Pacific jumping mouse 10

All species total 288

Results

A total of 7084 individuals of 23 species were caught over

2 on the 54 sites. Of years this total, 333 individuals of 17

species were caught on eight clearcut sites that were sampled

only in 1984 (table 2), and on one old-growth moderate site

Table 3Small mammals captured with snap and

pitfall traps on an old-growth moderate site in 1985

in the southern Washington Cascade Range (this

site was not sampled in 1984)

Common name Snap traps Pitfall traps

Montane shrew 6 10

Trowbridge’s shrew 0 1

Vagrant shrew 1 2

Coast mole 0 1

Deer mouse 5 6

Forest deer mouse 6 7

Totals 18 27

sampled in 1985 (table 3). The remaining 6751 captures of

20 species, made on 45 sites sampled in both years, constitute

the focus of this paper (table 1). A consistent feature of these

trapping returns is that they are numerically dominated by

four species (table 1). Trowbridge’s shrew was the most

common species (1946 captures), followed in abundance by

the southern red-backed vole (1512 captures), the montane

shrew (13 11 captures), and the forest deer mouse (9 14

captures). Insectivorous mammals (genera Sorex, Neurotrichus, and Scupanus) and rodents were roughly equal in the

sample, with 3670 insectivores to 3061 rodents (table 1).

Identification of all individuals to species is incomplete; 38

Sorex, one chipmunk, and eight microtine rodents, could not

be reliably identified (table 1). The largest group of unidentified animals are deer mice (156 young individuals, table 1).

Resolution of this problem awaits discovery of a reliable

morphological means of distinguishing juvenile and subadult forest deer mice from deer mice (Bangs 1898). Recent

work on this problem (Allard and others 1987, Gunn and

Greenbaum 1986) was conducted on adult animals, and as

the discriminating variables all depend on growth allometries,

they do not help with young animals. Adult deer mice were

assigned to species based on the tail-length criterion of

Allard and others (1987).

If captures for 1984 and 1985 are combined, at least 40 individuals were caught for 11 of the 20 species (fig, 1 and

table 1). The remaining nine species, although discussed

below with respect to their general habitat preferences, are

not considered further here. When compared over all 45 sites,

4 of the 11 species were caught with different frequency

between years (t-test, P < 0.05). The vagrant shrew and the

montane shrew were caught more frequently in 1985 than in

1984, but Trowbridge’s shrew and the forest deer mouse were

captured more frequently in 1984 than in 1985 (table 1).
MM” TRSH S”MD TOW OEM0 FDMO SRBY CR”0 PlMO The Gequency of capture differed between years for four speSpecies ties in the 36 sites comprising the chronosequence. The 

Species Response to Gradients of Forest Age and Moisture 

7 

Forest-age gradient--Only deer mice showed statistically 

0 Young significant differences between age-classes. The deer mouse 

q Matlm was more abundant in old-growth than in young forest, and 

. Old the forest deer mouse was more abundant in both mature and 

old-growth forest than in young forest (table 4). TWO species, 

the Townsend’s chipmunk and the Pacific jumping mouse, 

were caught in higher numbers in old forest than in young 

forest, although the differences were not statistically significant (table 4). 

vagrant shrew, the montane shrew, and the deer mouse wcrc 

caught more frequently in 1985 than in 1984. Trowbridge’s 

shrew was captured more frequently in 1984 than in 1985. 

Change in capture frequency between years was apportioned 

similarly across age-classes for the montane shrew, Trowbridge’s shrew, and the deer mouse, but the vagrant shrew 

was captured much more often in mature and old-growth 

sites in 1985 than in 1984 (table 4). 

Table 4-Means, standard deviations, and frequency of occurrence (percent) oismall mammal 

captures (number per 100 tiap-nights) in snap (July and August) and pitfall traps (October and 

early November) on 36 forested sites representing an age gradient in the southern Washington 

Cascade Range. (where ANOVA is significant, ditferences between means are indicated with 

different letters (A, B); c1 = 0.05) 

h 

i 

213 
Table &-continued 

Common name 

MWll 

S.D. 

