Relating geomorphic change and grazing to avian communities in riparian forests

Avian conservation in riparian or bottomland forests requires an understanding of the physical and biotic factors that sustain the structural complexity of riparian vegetation. Riparian forests of western North America are dependent upon flow-related geomorphic processes necessary for establishment of new cottonwood and willow patches. In June 1995, we examined how fluvial geomorphic processes and long-term grazing influence the structural complexity of riparian vegetation and the abundance and diversity of breeding birds along the upper Missouri River in central Montana, a large, flow-regulated, and geomorphically constrained reach. Use by breeding birds was linked to fluvial geomorphic processes that influence the structure of these patches. Species richness and bird diversity increased with increasing structural complexity of vegetation (F_1,32 = 75.49, p < 0.0001; F_1,32 = 79.76, p < 0.0001, respectively). Bird species composition was significantly correlated with vegetation strata diversity (r_s,33 = 0.98, p < 0.0001). Bird abundance in canopy and tall-shrub foraging guilds increased significantly with increasing tree cover and tall-shrub cover (F_1,22 = 34.68, p < 0.0001; F_1,20 = 22.22, p < 0.0001, respectively). Seventeen bird species, including five species of concern (e.g., Red-eyed Vireo [Vireo olivaceus]), were significantly associated (p < 0.10) with structurally complex forest patches, whereas only six bird species were significantly associated with structurally simple forest patches. We related the structural complexity of 34 riparian vegetation patches to geomorphic change, woody vegetation establishment, and grazing history over a 35-year post-dam period (1953–1988). The structural complexity of habitat patches was positively related to recent sediment accretion (t₃₃ = 3.31, p = 0.002) and vegetation establishment (t_20.7 = −3.63, p = 0.002) and negatively related to grazing activity (t_19.6 = 3.75, p = 0.001). Avian conservation along rivers like the upper Missouri requires maintenance of the geomorphic processes responsible for tree establishment and management of land-use activities in riparian forests.