Do invasive species provide a refuge from browsers? A test of associational resistance in a peri-urban habitat plagued by deer

Abstract

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 Sambucus canadensis (American elderberry) are now much less common, particularly in urban forests and forests with high deer densities. Here, we performed a short-term study in which we introduced propagations of a native shrub 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 S. canadensis fitness including leaf count and plant height over two growing seasons. Browsing substantially reduced survival 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, individuals within exclosures had over twice the number of leaves, were five times taller, and had much lower mortality versus those exposed to browsing. Removing invasive species did not change plant height, however, S. canadensis individuals experienced marginally higher leaf counts inside exclosures where we had removed invasive species. This trend was opposite in individuals outside of fences such that those growing within patches where invasive species remained had slightly higher leaf counts, especially during the first growing season. Until the final census date, elderberry growing within patches of invasive species had slightly lower mortality versus those growing in patches where invasive species had been removed. The results from our short-term study indicate that browsing is more inimical to a native shrub than competition from invasive species and we did not find evidence of associational resistance to browsing from invasive cover.