We addressed the following question: Do conifers within aspen stands (conifer invasion) increase bird species diversity in western landscapes? We tested the hypotheses that bird species diversity, measured as species richness or with the Shannon-Weiner diversity index, responds to aspen-conifer ratios (from 0% to 100% conifer) in a quadratic manner with a maximum occurring at an intermediate ratio of aspen and conifer. Extra sum-of-squares F tests comparing quadratic with linear models suggested that migratory bird diversity was inversely linearly related to the extent of conifer invasion. These linear responses were moderate (species richness: R2 ≥ 0.34, P < 0.01; Shannon-Weiner diversity index: R2 ≥ 0.34, P < 0.01). Resident species diversity appeared quadratically related to conifer invasion. However, variation was poorly described for species richness (R2 ≤ 0.13, P ≥ 0.09) and was marginal for the Shannon-Weiner diversity index (R2 ≤ 0.27, P ≤ 0.01). We concluded that mixed aspen-conifer stands do not have higher bird species diversity than pure aspen stands and that management activities should focus on heavily conifer-invaded stands to increase bird diversity in western landscapes and help reverse the decline of aspen habitat due to conifer invasion.
Hollenbeck, Jeff P. and Ripple, William J.
"Aspen and conifer heterogeneity effects on bird diversity in the northern Yellowstone ecosystem,"
Western North American Naturalist: Vol. 67
, Article 13.
Available at: http://scholarsarchive.byu.edu/wnan/vol67/iss1/13