White-tailed deer (Odocoileus virginianus) nutritional carrying capacity over two decades after exclusion

Studies on the impacts of white-tailed deer (Odocoileus virginianus; hereafter deer) herbivory on vegetation biomass and nutritional availability are limited. Considering deer can shift plant species composition and abundance through selective browsing, we sought to investigate whether deer could influence their own nutritional carrying capacity. Using paired control and exclosure experimental units constructed in 2000 on three Wildlife Management Areas in the Upper Coastal Plain, Lower Coastal Plain, and Mississippi Alluvial Valley (hereafter Delta) ecoregions of Mississippi, we collected biomass samples following methods initially used on the sites in 2005. Biomass data were collected during July - August of 2021 at 15 random sampling points per experimental unit. Additionally, representative samples of each plant species were collected and analyzed for crude protein content to determine nutritional carrying capacity. By analyzing biomass data from 2005 and 2021, we found between-year differences, but no differences by treatment (exclosure vs. control). Notably, there was a simultaneous increase in shade tolerant species and decrease in shade intolerant species from 2005 to 2021, and a decrease in preferred forage species. Nutritional carrying capacity was only determined using 2021 data and revealed a lack of treatment effects. The differences between ecoregions demonstrated abundant low quality forage on both Coastal Plain sites (Upper and Lower) and a comparatively low amount of forage that was higher in quality in the Delta. Results suggest changing understory light conditions and a need for additional management if increasing available forage for deer is a desired management outcome. Specifically, regional differences in nutritional availability between the Coastal Plain sites and the Delta site illustrate the potential for active management to aid in the promotion of herbaceous understory forage species.