% 

Rodents 

Townsend’s chipmunk 

MCXI 

S.D. 

% 

% 

Forest deer mouse 

MMem 

S.D. 

% 

Southern red-backed 

V& 

MeUl 

S.D. 

% 

Rodents 

creeping vole 

MWl 

S.D. 

% 

Pacific jumping mouse 

MeWI 

S.D. 

% 

Total mammals 

MWI 

S.D. 

Species richness 

MWI 

S.D. 
More total mammals were caught in mature. and old-growth 

forests than in young forests (table 4). Although not statistitally significant, the trend was toward more species in older 

forest age-classes (table 4, P = 0.07). 

caught in greater numbers on wet old-growth than on moderate sites, and showed a trend of higher captures on wet than 

dry sites (P = 0.07). The southern red-backed vole was caught 

more frequently in dry than moderate or wet old-growth forests (table 5). although this pattern is complicated by an 

Forest moisture gradient-Within the 27 old-growth forest 

sites, only the marsh shrew had an unambiguous response to 

the moisture gradient (table 5, fig. 2). The marsh shrew was 

interaction with the high elevation of dry sites as discussed 

below. 

Table S-Means, standard deviations, and frequency of occurrence (percent) of small mammal 

captures (number per 100 trap-nights) in snap (July and August) and pitfall traps (October and 

early November) on 21 forested sites representing 8 moishrre gradient within old-growth forest 

in the southern Washington Cascade Range (where ANOVA is significant, differences between 

means arc indicated with different letters; a = 0.05) 

1984 1985 

Insectivores 

vagrant shrew 

Mean 

S.D. 

% 

MWtl 

SD. 

% 

Marsh shrew 

MeSXl 

S.D. 

% 

Trowbridge’s shrewb 

MCXJI 

S.D. 

% 

Insectivores 

Shrew-mole 

MC.Wl 

SD. 

% 

Rodents 

Townsend’s chipmunk 

MeWI 

275 
Tablc S-continued 

1984 1985 

Gammon name 

Forest deer ,,,oux? 

MWI 

S.D. 

% 

Southern red-backed 

Vole 

MMl 

S.D. 

% 

Rodents 

creeping vole 

Me%8 

S.D. 

% 

Pacific jumping mouse 

MCUl 

S.D. 

90 

Total mammals 

MUII 

S.D. 

Species richness 

Mea 

S.D. 

The pattern of capture across moisture-classes was different 

between years for both deer mice species. Because of the sig1 nificant interaction of moisture-class by year, each yea was 

analyzed separately. Captures of the deer mouse in 1984 were 

not significantly different across moisture-classes, but deer 

mice were infrequently captured in wet (relative to moderate), 

old-growth forest in 1985 (table 5, P = 0.02), and tended to 

be captured in dry rather than wet old-growth forest as well 

(P = 0.08). In parallel fashion, captures of the forest deer 

mouse were not significantly different in 1984, but forest 

deer mice tended to be caught in wet relative to moderate 

old-growth forest (table 5, P = 0.06). 

The frequency of capture differed between years for three 

species in the 27 sites representing the moisture gradient 

(table 5). The montane shrew was caught more frequently in 

1985 than in 1984, but Trowbridge’s shrew and the forest 

deer mouse were captured more often in 1984 than in 198.5. 

As was true across forest age-classes, changes in capture 
Table 6-Multiple correlations between individual habitat variables and small mammal abuvdance and 

species richness for the southern Washington Cascade Raye (statistically significant regresston coefficients 

(I’< 0.05) are given for each variable; adjusted multiple R is listed at bottom of table) 

Variable 

BSNGSMT 

MCCTREE 

TREPIT 

MSHRUB 

CSNGL 

LCONIF 

LOGC 

ROCK 

FERN 

LOGAB 

MCDTREE 

WATER 

SlOPE 

ASTUMP 

BCSTUMP 

LICH 

SOIL 

BSNGL 

R* 

i 

Deer mouse 

Forest 

deer mouse 

SOUth~ 

red-backed 

Vole 

1984 198 

0.05 

-.06 

0.0 

.l 

-.C 

.C 

.30 .I 

frequency between years was apportioned similarly across 

moisture-classes for montane and Tmwbridge’s shrews 

(table 5). Tbe forest deer mouse, however, tended to be captured less frequently only in wet old growth. 

