Adaptive Management Monitoring of Spotted Owls

Larry L. Irwin and Dennis Rock, National Council for Air and Stream Improvement, Corvallis, OR

Over the next several decades, several million acres of timberlands in early- to mid-successional stages are likely to be thinned or partially harvested. Yet, there is little scientific guidance for conducting such intermediate manipulations relative to protecting Spotted Owls (Strix occidentalis). This study monitors Spotted Owl responses to recent and planned applications of such less-intensive forestry practices. We are monitoring habitat-use and home range configurations in response to thinning treatments in young forests (approx. 30-60 years of age) and to partial-cutting treatments in mixed-age forests. Seven study-area replicates have been initiated in western Oregon and northern California, each with 8-10 pairs of radio-tagged owls. In Oregon’s Douglas-fir Zone, thinning treatments leave approximately 120-150 trees per acre. In Mixed Coniferous forests, partial-harvesting treatments involve reductions to about 110-140 square feet per acre, leaving snags and coarse woody debris, and providing 40-60% canopy cover. The aim is to evaluate previous (i.e., retrostpective analysis) and planned treatments of up to 200 acres within 1000-acre “core areas”. Core areas occupy about 15-20% of annual home ranges, yet receive 60-80% of the use by owls. The study began in Spring 1998 and should extend at least through 2004. As of January 2002. About half of the silvicultural treatments have been implemented, and the remainder are expected to be completed by 2003. Currently, 104 individual owls are being radio-tracked at 62 owl sites. We have mapped 15, 905 radio-telemetry locations. Factors that influence habitat selection for foraging include elevation, topography, distance to riparian zones, basal area, quadratic mean diameter, snag density, and fuel loads. Cumulative home range size for birds with two years of data (using the 95% fixed-kernel algorithm) varies from 1,125 acres in the California redwood zone to 2,700 acres in the Douglas-fir zone, whereas core areas range from < 200 acres to 1,000 acres.