Influences of white-tailed deer on plant community assembly in northern temperate forests
The distribution of forest plants can relate to a variety of below- and aboveground factors such as the texture, chemistry and compaction of soils, invasion by non-native earthworms, light availability and competition. The objective of this work was to test how a keystone herbivore, the white-tailed deer, modified the importance of various abiotic and biotic variables in the assembly of understory communities. Our first project focused on plant composition and environmental differences inside and outside of 17 deer exclosures in northern Wisconsin and the Upper Peninsula of Michigan. For our second study, we established arrays of plots in four sites across northern Wisconsin. At each plot, we again measured environmental and vegetation factors plus deer visitation via video camera traps. The third project capitalized on a long-term canopy gap experiment with replicated deer exclosures in northcentral Wisconsin. Here, we sampled the environment, vegetation and deer usage at four different levels of canopy openness inside and outside of 10 exclosures. We used linear multiple regression, non-metric multidimensional scaling and nonparametric multiplicative regression to evaluate the influence of deer on the environment and vegetation as well as how deer-mediated changes in the environment indirectly affected the plant community.
Deer impacts were especially strong in open canopy plots where foraging activity was concentrated. Differences in browsing pressure on saplings and shrubs altered light availability for the ground layer, which correlated with variation in the abundance of exotic and shade tolerant species. Under low deer pressure, canopy openness played little role in community assembly but soil texture and chemistry exhibited strong relationships with composition. Increased environmental differentiation, where species’ ranges became more specialized, occurred for some plants in deer access plots. This may indicate that the diversity of community assemblages is declining across the region with deer overabundance.