Bird response to landscape pattern and disturbance across productivity gradients in forests of the Pacific Northwest

Abstract

Managing forest lands for biodiversity is a common goal in the public and private forests of the Pacific Northwest and is typically achieved through harvests that result in an array of vegetation structural conditions that provide suitable habitat for a number of species. The assumption is made that the causative factors of biodiversity do not vary geographically and that silviculture, as a management tool, can be applied similarly across different biophysical locales. The primary aim of this research is to better understand how species respond to both local and landscape-scale forest structural conditions in landscapes with different levels of productivity (e.g. gross primary productivity). We hypothesized that the influence of landscape effects on bird richness, abundance and community organization would be more pronounced in highly productive environments. We also hypothesized that species response to disturbance would differ across gradients in ecosystem productivity. We predicted that bird diversity would increase with increasing disturbance extent where favorable climatic conditions result in high levels of competitive exclusion. Alternatively, we predicted that bird diversity would decrease with increasing disturbance extent when factors other than competition limit or regulate bird species diversity.

We found that (1) a number of individual bird species respond significantly to landscape effects; (2) the slope of response to changes in edge density followed predictable patterns for bird canopy guilds; (3) many more species responded to landscape effects in a more productive setting; (4) landscape effects appear to be more pronounced at the community level in more productive settings; and, (5) bird species richness responded differently to increases in the amount of the landscape recently disturbed by timber harvest. These results support the premise that management of forest lands for bird diversity will be more effective if tailored to site conditions such as productivity. In productive landscapes, forest managers will likely increase landscape-scale diversity by providing all seral-stages and a range of forest structural complexity. In less productive settings, biodiversity will likely be maximized by managing local productive hotspots judiciously and adjusting harvest intensities in other locations to compensate for slower recovery and growth rates of vegetation following timber harvest.