Nobuya Suzuki, Evergreen
State College, Olympia, WA
Because of fires and
intensive logging practices, young forest stands dominate much of the landscape
of the Pacific Northwest. Most of young
stands were reforested with Douglas-fir (Pseudotsuga
menzeisii) trees at high densities.
During 1994–1996, we conducted experimental and observational studies to
assess effects of thinning intensity on abundance and reproduction of small
mammals in Douglas-fir forests of the Oregon Coast Range. In the experimental study, we assessed the
short-term effects of thinning stands to moderate (100 trees/acre) and to low
(50 trees/acre) tree densities on
small mammals during the first 2 years following thinning. In the observational study, we assessed
potential long-term effects of thinning by comparing relative abundance and
reproductive performance of small mammals in previously thinned (7-24 years
prior to the study) and unthinned stands.
Among the 12 species of small mammal we examined in the experimental
study, number of captures increased for 4 species and decreased for 1 within 2
years of thinning; however, responses were similar between moderately and
heavily thinned stands. Among the 9 species we examined in the observational
study, number of captures was greater for
5 species and lower for none in previously thinned than in unthinned
stands. Furthermore, total number of
small mammals captured was higher in previously thinned than in unthinned
stands. Effects of thinning on 2
species, creeping voles (Microtus oregoni)
and Pacific jumping mice (Zapus
trinotatus), were consistent in the short and long term; number of captures
for both species increased in the first 2 years following thinning and were
greater in stands thinned 7-24 years previously than unthinned stands. Number
of western red-backed voles (Clethrionomys
californicus) captured decreased within 2 years of thinning but was similar
in stands thinned 7-24 years previously and in unthinned stands. In the observational study, the reproductive
output of western red-backed voles was higher in thinned than in unthinned
stands. Overall, thinning did not have
substantial detrimental effects on any of the species we investigated and had
positive effects on several. We suggest
that potential negative effects of thinning could be minimized with a careful
planning based on ecological factors.
Keywords: abundance,
reproduction, small mammals, thinning.