Environmental Correlates With Mammalian Abundance 

Elevation-Site elevation was not strongly correlated with 

the abundances of most species. Only three species showed 

a significant association with elevation, and of these, only 

the southern red-backed vole showed a consistent relationship between years [Trowbridge’s shrew in 1984 only, 

I = a.417 (P < 0.01); the shrew-mole in 1984 only, 

r = 0.417 (P < 0.01); the southern red-backed vole in 1984, 

r = 0.533 (P < 0.001) and in 1985, r = 0.527 (P < O.OOl)]. 

Vegetative and phyaiographic variablesThree mammalian species and two community variables were significantly 

correlated with sets of vegetative and physiographic variables 

(table 6). Of the 18 vegetative and physiographic variables 

correlated with mammalian abundance, only six were 

correlated in more than one instance. 

decay class (BSNGSMT) and the percentage cover of midcanopy by coniferous trees (MCCTREE) were negatively 

correlated with deer mouse abundance, but deer mouse abundance in 1985 was positively correlated with the number of 

treepits QREPIT, or holes and root tangles associated with 

fallen trees), the percentage cover of mid-canopy shrubs 

(MSHRUB), and the number of large coniferous trees. Deer 

moose abundance in 1985 was negatively correlated with the 

number of large, well-decayed snags (CSNGL). Correlations 

between the forest deer mouse and vegetative and physiographic variables were consistent between years, with positive 

associations for the number of large coniferous trees 

(LCONIF) and the percentage cover of well-decayed logs 

@XC). In general, the weak nature of the correlations 

underscores the fact that although more abundant in older 

forests, both species also were found in younger forests. 

Although the abundance of the deer mouse was significantly 

correlated with vegetative and physiographic variables in each 

year, the variables were different between years (table 6). 

In 1984, the number of medium tall snags of intermediate 

The pattern of correlation for the southern red-backed vole 

was different in that some associations held between years 

but others did not. Vole abundance in 1984 was positively 

correlated with the percentage. cover of rock (ROCK) and 

the presence of water (WATER), but it was negatively correlated with the percentage cover of ferns (FERN), logs of 

early decay-classes (LGGAB), and mid-canopy deciduous 

tress (MCDTREE). In 1985, the correlations for WATER 

and MCDTREE remained, but new positive correlations were 

observed for the percentage cover of mid-canopy shrubs 

- 

271 
(MSHFWEQ, the number of large coniferous trees (LCONIF), 

and slope of the site (SLOPE), and a new negative correlation was seen for the percentage cover of well-decayed logs 

(LOGC). 

The total number of mammals captured (TMAM) and species richness (SPRICH) were not consistently correlated 

between years with individual vegetative and physiographic 

variables, although the multiple. correlations were significant 

in each year (table 6). 

Community Composition on Gradients of Forest Age and 

Moisture 

The composition of mammalian communities based on presence or absence of species was not clearly related to the 

forest-age gradient. The hierarchical clustering of sites using 

Jaccard’s coefficient of similarity on the age gradient (36 

sites) produced clusters of sites with similar mammalian composition along the left side of the dendrogmm in figure 3. 

Increasingly dissimilar sites (in terms of mammalian composition) were joined to the right side of the dendrogram. If 

forest age was a primary influence on mammalian composition, major clusters would be expected to reflect the age gradient In other words, different clusters should be composed 

of sites in different age-classes, which was not observed. 

Each of the five major clusters (cluster 1 = sites I-6; cluster 

2 = sites 7-14; cluster 3 = sites 15-20; cluster 4 = sites 21. 

