Effects of an invasive shrub, Lonicera maackii, and a generalist herbivore, white-tailed deer, on forest floor plant community composition

Abstract

Both overabundant ungulates and invasive shrubs negatively impact forest floor plants, but few studies have investigated their interaction. We tested for direct effects and interactions of white-tailed deer, Odocoileus virginianus, and the invasive shrub, Lonicera maackii, on forest floor plant richness and composition in five sites in southwest Ohio. In each site a deer exclosure and an unfenced plot (each 20×20m) were established in 2010; in half of each plot all L. maackii was removed. Each spring and summer, 2011–2014, percent cover of each plant species in 50×20cm subplots was recorded.Herb layer species richness was greater where L. maackii was removed, and this effect tended to be stronger where deer were excluded. Species composition in the spring was not affected by treatments, whereas summer species composition (in one year) was affected by L. maackii treatment. Cover of graminoids, spring herbs, and vines were lower where L. maackii was intact, whereas cover of shrubs and tree seedlings were lower, and cover of bare ground higher, where deer had access.Our findings reveal that deer and L. maackii negatively impact different components of the forest herb layer. However, the combined effects on species richness tended to be antagonistic (less than additive), possibly due to deer browse reducing the cover of this invasive shrub. This suggests that the diversity of the herb layer will be enhanced more by both removal of invasive shrubs and reduction of the deer population than by either management action alone.