Effects of proximity to riparian zones on avian species richness and abundance in montane aspen woodlands

Riparian zones often provide more food or nesting resources than surrounding ecosystems and thus support more species or a greater abundance of birds. However, the extent to which the positive effects of riparian zones extend into adjoining habitats has rarely been investigated. We examined bird species richness and abundance in aspen (Populus tremuloides) woodlands of the upper East River Valley in Colorado, a region with extensive stands of aspen located upslope from riparian zones dominated by willows (Salix spp.). Our objective was to determine how species richness and abundance in aspen stands located closer to riparian zones compared to that of aspen stands farther away. To control for elevation effects, we conducted point counts for birds and examined the effects of riparian proximity on avian species richness and abundance in three elevation belts (low, 2840–2875 m; middle, 2940–2960 m; and high, 3040–3060 m). We determined riparian proximity by measuring distances from each census site to the nearest body of water and to the nearest patch of willow. Proximity to major willow patches (≥6500 m2) had a stronger influence on species richness and abundance than did proximity to water or smaller patches of willows. Total species richness and abundance significantly increased with proximity to major willow patches at low elevation sites only. This relationship was driven by a greater prevalence of riparian-nesting species at lower elevation sites, where aspens were generally closer to riparian zones. The positive effects of willows diminished with abrupt changes in elevation. Our results highlight the importance of protecting willows around riparian zones in valley bottoms where habitat destruction caused by human land use is most prominent.