26; cluster 5 = sites 27-36) contain sites from all three ageclasses. Inspection of the dendrogram at higher levels of 

similarity yields some smaller clusters of similar age-class 

(for example, sites 9-13 and sites 27-31). but few clusters of 

similar age were found. K-means clustering (specifying three 

clusters) by the abundance of the most frequently captured 

species (11 species) showed the same lack of association 

between clusters and site age. The first cluster consisted of 

eight young, four mature, and nine old-growth sites; the 

second cluster of four mature and six old-growth sites; and 

the thiid cluster of one young, one mature, and three oldgrowth sites. 

Hierarchical clustering on the moisture gradient (27 sites) 

showed little correspondence between old-growth forest 

moisture-classes (dry, moderate, and wet) and the clusters of 

sites with similar mammalian species composition (fig. 4). 

No clusters consisted of just one moisture-class, although 

some clusters were composed of two classes, either moderate 

and wet (sites 15-21) or moderate and dry (sites 22-27). The 

K-means clustering procedure suggested that moisture status 

exerts somewhat more influence on community composition. 

Specifying three clusters, the fmt cluster was composed of 

two dry, eight moderate, and eight wet sites; the second 

cluster of five dry, two moderate, and one wet site; and the 

third cluster of just one moderate site. The grouping of five 

of the seven dry sites in the second cluster might indicate a 

tendency for similar mammalian communities on drier sitea. 

Sites in the second cluster supported rather high populations 

of red-backed voles. This relationship is clouded by the fact 

that several dry sites were at high elevation, and as discussed 

below, whether the primary influence is due to moisture or 

conditions related to high elevation is not clear. 

278 
I II 

Discussion 

Species Response to Gradients of Age and Moisture 

Forest age gradient-The relationships of deer mice to patterns of forest succession in Washington are being reevaluated in light of the recent elevation of the forest deer mouse 

to species states (Goon and Greenbaum 1986, Allard and 

others 1987). The forest deer mouse has mostly been considered a subspecies of the deer mouse (Osgood 1909, but 

see Liu 1954, Ingles 1965, and Sheppe 1961), and that view 

has enhanced the conception of the deer mouse as an extreme 

habitat generalist. A substantial part of the habitat thought to 

be occupied by the deer mouse was actually occupied by the 

forest deer mouse. As demonstrated in this study for the 

southern Washington Cascade Range, the forest deer mouse 

is the more abundant of the two species in forested habitat 

(tables 1,4; figs. 1 and 5). Research has just begun to describe the distributions and patterns of abundance of these 

species, and work is underway on the. taxonomic affinities of 

the forest deer mouse and other Peromyscus species in coastal 

British Colombia. The currently recognized range of the 

Species 

forest deer mouse may expand, a consequence of synonymizing some of the coastal forms with the forest deer mouse. 

Relative to the deer mouse, which has one of the widest geegraphic distributions of all species in North America and 

inhabits many different habitats, the forest deer mouse is a 

forest specialist. It is found in pre-canopy stages of forest 

succession (table 2, fig. 5), but is generally outnumbered by 

the deer mouse in such habitat. 

A positive relationship between abundance and forest age 

was an expected pattern for the forest deer mouse. As a forest specialist, the forest deer mouse would be expected to 

reach high abundance in the well-developed forests that, historically, were the prevailing forest condition in the Pacific 

Northwest. Observing the same pattern for the deer mouse, 

however, was a little surprising. Deer mice were also more 

abundant in old-growth than in matore or young forest on the 

western slopes of the central Oregon Cascades (Anthony and 

others 1987). and more abundant in riparian zones of oldgrowth forest than in mature- and sawtimber-classes (sites 

50-150 years) in northwestern California (Raphael 1988c). 

