Table l-continued

Common name 1984 1985 Total

RodentiaForest deer mouseb 539 375 914Unidentified deer mouse 96 60 156Southern red-backed vole 737 775 1512Long-tailed vole 0 4Creeping vole 2; 30 59Water vole 1 6 7Unidentified volePacific jumping mouse 3;

7 851 90

CarnivoraErmine 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 trapsduring 1984 on 8 clearcut areas in the southernWashington Cascade Range

Common name Total

fnsectivora:Vagrant shrew 2Montane shrew 34Marsh shrew 1Trowbridge’s shrew 7Unidentified shrew 3Shrew-mole 2

Rodentia:Yellow-pine chipmunk 19Townsend’s chipmunk 15Cascade golden-mantled ground squirrel 2Northern pocket gopher 4Deer mouse 61Forest deer mouse 36Unidentified deer mouse 64Southern red-backed vole 6Long-tailed vole 3Heather vole 6Creeping vole 12Unidentified vole 1Pacific jumping mouse 10

All species total 288

ResultsA total of 7084 individuals of 23 species were caught over2 on the 54 sites. Ofyears this total, 333 individuals of 17species were caught on eight clearcut sites that were sampledonly in 1984 (table 2), and on one old-growth moderate site

Table 3Small mammals captured with snap andpitfall traps on an old-growth moderate site in 1985in the southern Washington Cascade Range (thissite was not sampled in 1984)

Common name Snap traps Pitfall traps

Montane shrew 6 10Trowbridge’s shrew 0 1Vagrant shrew 1 2Coast mole 0 1Deer mouse 5 6Forest deer mouse 6 7

Totals 18 27

sampled in 1985 (table 3). The remaining 6751 captures of20 species, made on 45 sites sampled in both years, constitutethe focus of this paper (table 1). A consistent feature of thesetrapping returns is that they are numerically dominated byfour species (table 1). Trowbridge’s shrew was the mostcommon species (1946 captures), followed in abundance bythe southern red-backed vole (1512 captures), the montaneshrew (13 11 captures), and the forest deer mouse (9 14captures). Insectivorous mammals (genera Sorex, Neuro-trichus, and Scupanus) and rodents were roughly equal in thesample, with 3670 insectivores to 3061 rodents (table 1).

Identification of all individuals to species is incomplete; 38Sorex, one chipmunk, and eight microtine rodents, could notbe reliably identified (table 1). The largest group of unidenti-fied animals are deer mice (156 young individuals, table 1).Resolution of this problem awaits discovery of a reliablemorphological means of distinguishing juvenile and sub-adult forest deer mice from deer mice (Bangs 1898). Recentwork on this problem (Allard and others 1987, Gunn andGreenbaum 1986) was conducted on adult animals, and asthe discriminating variables all depend on growth allometries,they do not help with young animals. Adult deer mice wereassigned to species based on the tail-length criterion ofAllard and others (1987).

If captures for 1984 and 1985 are combined, at least 40 in-dividuals were caught for 11 of the 20 species (fig, 1 andtable 1). The remaining nine species, although discussedbelow with respect to their general habitat preferences, arenot considered further here. When compared over all 45 sites,4 of the 11 species were caught with different frequencybetween years (t-test, P < 0.05). The vagrant shrew and themontane shrew were caught more frequently in 1985 than in1984, but Trowbridge’s shrew and the forest deer mouse werecaptured 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 spe- Species 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 signifi- cant (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, Trow- bridge’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 statisti- tally 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 moder- ate 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 for- ests (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 sig-

1 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 cap- tured 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 relation- ship 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 mamma- lian 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 mid- canopy by coniferous trees (MCCTREE) were negatively correlated with deer mouse abundance, but deer mouse abun- dance 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 physiogra- phic 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 cor- related 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 correla- tion was seen for the percentage cover of well-decayed logs (LOGC).

The total number of mammals captured (TMAM) and spe- cies 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 pres- ence 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 com- position along the left side of the dendrogmm in figure 3. Increasingly dissimilar sites (in terms of mammalian compo- sition) were joined to the right side of the dendrogram. If forest age was a primary influence on mammalian composi- tion, major clusters would be expected to reflect the age gra- dient 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 age- classes. 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 old- growth 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 pat- terns of forest succession in Washington are being reevalu- ated 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 con- sidered 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 de- scribe 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 synonymiz- ing some of the coastal forms with the forest deer mouse. Relative to the deer mouse, which has one of the widest gee- graphic 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 for- est specialist, the forest deer mouse would be expected to reach high abundance in the well-developed forests that, his- torically, 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 old- growth 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 vegeta- tion 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, how- ever, 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 Pacificjumping mouse are rather common in habitats without closedforest canopies (table 2, fig. 5), and are known to inhabitmeadows and edge environments (Dalquest 1948). Their ten-dency toward higher abundance in old-growth forest mightwell be a response to the complex and patchy forest floor en-vironment of old forests, which may be more similar to theirpreferred habitats than ground conditions in young forests.Anthony and others (1987) found these species in greaternumbers in riparian zones of young, rather than mature orold-growth forests. This difference may have been due to theyounger 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 theWashington Cascades.

