Understanding the effects of tree retention on bird species richness in managed forests of the Pacific Northwest: Application of a hierarchical community model

Background/Question/Methods:  Tree retention in timber harvest units is a primary means by which industrial forest landowners ameliorate negative effects of forest management on wildlife habitat and biodiversity.  This activity is recognized as beneficial; however, there is little scientific support to guide the management prescriptions (e.g., patch sizes, distribution pattern). The objective of our study was to quantify the response of birds to tree retention in timber harvest areas located in the Pacific Northwest.  We used a hierarchical community model to examine how patch size and distance from forest edge of retained trees influenced the species richness of birds using the trees.  The modeling framework integrated multiple species-specific occurrence (use) models that accounted for imperfect detection to produce more precise estimates of species richness than those of traditional approaches.  We sampled a biogeoclimatic gradient by selecting four separate study areas (two in Washington, one each in Oregon and California) that support different forest types.  Selected harvest units (n=20) within each study area were controlled for size, elevation, tree species composition, and management/disturbance history.  Observations were conducted at a random selection of retained structures (e.g., patches, individual trees) within harvest units to record bird use in the breeding seasons of 2008 and 2009.

Results/Conclusions:   Estimated occurrence (use) probabilities differed by species and were influenced by both patch size (number of retained trees) and the distance from forest edge.  The community response to patch size was mostly consistent across all study areas and years – species richness estimates increased with patch size and approximated a species-area curve.  Species richness was not related to distance from forest edge except at distances >200m where richness estimates were relatively much lower.  These results suggest that the diversity of birds using tree retention in harvest units can be maximized at patches of 15-20 rotation age trees.