| Abstract |
- Silviculture systems that involve commercial thinning may provide higher quality
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 californicus) 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 were
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.
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