Tree invasion constrains the influence of herbaceous structure in grassland bird habitats

Trees and other woody plants threaten grassland obligate birds, as well as the biological integrity of grasslands around the world. Bird species associated with grasslands of southern mixed-grass prairie of North America have declined in abundance, whereas species associated with shrub-stage and woodland habitats have increased. Recent increases in the extent of eastern redcedar (Juniperus virginiana) in the southern Great Plains of North America explain some of the change in bird assemblages in landscapes composed of patches of grassland fragmented by cropland and stands of eastern redcedar. In this study, we determined the influence of eastern redcedar, relative to the influence of structural attributes of the herbaceous layer, on bird assemblages within individual patches of grassland habitat. We indexed bird abundance within the breeding season with point counts on grassland patches with varying levels of invasion of eastern redcedar. Canopy cover of eastern redcedar explained a greater proportion of the composition of bird communities in these grasslands than structure of herbaceous vegetation. Species associated with grassland habitats generally declined in abundance, whereas species associated with shrub and woodland habitats increased as cover of eastern redcedar increased. Perhaps more important to conservation ecology, our data indicate that as canopy cover of eastern redcedar increased, variation in abundance of grassland birds decreased, indicating that canopy cover of eastern redcedar may constrain the local influence of herbaceous habitat structure on bird assemblages.