Biodiversity potential in the Pacific and Inland Northwest : the relative importance of forest structure and available energy in driving species diversity

Abstract

Currently, the most common strategy of forest biodiversity managers in the Pacific and Inland Northwest (PINW) is to maintain structural complexity within forest stands and create the full range of seral stages across the landscape. Recent advances in ecological theory reveal that biodiversity at regional to continental scales is strongly influenced by available energy (i.e. factors influencing vegetative growth such as precipitation, temperature, radiation, soil fertility). We hypothesized bird species richness (BSR) exhibits a positive or unimodal relationship with energy across landscapes and the relationship between energy and BSR within a landscape is positive in energy limited landscapes and flat or decreasing in energy rich landscapes. Additionally, we hypothesized that structural complexity explains a lower percentage of the variation in BSR in energy limited environments and higher percentage in energy rich environments and that the slope of the relationship between structural complexity and BSR is greatest in energy rich environments.

Ours is the first study to test the relative influence of energy and vegetation structure on BSR at local scales, and to offer predictive theory on how these influences differ across energy gradients. We sampled bird communities and vegetation across a matrix of seral stages and biophysical settings at each of 5 landscapes. We analyzed the response of BSR to structural complexity and energy covariates at each landscape. We found that (1) BSR had a unimodal relationship with available energy across the PINW region, (2) the landscape-scale relationships between energy and BSR were positive or unimodal in energy limited locations and were flat or negative in energy rich locations, (3) forest structural complexity explained more of the variation in bird species richness in energy rich landscapes, and (4) the slope of the relationship between forest structural complexity and BSR was steepest in energy limited locations and negative in energy rich locations. In energy rich locations, forest managers will likely increase landscape-scale diversity by providing all seral-stages and a range of forest structural complexity. In low-energy environments, biodiversity will likely be maximized by managing local high-energy hotspots judiciously and adjusting harvest intensities in other locations to compensate for slower regeneration rates.