Evaluating the role of active management in mature Douglas-fir (Pseudotsuga menziesii) stands for songbird conservation
Forest birds, particularly those associated with late-successional forests, are of widespread conservation interest. Although birds are among the more widely studied taxa of forest wildlife, relatively few studies have examined the long-term effects of active management (i.e., intentional stand density reduction) on the forest bird assemblage. This is an important omission, as changes in stand structure and composition over the decades following harvesting may influence wildlife utilization of forest stands. We developed an observational study to evaluate forest bird response to stand structural development multiple decades following harvesting with differing levels of overstory density reduction, and in unmanaged stands. We focused, in particular, on songbirds and cavity excavating species associated with old-growth Douglas-fir forests, and the active management strategies that we examined – thinning and structural retention harvesting – were selected based on their potential to accelerate stand structural development. Our 18 mature stands (age range: 107–187 years) were located in the western hemlock zone of western Oregon, and bird surveys were conducted an average of 41 years and 22 years post-harvest in thinned and retention harvest stands, respectively. Poisson generalized linear models were formulated to evaluate the effects of management condition on forest birds. Although five species were associated with specific management conditions, relative abundance was not statistically different between unmanaged, thinned and retention harvest stands for a majority of the species in our analysis. Species richness was also relatively invariant across management conditions. Our results do suggest that retention harvesting adversely affects some songbird species associated with old-growth forests – Pacific-slope flycatcher ( Empidonax difficilis ) and Pacific wren ( Troglodytes pacificus ) for example - but our findings also indicate that retention harvesting has long-term benefits for birds associated with early-successional forests. This includes migrant species that have experienced significant population declines in Western North America over recent decades, such as Wilson’s and MacGillivray’s warblers ( Cardellina pusilla and Geothlypis tolmiei , respectively). Overall, our results imply that for many species of forest birds, including those associated with old-growth forests, managers have some flexibility in overstory density management where long-term species persistence is an objective. Equally significant, our results provide strong support for the application of variable retention harvesting as a tool for the conservation of bird species associated with early-successional forests, while also reinforcing the value of mature structural legacies for bird species associated with late-successional forests. While also reinforcing the value of mature structural legacies for birds associated with late-successional forests.