RESPONSE OF UNDERSTORY VEGETATION TO VARIABLE TREE MORTALITY FOLLOWING A MOUNTAIN PINE-BEETLE EPIDEMIC IN LODGEPOLE PINE STANDS IN NORTHERNUTAH

We examined the response of understory vegetation beneath monotypic, e ven-aged stands of lodgepole pine to increasing tree mortality followi ng an epidemic of mountain pine beetles. We hypothesized that understo ry biomass would increase continually as the tree canopy was reduced a nd competition with trees for light and soil moisture decreased, but t hat plant species diversity and heterogeneity would peak at intermedia te levels of beetle-caused tree mortality. Mean understory biomass cli pped from 50 1-m(2) circular plots/stand was an order of magnitude gre ater (40 g m(-2)) in beetle-killed stands, with typical levels of over story 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 le sser degrees indices of diversity that incorporate evenness, peaked in stands with moderate mortality. Measures of vegetation patchiness (th e coefficient of variation in mean plot biomass and an index of habita t interspersion) also peaked in stands with intermediate levels of dis turbance. The response of understory plant species diversity to increa sing disturbance severity is consistent with the pattern predicted by the intermediate disturbance hypothesis. However, other explanations o f this pattern are discussed. Although understory plant community rich ness was higher in beetle-killed stands than in unaffected stands, new species were not abundant and therefore did not contribute substantia lly to greater evenness in understory plant diversity.