EFFECTS OF BEAVER AND MOOSE ON THE VEGETATION OF ISLE ROYALE NATIONAL PARK
We studies the cumulative effect of browsing by moose (Alces alces) and tree-cutting by beaver (Castor canadensis) on tree density, basal area, and species composition on the northeastern end of Isle Royale National Park. Beaver-affected land (abandoned ponds, occupied ponds, and foraging areas) in one representative area from 26% of the land area in 1957 to 34% in 1978. Beaver significantly decreased aspen (Populus tremuloides) tree density from 140 ± 23 (SE) to 27 ± 10 stems/ha and basal area from 10.2 ± 1.6 to 3.6 ± 1.4 m2/ha adjacent to ponds and streams. Balsam fir (Abies lasiocarpa) tree density was significantly greater in beaver-cut areas than in uncut forest (417 ± 77 compared to 227 ± 55 stems/ha). Moose browsed preferentially on aspen in the summer and winter in both beaver-cut areas and uncut forest. Moose neither preferred nor avoided balsam fir in the beaver-cut area, but avoided it in the uncut forest. White spruce (Picea glauca) was not browsed. The density of coniferous species with diameter at breast heigh <15 cm was greater than 400 stems/ha, while stem densities of the heavily browsed deciduous species were less than 20 stems/ha in both beaver-cut areas and uncut forest. Changes in plant species composition affected by moose and beaver populations may decrease soil fertility and alter successional patterns in stream side areas and adjacent forests.
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