The influence of climate, soils, and land-use on primary productivity and cheatgrass invasion in semi-arid ecosystems

Abstract

Although the influence of climatic conditions and soil properties on ecosystem processes is well documented and generally accepted, land-use practices have only recently been recognized as a potential driver of ecosystem processes and global change. The overall goal of my dissertation is to increase our understanding of how land-use contributes to global change in semi-arid areas and to identify the importance of these contributions in relation to the known control exerted by climate and soils. Agricultural cultivation is a pervasive and widespread driver of global change in the U.S. Great Plains, yet few studies have estimated the consequences of cultivation for regional ecosystem processes. I quantified the ongoing impact of cultivation on net primary production and revealed that cultivation has substantially increased aboveground production while slightly decreasing belowground production, that the consequences of cultivation are concentrated in certain areas, and that a few specific crops account for much of the change in primary production. Comparing the influence of cultivation on spatial and temporal production and phenology with the influence of climate, soils and weather variations indicated that, despite a substantial importance of cultivation, climate retains primary control over spatial variability whereas cultivation and weather have roughly equivalent influence over temporal variation in ecosystem processes. In the Great Basin, where biological invasions are an important facilitator of global change, I studied the invasion of cheatgrass, an exotic annual grass from Asia that has successfully invaded much of the Western U.S. My results suggest that multi-date multi-spectral imagery has the potential to detect and map cheatgrass distribution, although the inclusion of land-use in formation did not contribute to detection success. This result may prove a useful tool in future management and research endeavors. I utilized simulation models to examine the importance of climate, soils, disturbance regime and propagule pressure on cheatgrass invasion and concluded that climate dictates areas that are susceptible to cheatgrass invasion, disturbance frequency influences how severe the invasion will be and propagule pressure determines the speed of invasion. These conclusions suggest a general framework for understanding the potential future of cheatgrass invasion in un-invaded areas. This work provides insight into the importance of land-use as a driver of global change processes in semi-arid ecosystems. The combination of these studies characterizes the important contributions of land-use to primary production and biological invasions in semi-arid regions, but also confirms the central role that climatic conditions play in influencing these ecosystem processes. This work represents one of the first attempts to directly compare the importance of human activities and environmental conditions on ecosystem processes across large scales.