Do white-tailed deer and the exotic plant garlic mustard interact to affect the growth and persistence of native forest plants?

Abstract

Herbivores and invasive plants strongly affect the persistence of individual species and the overall composition of forest plant communities. Although their impacts are usually studied separately, they often co-occur with potential synergistic effects on native plants. We used a 2×2 experimental design replicated across five locations in southern Wisconsin to assess the separate and combined effects of herbivory by white-tailed deer (Odocoileus virginianus) and competition from the weedy invader Alliaria petiolata (garlic mustard) on five native species. Deer and Alliaria often reduce the growth and survival of native plants in these forests. A fenced exclosure erected at each site surrounded one of two pairs of adjacent plots densely occupied by Alliaria. We cleared one plot (in- and outside each exclosure) of Alliaria and then planted seedlings of the native species into all four plots. Species were chosen to display different functional traits (Aster lateriflorus/Symphyotrichum lateriflorum, Carex blanda, Geranium maculatum, Quercus rubra, and Uvularia grandiflora). We then monitored native seedling growth and survival for 13months. Deer access depressed survival and branching in Uvularia, growth in Geranium and Quercus, and tillering in Carex. The presence of Alliaria reduced height and tillering in Carex and Geranium but increased branching, reproduction, and survival in Uvularia. Thus, Alliaria may sometimes act to protect taxa like Uvularia sensitive to deer herbivory. Alliaria reduced the growth of Carex, Geranium, and Quercus significantly more in the absence of deer. Such interactive effects suggest that when deer do not restrict growth, plants grow larger to a point where Alliaria’s competitive effects increase. The impacts of both deer and Alliaria varied by species and trait. Species experiencing regional declines (e.g., oak seedlings and the lily, Uvularia) were more affected by deer than by Alliaria. Oak seedlings suffer particularly from Alliaria competition when deer are absent. Managers seeking to sustain deer-sensitive species should first work to reduce deer densities before seeking to control weedy plant invasions. Alliaria reduced growth more in species that tolerate deer browsing (Carex and Geranium). These differential responses argue for tailoring local deer and weedy plant control efforts to the taxa of concern. Deer and weedy plants have cumulative effects on native plant species. Interactions between their effects will become more important as their populations and impacts increase.