Conserving avian richness through structure retention in managed forests of the Pacific Northwest, USA.

摘要

Structure retention is a practice used in managed forests to assist the conservation of biological diversity, whereby green trees, dead trees (i.e. snags), and downed wood are retained during timber harvest. This activity is recognized as beneficial. However, there is little scientific support to guide the management prescriptions (e.g. patch sizes, distribution pattern). We quantified the short-term response of birds to structure retention in timber harvest areas located in the Pacific Northwest. We used a hierarchical community model to examine how attributes of retention sites (number of trees and snags, distance to forest edge) were associated with the species richness of birds using the sites. The modeling framework integrated multiple species-specific occupancy models that accounted for imperfect detection to produce estimates of species richness. We sampled a biogeoclimatic gradient by selecting harvest units within four separate regions (two in Washington, one each in Oregon and California) that support different forest types. Observations were conducted at a random selection of retention sites (e.g. patches, individual trees) within harvest units to record bird use during the breeding seasons of 2008 and 2009. Estimated occupancy and detection probabilities differed by species and region. Retained tree count was associated with an increased occupancy probability for all observed species. The community response to tree count was consistent across all study regions and years-species richness estimates increased with tree count and approximated a species-area curve. Snag count and edge distance did not significantly affect occupancy probability for any observed species, and therefore, had no relationship with species richness. These results suggest that the diversity of birds using structure retention in harvest units can be maximized at patches of >10-15 rotation-age trees. Forest managers are encouraged to group green-trees around high-quality snags and other unique wildlife trees where possible, and to vary prescriptions across stands to provide habitat heterogeneity at the landscape scale.