Forest stand structure of the northern spotted owl's foraging habitat

Although the spotted owl's close association with old growth has been exten sively studied, it has been more difficult to identify and quantify the abu ndance of particular stand structures associated with preferred owl foragin g sites. Old-growth forests have a suite of characteristics that distinguis h them from younger forests but which also make it difficult to isolate ind ividual structural features important to the spotted owl. This study used a n analysis of use-only sites in areas where natural disturbance had created a gradient of old-growth structural characteristics. We used radio telemet ry data collected from reproducing owl pairs to locate sample stands and co mpute a relative measure of owl-use intensity in each stand. Snag volume an d tree height class diversity (a measure of canopy layering) were the stand structures significantly associated with owl foraging intensity. Stands wi th 142 m(3)/ha of intact, snags and a high diversity of tree heights had me dium or high foraging use by spotted owls. In these old-growth stands, biol ogical legacies (e.g., large trees and snags) produced by past disturbance provide important forest structures associated with spotted owl foraging us e.