Effects of silvicultural edges on terrestrial amphibian abundance and microclimate in northwestern California

Timber harvesting can be highly detrimental to terrestrial amphibians and their habitats. Clearcutting reduces relative abundance of amphibians, however, little is known about the effects of partial-harvesting methods and silvicultural edges on amphibian abundance. I compared the relative abundance of amphibians between clearcuts aged 10 — 25 years old, silvicultural edges, and late-seral forests, and, in a separate comparison, between thinned and unthinned late-seral forests. I developed models from habitat and microclimate parameters to explain amphibian abundance. I examined movement of Ensatina eschscholtzii and Plethodon elongatus to establish if these species are capable of moving between clearcuts and adjacent forests in response to seasonal climatic changes. Ten sites were sampled in northwestern California. I established two pitfall trap transects within each site, each containing seven pitfall trap stations, for a total of 14 stations per site. Each station contained 4 pitfall traps totaling 56 traps per site. Soil temperature was recorded near pitfall trap stations at four of the 10 sites. I collected data on macro- and microhabitat variables at each station. Sampling occurred October 1999 to June 2001. I found no statistically significant difference in amphibian abundance between clearcuts, silvicultural edges, and late-seral forests (P = 0.11). Amphibian abundance did not differ between thinned and unthinned late-seral forests (P = 0.87). For the most common species captured, E. eschscholtzii, I found similar results in a separate analysis with no significant difference in abundance between clearcuts, silvicultural edges, and late-seral forests (P = 0.16) and no difference in abundance between thinned and unthinned forests (P = 0.75). I found no difference in use of forests and clearcuts (P = 0.26) or thinned and unthinned late-seral forests (P = 0.92) by E. eschscholtzii and P. elongatus. There was no difference in use of forests and clearcuts (P = 0.74) or thinned and unthinned late-seral forests (P = 0.48) by age class in E. eschscholtzii. None of the habitat models explained a large proportion of the variation in amphibian capture rates by distance from the silvicultural edge. Only 18% of the variation in amphibian capture rates was attributable to the best model. Forty-two of 289 E. eschscholtzii were recaptured, for a recapture rate of 15%. Mean distance moved was 8.6 (± 16.6) m over the sampling period. I recaptured 15 of 53 P. elongatus, for a recapture rate of 28%. This species moved a mean distance of 4.4 (± 9.6) m. From these results I believe that animals from both species captured at a location are resident animals, and that it is unlikely that these salamanders move long distances in response to seasonal climatic changes. Although differences were not statistically significant, clearcuts within the study area had lower amphibian abundance than late-seral forests. However, based on the detection of all age classes of terrestrial salamanders within clearcuts and the relatively low vagility of these animals, it appears that they can survive in 10 to 25 year-old clearcuts within the study area. Abundances at silvicultural edges were nearly as high as within adjacent late-seral forests, and thinning of late-seral forests had no apparent long-term effect on amphibian abundance. Some previous researchers have shown that timber harvesting can have detrimental effects on terrestrial salamander populations. I suggest, based on my results, that clearcutting and thinning may not significantly reduce amphibian populations and that silvicultural edges may provide suitable habitat for terrestrial amphibians in my study area.