Vegetation of Beaver Impoundments

Abstract

We investigated the vegetation of beaver meadows and other beaver-flooded sites using a combination of field, greenhouse, model, and GIS studies. The beaver meadows were dominated by two graminoid plant species, Calamagrostis canadensis and Carex lacustris. Carex dominated in wetter sites along streams and the edges of residual beaver ponds which had lower nitrate availability (determined using in situ resin bags), whereas the ubiquitous Calamagrostis dominated in mesic to dry portions of beaver meadows that had higher nitrate availability. Greenhouse studies with these two species plus Scirpus cyperinus and Carex rostrata investigated the effects of fertilization and wetness: increased N supply increased plant height, biomass, leaf nitrogen, and the vertical distribution of photosynthetic machinery, effects that were accentuated at field moisture capacity (as opposed to saturation). In the field, light attenuation in various beaver meadow communities was primarily a function of canopy height. A model that combined the results of the greenhouse and light attenuation studies correctly predicted that Calamagrostis had a competitive advantage over Carex lacustris under high levels of nitrogen availability, whereas the reverse was true under low nitrogen, saturated conditions. Field observations of plant cover at 108 sample points in two Kabetogama beaver meadows were analyzed using agglomerative hierarchical clustering and non-metric multidimensional scaling to distinguish eight plant assemblages. The most diverse of these assemblages occurred along the upland edges of the beaver meadows, where forest species were beginning to encroach upon the meadows. The GIS analysis showed that “wet meadow/fen mosaic complex” was the most common beaver impoundment vegetation within the Kabetogama Peninsula, followed by deep marsh mosaic complex, speckled alder swamp, Canada bluejoint eastern meadow, and northern water lily aquatic wetland. Voles may affect the recolonization of beaver meadows by conifers due to their influence on ectomycorrhizae (symbiotic root-associated fungi).