The deer mouse may be more abundant in old rather than 

young forest because of the diversification of ground vegetation with forest succession (Spies, this volume). Older forests 

may more closely resemble the diversity and abundance of 

herbaceous plants characteristic of pre-canopy successional 

stages than those of many closed-canopy young and mature 

forests. This resemblance is partially related to the uneven 

nature of old-forest canopies, which allow the growth of 

ground-layer plant species that depend on increased light. 

Deer mouse patterns of peak abundance in old forests, however, are only relevant to the stages of forest succession 

that follow canopy closure. As in other regions (Anthony 

and others 1987, Raphael 1988c), the deer mouse is much 

more abundant in habitats that occur before canopy closure 

(table 2, fig. 5). 

279 
Like the deer mouse, Townsend’s chipmunk and the Pacific

jumping mouse are rather common in habitats without closed

forest canopies (table 2, fig. 5), and are known to inhabit

meadows and edge environments (Dalquest 1948). Their tendency toward higher abundance in old-growth forest might

well be a response to the complex and patchy forest floor environment of old forests, which may be more similar to their

preferred habitats than ground conditions in young forests.

Anthony and others (1987) found these species in greater

numbers in riparian zones of young, rather than mature or

old-growth forests. This difference may have been due to the

younger ages (25-50 years) of sites in their “young-forest”

age-class compared to those in this study (55-75 years).

Given younger sites, the same pattern may hold in the

Washington Cascades.

Forest moisture gradient-The marsh shrew frequents wet

areas (Cowan and Guiguet 1965, Pattie 1973), but it is not as

tied to standing water as the water shrew. I have taken marsh

shrews at a considerable distance from water in quite dry,

second-growth, Douglas-fir forests. Nonetheless, the species

appears to be most abundant in wet forest, regardless of forest age. Anthony and others (1987) caught six shrews in

riparian zones of the western Oregon Cascade Range, three

individuals in old-growth, one in mature, and two in young

forests.

At first inspection, the southern red-backed vole appears

strongly associated with dry old-growth forest (table 5). Although these data indicate that the voles can do well in dry

old-growth forests, dry conditions per se have not clearly

been shown to be critical. Southern red-backed voles were

caught more often at trap sites near water (table 6), and at

high elevation sites, patterns that held in both years. Dry sites

as a group tended to be at higher elevations (mean = 838 m)

than wet (762 m) or moderate sites (754 m). More important,

no dry sites were lower than 689 m in contrast to moderate

and wet sites with two sites less than 500 m, four less than

600 m, and five less than 660 m. Captures of red-backed

voles in these low elevation sites were few, averaging just

2.5 animals per site in 1984 and 0.5 animals in 1985. As a

further demonstration of this interaction, the correlation of

vole abundance for both years combined with the elevation

of dry sites was quite high (r = 0.83; P = 0.02). The primary

relationship may be with the environmental conditions related to high elevation, rather than to moisture. Six of the

seven dry sites were located south and southeast of Mount

Rainier, a region that yielded the highest capture totals for

red-backed voles. In fact, nine of the ten sites with the highest

capture totals for red-backed voles were from this region;

these included four dry and three moderate old-growth sites,

and two mature forest sites. Even though the relationship of

red-backed vole abundance to dry forest appeared to be positive, it is clearly not simple; elevation and biogeographic

factors must also be considered.

Both deer mice species shared the interaction between forest

moisture-class and years, and followed the same patterns of

capture in each moisture-class and year (table 5). For each

species, the major difference in capture frequency across

moisture-classes was the small number of captures in wet

sites during 1985. With only 2 years of data, determining

which, if either, of the 2 years represents the “usual” response to moisture condition is impossible. Both species may

have responded to a rather widespread influence. Meteorological data from the southern (Wind River), central (Packwood),

and northern (Longmire) portions of the study area indicate that 1984 was generally cooler and wetter than 1985

(Manuwal, pers. comm.). Given consistent meteorological

differences between years, these may conceivably have influenced population growth differentially across moistureclasses; however, speculations on mechanisms must await

clarification of the general response to moisture condition.

