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Abstract.-Relative abundance of 55 species of
amphibians, reptiles, and mammals was estimated
at 166 sites representing early clearcut through oldgrowth Douglas-fir forest in northwestern California.
Nine species were strongly associated with older
stands and 1 1 species were strongly associated with
younger stands. The remaining species were either
too rare to analyze statistically (22 species) or
exhibited no clear trends of abundance in relation
to stand age (1 3 species). Estimates of relative
abundance of each species in each stage, coupled
with data on historical, present, and future acreage
of timber in each seral stage, were used to
approximate the long-term impacts of timber
harvest on the fauna of the Douglas-fir region in
northwestern California.
rm Trends in
Abundance of Amphibians,
eptiles, and Mammals in
Douglas- Fir Forests of
tern California1
Martin 6.Raphael2
Management of old-growth Douglasfir (Pseudotsuga menziesii) forests is
controversial in the Pacific Northwest, primarily because of the possible value of old-growth as habitat
for certain wildlife species versus the
revenues represented by old-growth
trees (Meslow et al. 1981, Harris et al.
1982).Management to provide wildlife habitat requires an inventory of
associated wildlife species and an
assessment of their old-growth dependency. An analysis of the size
and distribution of habitat patches
necessary to support viable populations of those species is also critical
(Burgess and Sharp 1981, Rosenberg
and Raphael 1986, Scott et al. 1987).
This study describes the relative
abundance of amphibians, reptiles,
and mammals in six seral stages representing clearcuts, young timber
stands, and mature forest in northwestern California. These estimates
of relative abundance were used to
project probable long-term changes
in population size of amphibians,
reptiles, and mammals as each seral
'Paper presented at Symposium, Management of Amphibians, Reptiles and Small
Mammals in North America (Flagstaff,AZ,
July 19-27. 1988).
Research Ecologist, Forestry Sciences
Laboratory, USDA Forest Service, Rocky
Mountain Forest and Range Experiment
Station, 222 South 22nd Street, Laramie,
Wyoming 82070.
stage responds to forest management
practices.
METHODS
Stand Selection
Study stands were on the Six Rivers,
Klama th, and Shasta-Trinity National
Forests within a 50-km radius of Willow Creek, Calif. Forest cover was
dominated by Douglas-fir, usually in
association with an understory of
tanoak (Lithocarpus ensiflorus) and Pacific madrone (Arbutus menziesii). Elevations varied from 400 to 1300 m.
Stage
Seral state
1
Early
2
3
Late
Pole
4
5
Sawtimber
Mature
6
Old-growth
Age (yrs)
10-20
"O
20-50
50150
150-250
>250
Raphael and Barrett (1984) describe
methods for aging these stands.
Ground surveys were used to verify
stand conditions. Forest Service
stand designations were used to
guide stand selection, but the final
classification of each stand into seral
stages was based on measured vegeta tion characteristics.
The study region is characterized by
warm, dry summers and cool, wet
winters; total precipitation averages
60-170 cm per year.
After selecting potential study
stands using timber maps and aerial
photographs, I then located all stands
that were accessible by road, were
relatively homogeneous with respect
to tree cover, included no large clearings or other anomalous features,
and were free from scheduled timber
harvest for at least the next 3 years.
From this restricted subset of
stands, I randomly chose 10 to 15
stands representing each of six seral
stages:
>
>
>
Classification
Clearcut (brush/sapling)
Young forest (pole/sawtimber)
Mature forest
Vegetation Sampling
The structure and composition of
vegetation on each stand in the three
older seral stages was measured in
three, randomly selected, 0.04-ha circular subplots within a 90-m radius
of each plot center. Within each subplot, observers recorded species,
height, diameter at breast height
(d.b.h.) and crown dimensions of
each tree or shrub >2.0 m tall. In addition, all trees >S)O-cmd.b.h. were
counted on one 0.50-ha circular subplot centered on the plot. This
sample permitted a better estimate of
the density of large-diameter trees.
Numbers of larger ( S c m diameter)
logs and volume of other downed
woody debris were estimated along a
30-rn transect crossing the center of
each 0.04-ha subplot (Brown 1974).
Marcot (1984) sampled vegetation in
a similar manner on stands in the
three early-sera1 stages.
Vertebrate Sampling
All field data were collected by a
team of three to six biologists. We
used a variety of techniques to
sample various taxonomic groups.
