Table l-continued
Common name
Rodentia
Forest deer mouseb
Unidentified deer mouse
Southern red-backed vole
Long-tailed vole
Creeping vole
Water vole
Unidentified vole
Pacific jumping mouse
Carnivora
Ermine
All species totals
1984
1985
Total
539
96
737
3;
375
60
775
0
30
6
7
51
914
156
1512
4
59
7
8
90
5
13
18
3339
3412
6751
2;
1
Common name
Montane shrew
Trowbridge’s shrew
Vagrant shrew
Coast mole
Deer mouse
Forest deer mouse
Totals
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
Montane shrew
Marsh shrew
Trowbridge’s shrew
Unidentified shrew
Shrew-mole
2
34
1
7
3
2
Rodentia:
Yellow-pine chipmunk
Townsend’s chipmunk
Cascade golden-mantled ground squirrel
Northern pocket gopher
Deer mouse
Forest deer mouse
Unidentified deer mouse
Southern red-backed vole
Long-tailed vole
Heather vole
Creeping vole
Unidentified vole
Pacific jumping mouse
19
15
2
4
61
36
64
6
3
6
12
1
10
All species total
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)
288
Results
A total of 7084 individuals of 23 species were caught over
2 years
on the 54 sites. Of 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
Snap traps
Pitfall traps
6
0
1
0
5
6
10
1
2
1
6
7
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).
0 Young
q Matlm
. Old
7
MM”
TRSH
S”MD
TOW
OEM0
Species
FDMO
SRBY
CR”0
PlMO
Species Response to Gradients of Forest Age and Moisture
Forest-age gradient--Only
deer mice showed statistically
significant differences between age-classes. The deer mouse
was more abundant in old-growth than in young forest, and
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).
The Gequency of capture differed between years for four speties in the 36 sites comprising the chronosequence. The
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
CascadeRange. (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 speciesin older
forest age-classes(table 4, P = 0.07).
Forest moisture gradient-Within the 27 old-growth forest
sites, only the marsh shrew had an unambiguous responseto
the moisture gradient (table 5, fig. 2). The marsh shrew was
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 moderateor wet old-growth forests (table 5). although this pattern is complicated by an
interaction with the high elevation of dry sites as discussed
below.
Table S-Means, standard deviations,and frequencyof occurrence(percent)of small mammal
captures(number per 100trap-nights) in snap(July and August) and pitfall traps (Octoberand
early November)on 21 forestedsitesrepresenting8 moishrregradient within old-growth forest
in the southernWashingtonCascadeRange(whereANOVA is significant, differencesbetween
meansarc indicated with different letters; a = 0.05)
1984
1985
Insectivores
vagrantshrew
Mean
S.D.
%
MWtl
SD.
%
Marshshrew
MeSXl
S.D.
%
Trowbridge’sshrewb
MCXJI
S.D.
%
Insectivores
Shrew-mole
MC.Wl
SD.
%
Rodents
Townsend’schipmunk
MeWI
275
Tablc S-continued
1984
1985
Gammon name
Forest deer ,,,oux?
MWI
S.D.
%
Southern red-backed
Vole
MMl
S.D.
%
Rodents
creepingvole
Me%8
S.D.
%
Pacific jumping mouse
MCUl
S.D.
90
Totalmammals
MUII
S.D.
Speciesrichness
Mea
S.D.
of capture across moisture-classeswas different
between years for both deer mice species.Becauseof the significant interaction of moisture-classby year, each yea was
analyzed separately.Capturesof the deer mouse in 1984 were
not significantly different acrossmoisture-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, capturesof the forest deer
mousewere 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 pattern
1
The frequency of capture differed between years for three
speciesin 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 acrossforest age-classes,changesin capture
Table 6-Multiple correlations between individual habitat variables and small mammal abuvdance and
speciesrichness 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)
SOUth~
Deer mouse
Variable
1984
BSNGSMT
MCCTREE
TREPIT
MSHRUB
CSNGL
LCONIF
LOGC
ROCK
FERN
LOGAB
MCDTREE
WATER
SlOPE
ASTUMP
BCSTUMP
LICH
SOIL
BSNGL
R*
0.05
-.06
i
Forest
deer mouse
red-backed
Vole
198
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
Elevation-Site
Correlates With Mammalian Abundance
elevation was not strongly correlated with
the abundancesof most species.Only three speciesshowed
a significant association with elevation, and of these,only
the southern red-backedvole 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-backedvole in 1984,
r = 0.533 (P < 0.001) and in 1985, r = 0.527 (P < O.OOl)].
