We examined the response of understory vegetation beneath monotypic, even-aged stands of lodgepole pine to increasing tree mortality following an epidemic of mountain pine beetles. We hypothesized that understory biomass would increase continually as the tree canopy was reduced and competition with trees for light and soil moisture decreased, but that plant species diversity and heterogeneity would peak at intermediate levels of beetle-caused tree mortality. Mean understory biomass clipped from 50 1-m(2) circular plots/stand was an order of magnitude greater (40 g m(-2)) in beetle-killed stands, with typical levels of overstory tree mortality (50-75%), than in unaffected stands (4 g m(-2)); and it increased exponentially with disturbance severity. Frequency of fruit occurrence was positively related to increasing tree mortality, but was highly variable...