20-year effects of white-tailed deer (Odocoileus virginianus) herbivory on vegetation composition and structure

Abstract

Throughout the southeastern U.S., white-tailed deer (Odocoileus virginianus; hereafter deer) are the dominant herbivores and have the potential to impact vegetation communities through selective browse pressure. Previous studies regarding deer herbivory impacts focused on northeastern and midwestern U.S. forests. Additionally, most studies have observed herbivory effects over short timescales (i.e., <10 years). Therefore, we sought to determine deer herbivory effects on vegetation structure and composition among three different ecoregions in Mississippi at 0, 5, and ∼ 20 years after exclusion. In 2000, 3 pairs of experimental units were identified in each of the Upper Coastal Plain, Lower Coastal Plain, and Mississippi Alluvial Valley ecoregions. An exclosure treatment was randomly assigned within each pair (averaging 2.7 ha) with the other experimental unit a control. From May-June, vegetation structure and composition were measured among 10 systematic sampling points per plot during 2000, 2005, and 2021. While no treatment effects on vegetation composition and structure were observed, among-year differences suggested a shift in overstory canopy conditions between 2005 and 2021 towards greater basal area and canopy coverage resulting in a decrease in forbs, legumes, shade intolerant species, and high year-round deer-preferred forage species. Deer herbivory effects could have been obscured by overstory conditions, low deer densities, and similar baseline conditions reflecting past herbivory effects. Regional differences in forest structure and composition and exclosure breaches may also be responsible for the lack of treatment effects. Future studies should consider utilizing disturbance to offset prior herbivory impacts and to standardize overstory conditions.