Forty-seven year changes in vegetation at the Apostle Islands: Effects of deer on the forest understory

Background/Question/Methods

White-tailed deer (Odocoileus virginianus Zimmerman) have been labeled a keystone herbivore in forests of the Midwestern United States, particularly as deer have increased over the past century due to forest fragmentation, reduction of natural predators, reduced hunting, and mild winters.  Deer browsing in the Apostle Islands National Lakeshore has had a pronounced effect on formerly large continuous patches of reproductive Canada yew (Taxus canadensis Marshall) found there.  In this study we resurveyed 32 forest sites on the Apostle Islands to compare understory plant communities in 2005 to those sampled in 1958.  Some sites remained free of deer throughout the study period, others retained constant deer pressure, while the remaining sites gained or lost deer.  
Results/Conclusions

Four new understory species have colonized these sites (including Arisaema triphyllum L. and Maianthemum racemosum L.), four other species increased by more than 20% (including Rubus idaeus L. and Equisetum sylvaticum L.), while 12 species decreased by more than 20%.  Several of these species appear to have shifted their abundance in response to deer browse.  Multivariate analyses reveal that deer have strong effects on both the rate and type of change in the forest understory.  As deer remove the dominant shrub Taxus, other species, particularly grasses and sedges, are favored.  Perennial forbs may be threatened on islands with consistent deer pressure.  The impacts of deer on these understory plant communities appear to be increasing over time.