| In the last 150 years, explosive growth in global industry, human populations, and consumption has led to changes in the world's atmosphere and climate. Rising atmospheric CO2 is driving global increases in temperature and continental precipitation. Anthropogenic nitrogen fixation and deposition further contribute to these climate changes. My dissertation examines the effects of these four, linked global changes—elevated CO2, warming, increased precipitation, and nitrogen deposition—on the composition and function of a natural ecosystem, the California grassland. I conducted my research in and near the Jasper Ridge Global Change Experiment, a long-term study exposing annual grassland plots to treatments simulating all possible combinations of these four global changes. I followed the effects of global change treatments for two years on grassland invasion by the shrub Baccharis pilularis (chapter 1). Climate changes enhanced shrub establishment. These findings, coupled with the results of a chronosequence study of Baccharis' effects on C pools and other ecosystem properties (chapter 2), illustrate the potential for strong, vegetation-mediated effects of global changes on ecosystem function. To further explore the potential importance of species-mediated effects of global changes, I followed the responses of plant diversity, community composition, and productivity by different plant guilds to three years of global change treatments (chapters 4–5). Strong grassland diversity responses illustrate the sensitivity of this ecosystem to human-caused climate and atmospheric changes and pinpoint the potential for species composition changes to mediate longer-term effects on ecosystem function. The effects of global changes on the availability of key resources such as soil moisture (chapter 3) may play important roles in community responses to these climate and atmospheric changes. |
