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Use of Woody Debris by
Plethodontid Salamanders in
Douglas-Fir Forests in
Washington
Abstract.- Ensafina eschscholfiiwas found most
often under ~ i e c e s
of bark, whereas Plefhodon
vehiculum occurred primarily under logs. Captures
of both species were highest in young stands, but
occurred in all age classes. Our results suggest that
the retention of coarse woody debris in managed
forests would provide for the habitat needs of these
species.
Keith B. Aubry,' Lawrence L. C. J ~ n e sand
,~
Patricia A. Hail3
The harvesting of old-growth
Douglas-fir (Pseudotsuga menziesii)
forests in the Pacific Northwest, and
its potential effects on wildlife species has been the focus of much concern in recent years (e.g., Lumen and
Nietro 1980, Franklin et al. 1981,
Meslow et al. 1981, Meehan et al.
1984, Gutierrez and Carey 1985).
Most of this at ten tion has been directed towards birds and mammals
such as the spotted owl (Sfrix occidentalis), Vaux's swift (Chaetura vauxi),
northern flying squirrel (Glaucomys
sabrinus), and red tree vole (Arborimus longicaudus); little concern
has been expressed about amphibians and reptiles. These groups have
not been studied extensively in the
Pacific Northwest. Only recently has
research been conducted on habitat
associations among different forest
age classes (Raphael 1984, Raphael
and Barrett 1984, Ruggiero and
Carey 1984).
From 1983 to 1986, the USDA Forest Service and USDI Bureau of Land
Management funded a major research effort aimed at identifying
wildlife species that occur in highest
abundances in old-growth Douglasfir forests and investigating the ecological basis of observed patterns of
association
Amphibian communities were
sampled using pitfall traps, stream
surveys, and time-constrained
searches (Standard Sampling Protocols on file at the Forestry Sciences
Laboratory, Olympia, WA). Some of
the results of these studies are reported elsewhere in this volume
(Bury and Corn 1988, Welsh 1988).
Here, we report the results of timeconstrained searches conducted in
southern Washington in 1984. Our
objectives are to (1) identify potential
habitat associations, (2) examine patterns of cover object use, and (3)
evaluate the efficacy of this technique
for studying amphibians in this region.
MT. ST. HELEKS
NATIONAL
0 Old-growth
O
Mature
Study Area
Paper presented at symposium, Management of Amphibians, Reptiles, and
Small Mammals in North America. (Flagstaff, AZ,July 19-21, 1988).
2ResearchWildlife Biologist, USDA Forest
Service, Pacific Northwest Research Station,
3625 93rd Ave. SW, Olympia, WA 98502.
3Biological Technician, USDA Forest Service, Pacific Northwest Research Station,
3625 93rd Ave. SW, Olympia, WA 98502.
4WildlifeBiologist, USDA Forest Service,
Pacific Northwest Research Station, 3625
93rd Ave. SW, Olympia, WA 98502,
Forty-five forest stands were
sampled in the southern portion of
the Cascade Range in Washington
(fig. 1). Stands ranged in age from 55
to 730 yr and were at least 20 ha in
size. All stands were located within
the western hemlock (Tsuga heterphylla) zone and lower elevations of
the Pacific silver fir (Abies amabilis)
zone (Franklin and Dyrness 1973),
which are characterized by a wet and
Figure 1 .-Location of study stands by age
class in the southern Washington Cascade
Range.
mild maritime climate. Snow rarely
accumulates at our sites.
Old-growth stands (210-730 yr)
typically contained high proportions
of Douglas-fir and western hemlock
and, in wet sites, western redcedar
(Thuja plicata). Mature (95-190 yr)
and young (55-80 yr) stands were
dominated by Douglas-fir. In all age
classes, other species such as red
alder (Alnus rubra), vine maple (Acer
circinafum), bigleaf maple (A. macrophyllurn), Pacific silver fir, and western hemlock occurred in lesser
amounts.
Average age of each stand was
determined through growth ring
counts, either by increment coring or
examination of cut stumps in nearby
stands. Old-growth stands were classified into wet, moderate, and dry
moisture classes on the basis of floristic and physiographic characteristics; all young and mature stands
were in the moderate moisture class
(T. A. Spies, unpubl. data). All stands
had resulted from natural regeneration following fires; none had undergone silvicultural treatments.
Methods
Surveys for terrestrial amphibians
were conducted from 16 April to 12
June 1984; all but four high-elevation
stands were sampled by 4 May. A
time- constrained search method was
used (Campbell and Christman
1982). A crew of two to four persons
actively searched each stand for amphibians for a total of 4 personhours. An initial search area was selected at least 50 rn within the stand
to avoid edge effects.
In general, woody debris such as
logs, snags, and pieces of bark was
abundant in each stand and constituted virtually all potential cover objects. An area was searched for 0.5
person-hours, after which we moved
a minimum of 25 m to search another
suitable area; sampling areas were
not spatially constrained. This was
repeated until the sampling period
was over. All potential cover objects
were searched by hand or with potato rakes, but no single object was
searched for more than 20 min. Logs
of all sizes in advanced stages of decomposition were pulled apart with
potato rakes. Areas beneath large
undecomposed logs could not be
searched in most cases. Little effort
was expended searching leaf litter, as
this has been shown to be relatively
ineffective when sampling amphibians in Douglas-fir forests (Bury and
Raphael 1983).Areas near seeps,
streams, ponds, rock outcrops, and
other areas not representative of the
stand were avoided.
