rm Trends in Abundance of Amphibians,

eptiles, and Mammals in Douglas- Fir Forests of

tern California1

Martin 6. Raphael2

Management of old-growth Douglas- fir (Pseudotsuga menziesii) forests is controversial in the Pacific North- west, primarily because of the pos- sible value of old-growth as habitat for certain wildlife species versus the revenues represented by old-grow th trees (Meslow et al. 1981, Harris et al. 1982). Management to provide wild- life habitat requires an inventory of associated wildlife species and an assessment of their old-growth de- pendency. An analysis of the size and distribution of habitat patches necessary to support viable popula- tions 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 rep- resenting clearcuts, young timber stands, and mature forest in north- western 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, Man- agement 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.

Abstract.-Relative abundance of 55 species of amphibians, reptiles, and mammals was estimated at 166 sites representing early clearcut through old- growth 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.

stage responds to forest management practices.

METHODS

Stand Selection

Study stands were on the Six Rivers, Klama th, and Shasta-Trini ty National Forests within a 50-km radius of Wil- low Creek, Calif. Forest cover was dominated by Douglas-fir, usually in association with an understory of tanoak (Lithocarpus ensiflorus) and Pa- cific madrone (Arbutus menziesii). Ele- vations varied from 400 to 1300 m.

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 clear- ings 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:

Stage Seral state Age (yrs) Classification

1 Early

"O > Clearcut (brush/sapling) 2 Late 10-20 3 Pole

20-50 > Young forest (pole/sawtimber) 4 Sawtimber 50- 1 50 5 Mature 150-250 > Mature forest 6 Old-growth >250

Raphael and Barre tt (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 vege- ta tion characteristics.

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 cir- cular subplots within a 90-m radius of each plot center. Within each sub- plot, observers recorded species,

height, diameter at breast height (d.b.h.) and crown dimensions of each tree or shrub >2.0 m tall. In ad- dition, all trees >S)O-cm d.b.h. were counted on one 0.50-ha circular sub- plot 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 insecti- vores), 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 20- m 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 ani- mals were marked and released; re- captures were excluded from analy- ses. 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) dem- onstrated 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 oper- ated 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, Ra- phael and Barrett 1981, Raphael et al. 1986, Taylor and Raphael 1988). Based on results of a pilot study (Ra- phael 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, ma- ture, and old-growth stages, respec- tively. The proportion of stations in each seral stage on which a species occurred was as an index of that spe- cies' 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 Sher- man 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). Re- sults 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 two- person team searched under all mov- able 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 proce- dures. We tallied observations to cal- culate 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 com- bined similar pairs of seral stages into three generalized stages repre- senting brush/sapling, pole/sawtim- ber, and mature timber. I then com- puted 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 esti- mates for historical, present, and fu- ture 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.

RESULTS

Vegetation Structure

Comparisons of vegetation structure among the seral stages (table 1) showed that older stands had greater

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 sawtim- ber 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 ridg- etops). Stands in the two earliest seral stages, also because of logging, were smaller in area than stands in the four older stages.

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 sur- face searches; 3,066 small mammal captures during 35,070 trapnights from livetrap grids; and 510 detec- tions of larger mammals from track stations. Relative abundances of 55 species, based on the most appropri- ate sampling method for each spe- cies, are summarized in table 2. Val- ues are comparable across stages but not among taxa if different sampling methods were used. Amphibians were much more abundant in for- ested than in clearcut stands, whereas reptiles were more abun- dant in clearcuts. None of the am- phibians and reptiles [except rarer species such as northwestern sala- mander (see appendix for scientific names of vertebrates)] was absent from any stage.

Mammals exhibited a greater vari- ety of responses to seral stage. Some (e.g., Douglas' squirrel, western red- backed vole) increased in abundance from earliest to latest seral stages; others (e.g., deer mouse) decreased along this gradient. A number of spe- cies (eg., Allen's chipmunk, dusky- footed woodrat, pinyon mouse, Cali- fornia vole) were most abundant both in late shrub-sapling and ma- ture 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-Term Trends

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 old- growth 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 pat- terns of the future (table 3).

The implications of these changing distributions of seral stages for am- phibians, 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. Am- phibians showed an opposite pattern.

Four of the eight species are pres- habitat than early brush/sapling, ently less abundant and five of the pole, and sawtimber stages. Among eight may be less abundant in the fu- ture. 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

amphibians, only ensatinas were cap- tured 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 con- clusions.

With the exception of the deer mouse, small mammals were more abundant on late bmsh/sapling sites. Dusky-footed woodrats were of spe- cial interest in this regard as we ob- served many woodrat nests built among the stems of tanoak and Pa- cific madrone in late brush/sapling sites. The combination of abundant mast, good nesting substrate, and protection from predation (spotted owls rarely forage in old, brush- dominated clearcuts) provided by the dense, brushy cover were proba- bly the reasons that woodrats and other small mammals were more numerous in late clearcut sites (Ra- phael 1987).

