Graduate Thesis Or Dissertation
 

Created Snag Dynamics and Influence on Cavity-nesting Bird Communities over 25 years in Western Oregon

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/c821gn068

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  • Snags provide critical habitat for nearly one-third of wildlife species in forests of the Pacific Northwest, so historic declines in snags are thought to have had a strong impact on biodiversity. Resource managers often create snags to mitigate the scarcity of snags within managed forests, but information regarding the function and structure of created snags across long time periods (>20 years) is absent from the literature. Using snags that were created by topping mature Douglas-fir trees (Pseudotsuga menziesii) as part of the OSU College of Forestry Integrated Research Project, we measured characteristics of 731 snags and quantified foraging and breeding use of snags by birds 25-27 years after their creation. We also examined whether different harvest treatments (i.e., group selection, two-story, clearcut) and snag configurations (i.e., scattered and clustered) influenced snag characteristics or avian use for foraging and nesting. In addition, we conducted point count and call play-back surveys to calculate naïve occupancy estimates for cavity-nesting bird species. Finally, we compared current estimates of snag condition and avian use to estimates from historic surveys to assess changes over time. We found that 91% of created snags remained standing and 65% remained unbroken during the period of our study. We also found that 54% of snags had bark peeling away from the bole of the snag and mean bark cover was 82%. Relative to historic surveys of the same pool of snags, we found that breaking and peeling bark has greatly increased compared with historic surveys. Furthermore, decay characteristics and historic use of snags (cavity-cover) differed among the three harvest treatments. Snags created in the group selection treatment exhibited lower amounts of bark loss and cavity cover than snags created in the two-story or clearcut treatments, whereas characteristics between snag configurations were generally similar. Despite observing snags for >750 h throughout the course of two breeding seasons, we observed that only 11% of snags were used by cavity-nesting birds for nesting across harvest treatments. Of eight bird species detected during surveys, only four species were detected using created snags for nesting (n=36 nests). Compared with historic surveys, use of snags for nesting has decreased in both proportion of snags used and species richness. In particular, the proportion of woodpecker species using snags for nesting has decreased from 23% in 1996 to 3% by 2016. Additionally, we observed foraging by nine bird species although the rate of foraging observations was low (0.05 observations/h). Extent of foraging also decreased 8× from the most recent historic surveys conducted in 2001. Our results suggest that ≥25-year old created snags in managed forests provided limited nesting and foraging opportunities for cavity-nesting birds, and their use has declined markedly since 2001. If management for biodiversity is a goal of snag creation, then retaining or creating a variety of age classes of snags on the landscape is expected to support the full diversity of species that depend on snags for nesting or foraging.
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  • 2017-08-04 to 2018-04-07

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