Modifications of methods developed by Raphael (1984) were used to
describe capture sites. The following
information was recorded for each
individual captured: species, vertical
position in relation to cover object,
snag or log decay class, length and
width of cover object, and slope and
aspect of captire site. All amphibians
were collected, measured, and preserved, usually on the same day.
Snout-vent length (to anterior margin
of vent), total length, and weight
were recorded. Specimens were deposited in the Museum of Vertebrate
Zoology, University of California,
Berkeley.
Results
Captures
A total of 214 amphibians, including
6 species of salamanders and 3 species of frogs, were captured; no reptiles were encountered (table 1). Only
two species of plethodontid salamanders, the ensatina (Ensatina eschscholtzii) and western redback
salamander (Plefhodon vehiculum),
were captured in sufficient numbers
(141 and 50, respectively) to permit
comparisons of abundance among
stand types or to conduct analyses of
cover object use.
Habitat Occupancy
Ensatinas and redback salamanders
occurred in all forest age and moisture classes. Although both species
were most abundant in young forests
(table I), a one-way ANOVA revealed no significant differences
among stand types. Mean captures
for both species were lowest in wet
old-growth stands. The proportion of
stands containing ensatinas was also
relatively low in wet old growth:
fewer than 20% of wet old-growth
stands sampled contained ensatinas,
whereas all other stand types had a
frequency of occurrence of 65% or
greater (fig. 2). The proportion of
stands containing redback salamanders was generally low in all stand
types (fig.2), suggesting that at the
time of our sampling, redback salamanders were less abundant or more
clumped in distribution than ensatinas. We found no amphibians in 67%
of old-growth wet stands, 11% of
young and mature stands, 12%of
moderate old-growth stands, and 0%
of dry old-growth stands.
Use of Woody Debris
Cover object selection varied between the two species. Ensatinas
Figure 3.-Use of cover objects by Ensafina eschscholfzii (ENES) and Plefhdon vehiculum
(PLVE) in the southern Washington Cascade Range.
ENES
lg PLVE
YNG
MAT
(N=9)
(Nag)
OGW
(N=6)
STAND TYF'E
OGM
(N= 17)
OGD
(N=4)
Figure 2.-Proportion of stands in each stand type with captures of Ensafina eschschoifzii
(ENES) and Plethodon vehiculum (PLVE) in the southern Washington Cascades Range. Stand
type YNG=Young, MAT=Mature, OGW=Wet Old Growth, OGM=Moderate Old Growth,
OGD=Dry Old Growth.
were most often found under pieces
of bark (generally within 1 m of a
snag or log) and secondarily under
logs (fig. 3). The pattern was reversed for redback salamanders. Neither species was found under bark
on snags. When found under pieces
of bark, ensatinas most often occurred in bark piles at the base of
moderately decayed snags (see Thomas et al. 1979, p. 64). Seventy-four
percent of these captures occurred
next to snags in which the top had
broken off, the wood was soft, and
most or all of the bark had sloughed
onto the ground. Logs where ensatinas and redback salamanders were
captured were most often 10- 30 cm
in diameter (fig. 4), Both species were
captured in low numbers in association with very large logs (diameter
>30 cm), but our inability to adequa tely search this cover type may
account for these results. Virtually all
logs where ensatinas and redback
salamanders were found were in intermediate stages of decay (fig. 5)
(see Maser et al. 1979, p. 80). Only a
few captures of either species occurred in association with intact or
extensively decomposed logs. Neither species was commonly found
under rocks, but this cover type is
relatively rare in Douglas-fir forests.
No correlations between slope or aspect and amphibian capture sites
could be detected.
Discussion
Old-growth forests do not appear to
provide unique habitat for either ensatinas or western redback salamanders; both species were well-represented in all age classes. Our results
suggest that abundance levels of
these salamanders are more likely a
function of the availability of woody
debris for cover than age of the overstory. Wet old-growth stands in
southern Washington, however, apparently provide low quality habitat
for these plethodontids, especially
ensatinas (table 1, fig. 2). Soils in
these stands were often saturated
Figure 5.-Use of logs by Ensatina eschscholtzii (ENES) and Pleihodon vehiculum (PLVE) by
decay class In the southern Washington Cascade Range.
with water, and such conditions may
reduce the availability of microenvironments suitable for cover, maintenance of water balance, and successful reproduction. In addition, these
Figure 4.-Use of logs by Ensatina eschscholfzii (ENES) and Plefhodon vehiculum (PLVE) by
diameter class in the southern Washington Cascade Range.
