Snags in Beaver Ponds and Indications of Use by Primary Cavity-Nesting Birds in the Western Oregon Cascades

Jacqueline I. Fern

Master of Science in Environmental Science

Portland State University

Presented July 18, 2001.

Abstract

Dead trees, or snags, are used by primary cavity-nesting birds for nesting, foraging, and roosting and are essential habitat for these species. Snags formed in beaver ponds due to flooded conditions are utilized by a variety of woodpecker species. In this study I quantified and compared snag density, size, decay characteristics, and excavations in beaver ponds and in forested riparian sites without beaver influence (reference sites) in the western Oregon Cascades. Beaver ponds were treated as a pooled group (n=8) and also categorized into old (n=5) and new (n=3) classes based on decay indicators. Reference sites (n=8) were treated as a pooled group.

Snag density was significantly higher in pooled and new beaver ponds (P = 0.014 and P < 0.001) compared to reference sites. In general, reference site snags had larger diameters (P < 0.001) and were in more advanced decay stages (P < 0.001) than beaver pond snags. The proportion of snags excavated was significantly higher in reference sites compared to pooled beaver ponds and new ponds (P = 0.007 and P < 0.001, respectively). In comparisons between reference sites and old beaver ponds, neither snag density (P = 0.104) nor proportion of snags excavated (P = 0.065) differed significantly. In new ponds, snag density was high, snags were in early decay stages and utilization by primary cavity-nesting birds was low. As ponds aged, snag density decreased, remaining snags were larger and more decayed, and utilization by primary cavity-nesting birds was higher. In riparian habitat without beaver influence, snag density was low, snags were large and in advanced stages of decay, and utilization was high. These findings suggest that snags in beaver ponds form and reach optimal stages of decay quickly, yet also decompose rapidly. In riparian forests without beaver influence snags form and reach optimal stages of decay more slowly, yet remain on the landscape longer.