SHRUB EXPANSION IN MONTANE MEADOWS: THE INTERACTION OF LOCAL-SCALE DISTURBANCE AND SITE ARIDITY

摘要: Montane meadows in the Sierra Nevada of California have experienced dra- matic expansion shrubs (Artemisia rothrockii) and reduction herbaceous species cover since introduction livestock late 1800s. Increases meadow aridity due to use has been proposed as primary factor facilitating sagebrush dominance these areas. However, our data suggest that can readily expand into moist areas where water table is shallow. We explored how relative importance local processes influencing seedling establishment vary with changes site aridity. quantified patterns abundance relation depth surface soil moisture occurrence distance from reproductive plants presence gopher disturbance. tested independent interactive effects vegetation clipping disturbances on germination, survival, growth using experiments established four types differed depth, moisture, cover. Experiments were con- ducted over two growing seasons availability. Our results small ( , 1m 2 ) promote germination seedlings intact, ungrazed, moist, In absence distur- bance, dense herbs, whether clipped or not, prevented germination. The bance strongest sites support a canopy less important lower surface-soil estab- lishment rates low despite abundant exposed soil. spatial distribution consistent experimental results. Sagebrush density decreased dramatically shrubs, almost always preferentially associated mounds Clipping above- ground biomass herbs relatively scale (4 m had no effect early growth; however, survival larger trans- planted was enhanced by clipping. conclude that, while traditionally increased aridity, it exhibits greatest potential for growth, mesic, rather than xeric, sites. Realization this dependent confluence soil, nearby seed source, aboveground herb biomass.