The effects of deer and an invasive shrub, Lonicera maackii, on forest understory plant composition

Overabundant generalist ungulates and invasive plants negatively impact forest plants, but few studies have investigated how these stressors interact. We tested the impacts after 5-6 years of exclusion vs. access of white-tailed deer and the presence or removal of an invasive shrub, Lonicera maackii (Amur honeysuckle), on forest understory plant composition in Ohio, USA. Exclusion of deer resulted in increased tree seedling density and species richness, and increased shrub basal area, but decreased species richness of the forest floor layer, as well as decreased cover of bare ground and annual plants. The removal of L. maackii resulted in impacts on a broader range of understory plants, including greater species richness and cover of the forest floor layer, and greater cover of graminoids and vines specifically. There were also interaction effects between deer and L. maackii. Where deer were excluded and L. maackii was removed, there was greater cover of tree seedlings, vines, and spring perennials, and a tendency for greater native species richness. These findings reveal that deer and invasive shrubs have synergistic effects on forest understories, indicating that management of both is warranted.