Balsam Fir on Isle Royale: Effects of Moose Herbivory and Population Density

Balsam fir (Abies balsamea) population parameters were analyzed in Isle Royale National Park, Michigan, to asses moose (Alces alces andersoni) herbivory in relation to the population density of both fir and moose. Fir population parameter were determined at nine study sites, each representing a different combination of relative fir and moose density (high, medium, and low levels, respectively). In general, moose suppression of fir height growth and recruitment to the canopy increased with increasing moose density and decreasing fir density. Where fir densities were low, sapling heights were restricted to <1 m regardless of moose density. In such sites, sapling growth suppression has been continuous since a peak in the moose population in the 1920s. Canopy fir trees at most sites established prior to arrival of moose on Isle Royale around 1910. At high fir density release from growth inhibition was common following a period of low moose numbers from the mid—1970s to early 1980s. Here this release should facilitate recruitment of fir and effect replacement of canopy losses due to senescene. In other areas, however, continuing canopy looses and lack of recruitment due to intensive herbivory by moose will greatly reduce the fir component of the canopy, effectively preventing any return to the fir—dominated forests of the past.