Differences in bird species richness and abundance among three successional stages of aspen-dominated boreal forests

Abstract

Birds and vegetation were surveyed in young, mature, and old aspen-dominated boreal forests in Alberta. Height and size of live trees, density of large dead trees, and volume of downed woody material increased during succession, whereas density of live trees decreased. Canopy heterogeneity had a bimodal relationship with succession: old forests had the highest, mature forests the lowest, and young forests an intermediate canopy heterogeneity. Old forests had greater bird species richness than young forests, which in turn had greater richness than mature forests. Twenty-seven, 3, and 10 bird species had their highest abundances in old, mature, and young forests, respectively. Seven bird species that nest and forage in canopy gaps and three bird species that nest and forage in large trees and snags were more abundant in young and old forests than in mature forests. Contrary to our predictions, patterns of richness and abundance for bird species that nest and forage in the canopy or in tree cavities were similar to those for bird species that nest or forage in the lower strata. Bird species preferring coniferous forests tended to be more abundant in old than in young or mature aspen-dominated forests, possibly because old aspen forests had more conifers than younger aspen forests.