Characteristics of Forests at Spotted Owl Nest Sites in the Pacific Northwest

Abstract

We describe the structure of forests at 105 nest sites of northern spotted owls (Strix occidentalis caurina) in the Klamath, Coast, and Cascade provinces of western Oregon and the Olympic province of Washington. This information is critical for management and recovery of this threatened species. We compared forest stand data at nest sites with data from 105 random sites, using logistic regression for 1:1 matched pairs. All random sites were located in older forests (overstory trees >50 cm diameter at breast height (dbh)) within owl home ranges. Most nests in Oregon were in Douglas-fir (Pseudotsuga menziesii) trees (88%), whereas nests in the Olympic province were equally divided among Douglas-fir, western hemlock (Tsuga heterophylla), and western redcedar (Thuja plicata). In all 4 provinces, nests were most often located in live trees (73-97%) with broken tops (60-93%), most of which were fire scarred (77-83%). Mean diameter of all nest trees (n = 105) was 139.4 ? 5.2 cm ( ?_ SE). Most nests (83%) were in cavities; of the 17% nests that were in platforms, most (67%) were in the Klamath province. The majority of nest sites (95%) were found from the middle to the bottom of slopes. Mean aspects at nest sites were southerly in Oregon and northwesterly in the Olympics. Elevations at nest sites were lower than at their paired random sites, and evidence of fire was present at 86% of nest sites. Univariate analyses indicted nest sites were associated with structurally diverse older forests exhibiting characteristics typical of old-growth forests in the Pacific Northwest. Mean diameter of large trees (>100 cm dbh) was greater at nest than random sites (130.6 ? 1.8 cm vs. 123.1 ? 1.3 cm; P 53.3 cm dbh with 1 or more secondary crowns) were also greater at nest sites than at random sites in all physiographic provinces (P < 0.001), as was the volume of logs in Decay Classes 4 and 5 (P < 0.029). The best multivariate model based on the lowest Akaike's Information Criterion (AIC) value indicated the combination of greater basal area of broken-top trees, greater basal area of small trees in Class 1, larger volumes of logs in Decay Class 5, and lower elevations best distinguished nest sites from random sites. If silvicultural prescriptions are designed in an attempt to produce the stand structure of nesting habitat, they must consider the roles fire and other disturbances have played in creating the diverse stand structure found at nest sites.