Density and Diversity Responses of Summer Bird Populations to the Structure of Aspen and Spruce-Fir Communities on the Wasatch Plateau, Utah

Sixteen stands representing a range of structural variation in aspen, mixed aspen-conifer, and spruce-fir communities of the Wasatch Plateau, Utah, were censused by the sample count method. The stands were classified as eleven community types based on the understory dominants or indicator species and the cover types. Fifty bird species were recorded during the two seasons; thirty-two occurred in aspen cover, forty-four in mixed aspen-conifer cover, and twenty-two in spruce-fir. Comparisons of the composition and density of bird populations were made between uniform stands of a single life form and more structurally complex stands of either single or mixed life forms. Limiting factors in the structural characteristics of the stands were identified for birds restricted to particular stands. Low avian similarities between some aspen stands were attributed to the differences in structure between the stands. Bird species which favored the deciduous life form tended to decrease in abundance in the mixed stands as the canopy coverage of conifers increased, and they were absent in the spruce-fir stand. Coniferous forest bird species were more abundant in mixed stands with high coniferous coverage than in the aspen-dominated stands. Low individual bird numbers were found in the conifer stand of uniform small trees. Several vegetational characteristics of the stands were evaluated to determine if any was an index of forest heterogeneity predictive of bird species diversity. The habitat features of ecological relevance to most of the bird species were the size, spacing, and life form of the trees. The diversity of the distribution of diameter measurements at breast height for the tree species was predictive of bird species diversity. High diversity in the distribution of tree measurements at breast height was correlated with variation in tree height, tree canopy diameter, and the spacing of the life forms. It was therefore an index of three dimensional environmental patchiness, easily visualized by the variation in life forms and the number of stories within the stand