The influence of moose on the composition and structure of Isle Royale forests

Abstract

The long-term influence of moose (Alcesalces) on the structure and species composition of three forest types in Isle Royale National Park, Michigan was evaluated using four moose-proof exclosures established in 1949–1950. Results obtained during the present study were compared with the original vegetation at each study site and with data collected by Krefting (1974) in order to discriminate between the effects of moose browsing and forest community succession. In the spruce–birch–fir climax and aspen–birch–conifer forest study sites, self-thinning and reduced light availability reduced the density of woody plant stems in exclosure areas. In the adjacent control areas, moose browsing has slowed the rate of succession. Repeated browsing by moose on palatable species has retarded vertical growth and has prevented stems from growing beyond reach. Effects of moose browsing on succession in the aspen–birch study sites in the 1936 burn were less clear. In all three forest types, stem densities of most species were higher in control areas than in exclosures. Species diversity was also greater in control areas. American yew (Taxuscanadensis) and red-osier dogwood (Cornusstolonifera) were the only species that declined in abundance as a result of browsing by moose.