Pitfall Arrays
We used pitfall arrays to capture
small mammals (especially insectivores), reptiles, and salamanders. An
array was composed of ten 2-gallon
plastic buckets buried flush with the
ground and covered with plywood
lids, arranged in a 2 x 5 grid with 20m spacing. We placed one array
within each stand center and checked
traps at weekly to monthly intervals
from December 1981 (sawtimber,
mature, old-growth; n = 27,56, and
52 sites in each stage, respectively) or
August 1982 (early shrub-sapling,
late shrub-sapling, pole; n = 10 sites
each) until October 1983. All live animals were marked and released; recaptures were excluded from analyses. Dead animals were collected and
prepared for permanent deposit in
museum collections. Results for each
species were expressed as captures
per 1000 trapnights on each stand.
Raphael and Rosenberg (1983)demonstrated that abundance estimates
(capture rates) had stabilized after 15
months of continuous trapping.
Drift Fence Arrays
To better sample snakes, we installed
a drift fence array (Campbell and
Christman 1982, Vogt and Mine 1982)
on each of 60 randomly selected
stands (10 of each of the three early
stages and sawtimber, 8 mature, and
12 old-growth). An array consisted of
two 5-gallon buckets placed 7.6 m
apart and connected by an aluminum
fence 7.6 m long and 50 cm tall with
two 20 x 76 crn cylindrical funnel
traps, one on each side of the center
of the fence. These fences were operated from May through September
1983. All captures were combined
with those from the pitfall arrays
along with the associated trapnights
from each stand.
Track Stations
Tracks of squirrels and other larger
mammals were recorded on each site
on a smoked aluminum plate baited
with tuna pet food (Barrett 1983, Raphael and Barrett 1981, Raphael et al.
1986, Taylor and Raphael 1988).
Based on results of a pilot study (Raphael and Barrett 1981), observers
monitored each station for 8 days in
August or September in 1981-1983,
sampling 20 stations in each of the
three early stages and 81,168, and
157 stations in the sawtimber, mature, and old-growth stages, respectively. The proportion of stations in
each seral stage on which a species
occurred was as an index of that species' abundance.
Livetrap Grids
To better estimate abundance of
small mammals that were liable to
escape from pitfalls, we established
27 livetrap grids (3 in each of the
three earliest stages and 5,7, and 6 in
the three later stages), each of which
usually consisted of 100 25-cm Sherman livetraps arranged in a 10 x 10
grid with 20-m spacing. Other grid
sizes or shapes were used when the
plot configuration would not contain
the standard grid. Traps were
checked each day for 5 days (based
on pilot studies, Raphael and Barrett
1981) during July in 1981 (late stages
only), 1982, and 1983 (all stages). Results for each species were expressed
as mean number of captures per 100
trapnights.
Surface Search
To better sample certain amphibian
species, we conducted time- and
area-constrained searches (Bury and
Raphael 1983, Raphael 1984) on a
subset of sites in 1981 (late stages),
1982, and 1983 (all stages). A twoperson team searched under all movable objects and within logs on three
randomly located 0.04-ha circular
subplots (fall 1981,1982)or within a
1-ha area for 4 working hours (spring
1983). We conducted 20 surveys in
each of the three early stages and 29,
39, and 48 surveys in the three late
stages.
Opportunistic Observations
Observers recorded the presence of
vertebrates or identifiable vertebrate
sign incidental to the above procedures. We tallied observations to calculate frequency of occurrence of
rarer species within each stage.
Forest Area Trends
Estimates of historical, current, and
future acreage in each seral stage
were taken from Raphael et al. (in
press). For these analyses, I combined similar pairs of seral stages
into three generalized stages representing brush/sapling, pole/sawtimber, and mature timber. I then computed relative abundance of each
vertebrate species in these three
stages using a weighted average
(weights based on sampling effort)of
estimates from each of the two stages
forming the pair. Population estimates for historical, present, and future time periods were computed
using the formula:
where Pit was the relative population
size of the ith vertebrate species at
time t, Dij was the relative abundance
of the ith vertebrate in the jth seral
stage, and A, was the total area of
each of the three seral stages at time
t.
canopy volume, basal area, litter
depth, and density of Douglas-fir
stems >9O cm d.b.h. Downed wood
mass differed among stages, but the
greatest volume occurred in the
youngest stands, probably in the
form of logging slash, and the lowest
volume occurred in pole and sawtimber stages. Early-sera1 stands were
higher in elevation than older stands,
probably because of the logistics of
timber harvest in the area (most
clearcuts were located along ridgetops). Stands in the two earliest
seral stages, also because of logging,
were smaller in area than stands in
the four older stages.
