White-tailed deer (Odocoileus virginianus) limit the abundance and persistence of many herbaceous species susceptible to herbivory, and can promote exotic species invasions, with downstream impacts on many other species. Here, we explore the utility of using plant functional traits to assess long-term deer impacts in plant communities by asking how deer herbivory affects functional trait composition and diversity in forest plant communities. Functional traits reflect key ecophysiological, morphological and life-history strategies that affect the ways organisms interact with the environment and other species. In 2011, we surveyed plant communities in- and outside 20 paired deer exclosures in Northern Wisconsin and Michigan's Upper Peninsula. We then compared community-weighted means for 30 functional traits in- and outside the exclosures to assess how deer select for and against these traits. We also compared functional trait diversity in protected vs. exposed communities and characterized multivariate community trait differences using discriminant function analysis.
Results/Conclusions
Our trait data covered 83% of species occurrences in these communities. Community-weighted mean leaf carbon content, leaf dry matter content, plant height, and leaf lignin content were all higher in unprotected areas outside deer exclosures. In contrast, mean specific leaf area and neutral detergent fiber increased inside the exclosures. We found higher functional divergence inside the exclosures, but this difference was small with weak significance. We found no differences in overall functional richness, evenness, or dispersion in the communities in- vs. outside the exclosures. Nevertheless, our discriminant function correctly categorizes functional trait distributions in- vs. outside deer exclosures 75% of the time. Functional traits thus show considerable promise as indicators of deer impacts particularly in the form of community-weighted means or combined into a discriminant function. Whether these trait differences are general or specific to particular regions should now be explored. These tools, perhaps in conjunction with others used to assess deer impacts, can be used to assess likely effects on forest species’ distribution and abundance, and to inform landscape-level herd management across ecological gradients and shifts in plant species distributions.