Cooperative Fish and Wildlife Research Units Program: Montana Wildlife
Education, Research and Technical Assistance for Managing Our Natural Resources

Aspen (Populus tremuloides) forests hold a high diversity of organisms but are increasingly invaded and replaced by conifers. The consequences of this habitat change for diversity and demography of organisms, such as birds, remains poorly understood. Conifer encroachment of an aspen stand may increase bird diversity by increasing vegetation complexity. However, the presence of conifers may also impact the reproductive success of birds via addition of conifer-dependent nest predators or via changes in habitat structural complexity. We examined the effects of conifer encroachment on the diversity, reproductive success, and population growth rates of birds in fourteen aspen stands that differed in the extent of conifer encroachment in Montana, USA. Bird species diversity increased with aspen stand area but was not correlated with proportion of conifer trees in a stand. Nest predator density increased with proportion of conifer trees in a stand. Conifer encroachment directly increased nest predation rates for four of ten open-cup nesting species and indirectly increased nest predation rates for three additional open-cup nesting species. Furthermore, increased nest predation with greater conifer encroachment transformed source breeding habitat (λ > 1) into sink habitat (λ < 1) for these seven species. Based on these results, conifer encroachment into aspen stands has the potential to decrease diversity, reproductive success, and population growth rates of songbirds, which emphasizes the value of large, relatively pure aspen stands on the landscape.