Large ungulates are an important driver of plant community composition and structure. In eastern North America, white-tailed deer (Odocoileus virginianus) thrive in agricultural mosaics and fragmented forested landscapes, at times reaching unprecedented densities. Nevertheless, few long-term data sets are available that allow an assessment of the long-term consequences of chronic herbivory. We quantified herbaceous-layer change over a 26 y period in Cades Cove, Great Smoky Mountains National Park, USA. Cades Cove has a long and well-documented history of deer overabundance, with densities reaching 43 deer km⁻² during the late 1970s. Over the 26 y sampling interval, mean coverage of herbaceous species declined significantly (P < 0.001) in the forests bordering Cades Cove. Although most plots only lost 1–2 species during the interval, 46 herbaceous species recorded on plots during the 1970s were wholly absent in 2004 (63% of which were forest species). Additionally, the herbaceous layer has become significantly more homogeneous over time. In contrast, species richness and cover on reference plots increased by 106 and 183%, respectively, over a similar time interval. Whereas some compositional changes were associated with forest succession, proximity to the Cove's edge environments was the most informative environmental gradient, lending support to the hypothesis that deer foraging behavior results in a biotic edge effect in fragmented landscapes. Chronic herbivory may result in impoverishment and simplification of herbaceous layers in forests otherwise protected from habitat degradation and loss.