The effects of changes in timing and amounts of precipitation on vegetation dynamics and nitrogen mineralization in a sagebrush-steppe ecosystem.

摘要

Sagebrush steppe, a semiarid, cold desert, is endangered as a result of land use practices and invasion by exotic species. Semiarid ecosystems are thought to be highly susceptible to climate change. Climate change scenarios predict changes in the amount and timing of precipitation. For the Snake River Plain, there are two predicted scenarios of precipitation change, increased precipitation in the summer and increased precipitation in the winter/spring. Increased summer precipitation would wet only surface layers of the soil, while increased winter/spring precipitation would lead to deeper soil recharge. The objective of this study was to assess the effects of changes in the amount and timing of precipitation on vegetation dynamics and nitrogen mineralization in sagebrush steppe.; Two sets of study plots were used for this research, natural vegetation plots established in native sagebrush steppe and the Protective Cap/Biobarrier Experiment (PCBE) plots, which consist of subplots planted in either native vegetation or crested wheatgrass monocultures. Study plots received one of three irrigation treatments, additional summer, additional winter, or ambient precipitation.; Each precipitation scenario had different effects on the sagebrush steppe community. Additional summer precipitation increased the amount of perennial grasses, with the proportional abundance of rhizomatous grasses increasing as well. The amount of nitrogen mineralized tended to increase as the period of mineralization was extended into the summer months. These changes may lead to a more grass-dominated ecosystem, especially one dominated by sod-forming rhizomatous grasses. Additional winter precipitation, applied in the fall, increased perennial grasses as well. However, this fall precipitation caused an exponential-like increase in Bromus tectorum, an exotic annual grass. There was some evidence suggesting that fall precipitation will increase nitrogen mineralization. However, it is unlikely this nitrogen would be available to plants in the spring, since increased soil moisture may cause more denitrification and leaching. Fall precipitation, coupled with increased nitrogen mineralization may allow Bromus tectorum to become dominant in sagebrush steppe susceptible to invasion.; The PCBE plots provided an opportunity to study how different landfill cap configurations affected vegetation composition under ambient and augmented precipitation, as well as the effects of augmented precipitation on nitrogen mineralization in native vegetation and grass monocultures. Soils with an impermeable layer at 1 m had increased rhizomatous grass cover and a reduction in shrubs. Gravel/cobble layers in the soil profile allowed root penetration of deeply rooted shrub species, giving Chrysothamnus nauseosus and Artemisia tridentata an advantage over grasses, which may not extend as many roots through the gravel/cobble layer. The greatest changes in shrub cover occurred due to differential responses to the precipitation timing for each cap configuration. Increased winter precipitation was most responsible for the increase in shrubs, specifically C. nauseosus and A. tridentata on plots with gravel/cobble layers, while it caused a reduction in cover of those shrubs on EPA plots. Increased N mineralization occurred one summer in response to summer irrigation on the PCBE plots. Increased mineralization only occurred during the winter/spring in response to additional winter/spring precipitation during 1999 and 2000, two dry years.; These results suggest that amount and timing of precipitation are important in determining plant species composition and amount of nitrogen mineralized in sagebrush steppe.