Riparian edge effects on boreal balsam fir bird communities

Abstract

Riparian forests are often identified as prime habitat for wildlife because of the presence of particular plant communities and edges creating a highly developed and diversified vegetation structure. However, in the northeastern boreal forests of Canada, where narrow land–water ecotones with abrupt edges are quite common, the relative habitat value of riparian forests remains to be demonstrated. We compared bird communities of eight pairs of riparian and nonriparian plots, similar in vegetation structure and composition, to verify the relative value for breeding birds of typical coniferous riparian forest stands of the southern boreal region of eastern Quebec. Bird abundance (P = 0.02), richness (P = 0.03), and diversity (P = 0.02) were significantly higher in the riparian stands, where the spatial sequence of three distinct habitats (a balsam fir (Abiesbalsamea (L.) Mill.)–northern white cedar (Thujaoccidentalis L.) forest, a narrow alder (Alnusrugosa (Du Roi) Spreng.) - grass wetland, and water) created high horizontal vegetation diversity. In riparian stands median bird richness and density were, respectively, 23.5 species and 437.5 territories/km2 compared with 19.0 species and 348.2 territories/km2 for interior stands. Nine species were observed exclusively in riparian plots. In addition to the species usually found in the studied nonriparian forests, the riparian plots were used by species typically related to the water edge such as the Northern Waterthrush (Seiurusnoveboracensis Gmelin) and Rusty Blackbird (Euphaguscarolinus Miiller) as well as species associated with the shrub and grass wetland such as the American Robin (Turdusmigratorius L.), the Veery (Catharusfuscescens Stephens), and the Common Yellowthroat (Geothlypistrichas L.). The shrub-foraging guild showed higher abundance in riparian stands. The tree-foraging and tree-bole foraging guilds, however, were comparable in both groups of stands. In this study, the natural conditions prevailing along he riparian sites appeared mostly positive for the breeding-bird community; it created what is perceived as being an "edge effect." The edge effect can be defined, in this case, as being the additional density and number of species induced by the added horizontal vegetation diversity created by the close association of three extremely different ecosystems: a forest stand, an aquatic ecosystem, and a narrow but distinct shrub-grass wetland. This also confirms the necessity of distinguishing natural edges that are permanent features of the landscape from induced edges created by human activity.