Woody species response to altered herbivore pressure at Isle Royale National Park

Abstract

AbstractHerbivores shape vegetative communities via numerous mechanisms, including browse. We used vegetation monitoring data from Isle Royale National Park to examine woody species change across a nine‐year interval, coinciding with herbivore escalation. Here, moose and snowshoe hare are the dominant herbivores, while the gray wolf is the apex predator. Our initial sampling period (2010) followed six years of low moose abundance, while our second sampling event (2019) followed a nine‐year escalation in moose density. We tested for change in both saplings and shrubs and compared diameter size distributions of common tree species in three island sections. We found a decline in large saplings of sugar maple, a species limited to the west section of the island. We also saw declines in small saplings of sugar maple, paper birch, and trembling aspen. For some species, including black spruce and white spruce, taxa that are unpalatable to moose, diameter distributions were proportionally larger (i.e., indicative of fewer small individuals) in 2019 than in 2010. In contrast, diameter distributions of black ash in the central section and trembling aspen in the central and east sections of the island were proportionally smaller (indicative of more small individuals) in 2019 than in 2010. We found that ~50% of common shrub taxa were more abundant during the second sampling event, while none declined. Our work here brings to light several unanticipated results. While we demonstrated partial recovery of trembling aspen, likely originating during the earlier period of low moose abundance, our results suggest further changes may be attributable to other island herbivores. The response of black ash, a hydric species, may be stemming from a sixfold increase in active beaver sites, while that of spruce may be attributable to a historic spike in snowshoe hare density. We currently have only a limited understanding of the impacts of the island's lesser studied herbivores on overstory regeneration. Further research on these linkages can inform decisions addressing controls on forest structure.