Response of terrestrial small mammals to varying amounts and patterns of green-tree retention in Pacific Northwest forests

To sustain native species in managed forests, landowners need silvicultural strategies that retain habitat elements often eliminated during traditional harvests such as clearcut logging. One alternative is green-tree or variable retention. We investigated the response of terrestrial small mammals to experimental harvests that retained large live trees in varying amounts (approximately 100, 75, 40, and 15% of original basal area) and patterns (aggregated versus dispersed) in mature coniferous forests of western Oregon and Washington. Treatments were applied in 36, 13-ha experimental units. We used pitfall traps to sample small mammals for 4 weeks each autumn during 2 years before and 2 years after treatments. We captured 21,351 individuals of 32 species. We analyzed effects of treatments on relative abundance of 12 species. As level of retention declined, we expected species associated with closed-canopy forests to decrease (Sorex trowbridgii, Neurotrichus gibbsii, Peromyscus keeni, Myodes [Clethrionomys] californicus, and M. gapperi); species associated with early successional habitats to increase (S. vagrans, P. maniculatus, Microtus longicaudus, and Microtus oregoni); and habitat generalists to show little response (S. monticolus, S. pacificus, and S. sonomae). As expected, M. californicus declined after harvest, and P. maniculatus and M. longicaudus increased. Sorex sonomae showed an unpredicted decrease. Other species did not show consistent changes. Responses of S. monticolus, S. sonomae, and M. gapperi varied among study areas. For M. gapperi, this variation was not explained by differences in habitat structure among areas. For all species, capture rates were similar in dispersed- and aggregated-retention units. Similarity in species composition between harvested sites and controls decreased with decreasing retention. Future sampling of these treatments is needed to assess long-term responses. Based on our initial results, green-tree retention strategies need to be sensitive to regional variation in environmental characteristics and small mammal community composition.