Table S-Contribution
of landscape variables to squared multiple R (R' change) in stepwise regressions of
vertebrate richness and log. (abundance) with stand and landscape variables (alpha to enter and remove in
rerression = 0.15)
Richness
Abundance
Sd%%Sd
variables
Model R2
Regression
P
2
0
0
Stand buffer composition
Clewcut
Old growth
Late successional forest
1
1
0
Landscape composition
Clearcut
Old growth
Late successional forest
3
0
1
Landscape pattern indices
FrtXtal
DOminance
Point diversity
Disturbance
1
Table 6Speannan
correlations of bird, small mammal, and amphibian
abundance in old-growth stands with landscape variables
Species richness
species richness and
Abundance
Sample stand
Area
P&II&T
Edge index
EkVtXiO”
Stand buffer composition
Clearcut
Old growth
Late successional
forest
Landscape composition
Clearcut
Old gmwth
Late successional forest
435
Table 6-continued
Abundance
Species richness
Birds
Landscape pattern indices
Fractal
Dominance
Point diversity
Disturbance
-.44***
-03
.08
.29*
Mammals Amphibians
.41***
-.20*
.16
-.22
-.29*
.15
-.03
-.09
Birds
-.03
-.33**
.39**
.28*
Mammals Amphibians
-.10
.42***
-.14
-.37**
-.40***
.19
-.14
.OO
* P < 0.10; ** P < 0.05; *** P < 0.01; **** P < 0.001.
Individual bird species-Few strong associations between
the abundance of 22 bird species and landscape variables
were revealed by partial correlation (table 7). Most correlations were < 0.50, indicating a weak association with less
than 25 percent shared variation. Moreover, the number of
significant correlations may not be different than expected by
chance alone (P < 0.10). Dark-eyed juncos were the only
species that showed an association with stand-scale variables:
abundance increased with stand area, perimeter, and edge,
which were all somewhat collinear. Red-breasted nuthatches
and rufous hummingbirds were associated with clearcuts in
the buffer and negatively associated with complex patchshapes measured by the fractal index. Consistent with these
neighborhood relations, red-breasted nuthatches increased
with clearcut area in the landscape. Rufous hummingbirds
however, were positively correlated with old-growth area in
the landscape.
The abundance of winter wrens was associated with undisturbed buffer zones: abundance decreased with clearcut area
and increased with late-successional-forest area in stand
buffers. Their abundance was not associated with the wholelandscape variables. Black-throated gray warbler abundance
showed a very strong negative correlation with late-successional forest in stand buffer-zones. The combined abundance
of hermit and Townsend’s warblers was similarly associated
with young forest: abundance decreased with old-growth area
at the buffer and landscape scales. Hermit and Townsend’s
warblers were grouped into one functional species because
hybridization in southern Washington made them indistinguishable in the field (see Manuwal, this volume).
Cavity-nesting birds-Several abundance relations were
graphically examined for cavity-nesting birds (hairy woodpecker, red-breasted nuthatch, chestnut-backed chickadee,
brown creeper) based on the hypothesis that these birds
would be negatively affected by the loss of vertical structural
diversity from clearcut logging and snag-reduction policies in
remaining forest stands (Manuwal, this volume).
436
Our questions were:
l
Does abundance increase with the size of the sample
stand?
l
Is abundance in young stands greater when surrounded
by old growth (may abundance in young stands be subsidized by adjacent old forest)?
l
Is abundance less in stands that have a clearcut neighborhood (buffer zone)?
l
Is abundance less in stands embedded in a clearcut
landscape?
Cavity-nesting bird abundance showed no significant relation
to stand area (fig. 9A). Abundance increased with clearcut
area in the buffer in a weak (R2 = 0.102), but significant
(P = 0.033), linear relationship (fig. 9B). The linear relation
between abundance and total clearcut area (fig. 9D) was not
significant (P = 0.127), however. The hypothesis that abundance is higher in young stands surrounded by old growth is
supported (fig. 9C) with a significant linear regression (P =
0.011) and high R = 0.76, if the apparent outlier in the top
left comer is deleted.
Individual small mammal species-Some strong partial
correlations were found between individual small mammal
species’ abundance and stand or landscape variables, but
these correlations could have occurred by chance (P < 0.10)
(table 8). The creeping vole was the only species showing an
association with stand variables: abundance was negatively
correlated with stand perimeter and the edge index. Northern
flying squirrels and marsh shrews both were negatively
associated with late-successional forest in the buffer zones,
but the water shrew was very strongly associated with latesuccessional forest in the buffer. Marsh shrew abundance
also was positively correlated with landscape clearcut area
and the disturbance index, and negatively associated with
late-successional-forest area. Ermine were associated with
high point diversity.
Table ‘I--Partial correlation coefficients (stand age and elevation constant) of bird species’ abundance with landscape variables
(data for 1984 and 1985 were combined)
N
-0.34
RRCR
CBCH
DEm
EVGR=
GCKI
GFUA
HAFL’
HAWOc
HETH’
HETO
PISS
RDN”
RUHU’
.27
-.16
-.,6
.,I
.m
-.21
.24
amphibian species-A few significant partialcorrelations were strong enough to be unlikely due to chance
(table 8). Amphibians respondedmore strongly to stand-scale
variables and more weakly to neighborhood- and landscapescale variables than birds or small mammals.Abundance of
the western redback salamanderwas associatedwith more
variables than other species:abundancewas positively COTrelated with stand arca and the associatedvariables of perimeter and edge, and negatively associatedwith old growth in
the buffer. The abundanceof western redback salamanders
was also correlated with habitat dominance. The abundance
of tailed frogs was negatively correlated with stand edge and
complex patch-shapesrepresentedby the fractal index.
