Herbivory serves as a critical top-down mechanism within plant communities by regulating biodiversity, productivity, and ecosystem function. While top-down impacts of mammalian herbivores have garnered significant attention in the literature, fewer studies have investigated the role of herbivorous reptiles in regulating plant communities, which can serve as crucial herbivores in some ecosystems. In Southeastern United States coastal plain long-leaf pine forests, the gopher tortoise (Gopherus polyphemus) is a keystone species that may suppress plant productivity and promote local diversity within hyper-diverse understory plant communities. In January 2019, we established tortoise-exclusion plots within an active gopher tortoise population at Splinter Hill Bog Preserve in southwest Alabama, USA. We measured the response of plant species diversity, composition. and productivity through a single growing season to quantify the short-term impacts of tortoise-excluded plots on understory plant communities. We found that plots excluded tortoises had 35% more plant cover with a 15% reduction in plant richness, 6% reduction in evenness, and 12% reduction in Shannon’s diversity relative to control plots. Within a single growing season, we found that tortoise exclusion explained 5% of the variation within overall plant community composition. Our results provide clear evidence on the role of gopher tortoise herbivory for the maintenance of plant diversity within the species-rich longleaf pine forests of the Southeastern United States, even at short time scales and in an heterogeneous environment.