Trends in Other Species

Nine species poorly represented in the capture totals were

not tested statistically for their numeric responses to forest

age and moisture gradients. Because species that occur in

low abundance are a special concern, noting that the small

number of captures for these species (table 1: water shrew,

coast mole, yellow-pine chipmunk, pika, northern flying

squirrel, northern pocket gopher, water vole, long-tailed vole,

and ermine) does not necessarily indicate regional scarcity is

important. Most species either reach maximum abundance in

adjacent habitats or were not effectively sampled by the techniques targeted for forest-floor mammals. The yellow-pine

chipmunk, the northern pocket gopher, the water vole, and

the pika are all found more commonly at higher elevations

than were sampled in this study (Dalquest 1948). The coast

mole is present in many habitats (Dalquest 1948; Hartman

and Yates 1985, Maser and others 1981) and is probably

most common in habitats without closed canopies at lower

elevations. It was not readily caught by pitfall and snap traps

and is no doubt more abundant in these forests than the capture returns would indicate. The coast mole can be sampled

more effectively with traps designed to capture moles, or by

indexing activity via mole-run counts (West, in press). The

ermine, although typically not an abundant species, is found

in a great variety of habitats (Cowan and Guiguet 1965,

Maser and others 1981).

Other species that were poorly represented in the capture

totals are of interest for different reasons. The northern flying

squirrel has received attention because of its importance as

prey for the spotted owl. As expected, pitfall or snap traps

did not adequately sample this species. Track plates were

used to index flying squirrel activity, and the resulting

frequency-counts were not positively associated with forest

age or moisture condition. Whether track frequencies are

highly correlated with squirrel abundance is not clear, however (Carey, pers. comm.). Recent work on the flying

280
squirrel by the Forestry Sciences Laboratory (USDA Forest

Service, Olympia) in both Oregon and the Olympic Peninsula

of Washington should help clarify patterns of squirrel abundance and their association with gradients of forest age or

moisture.

The habitat affinities of the long-tailed vole are poorly understood in Washington. This species is captured sporadically,

although it has been recorded from many different habitats

(Cowan and Guiguet 1965, Dalquest 1948, Maser and others

1981, Randall and Johnson 1979). It has been found most

commonly in forest-edge environments and brushy riparian

zones where grass cover is usually present. I collected three

individuals in clearcut areas (table 2), one in a young, one

in an old-growth wet, and two in old-growth dry forests

(table 1). Although it is not a species that is closely associated with old forests, it needs further ecological study.

The water shrew is a riparian specialist. The techniques used

in this study probably would be effective in sampling the

water shrew if they were concentrated along permanent

watercourses, but because the sampling did not focus on

riparian strips, captures of this species were few. The water

shrew is also more common at elevations higher than most

sites in this study (Dalquest 1948). Its requirements for water

are understood, but whether it is more abundant in riparian

zones in old rather than young forests is unclear. In this

study, two shrews were caught in young forests, four in oldgrowth moderate, and three in old-growth wet forests. In a

study focused on riparian habitat, Anthony and others (1987)

caught two shrews in old-growth, one in mature, and none in

young forest. All three individuals were caught at streamside

(about 1 m from water), rather than in adjacent riparian habitat (15-25 m from water).

Community Patterns in the Southern Washington

Cascade Range

The mid-elevation forests of the southern Washington Cascades are inhabited by a small mammal fauna that is broadly

adapted to naturally regenerated forests. Unique small

mammal communities were not seen in forests of different

age or in old-growth forests of different moisture condition.