35
stands were located in topographically low sites where cold air accumulates, which may create unfavorable microclimatic conditions for plethodontid salamanders. Our results
also suggest that plethodontid salamanders may prefer certain types of
woody debris as cover, especially
those associated with large, moderately to well-decomposed snags and
logs. Captures of ensatinas were
most common under pieces of bark,
especially in bark piles at the base of
well- decayed snags (fig. 3). Snags in
the early stages of decomposition
with shallow or no bark piles at their
bases provide few suitable microhabitats for salamanders. Depth
of these bark piles increases as
sloughing continues until all bark has
fallen off. Later stages of snag decomposition provide no additional
bark to the pile and habitable spaces
become compressed as the lower layers of bark decay and mix with the
underlying substrate.
Bark microhabi ta ts formed by the
deterioration of snags differ in structure from those formed by the de-
composition of logs. As logs decay, a
single layer of bark is deposited on
the forest floor, whereas bark sloughing from snags forms multilayered,
structurally complex cover. Such
bark piles could provide microclimatic conditions more resistant to
fluctuations in temperature and
moisture than those found under
bark on the ground. Additional foraging habitat may also be available.
Redback salamanders, on the
other hand, were most often found
under moderately decayed logs 10-30
cm in diameter (figs. 3-5). In the early
stages of decay, bark has not begun
to slough and branches suspend the
log above the ground. As the bark
begins to slough and branches deteriorate, increased cover and moisture
are provided along the length of the
bole where it comes in contact with
the forest floor (Maser and Trappe
1984).The quality of this environment for salamanders continues to
improve with further decay until the
organic matter becomes incorporated
into the underlying substrate and
habitable interstices become compressed in the advanced stages of
decomposition.
All known nest sites of ensatinas
in the Pacific Northwest have been
found in association with large, moderately decayed logs (Norman and
Norman 1980, Maser and Trappe
1984, Jones and Aubry 1985, Norman
1986, L. L. C. Jones unpubl. data).
This habitat feature may be important for the persistence of ensatinas
in these forests. We do not know to
what extent coarse woody debris
may be important for reproduction
of redback salamanders in Douglasfir forests; only one nest site has been
found, and this was in moist talus in
the Oregon Coast Range (Hanlin et
al. 1978).
In Douglas-fir stands of the Cascade Range that have regenerated
after catastrophic fires, levels of
coarse woody debris (CWD) (logs
and snags > 10 cm in diameter) are
moderate in young stands, lowest in
mature stands, and highest in old-
growth stands (Spies et al. in press).
In general, this is due to the inheritance of high levels of CWD in young
stands from the preceding oldgrowth stands, a low accumulation
of CWD in mature stands as CWD
decays but inputs are low, and high
inputs of CWD in older stands as the
large Douglas-firs die and accumulate as snags and logs. Intensive forest management results in levels of
CWD substantially lower than that
encountered in unmanaged forests
(Spies and Cline in press). This is because plantations inherit little CWD
from the preceding stand when it is
clearcut and existing CWD is removed and fragmented. In addition,
thinning operations reduce the input
of CWD from suppression mortality
and short rotations prevent the accumulation of CWD. Maintaining even
moderate amounts of CWD in managed forests will require modifications of current harvesting and
silvicultural practices (Himon et al.
1986, Spies et al. in press).
Virtually all available cover objects we encountered were woody
debris, and both species were found
most often in association with large,
moderately decayed logs and snags.
Our results suggest that the availability of coarse woody debris may be
important for maintaining salamander populations in Douglas-fir forests. Additional studies of terrestrial
salamanders in managed vs. unmanaged forests are necessary to determine the extent to which they may be
affected by intensive forest management.
In general, our study yielded a
relatively low number of captures.
Only two common species (Nussbaum et al. 1983) were captured in
high enough numbers to permit
analyses of the data; captures of all
other species were incidental. The
total number of species detected was
also low in relation to known species
richness: pitfall trapping for approxima tely 1000 trap nights in each of the
same study sites in the fall of 1984
yielded 916 captures of 13 species (K.
B. Aubry unpubl. data). Research using time-constrained searches to
study all but the most common species in this region would require substantially more search time. Sampling should also be conducted during all seasons of the year to detect
seasonal shifts in habitat selection or
cover object use, and to sample species that are active at other times of
the year.
Acknowledgements
We thank R. W. Lundquist, J. B.
Buchanan, B. A. Schrader, A. B.
Humphrey, M. Q. Affolter, M. J.
Reed, B. F. Aubry, and M. J. Crites
for assistance. T. A. Spies at the Forestry Sciences Laboratory, Corvallis,
OR provided data on stand characteristics. R. W. Lundquist provided
the map used in figure 1. This study
was funded under USDA Forest
Service Cooperative Agreement
PNW-83- 219 to S. D. West and D. A.
Manuwal at the TJniv. of Washington. We thank personnel of the Gifford Pinchot National Forest and
Mount Rainier National Park for
their cooperation and support. M. G.
Raphael, K. E. Severson, T. A. Spies,
and A. B. Carey provided constructive comments on a previous draft of
the manuscript. This paper is Contribution No. 65 of the Old-growth Forest Wildlife Habitat Project, USDA
Forest Service, Pacific Northwest Research Station, Olympia, WA.
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