Individual
Pond-breeding amphibians-We
hypothesized that the
abundanceof pond-breeding amphibians (northwestern salamander, roughskin newt, red-legged frog, Cascadesfrog)
would decline with fragmentation as a result of the increased
isolation of subpopulations and postbrceding dispersal mortal.
ity in clearcut barriers. The abundance-area.
relationship was
not significantly different from zero (fig. 10A). Abundance
-.13
-.18
-.14
.06
.36
.I7
-03
.16
39
.w
.2O
-.28
-.,I
.19
-.04
43
-.lS
33
-3l
.Ol
.O4
-.I6
.O7
.I6
.10
30
.19
.I6
-.04
.2O
.rn
-.a7
.02
.2O
46
15
8
88
91
83
12
92
64
23
26
39
78
31
72
31
22
17
87
84
9
24
91
appearedto increasewith clearcut area in the buffer zone,
but the linear relation was not significant. Abundance also
was not associatedwith clearcut area in the landscapein a
significant linear fashion. Abundance in young stands,however, showed a strong negative relation to buffer-zone oldgrowth, with a significant linear regression model (P S 0.05;
R2 = 0.55).
Discussion
Limitations
of the Data Set
The stand-scalevertebrate data set is limited in several ways
for a landscape-scaleanalysis. The criterion that study stands
be larger than 40 ha restricted the examination of speciesarea relations to relatively large stands,where the effects of
size may be less important. Rosenbergand Raphael (1986)
found that forest stands~20 ha in northern California had
less than the full complement of species.The minimum-size
criterion also may have resulted in the selection of landscapes
with less fragmentation than might have occurred had selection been unconstrained.
431
Area (hectare)
L
00
Old-growth (proportion)
Clearcut (proportion)
Clearcut (proportion)
Sampling bias for relatively large standsmay have excluded
highly fragmentedareasfrom the analysis, resulting in conservative estimatesof fragmentation. Study landscapeson
National Forest land ranged from 0 to 48 percent clearcut,
with most landscapesless than 30 percent cutover. The mean
cutover percentagewas 16 percent. The mean percentage
cutover for the entire Forest, basedon data from the TRI
database(G. Gmelich, unpubl. data), was also 16 percent,
with a range of 14 to 19 percent cutover by Ranger District.
The similarity of the meansmay be misleading, however,
becauseForest-wide estimatesare averagesmade for a larger
Disbict scale of measurement.Proper comparison requires
estimates from a random sample of 2025ha landscapes
acrossthe Forest, which is a project beyond the scopeof
our study.
The vertebmte sampling design also had limitations for detecting uncommon and medium- to large-bodied speciesthat
may be most influenced by fragmentation (Diamond 1984;
Lehmkuhl and Ruggiem, this volume: Pimm and others 1988;
Terborgh and Winter 1980). The nine speciessuggestedby
Rosenbergand Raphael (1986) as most sensitive to fragmentation are all uncommon with medium-to-large body-size for
their taxa: we examined data for only three of these species.
Also, sampling area for vertebrateswas constant in all stands
so that large standshad relatively less area sampled than
small stands.An attempt was made to distribute the sampling
effort throughout the stand,but our sampling in large stands
with high habitat-heterogeneityand associatedheterogeneous
speciesdistributions may not have detectedall of the species
present (Robbins and others 1989). Thus, species-arearelations may be more underestimatedby the relatively smaller
sampling areasin large standsthan in small stands(Wilcox
Table &Partial correlation coefficients (stand age and elevation constant) of small mammal and amphibian species’ abundance with
landxape variables (data for 1984 and 1985 were combined)
N
COLMOC
MASHC
Maw
WASH=
TRSH
VASHS
438
56
.03
-.14
.oo
~06
-18
-.ll
-.18
83
.47
20
.,I
-3
-.I”
-28
-.a
-..I5
-.03
6
87
21
Table &--(continued)
Amphibian/
hWSA
TLFR
ENS.4
RBSA”
RLF’R’
CAFF
RASP=
RSNE=
.n
-.2@’
-.03
.,3
49
2,
.26
.09
.22
.w
-.23
.st**
.m
47
.32
.32*
-.08
.*o
-.,2
.0367
43
.02
-.04
.,6
3,
-.02
-45
.07
-.23
.,4
.I6
-.24
-.29’
69
20
33
9
2s
34
1980). Finally, interpretations of abundancein young or
mature standswas constrained by sample sizes less than half
(n = 11) the number of old-growth stands(n = 26).
Despite theselimitations, we believe our studies provide
valuable insight for examining the effects of landscapecontext on vertebraterichness and abundancein the communitystudy stands.
Landscape Dynamics
Habitat-Area Relations
The reciprocal relation of late-successionalforest and clexcut areasin the landscapeconformed to the Franklin and
Forman (1987) checkerboardmodel: the composition of the
landscapemat& switched from late-successionalforest to
clearcut at 50 percent cutover (fig. 1). The model did not
hold for old-growth and clearcut areas,however: clearcut
area exceededold-growth area earlier in the harvest regime
when the landscapewas 30 percent cutover. Timber harvest
apparently was not restricted to old-growth stands,and the
amount of clearcutting in an area did not indicate the loss of
old-growth forest, but rather the loss of late-successionalforest. The 30.percent cutover threshold meansthat old-growth
patches and populations of closely associatedspecieswill be
isolated more rapidly than will late-successionalforest
patchesand animals as cutting continues.
Area
Old-growth
(hectare)
(proportion)
Clearcut
(proportion)
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