EFFECTS OF MOOSE BROWSING ON VEGETATION AND LITTER OF THE BOREAL FOREST, ISLE ROYALE, MICHIGAN, USA

Large mammalian herbivores can influence the dynamics and structure of ecosystems by selectively removing tissues of specific plant species. The plant community composition can be altered as animals feed on some species but not others, changing the biomass, production, and nutrient cycling of an entire ecosystem. We used four paired moose (Alces alces) exclosures and browsed plots (built between 1948 and 1950) on Isle Royale, Michigan, to examine the influence of moose on aboveground biomass, production, and annual litterfall of boreal vegetation in 1987. Tree biomass was significantly greater (XBAR = 230 vs. 150 Mg/ha, df = 3, P < .05), shrub biomass was significantly less (XBAR = 1.9 vs. 3.1 Mg/ha, P < .05), and herb biomass was significantly less (XBAR = 0.2 vs. 0.8 Mg/ha, P < .05) in exclosures than in browsed plots. Tree production was greater in exclosures than in browsed plots (XBAR = 7.9 vs. 5.0 Mg-ha-1.yr-1, P = .05), but there was no difference in the production per unit biomass between exclosures and browsed plots. Shrub production in exclosures was similar to that of browsed plots (XBAR = 0.4 vs. 0.3 Mg.ha-1.yr-1, P > .05). Litter production was greater in exclosures than in browsed plots (XBAR = 3.5 vs. 2.3 Mg.ha-1.yr-1, P < .05), despite total vegetation biomass differences between paired plots. There was significantly greater herb litter produced in the browsed plots than in the exclosures (XBAR = 0.7 vs. 0.1 Mg.ha-1.yr-1, P < .05). Moose browsing prevented saplings of preferred species from growing into the tree canopy, resulting in a forest with fewer canopy trees and a well-developed understory of shrubs and herbs. In addition, browsing may have altered the eventual balance of white spruce (Picea glauca) was balsam fir (Abies balsamea), causing an increase in the former and a decrease in the latter. Thus, browsing by moose influences the long-term structure and dynamics of the boreal forest ecosystem, which has important implications for forest ecosystem management, especially where the population dynamics of moose are regulated.