Does available energy influence avian response to key forest stand habitat features

Background/Question/Methods

Private forest managers are often charged with responsibility for maintaining structural complexity, biological diversity, and ecosystem services on their lands.  However, little consideration has been given to potential geographic variation in the nature of relationships between elements of forest structure and biodiversity.  Decomposition rates of snags and coarse woody debris are mediated by available energy, and the state of decay in snags and coarse woody debris affects their use by birds and other wildlife.  Accordingly, the value of stand level structural components to wildlife species may depend on the amount of energy available at the local scale.  We sampled montane avian communities across an energy gradient in the Cascades Mountains of Oregon and Washington, USA.  We predicted avian functional diversity would be positively associated with large snag density in a high energy landscape in Oregon and decrease in a low energy landscape in Washington.  We also predicted abundance and richness of species within ground foraging guilds would increase with increasing coarse woody debris in higher energy settings, but that coarse woody debris would not be a significant factor in lower energy locations. We fit a Bayesian multispecies site occupancy model to estimate species level covariate effects as well as population level measures of occupancy, including species richness.  For abundance data, we fit a Bayesian multispecies version of the N-mixture model. 

Results/Conclusions

We found nearly all foraging guilds were represented in each stand.  As a result, the strength of association between avian functional diversity, snag and coarse woody debris density was small in both high and low energy landscapes.  A large and precise negative response of key foraging guild bird abundance to large snag density was observed in the lower energy site. 95 percent posterior credibility intervals for the community-level hyper-parameters for snag density at the low energy landscape did not include zero, indicating a defined overall population-level trend.  Large but less precise negative responses of primary guild bird abundance to coarse woody debris density were observed at both high and low energy landscapes.  In this case, 95 percent posterior intervals for the community hyper-parameters included zero at both landscapes, indicating little overall population-level trend with respect to coarse woody debris.  We discuss how residual forest structure can be managed across a gradient in available energy to conserve avian diversity in forested settings.