Forest moisture gradient-The marsh shrew frequents wetareas (Cowan and Guiguet 1965, Pattie 1973), but it is not astied to standing water as the water shrew. I have taken marshshrews at a considerable distance from water in quite dry,second-growth, Douglas-fir forests. Nonetheless, the speciesappears to be most abundant in wet forest, regardless of for-est age. Anthony and others (1987) caught six shrews inriparian zones of the western Oregon Cascade Range, threeindividuals in old-growth, one in mature, and two in youngforests.

At first inspection, the southern red-backed vole appearsstrongly associated with dry old-growth forest (table 5). Al-though these data indicate that the voles can do well in dryold-growth forests, dry conditions per se have not clearlybeen shown to be critical. Southern red-backed voles werecaught more often at trap sites near water (table 6), and athigh elevation sites, patterns that held in both years. Dry sitesas 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 moderateand wet sites with two sites less than 500 m, four less than600 m, and five less than 660 m. Captures of red-backedvoles in these low elevation sites were few, averaging just2.5 animals per site in 1984 and 0.5 animals in 1985. As afurther demonstration of this interaction, the correlation ofvole abundance for both years combined with the elevationof dry sites was quite high (r = 0.83; P = 0.02). The primaryrelationship may be with the environmental conditions re-lated to high elevation, rather than to moisture. Six of theseven dry sites were located south and southeast of MountRainier, a region that yielded the highest capture totals forred-backed voles. In fact, nine of the ten sites with the highestcapture 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 ofred-backed vole abundance to dry forest appeared to be posi-tive, it is clearly not simple; elevation and biogeographicfactors must also be considered.

Both deer mice species shared the interaction between forestmoisture-class and years, and followed the same patterns ofcapture in each moisture-class and year (table 5). For eachspecies, the major difference in capture frequency acrossmoisture-classes was the small number of captures in wetsites during 1985. With only 2 years of data, determiningwhich, if either, of the 2 years represents the “usual” re-sponse to moisture condition is impossible. Both species mayhave responded to a rather widespread influence. Meteorolog-ical data from the southern (Wind River), central (Packwood),and northern (Longmire) portions of the study area indi-cate that 1984 was generally cooler and wetter than 1985(Manuwal, pers. comm.). Given consistent meteorologicaldifferences between years, these may conceivably have in-fluenced population growth differentially across moisture-classes; however, speculations on mechanisms must awaitclarification of the general response to moisture condition.

Trends in Other Species

Nine species poorly represented in the capture totals werenot tested statistically for their numeric responses to forestage and moisture gradients. Because species that occur inlow abundance are a special concern, noting that the smallnumber of captures for these species (table 1: water shrew,coast mole, yellow-pine chipmunk, pika, northern flyingsquirrel, northern pocket gopher, water vole, long-tailed vole,and ermine) does not necessarily indicate regional scarcity isimportant. Most species either reach maximum abundance inadjacent habitats or were not effectively sampled by the tech-niques targeted for forest-floor mammals. The yellow-pinechipmunk, the northern pocket gopher, the water vole, andthe pika are all found more commonly at higher elevationsthan were sampled in this study (Dalquest 1948). The coastmole is present in many habitats (Dalquest 1948; Hartmanand Yates 1985, Maser and others 1981) and is probablymost common in habitats without closed canopies at lowerelevations. It was not readily caught by pitfall and snap trapsand is no doubt more abundant in these forests than the cap-ture returns would indicate. The coast mole can be sampledmore effectively with traps designed to capture moles, or byindexing activity via mole-run counts (West, in press). Theermine, although typically not an abundant species, is foundin a great variety of habitats (Cowan and Guiguet 1965,Maser and others 1981).

Other species that were poorly represented in the capturetotals are of interest for different reasons. The northern flyingsquirrel has received attention because of its importance asprey for the spotted owl. As expected, pitfall or snap trapsdid not adequately sample this species. Track plates wereused to index flying squirrel activity, and the resultingfrequency-counts were not positively associated with forestage or moisture condition. Whether track frequencies arehighly correlated with squirrel abundance is not clear, how-ever (Carey, pers. comm.). Recent work on the flying

280

squirrel by the Forestry Sciences Laboratory (USDA ForestService, Olympia) in both Oregon and the Olympic Peninsulaof Washington should help clarify patterns of squirrel abun-dance and their association with gradients of forest age ormoisture.

