Ecological forestry to enhance stand complexity: Songbird community response in a red pine (Pinus resinosa) forest

Structural complexity has long been recognized as important for maintaining songbird diversity. In recognition of the simplified condition of many managed forests, forest scientists have initiated efforts aimed at increasing the structural and compositional complexity of forest stands. Songbird community assemblage following timber harvest has been studied to assess these enhancement efforts, though current literature is largely limited to short-term responses and may not reflect longer-term patterns in response. Through a designed, replicated (n = 4) experiment located in northern Minnesota, we evaluated bird community response to differences in forest structure created through retention harvesting over a ten-year period. Treatments include three overstory manipulations (dispersed retention; aggregated retention with small harvest gaps; aggregated retention with large harvest gaps), one understory manipulation (mechanical brush removal), and controls (unharvested overstory; and/or unbrushed understory). Treatments were designed to alter forest structural complexity compared to controls and to restore more open understory conditions that resulted from now-suppressed surface fires. I hypothesized that songbird response would be different in treatment stands than in unharvested controls as a result of increased structural heterogeneity in harvested areas. Surveys were conducted in 2003, each odd year thereafter through 2011, and in 2012. Over this period avian community abundance and richness increased in overstory treatments compared to unharvested controls; no community-level differences were found between understory treatments. Foliage gleaning and tree- or shrub-nesting guilds represented the greatest number and abundance of species; cavity nesting guild abundance and richness were comparably low. Species positively responding to overstory treatments were generally associated with early seral and edge habitats (e.g., American Redstart [Setophaga ruticilla], Chestnut-sided Warbler [Setophaga pensylvanica], and Mourning Warbler [Geothlypis philadelphia]) or a mature overstory component (e.g., Yellow-rumped Warbler [Setophaga coronate]). Ovenbirds (Seiurus aurocapilla) were found at higher densities in the unharvested controls and within the matrix areas of aggregate retention treatments compared to dispersed treatments and gaps. American Redstarts were more abundant at unbrushed understory treatments. Additionally, a blocking effect was found for this species that appears related to vegetative compositional differences measured between experimental blocks. My results suggest that forest birds respond to increased structural complexity created through retention-based timber harvesting, and that forest stand composition is an important habitat consideration for certain bird species. Further, divergence in avian community composition between treatment sites and unharvested controls suggest an important component of a biodiverse forest landscape includes areas of mature, uncut forest