We studied understory vegetation response to complete overstory mortality in a xeric oak (Quercus) forest in Oklahoma to determine the magnitude of changes in understory species composition, richness, cover, and biomass. Graminoid species showed the largest increases in species richness, cover, and biomass following forest canopy decline, mainly attributable to C4 species. The increase in richness of forb species might have been due to the large number of annuals with light, wind-disseminated seeds. Woody and legume species did not respond to forest canopy decline, but their capacity to sprout following disturbance ensured their continued presence. Although graminoid and forb species increased dramatically in response to forest canopy decline, woody plants might eventually recover and produce a forest canopy similar to the canopy before forest decline, unless prevented by annual prescribed burns.