Response of understory vegetation to variable tree mortality following a mountain pine beetle epidemic in lodgepole pine stands in northern Utah

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. Understory plant species richness and, to lesser degrees indices of diversity that incorporate evenness, peaked in stands with moderate mortality. Measures of vegetation patchiness (the coefficient of variation in mean plot biomass and an index of habitat interspersion) also peaked in stands with intermediate levels of disturbance. The response of understory plant species diversity to increasing disturbance severity is consistent with the pattern predicted by the intermediate disturbance hypothesis. However, other explanations of this pattern are discussed. Although understory plant community richness was higher in beetle-killed stands than in unaffected stands, new species were not abundant and therefore did not contribute substantially to greater evenness in understory plant diversity.