Effects of white-tailed deer and invasive garlic mustard on native tree seedlings in an urban forest

Abstract

An ongoing goal in forest ecology is to understand how animal populations and invasive plants affect forest communities. Reduced numbers of native tree saplings in urban forests have driven concerns that overabundant white-tailed deer (Odocoileus virginianus) populations and invasive plants are both suppressing the size and survival of native tree seedlings. Previous work has focused on the individual effects of either deer or invasive plants, but few studies have examined the combined effects of these two stressors. In this study, I examined the effects of white-tailed deer and garlic mustard (Alliaria petiolata) on transplanted seedlings of three native tree species: northern red oak (Quercus rubra), black cherry (Prunus serotina), and eastern hemlock (Tsuga canadensis). To test for their individual and combined effects on seedlings, I designed a factorial experiment where I manipulated the level of deer presence with exclosures and the level of garlic mustard presence by weeding. To measure the impacts on seedlings, I scored browse damage, tracked survival, and measured multiple size traits for each study species. I found that deer presence reduced seedling size but did not affect mortality within the short duration of the study. In addition, seedlings were not affected by garlic mustard removal, nor did the effects of deer depend on garlic mustard presence. While all species were impacted by deer, seedling browsing patterns varied among tree species. Moreover, I found that browsing by deer is changing the species composition within McMaster Forest to favour browse tolerant tree species. These results have implications for management strategies in forests with high deer densities which should prioritize controlling the deer population in addition to invasive plants and should be tailored to conserve the most sensitive species.