Habitat Configuration around Spotted Owl Sites in Northwestern California

Abstract

During each breeding season between 1988 and 1992, nests and daytime roosts were located for most territorial members of a contiguous population of Northern Spotted Owls (Strix occidentalis caurina) in northwestern California. Using Landsat imagery, we compared the amount of five land cover types, mature and old-growth forest fragmentation, and seral stage heterogeneity within 800 m (200 ha) circular plots around nest, roost, and random sites. This plot size was based on the observed spatial distribution of owl sites (one-half the average nearest-neighbor distance between territory centers) within the study area. Nest and roost sites were characterized by lower amounts of nonvegetated and herbaceous land cover, and greater amounts of mature and old-growth coniferous forest which was less fragmented than random sites. Mean amounts of mature and old-growth forest within 200 ha radius plots were 94.1 ha, 92.0 ha, and 71.8 for nest, roost, and random sites, respectively. The area of other land cover types was similar between owl and random sites. All habitat variables were similar at nest and roost sites. To evaluate the influence of spatial scale, habitat variables around nest and random sites also were estimated within eight concentric circular plots ranging from 800 to 3,600 m radii. Differences between nest and random sites in the amount and fragmentation of mature and old-growth forest were significant (P < 0.01) out to 1,200 m. Differences in the amount of nonvegetated and herbaceous, and seral stage heterogeneity were significant (P < 0.05) out to 1,200 m and 800 m, respectively. These results indicate that spatial scale of sampling is important and will affect analytical results. Our findings from the Klamath Physiographic Province of California were similar to results from comparable studies in Oregon and Washington.