RESULTS
Vegetation Structure
Comparisons of vegetation structure
among the seral stages (table 1)
showed that older stands had greater
Vertebrate Abundance and
Diversity
Among all plots and years of study,
we recorded 9,928 captures of all
species during 898,431 trapnights
from pitfalls and drift fences; 1,636
captures of amphibians during surface searches; 3,066 small mammal
captures during 35,070 trapnights
from livetrap grids; and 510 detections of larger mammals from track
stations. Relative abundances of 55
species, based on the most appropriate sampling method for each species, are summarized in table 2. Values are comparable across stages but
not among taxa if different sampling
methods were used. Amphibians
were much more abundant in forested than in clearcut stands,
whereas reptiles were more abundant in clearcuts. None of the amphibians and reptiles [except rarer
species such as northwestern salamander (see appendix for scientific
names of vertebrates)] was absent
from any stage.
Mammals exhibited a greater variety of responses to seral stage. Some
(e.g., Douglas' squirrel, western redbacked vole) increased in abundance
from earliest to latest seral stages;
others (e.g., deer mouse) decreased
along this gradient. A number of species (eg., Allen's chipmunk, duskyfooted woodrat, pinyon mouse, California vole) were most abundant
both in late shrub-sapling and mature or old-growth stands.
Mean numbers of mammal and
reptile species recorded per stand
differed among seral stages, but
mean numbers of amphibian species
did not differ significantly (fig. 1).
Among mammals, mean numbers of
species were greatest in mature and
old-growth stages. In contrast, mean
numbers of rep tile species were
greatest in the two earliest stages.
Long-TermTrends
Estimates of land area in each seral
stage through time (table 3) indicate
more area is occupied by early seral
stages currently than during historic
or future times. Mature and oldgrowth stages currently occupy
about half of historic acreage, and
these stages will probably occupy
only about 30% of current acreage
under the most likely harvest patterns of the future (table 3).
The implications of these changing
distributions of seral stages for amphibians, reptiles, and mammals are
summarized in figure 2. Nearly equal
numbers of species are likely to have
increased or decreased by more than
25% relative to historic abundance at
present and in the future. Three of
the five reptile species are presently
more abundant than in historic times
and all five species will likely be
more abundant in the future. Amphibians showed an opposite pattern.
Four of the eight species are presently less abundant and five of the
eight may be less abundant in the future. Among the 20 mammal species,
seven are presently less abundant
than in historic times whereas five
are more abundant. Eight species
will probably be less abundant in the
future and six more abundant.
DISCUSSION
Abundance in Seral Stages
Results suggest late brush/sapling
and mature/old-growth seral stages
provided more productive wildlife
habitat than early brush/sapling,
pole, and sawtimber stages. Among
amphibians, only ensatinas were captured frequently in pole sites.
Clouded salamanders were generally
under bark or inside downed logs
and persisted in clearcut stands as
long as adequate numbers of logs
were retained, especially in late sites
(Raphael 1987, Welsh, this volume).
Lizards were more abundant in
earlier seral stages than in pole and
mature stages. Among snakes, only
sharp-tailed snakes were observed
on early sites; other species occurred
on later sites. However, sampling
was not sufficient for definitive conclusions.
With the exception of the deer
mouse, small mammals were more
abundant on late bmsh/sapling sites.
Dusky-footed woodrats were of special interest in this regard as we observed many woodrat nests built
among the stems of tanoak and Pacific madrone in late brush/sapling
sites. The combination of abundant
mast, good nesting substrate, and
protection from predation (spotted
owls rarely forage in old, brushdominated clearcuts) provided by
the dense, brushy cover were probably the reasons that woodrats and
other small mammals were more
numerous in late clearcut sites (Raphael 1987).
Tree squirrels were most abundant in mature forest sites and
ground squirrels were more abundant in early clearcut sites. Chipmunks were the only squirrel that
reached peak abundance in early
sera1 sites. Their abundance was correlated with the cover of tanoak in
the understory (Raphael 1987). Management actions, such as herbicide
treatments, that shorten or delete the
late brush/sapling stage are probably
detrimental to chipmunks, woodrats,
and certain other rodents.
Several carnivorous mammals
were abundant in the late brush/sapling stage. Greater prey density in
late compared to early and pole sites
may explain this higher frequency of
carnivores although more data will
be necessary to confirm this observation.
