A HYPOTHESIS OF THE EFFECTS OF MOOSE AND BEAVER FORAGING ON SOIL NITROGEN AND CARBON DYNAMICS, ISLE ROYALE

Abstract

Selective foraging by moose (Alces alces) and beaver (Castor canadensis) alter the species composition of forests. We hypothesize that this affects the types of litter returned to the soil, soil carbon and nitrogen pools and soil nitrogen availability. Preliminary analyses of soils collected from areas with various combinations of moose and beaver foraging on Isle Royale, Michigan, support this hypothesis. Soil carbon and nitrogen pools and nitrogen availability are significantly greater where beaver cutting of large diameter (> 10 cm) aspen (Populus tremuloides), mountain ash (Sorbus americana) and other hardwoods maintains their dominance because they sprout prolifically from roots and stumps. These species have easily decomposible litter which increases soil nitrogen availability. In contrast, moose browsing of young hardwoods suppresses their growth, allowing unbrowsed spruce (Picea glauca, P. mariana) to gain dominance. Spruce litter is low in nitrogen and difficult to decompose. Soil nitrogen pools and nitrogen availability are significantly lower in areas of heavy moose browsing. Because nitrogen is the limiting nutrient in boreal forests, moose and beaver lay have long-term effects on ecosystem properties.