The habitat affinities of the long-tailed vole are poorly under-stood in Washington. This species is captured sporadically,although it has been recorded from many different habitats(Cowan and Guiguet 1965, Dalquest 1948, Maser and others1981, Randall and Johnson 1979). It has been found mostcommonly in forest-edge environments and brushy riparianzones where grass cover is usually present. I collected threeindividuals in clearcut areas (table 2), one in a young, onein an old-growth wet, and two in old-growth dry forests(table 1). Although it is not a species that is closely associ-ated with old forests, it needs further ecological study.

The water shrew is a riparian specialist. The techniques usedin this study probably would be effective in sampling thewater shrew if they were concentrated along permanentwatercourses, but because the sampling did not focus onriparian strips, captures of this species were few. The watershrew is also more common at elevations higher than mostsites in this study (Dalquest 1948). Its requirements for waterare understood, but whether it is more abundant in riparianzones in old rather than young forests is unclear. In thisstudy, two shrews were caught in young forests, four in old-growth moderate, and three in old-growth wet forests. In astudy focused on riparian habitat, Anthony and others (1987)caught two shrews in old-growth, one in mature, and none inyoung forest. All three individuals were caught at streamside(about 1 m from water), rather than in adjacent riparian habi-tat (15-25 m from water).

Community Patterns in the Southern WashingtonCascade Range

The mid-elevation forests of the southern Washington Cas-cades are inhabited by a small mammal fauna that is broadlyadapted to naturally regenerated forests. Unique smallmammal communities were not seen in forests of differentage or in old-growth forests of different moisture condition.With the exception of the deer mice, no species was signifi-cantly correlated with the forest-age gradient. Four commonspecies (Trowbridge’s shrew, southern red-backed vole, mon-tane shrew, and forest deer mouse) and a few less commonspecies typically were found at a given site (table 4 andfig. 1). This pattern resulted in a very similar ranking of spe-cies by abundance in each year. Clustering of similar sites bysmall mammal presence or absence and by abundance ofcommon species yielded clusters that related poorly to theage gradient (fig. 3). Similarly, only the marsh shrew was un-ambiguously related to the moisture gradient,, and clusteringof sites by abundance and by small mammal presence orabsence produced clusters not clearly related to the moisture

gradient (fig. 4). In concordance with these findings, corre-lations between small mammals and variable means describ-ing the forest environment were generally weak. Statisticallysignificant correlations, especially those similar in both years,were restricted to a few species which generally occurredwidely across both gradients (table 6). These patterns are es-sentially ones found in all three provinces (Oregon Cascades,Oregon Coast Range, and southern Washington Cascades) inthis study (Aubry and others, this volume; Corn and others1988). Aside from differences in species composi-tion related to geographic distribution, small mammal com-munities of Douglas-fir forests in this region are structuredsimilarly.

Community composition did not vary appreciably with forestage, but total mammalian abundance was higher in old-growth than in young forests (table 4). The higher abundancepartially resulted from the presence of more species in oldforest (table 4). Higher abundance may be linked to age-related differences in the structure and productivity of theground environment, which is more diverse in old forests.Whether the average number of species is typically greater inold-growth forests is uncertain, as is the associated questionof whether variation in species number is related to the agegradient. In this study, coefficients of variation (C.V.) forspecies richness were similar between years on the age gradi-ent (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, and22.1). Variation in species number might be expected to behigher in young than in old-growth forest, as a reflection ofthe higher proportion of dispersal sinks in young forest, andperhaps as a function of the less-diverse and productiveresource-base at ground level and the less physically bufferedenvironment. Such a trend may exist, although slight, in theabove coefficients, but a thorough answer to these questionsrequires long-term study. Of particular interest would be thevariation in species number with season in young versus oldforest. The initial design of this study, which was not fullyimplemented, included a spring and fall sampling-period foreach site as a way of addressing this question. This study,therefore, cannot characterize the magnitude of seasonalchanges. That snap-trapped animals were breeding is impor-tant to note; however, site occupancy was thus not simplydue to the capture of individuals dispersing from favored orsurvival habitats (Anderson 1980). This spilling-over effectthat resulted from the movement of individuals from high-density to low-density habitats (Fretwell 1972), or fromsaturation dispersal (Lidicker 1975), would be more evidentlater in the fall and early winter.