Of the 55 species observed, 20
were strongly associated with either
older (9 species) or younger (11 species) stands (table 4). Three salamanders and six mammals were associated with older stands. One toad,
one frog, five lizards, and four mammals were associated with younger
stands. Five species associated with
old-growth were also abundant in
late (brushy) clearcut stages (table 2).
These species peak in abundance in
old stands and late clearcuts, with
low abundance in intermediate age
classes.
L
I
1
1
I
I
4
5
1
SEW STAGE
n
n
-3
-2s
D
a
so
n
>n
CHANCl N MMwuf ( X )
Figure 1 .-Mean numbers of amphibian,
reptile, and mammal species observed in
serel stages of Douglas-firforest, northwestern California, 1981-1 983. Sera1 stages (and
numbers of stands sampled) are: 1 - early
brush/sapling (n = 10); 2 late brushlsapling (n = 10); 3 - pole (n = 10); 4 sawtimber
(n = 27); 5 mature (n = 56); 6 old-growth
(n = 53). Vertical lines indicate 9Sohconfidence intervals.
-
-
-
-
Figure 2.-Percent change in population
size of amphibian, reptile, and mammal
species at present and in the future relative
to estimated historical populations. Histograms represent the numbers of species
increasing or decreasing by specified percentages.
I examined habitat associations
among each of the above 9 species by
computing correlations of their abundance with specific habitat components (table 5). Density of large trees
and hardwood volume were correlated with the abundance of most
species. Moisture, as measured by
the presence of surface water, moisture-loving tree species, or north-facing slopes, was important for most
mammals and one salamander species. Four mammal species were signi ficantl y more abundant on higher
elevation stands. Downed wood volume also was significantly and positively correlated with abundance of
four amphibian and mammal species. The abundance of hardwoods in
the understory was important for
many species in each group. In contrast, snag density was not positively
correlated with the abundance of any
species.
Long-Term Trends
The list of sensitive species (table 4)
is tentative pending results of addi-
tional surveys and more intensive,
species-specific research. The projections, although based on an intensive
sampling effort, are highly speculative. Three assumptions must be recognized to interpret these results.
First, I assumed that greater relative
abundance in a seral stage indicates a
species' preference for that stage and
that preferences remain constant
with shifting distri5ution of acreage
in each stage. Some species have (or
could) adapt to new stages over time.
Second, I assumed total acreage of
each seral stage can be used to estimate responses of vertebrates without regard to size and juxtaposition
of stands comprising each stage.
However, continued fragmentation
of forest habitats may result in disjunct patches so small they cannot
support a species that would otherwise find the habitat suitable. Rosenberg and Raphael (1986)found that
at least eight species of amphibians
(2), reptiles (21, and mammals (4)
were significantly less abundant in
stands el0 ha in size than in larger
stands. Some of these (e.g., western
gray squirrel) were not listed in this
study among the sensitive species
(table 41, but the effects of habitat
fragmentation may nonetheless be
cause for concern.
A third assumption is that young
forested stands (pole, sawtimber) in
this study represent young stands of
the future. Naturally occurring pole
and sawtimber stands contain some
large Douglas-fir stems and a substantial amount of standing and
downed wood (table 1).If future
management activities result in fewer
large live trees, snags, and downed
logs, the abundance of vertebrates
associated with these habitat components may also decline. In this case,
responses of vertebrates to forest
management may be more extreme
than those projected.
The overall trend is for increased
abundance among species of southern affinity that are associated with
open, drier habitats in other parts of
their ranges, and decreased abundance among species of boreal affinity that are primarily associated with
moist coniferous forest throughout
their ranges. Furthermore, most of
the increasers are widespread species
with large distributions that include
many nonthreatened habitats. In contrast, the decreasers are almost all
species with rather restricted total
ranges, most of which are in threatened habitats. Therefore, even
though total numbers of increasers
and decreasers are nearly equal, the
effects of old-growth reduction
should not be viewed as neutral.
Bccause many of the decreasers
are affected by soil moisture and
other microclimatic conditions, management to protect stream edges,
moist ravines, and other moist sites
may provide refuges for species that
can later recolonize maturing stands.
Management efforts to retain (or recreate) natural components of regenerating stands, such as hardwood
understory, snags, and logs, may
help mitigate against wildlife losses
in future forests. It is not stand age,
per se, but the structural characteristics of forests sf various ages that are
important to survival of most species.
