干旱区生态环境知识资源中心

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