What does the apparent lack of strong community patternson gradients of forest-age and moisture mean? Is it reallytrue that the small mammals of this region are insensitive tovariation in forest structure? Not at all; noting that thissurvey was done exclusively on naturally regenerated stands,

281

clearly-from this study, from work in Oregon (Corn andBury, this volume a; Gilbert and Allwine, this volume a), andfrom work on the successional changes in the compositionaland structural features of these forests (Spies and Franklin,this volume; Spies and others 1988~thresholds of criticalhabitat-variables are met for most forest-dwelling speciessoon after canopy closure.

In a naturally regenerating landscape, the major change inthe species composition of small mammals occurs before andduring canopy closure. At the transition from grass- and forb-dominated plant communities to later stages dominated byshrubs and young trees, small mammal communities charac-terized by the deer mouse, several species of meadow voles,Townsend’s and yellow-pine chipmunks, the Pacific jumpingmouse, and shrews of open habitats (the vagrant shrew andthe montane shrew) begin to give way to communities of theclosed-canopy forest described in this study. Rather minorchanges in species composition and relative abundanceamong species takes place in subsequent successional stages.As a consequence of this pattern of mammalian speciesreplacement, we would expect poor correlations betweensmall mammal abundance and habitat elements in studiesthat use a data set based on naturally regenerated forests, aswas observed.

If thresholds for limiting habitat-elements for small mammalsare usually met or exceeded in naturally regenerated forests,this data set presents an interesting problem. In terms of in-formation to guide management on forests regenerated aftertimber harvest, this data set is not usable for identifying criti-cal habitat-elements, or to assess their threshold values-astraightforward consequence of the fact that the range of var-iation in these variables was insufficient to provide strongcorrelations or to indicate threshold values. For such a pur-pose, knowing under what conditions a species is absent isjust as important as knowing the conditions under which it is

present. On the one hand, this study shows that most specieshave a fair amount of adaptability in their use of forest habi-tats, but on the other, it tells us little about sufficient valuesfor habitat elements. If forests managed for timber productionare to retain most of their native small mammals, we need tobegin the process of identifying critical habitat-elements andassessing their minimum threshold values. That second- andthird-rotation forests would resemble naturally regeneratedforests in a number of respects seems unlikely, and they cer-tainly would exceed the range of variation in habitat vari-ables sampled in this study. The required information, conse-quently, can only come from a data set based on stands thatare intensively managed for timber production, or an array ofexperimental sites designed to mimic such conditions. Tothis end, manipulative field experiments conducted on spatialscales sufficiently large to avoid the problem of animal move-ment from adjacent edges probably hold the most promise.Because of the need for large scale, such studies might bestbe undertaken cooperatively between the scientific commu-nity and government agencies or private companies.

AcknowledgmentsIn the course of this study, over 60 people contributed to thefield work and data analyses. Field work required long hoursunder hard conditions, and I am indebted to these people forcompleting it. Diane Converse deserves special thanks forher leadership and perseverence over the course of the study.Funding was provided by the USDA Forest Service, PacificNorthwest Research Station. I thank personnel of the WindRiver, Mount Adams, Packwood, and Randle RangerDistricts of the Gifford Pinchot National Forest, the WliiteRiver Ranger District of the Mount Baker-SnoqualmieNational Forest, and Mount Rainier National Park for theiradvice and logistical help.

This paper is contribution 125 of the Wildlife Habitat Rela-tionships in Western Washington and Oregon ResearchProject, Pacific Northwest Research Station, USDA ForestService. 0

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AppendixTable 7-Physiographic and vegetative variables thatdescribe habitats in the southern Washington CascadeRange

Variable Description

SLOPEWATERROCKGRASSHERBSOILMOSSLICHLITTLITDEPFERNSHRUBMSHRUBMCCTREE

MCDTREE

CONIFSM

DECIDSM

LCONIFLDECIDSCANTREPITASTUMPBCSTUMPLGGAB

LOGCASNGSMT

BSNGSMT

CSNGSM

ASNGL

BSNGL

CSNGL

Percentage slopePresence of permanent waterPercentage cover of rockPercentage cover of grassesPercentage cover of herbsPercentage cover of mineral soilPercentage cover of mossPercentage cover of lichensPercentage cover of fine litter (<10-cm diameter)Litter depth (cm)Percentage cover of fernsPercentage cover of ground-layer shrubsPercentage cover of mid-canopy shrubsPercentage cover of canopy and mid-canopy

coniferous treesPercentage cover of canopy and mid-canopy

deciduous treesNumber 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 treesNumber of tree-fall pitsNumber of slightly decayed stumpsNumber of moderately to well-decayed stumpsNumber of slightly to moderately decayed logs,

d.b.h. > 10 cmNumber of well-decayed logs, d.b.h > 10 cmNumber of slightly decayed small and medium-

diameter (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