Do invasive species provide a refuge from browsers? Evidence for associational resistance in a habitat plagued by deer

Tiffany Betras, Ali Carroll and Walter P. Carson, Biological Sciences, University of Pittsburgh, Pittsburgh, PA, Esmée DeCortie, Chatham University, Pittsburgh, PA, Ryan Utz, Falk School of Sustainability, Chatham University, Gibsonia, PA

Background/Question/Methods
White-tailed deer (Odocoileus virginianus) overbrowsing over the past several decades has caused substantial changes to plant communities in eastern deciduous forests. Deer-preferred species have declined or become locally extirpated in many areas while deer-tolerant species have greatly increased in abundance. Moreover, the abundance of nonnative invasive species has also increased over the last few decades, outcompeting many native species. Native shrubs such as American elderberry (Sambucus canadensis) are now much less common, particularly in forests with high deer densities. Here, we introduced American elderberry seedlings into a replicated field experiment in which we factorially manipulated deer presence or absence and invasive species presence or absence by using exclosures and removing invasive species. We monitored metrics of elderberry fitness including leaf count and plant height over two growing seasons. We tested four hypotheses: (1) Deer browsing would reduce the performance of elderberry seedlings, (2) Dense layers of nonnative invasive species would reduce the performance elderberry seedlings, (3) The combination of browsing and nonnative species together interact (non-additive) to reduce elderberry performance more than either factor alone, and (4) Alternatively, dense layers of invasive species “hide” elderberry seedlings from browsers (i.e. associational resistance).
Results/Conclusions
Browsing substantially reduced the performance of elderberry whereas invasive species provided a modest degree of associational resistance. Browsing significantly decreased both leaf count and plant height. By the end of the study, elderberry seedlings within exclosures had twice the number of leaves, were three times taller, and had much lower mortality versus those exposed to browsing. Removing invasive species did not change leaf count or plant height. However, elderberry growing within patches of invasive species had slightly lower mortality versus those growing in patches where invasive species had been removed (marginally significant browsing x removal interaction). Our results demonstrate that, at least over several years, browsing is more inimical to a native shrub than competition from invasive species. Moreover, we detected evidence for associational resistance where native species may get some relief from browsers by “hiding” among unpalatable plant species. This warrants further investigation because it suggests that patches of invasive species may be browsing refugia and a potential way to manage native plants subject to chronic levels of overbrowsing.