Finally, results of this study address another important forest management issue in the northwest;
What should managers use as a
baseline for evaluation of impacts:
historic or present conditions? It is
apparent that many species are presently much less abundant compared
with historic numbers (fig. 2). Additional reductions because of continued timber harvest will cause further
declines in some species but most
major declines have already occurred. Therefore, I believe that estimates of historic populations should
be used as baselines for monitoring
biological diversity, rather than present populations.
ACKNOWLEDGMENTS
Field studies were funded by the Pacific Southwest Region and the Pacific Southwest Forest and Range
Experiment Station of the USDA Forest Service and by the University of
California, Agricultural Experiment
Station 3501 MS. I especially thank
my field assistants (Paul Barrett, John
Brack, Cathy Brown, Christopher
Canaday, Lawrence Jones, Ronald
Lavalley, Kenneth Rosenberg, and
Cathy Taylor) for their dedication
and blisters; R. H. Barrett, C. J.
Ralph, and J. Verner for their support; Bruce G. Marcot for freely sharing information from his studies and
for valuable discussions; and Kenneth V. Rosenberg, Fred B. Samson,
and Hobart M. Smith for their commen ts on an earlier draft of this
manuscript.
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Appendix
Common and scientific names of vertebrates mentioned in text (nomenclature follows baudenslayer and
Grenfell (1 983)).
Salamanders
Northwestern salamander ........................ Ambystoma gracile
Pacific giant salamander .......................... Dicamptodon ensatus
Olympic salamander .................................. Rhyacotriton olympicus
Rough-skinned newt ..................................Taricha granulosa
Del Norte salamander ................................Plethodon elongatus
h s a tina ........................................................
Ensatina eschscholtzi
Black salamander ........................................Aneides flavipunctatus
Clouded salamander .................................. Aneides ferreus
Frogs and toads
Tailed frog ....................................................
Ascaphus truei
Western toad ................................................ Bufo boreas
Pacific treefrog ............................................ Hyla regilla
Foothill yellow-legged frog ......................Rana boylei
Bullfrog ........................................................ Rana catesbeiana
Turtles
Western pond turtle
.................................. Clemmys marmorata
Lizards
Western fence lizard .................................. Sceloporus occidentalis
Sagebrush lizard ........................................ Sceloporus graciosus
Western skink .............................................. Eumeces skiltonianus
Southern alligator lizard .......................... Gerrhonotus multicarinatus
Northern alligator lizard .......................... Gerrhonotus coeruleus
Snakes
Rubber boa .................................................. Charim bottae
Ringneck snake ............................................ Diadophis pnctatus
Sharp-tailed snake ...................................... Phyllorhynchus decurtatus
Racer .............................................................. Coluber constrictor
Gopher snake .............................................. Pituophis melanoleucus
Common kingsnake .................................... Lampropeltis zonata
Common gartersnake ..................................
Thamnophis sirtalis
Western terrestrial gartersnake ................Thamnophis elegans
Western rattlesnake ....................................
Crotalis viridis
Mammals
Pacific shrew ......................................... Sorex pacificus
Trowbridge's shrew ....................................Sorex trowbridgii
Shrew-mole .................................................. Neurotrichus gibbsii
Coast mole ....................................................
Scapanus orarius
Allen's chipmunk ........................................
Tamias senex
Western gray squirrel ..................................Sciurus griseus
Douglas' squirrel ..........................................
Tamiasciurus douglasii
Northern flying squirrel ............................ Glaucomys sabrinus
Deer mouse ................................................. Peromyscus maniculatus
Brush mouse ..................................................Peromyscus boy1ii
Pinyon mouse .............................................. Peromyscus truei
Dusky-footed woodrat ................................ Neotoma fuscipes
Western red-backed vole ............................ Clethrionomys californicus
Red tree vole ................................................ Arborimus longicaudus
California vole ........................................ Microtus califomicus
Creeping vole ................................................Microtus oregoni
Western jumping mouse ............................ Z a p s princeps
Coyote .......................................................... Canis Iatrans
Gray fox ......................................................... Urocyon cineremrgenteus
Black bear ......................................................Ursus americanus
Ringtail .......................................................... Bassariscus astutus
Raccoon ......................................................... Procyon lotor
Fisher ............................................................. Martes pennanti
Ermine ............................................................ Mustela erminea
Western spotted skunk .............................. Spologale gracilis
Striped skunk ................................................Mephitis mephitis
Bobcat .......................................................... Lynx rubs