Habitat relationships of small terrestrial vertebrates and invertebrates in managed forests in the southern Oregon Cascades

approved: AN ABSTRACT OF THE THESIS OF Kirsten Ayn McDade for the degree of Master of Science in Forest Science presented on July 10, 2001. Title: Habitat Relationships of Small Terrestrial Vertebrates and Invertebrates in Managed Forests in the Southern Oregon Cascades. Signature redacted for privacy. Chris C. Maguire Silviculture systems that involve commercial thinning may provide higherquality wildlife habitat than traditional clearcut systems, yet such systems have not been vigorously tested. This prospective study examined forest floor animal abundance and habitat relationships under three different silviculture conditions: clearcuts, commercial thins, and uncuts. Eighteen stands, six per treatment, were selected in the Umpqua National Forest, in the southern Oregon Cascades, and surveyed for ground-dwelling fauna. A total of 3,359 arthropods, 202 gastropods, 212 amphibians, and 456 small mammals were captured in pitfall traps; 130 gastropods and 19 salamanders were located during transect searches. Pitfall traps were open for 28 consecutive days in fall 1999; because of large numbers captured arthropods were counted only on the first nine sample days. Western-red backed vole (Clethrionomys calfornicus) and overall amphibian captures, and gastropod species richness and diversity decreased with harvest intensity. Small mammal and amphibian species richness and northwestern salamander (Ambystoma gracile) captures were significantly higher in uncut stands than either thins or clearcuts. Ensatina (Ensatina eschscholtzii) and Haplotrema snail captures were significantly higher in thin stands than clearcut stands, though, captures did not differ between clearcuts and uncuts. Small mammal, Trowbridge shrew (Sorex trowbridgii), gastropod, and cricket (Gryllidae/Gryllacrididae) captures were similar between thin and uncut stands but there were significantly fewer captures in clearcuts than uncuts. Vagrant shrew (S. vagrans), grasshopper (Acrididae), beetle (Coleoptera), and millipede (Diplopoda) captures were highest in clearcut stands. No treatment differences werç observed for deer mice (Peromyscus maniculatus), Oregon voles (Microtus oregoni), Vespericola snails, reticulated tail dropper slugs (Prophysaon andersoni), overall arthropods, or spiders (Araneae). Multiple linear regression results showed that canopy cover and basal area were important habitat variables for Trowbridge shrews, Haplotrema snails, and crickets, which suggests that these animals may be sensitive to tree removal. Other important habitat variables were down wood volume (Haplotrema and cricket), shrub cover (Haplotrema), duff and litter depth (ensatina and Vespericola) and gravely soils (Haplotrema). Trowbridge shrew, Ensatina, and Haplotrema detections were positively correlated with relative humidity and rainfall. Grasshoppers were the only animal analyzed for habitat relationships that were correlated to open forest habitat. In sum, 4 of the 15 ground-dwelling fauna species analyzed for treatment differences were found more frequently in uncuts than clearcuts. Four species had higher capture rates in thins than clearcuts and only 1 species (northwestern salamander) was captured more frequently in uncuts than thins. These results suggest that commercial thinning does not affect forest floor animal populations as severely as clearcut harvesting and may be a useful silvicultural tool that will provide both wildlife habitat and a sustainable timber supply. Habitat Relationships of Small Terrestrial Vertebrates and Invertebrates in Managed Forests in the Southern Oregon Cascades by Kirsten Ayn McDade