Graduate Thesis Or Dissertation
 

Characteristics of forests at spotted owl nest sites in the Pacific Northwest

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/mw22v987x

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  • This study describes the composition and structure of forests within the immediate vicinity of Northern spotted owl (Strix occidentalis caurina) nest sites in the Klamath, Coast, and Cascade provinces of western Oregon and the Olympic province, Washington. I compared forest stand data collected at 105 nest sites with data from 105 random sites located in older forests within the owl's home range using paired t-tests and multiple logistic regression. Most nests in Oregon were in Douglas-fir (Pseudotsuga menziesii) trees (88%) whereas in the Olympics, they were equally divided among Douglas-fir, western hemlock (Tsuga heterophylla), and western redcedar (Thuga plicata). Mean dbh of all nest trees (n=105) was 139.4 cm (SE=5.2 cm). Most nests were in cavities (83%), and of the 17% that were on platforms, most were in the Klamath province. The majority of nest sites were found from the middle to the bottom of slopes. Mean aspects at nest sites were southerly in Oregon and northeasterly in the Olympics. Elevations at nest sites were lower than their paired random sites and evidence of fire was present at 86% of nest sites. Logistic regression and univariate analyses indicted that spotted owl nest-sites were associated with structurally diverse, decadent older forests. Nest sites had more densely multilayered canopies than random sites as evident from the greater density of trees, especially trees <53 cm dbh and <38 m in height. Basal area of broken-top trees and volume of decay class 5 logs were also greater at nest sites than at random sites in all physiographic provinces, (P<0.0001 and P<0.0293 respectively), and were indications of the greater decadence found at nest sites. Silvicultural prescriptions designed to produce the stand structure of nesting habitat must consider both the role fire and other disturbances have played to create the diverse species composition and stand structure found at nest sites and the importance of stand decadence in nest-site selection by spotted owls.
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