Vegetative and phyaiographic
variablesThree
mamma-
decay class (BSNGSMT) and the percentagecover of midcanopy by coniferous trees (MCCTREE) were negatively
correlated with deer mouseabundance,but deer mouseabundancein 1985 was positively correlated with the number of
treepits QREPIT, or holes and root tangles associatedwith
fallen trees), the percentagecover of mid-canopy shrubs
(MSHRUB), and the number of large coniferous trees. Deer
mooseabundancein 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
associationsfor the number of large coniferous trees
(LCONIF) and the percentagecover of well-decayed logs
@XC). In general, the weak nature of the correlations
underscoresthe fact that although more abundant in older
forests,both speciesalso were found in younger forests.
lian speciesand two community variables were significantly
correlated with setsof 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.
The pattern of correlation for the southernred-backed vole
was different in that some associationsheld between years
but others did not. Vole abundancein 1984 was positively
correlated with the percentage.cover of rock (ROCK) and
the presenceof water (WATER), but it was negatively correlated with the percentagecover of ferns (FERN), logs of
Although the abundanceof the deer mousewas significantly
correlated with vegetative and physiographic variables in each early decay-classes(LGGAB), and mid-canopy deciduous
tress (MCDTREE).
In 1985, the correlations for WATER
year, the variables were different between years (table 6).
and MCDTREE remained, but new positive correlations were
In 1984, the number of medium tall snagsof intermediate
observedfor the percentagecover 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 seenfor the percentagecover 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 basedon presence or absenceof specieswas not clearly related to the
forest-agegradient. The hierarchical clustering of sites using
Jaccard’scoefficient 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 abundanceof the most frequently captured
species(11 species)showed the samelack of association
between clusters and site age. The first cluster consisted of
eight young, four mature, and nine old-growth sites; the
secondcluster 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 correspondencebetween old-growth forest
moisture-classes(dry, moderate,and wet) and the clusters of
sites with similar mammalian speciescomposition (fig. 4).
No clusters consisted of just one moisture-class,although
some clusters were composedof two classes,either moderate
and wet (sites 15-21) or moderate and dry (sites 22-27). The
K-means clustering procedure suggestedthat moisture status
exerts somewhatmore influence on community composition.
Specifying three clusters, the fmt cluster was composedof
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
278
of the seven dry sites in the secondcluster might indicate a
tendency for similar mammalian communities on drier sitea.
Sites in the secondcluster supported rather high populations
of red-backedvoles. 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.
Species
I II
forest deer mouse may expand, a consequenceof 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 speciesin North America and
inhabits many different habitats, the forest deer mouse is a
forest specialist. It is found in pre-canopy stagesof forest
succession(table 2, fig. 5), but is generally outnumberedby
the deer mouse in such habitat.
A positive relationship between abundanceand forest age
was an expectedpattern for the forest deer mouse. As a forest specialist, the forest deer mousewould be expectedto
reach high abundancein the well-developed forests that, historically, were the prevailing forest condition in the Pacific
Northwest. Observing the samepattern for the deer mouse,
Discussion
however, was a little surprising. Deer mice were also more
abundant
in old-growth than in matore or young forest on the
Species Response to Gradients of Age and Moisture
western slopes of the central Oregon Cascades(Anthony and
Forest age gradient-The
relationships of deer mice to patothers 1987). and more abundant in riparian zones of oldterns of forest successionin Washington are being reevalugrowth forest than in mature- and sawtimber-classes(sites
ated in light of the recent elevation of the forest deer mouse
50-150 years) in northwestern California (Raphael 1988c).
to speciesstates (Goon and Greenbaum 1986, Allard and
The deer mouse may be more abundant in old rather than
others 1987). The forest deer mousehas mostly been conyoung forest becauseof the diversification of ground vegetasidered a subspeciesof the deer mouse (Osgood 1909, but
tion with forest succession(Spies, this volume). Older forests
see Liu 1954, Ingles 1965, and Sheppe 1961), and that view
may more closely resemble the diversity and abundanceof
has enhancedthe conception of the deer mouse as an extreme herbaceousplants characteristic of pre-canopy successional
habitat generalist. A substantial part of the habitat thought to
stagesthan those of many closed-canopy young and mature
be occupied by the deer mouse was actually occupied by the
forests. This resemblanceis partially related to the uneven
forest deer mouse. As demonstratedin this study for the
nature of old-forest canopies,which allow the growth of
southern Washington CascadeRange, the forest deer mouse
ground-layer plant speciesthat depend on increasedlight.
is the more abundant of the two speciesin forested habitat
Deer mousepatterns of peak abundancein old forests,how(tables 1,4; figs. 1 and 5). Researchhasjust begun to deever, are only relevant to the stagesof forest succession
scribe the distributions and patterns of abundanceof these
that follow canopy closure. As in other regions (Anthony
species,and work is underway on the.taxonomic affinities of
and others 1987, Raphael 1988c), the deer mouse is much
the forest deer mouse and other Peromyscusspeciesin coastal more abundant in habitats that occur before canopy closure
British Colombia. The currently recognized range of the
(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.
280
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
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
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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
Appendix
Table 7-Physiographic and vegetative variables that
describe habitats in the southern Washington Cascade
Range
Variable
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
Description
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
Continue
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