Influence of beaver ponds, beaver herbivory, and fire on boreal forest succession

Boreal forests are subjected to a variety of natural disturbances that affect their species composition and spatial pattern. Beavers (Castor canadensis) influence boreal forests through selective herbivory and by building ponds that flood riparian forests, while fires kill non-resistent tree species and alter germination. Forest stands at Voyageurs National Park were mapped using aerial photography from 1940, 1961, and 1988 and analyzed with a GIS to determine changes in forest composition and patchiness due to beaver activity and an extensive fire that burned the region in 1936. Beaver pond building initially created many small (approx. 4 ha) non-forested patches scattered throughout the forest matrix. Over time, ponds coalesced into interconnected networks as beaver dams altered entire drainageways. Beaver foraging altered forest composition and decreased biomass within narrow (approx. 80 m) bands adjacent to the ponds. The 1936 fire burned 56% of the park, about twice as much as that altered by beaver; most of the area burned was in two large patches. These disturbances operate at much different spatial and temporal scales, which are reflected in the resultant forest.