Tree squirrels were most abun- dant in mature forest sites and ground squirrels were more abun- dant in early clearcut sites. Chip- munks were the only squirrel that reached peak abundance in early sera1 sites. Their abundance was cor- related with the cover of tanoak in the understory (Raphael 1987). Man- agement 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/sap- ling 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 observa- tion.

Of the 55 species observed, 20 were strongly associated with either older (9 species) or younger (11 spe- cies) stands (table 4). Three salaman- ders and six mammals were associ- ated with older stands. One toad, one frog, five lizards, and four mam- mals 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.

I L I I 1 1 4 5 1

SEW STAGE

Figure 1 .-Mean numbers of amphibian, reptile, and mammal species observed in serel stages of Douglas-fir forest, northwest- ern California, 1981 -1 983. Sera1 stages (and numbers of stands sampled) are: 1 - early brush/sapling (n = 10); 2 - late brushlsap- ling (n = 10); 3 - pole (n = 10); 4 - sawtimber (n = 27); 5 - mature (n = 56); 6 - old-growth (n = 53). Vertical lines indicate 9Soh confi- dence intervals.

n n -3 -2s D a so n >n CHANCl N MMwuf ( X )

Figure 2.-Percent change in population size of amphibian, reptile, and mammal species at present and in the future relative to estimated historical populations. Histo- grams represent the numbers of species increasing or decreasing by specified per- centages.

I examined habitat associations among each of the above 9 species by computing correlations of their abun- dance with specific habitat compo- nents (table 5). Density of large trees and hardwood volume were corre- lated with the abundance of most species. Moisture, as measured by the presence of surface water, mois- ture-loving tree species, or north-fac- ing slopes, was important for most mammals and one salamander spe- cies. Four mammal species were sig- ni fican tl y more abundant on higher elevation stands. Downed wood vol- ume also was significantly and posi- tively correlated with abundance of four amphibian and mammal spe- cies. The abundance of hardwoods in the understory was important for many species in each group. In con- trast, 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 projec- tions, although based on an intensive sampling effort, are highly specula- tive. Three assumptions must be rec- ognized 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 esti- mate responses of vertebrates with- out regard to size and juxtaposition of stands comprising each stage. However, continued fragmentation of forest habitats may result in dis- junct patches so small they cannot support a species that would other- wise find the habitat suitable. Rosen- berg 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 sub- stantial 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 compo- nents 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 south- ern affinity that are associated with open, drier habitats in other parts of their ranges, and decreased abun- dance among species of boreal affin- ity 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 con- trast, the decreasers are almost all species with rather restricted total ranges, most of which are in threat- ened 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, man- agement 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 rec- reate) natural components of regen- erating 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 characteris- tics of forests sf various ages that are important to survival of most spe- cies.

Finally, results of this study ad- dress another important forest man- agement 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 pres- ently much less abundant compared with historic numbers (fig. 2). Addi- tional reductions because of contin- ued timber harvest will cause further declines in some species but most major declines have already oc- curred. Therefore, I believe that esti- mates of historic populations should

be used as baselines for monitoring biological diversity, rather than pre- sent populations.

ACKNOWLEDGMENTS

Field studies were funded by the Pa- cific Southwest Region and the Pa- cific Southwest Forest and Range Experiment Station of the USDA For- est 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 sup- port; Bruce G. Marcot for freely shar- ing information from his studies and for valuable discussions; and Ken- neth V. Rosenberg, Fred B. Samson, and Hobart M. Smith for their com- men 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 ........................ Pacific giant salamander .......................... Olympic salamander .................................. Rough-skinned newt .................................. Del Norte salamander ................................ h s a tina ........................................................ Black salamander ........................................ Clouded salamander ..................................

Frogs and toads Tailed frog .................................................... Western toad ................................................ Pacific treefrog ............................................ Foothill yellow-legged frog ...................... Bullfrog ........................................................

Turtles Western pond turtle ..................................

Lizards Western fence lizard .................................. Sagebrush lizard ........................................ Western skink .............................................. Southern alligator lizard .......................... Northern alligator lizard ..........................

Snakes Rubber boa .................................................. Ringneck snake ............................................ Sharp-tailed snake ...................................... Racer .............................................................. Gopher snake .............................................. Common kingsnake ....................................

Ambystoma gracile Dicamptodon ensatus Rhyacotriton olympicus Taricha granulosa Plethodon elongatus Ensatina eschscholtzi Aneides flavipunctatus Aneides ferreus

Ascaphus truei Bufo boreas Hyla regilla Rana boylei Rana catesbeiana

Clemmys marmorata

Sceloporus occidentalis Sceloporus graciosus Eumeces skiltonianus Gerrhonotus multicarinatus Gerrhonotus coeruleus

Charim bottae Diadophis pnctatus Phyllorhynchus decurtatus Coluber constrictor Pituophis melanoleucus Lampropeltis zonata

Common gartersnake .................................. Thamnophis sirtalis Western terrestrial gartersnake ................ Thamnophis elegans Western rattlesnake .................................... Crotal is 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 boy1 ii 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