With the exception of the deer mice, no species was significantly correlated with the forest-age gradient. Four common

species (Trowbridge’s shrew, southern red-backed vole, montane shrew, and forest deer mouse) and a few less common

species typically were found at a given site (table 4 and

fig. 1). This pattern resulted in a very similar ranking of species by abundance in each year. Clustering of similar sites by

small mammal presence or absence and by abundance of

common species yielded clusters that related poorly to the

age gradient (fig. 3). Similarly, only the marsh shrew was unambiguously related to the moisture gradient,, and clustering

of sites by abundance and by small mammal presence or

absence produced clusters not clearly related to the moisture

gradient (fig. 4). In concordance with these findings, correlations between small mammals and variable means describing the forest environment were generally weak. Statistically

significant correlations, especially those similar in both years,

were restricted to a few species which generally occurred

widely across both gradients (table 6). These patterns are essentially ones found in all three provinces (Oregon Cascades,

Oregon Coast Range, and southern Washington Cascades) in

this study (Aubry and others, this volume; Corn and others

1988). Aside from differences in species composition related to geographic distribution, small mammal communities of Douglas-fir forests in this region are structured

similarly.

Community composition did not vary appreciably with forest

age, but total mammalian abundance was higher in oldgrowth than in young forests (table 4). The higher abundance

partially resulted from the presence of more species in old

forest (table 4). Higher abundance may be linked to agerelated differences in the structure and productivity of the

ground environment, which is more diverse in old forests.

Whether the average number of species is typically greater in

old-growth forests is uncertain, as is the associated question

of whether variation in species number is related to the age

gradient. In this study, coefficients of variation (C.V.) for

species richness were similar between years on the age gradient (from table 4: C.V. 1984 = 21.9, 19.1, and 20.4 for young,

mature, and old-growth forests; C.V. 1985 = 26.5,21.4, and

22.1). Variation in species number might be expected to be

higher in young than in old-growth forest, as a reflection of

the higher proportion of dispersal sinks in young forest, and

perhaps as a function of the less-diverse and productive

resource-base at ground level and the less physically buffered

environment. Such a trend may exist, although slight, in the

above coefficients, but a thorough answer to these questions

requires long-term study. Of particular interest would be the

variation in species number with season in young versus old

forest. The initial design of this study, which was not fully

implemented, included a spring and fall sampling-period for

each site as a way of addressing this question. This study,

therefore, cannot characterize the magnitude of seasonal

changes. That snap-trapped animals were breeding is important to note; however, site occupancy was thus not simply

due to the capture of individuals dispersing from favored or

survival habitats (Anderson 1980). This spilling-over effect

that resulted from the movement of individuals from highdensity to low-density habitats (Fretwell 1972), or from

saturation dispersal (Lidicker 1975), would be more evident

later in the fall and early winter.

What does the apparent lack of strong community patterns

on gradients of forest-age and moisture mean? Is it really

true that the small mammals of this region are insensitive to

variation in forest structure? Not at all; noting that this

survey was done exclusively on naturally regenerated stands,

281
clearly-from this study, from work in Oregon (Corn and

Bury, this volume a; Gilbert and Allwine, this volume a), and

from work on the successional changes in the compositional

and structural features of these forests (Spies and Franklin,

this volume; Spies and others 1988~thresholds of critical

habitat-variables are met for most forest-dwelling species

soon after canopy closure.

In a naturally regenerating landscape, the major change in

the species composition of small mammals occurs before and

during canopy closure. At the transition from grass- and forbdominated plant communities to later stages dominated by

shrubs and young trees, small mammal communities characterized by the deer mouse, several species of meadow voles,

Townsend’s and yellow-pine chipmunks, the Pacific jumping

mouse, and shrews of open habitats (the vagrant shrew and

the montane shrew) begin to give way to communities of the

closed-canopy forest described in this study. Rather minor

changes in species composition and relative abundance

among species takes place in subsequent successional stages.

As a consequence of this pattern of mammalian species

replacement, we would expect poor correlations between

small mammal abundance and habitat elements in studies

that use a data set based on naturally regenerated forests, as

was observed.

