Effects of Thinning on Small Mammals in Oregon Coastal Forests

Abstract

Because of fires and intensive logging practices, young forest stands dominate much of the landscape of the Pacific Northwest. Most young stands were reforested with Douglas-fir (Pseudotsuga menzeisii) trees at high densities. Researchers have proposed thinning of the densely stocked young stands as a means to improve habitats for vertebrates. However, effects of thinning intensity on forest-floor small mammals are not well understood. During 1994-1996, we conducted experimental and retrospective studies using pitfall trapping to assess effects of thinning intensity on abundance and reproduction of small mammals in Douglas-fir forests of the Oregon Coast Range, USA. In the experimental study, we assessed the short-term effects of thinning stands to moderate and to low tree densities on small mammals during the first 2 years following thinning. In the retrospective 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 mammals 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 retrospective 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. The number of captures for both species increased in the first 2 years following thinning and was 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. Reproductive performances of deer mice (Peromyscus maniculatus) and creeping voles improved following thinning in the short term. In the retrospective study, reproductive performance 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 thinning is a viable option to enhance habitat quality for several species of forest-floor small mammals in densely stocked, young Douglas-fir stands.