Graduate Student Theses Supported by DOE ' s

This dissertation describes work I conducted in collaboration with many others to examine the response of cloud forest ecosystems to global change and the role that photosynthetic pathways play in the carbon cycle. In the second chapter, my co-authors (Pru Foster and Stephen Schneider) and I examine the potential impacts of climate change on tropical montane cloud forests. These forests are critically endangered ecosystems that provide a range of ecosystem services, from serving as watersheds to regulating seasonal water releases. They are also very rich in endemic species, perhaps in response to the unique microclimates resulting from frequent cloud contact. We predict changes in cloud formation heights in a doubled-CO 2 world, with attendant impacts on these unique ecosystems. In the third chapter, I describe a global distribution of C 3 and C 4 plants I developed for use in carbon cycle studies. Because of their distinct morphology and biochemistry, C 4 plants respond very differently to light, temperature, and carbon dioxide than do C 3 plants. A better understanding of the terrestrial carbon cycle using forward and inverse modeling techniques therefore requires knowledge of the spatial and temporal extent of both photosynthetic types. The global distribution I developed combines new remote sensing products with physiological modeling. With this approach, I have simulated the carbon fluxes associated with each photosynthetic type using the SiB2 land surface model. This distribution predicts the areal coverage of C 4 vegetation to be 18 million km 2 (~10% of the land surface), and C 4 gross primary productivity to be 20% of global productivity. The fourth chapter concerns work at the regional scale, where I have used isotopic techniques to estimate the C 4 /C 3 mixture of a tallgrass prairie site in Oklahoma. The photosynthetic mixture is required for understanding the physiological controls on carbon, water and energy fluxes measured at the site with an eddy covariance system. Results suggest seasonal changes in this mixture: in