If thresholds for limiting habitat-elements for small mammals

are usually met or exceeded in naturally regenerated forests,

this data set presents an interesting problem. In terms of information to guide management on forests regenerated after

timber harvest, this data set is not usable for identifying critical habitat-elements, or to assess their threshold values-a

straightforward consequence of the fact that the range of variation in these variables was insufficient to provide strong

correlations or to indicate threshold values. For such a purpose, knowing under what conditions a species is absent is

just as important as knowing the conditions under which it is

present. On the one hand, this study shows that most species

have a fair amount of adaptability in their use of forest habitats, but on the other, it tells us little about sufficient values

for habitat elements. If forests managed for timber production

are to retain most of their native small mammals, we need to

begin the process of identifying critical habitat-elements and

assessing their minimum threshold values. That second- and

third-rotation forests would resemble naturally regenerated

forests in a number of respects seems unlikely, and they certainly would exceed the range of variation in habitat variables sampled in this study. The required information, consequently, can only come from a data set based on stands that

are intensively managed for timber production, or an array of

experimental sites designed to mimic such conditions. To

this end, manipulative field experiments conducted on spatial

scales sufficiently large to avoid the problem of animal movement from adjacent edges probably hold the most promise.

Because of the need for large scale, such studies might best

be undertaken cooperatively between the scientific community and government agencies or private companies.

Acknowledgments

In the course of this study, over 60 people contributed to the

field work and data analyses. Field work required long hours

under hard conditions, and I am indebted to these people for

completing it. Diane Converse deserves special thanks for

her leadership and perseverence over the course of the study.

Funding was provided by the USDA Forest Service, Pacific

Northwest Research Station. I thank personnel of the Wind

River, Mount Adams, Packwood, and Randle Ranger

Districts of the Gifford Pinchot National Forest, the Wliite

River Ranger District of the Mount Baker-Snoqualmie

National Forest, and Mount Rainier National Park for their

advice and logistical help.

This paper is contribution 125 of the Wildlife Habitat Relationships in Western Washington and Oregon Research

Project, Pacific Northwest Research Station, USDA Forest

Service. 0

282
Appendix

Table 7-Physiographic and vegetative variables that

describe habitats in the southern Washington Cascade

Range

Variable Description

SLOPE

WATER

ROCK

GRASS

HERB

SOIL

MOSS

LICH

LITT

LITDEP

FERN

SHRUB

MSHRUB

MCCTREE

MCDTREE

CONIFSM

DECIDSM

LCONIF

LDECID

SCAN

TREPIT

ASTUMP

BCSTUMP

LGGAB

LOGC

ASNGSMT

BSNGSMT

CSNGSM

ASNGL

BSNGL

CSNGL

Percentage slope

Presence of permanent water

Percentage cover of rock

Percentage cover of grasses

Percentage cover of herbs

Percentage cover of mineral soil

Percentage cover of moss

Percentage cover of lichens

Percentage cover of fine litter (<10-cm diameter)

Litter depth (cm)

Percentage cover of ferns

Percentage cover of ground-layer shrubs

Percentage cover of mid-canopy shrubs

Percentage cover of canopy and mid-canopy

coniferous trees

Percentage cover of canopy and mid-canopy

deciduous trees

Number of small and medium-diameter coniferous

trees (l-50 cm)

Number of small and medium-diameter deciduous

trees (l-50 cm)

Number of large-diameter coniferous trees (>50 cm)

Number of large-diameter deciduous trees (>50 cm)

Presence of supercanopy trees

Number of tree-fall pits

Number of slightly decayed stumps

Number of moderately to well-decayed stumps

Number of slightly to moderately decayed logs,

d.b.h. > 10 cm

Number of well-decayed logs, d.b.h > 10 cm

Number of slightly decayed small and mediumdiameter (l-50 cm) short (<l.5 m), medium (5-15 m),

and tall (>I5 m) snags

Number of moderately small and medium-diameter

(l-50 cm) short (<l.5 m), medium (5-15 m), and tall

(>15 m) snags

Number of well-decayed small and medium-diameter

(l-50 cm) short (<l.5 m) and medium (5-15 m) snags

Number of slightly decayed large-diameter (>50 cm),

medium and tall snags

Number of moderately decayed large-diameter

(>50 cm). medium and tall snags

Number of well-decayed large-diameter (>50 cm),

medium and tall snags

283