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

摘要

We examined the response of understorey vegetation beneath monotypic, even-aged stands of lodgepole pine (Pinus contorta) to increasing tree mortality following an epidemic of mountain pine beetles (Dendroctonus ponderosae). It was hypothesized that understorey 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 understorey biomass clipped from fifty 1-m circular plots/stand was an order of magnitude greater (40 g/m) in beetle-killed stands (with typical levels of overstorey tree mortality 50-75%), than in unaffected stands (4 g/m), and it increased exponentially with disturbance severity. Frequency of fruit occurrence was positively related to increasing tree mortality, but was highly variable. Understorey 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 understorey 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 understorey 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 understorey plant diversity.