Pastore, Melissa2022-11-142022-11-142020-08https://hdl.handle.net/11299/243125University of Minnesota Ph.D. dissertation. 2020. Major: Ecology, Evolution and Behavior. Advisors: Sarah Hobbie, Peter Reich. 1 computer file (PDF); 165 pages.A key uncertainty in ecology is how concurrent global change factors will interact to affect terrestrial ecosystems. Humans have altered Earth’s carbon dioxide (CO2) concentrations, climate, nutrient levels, and biodiversity, all of which affect plant communities and ecosystem function. Yet, few multi-factor field studies exist to examine interactive effects of global changes on plants and ecosystems. I characterized the physiological responses of perennial grassland species from four plant functional groups (C3 grasses, C4 grasses, nitrogen-fixing leguminous forbs, and non-leguminous forbs) to single and interactive global changes including elevated carbon dioxide, increased soil nitrogen supply, reduced rainfall, and climate warming. I also determined how elevated CO2, increased soil nitrogen supply, and planted species richness affected total ecosystem carbon (C) storage over 19 years. These studies took place in the open-air, global change grassland ecosystem experiment, BioCON (Biodiversity x CO2 x Nitrogen), located at the Cedar Creek Ecosystem Science Reserve in Minnesota, USA. I present evidence that (1) the ability of plants to capture additional C as atmospheric CO2 rises via photosynthesis may be more limited than traditionally believed; (2) substantial, sustained declines in stomatal conductance and increases in water-use efficiency under elevated CO2 occur widely among grassland species; (3) global change factors interact in complex ways to affect photosynthesis, and how factors interact varies among grassland species; and (4) declines in biodiversity may influence ecosystem C storage more than a 50% increase in CO2 or high rates of nitrogen deposition in perennial grassland systems. These findings show that simple predictions of plant physiological responses to global changes based on theoretical expectations of isolated effects and on functional classifications of species are not sufficient – global changes and other environmental factors interact in complex ways to impact responses of species. These results also highlight the importance of biodiversity in promoting ecosystem function and call into question whether elevated CO2 will increase the C sink in grassland ecosystems and help to slow climate change.enclimate changeelevated CO2functional groupsglobal changegrasslandsnitrogenThesis or Dissertation