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-CO2 world, with attendant impacts on these unique ecosystems.

In the third chapter, I describe a global distribution of C3 and C4 plants I developed for use in carbon cycle studies. Because of their distinct morphology and biochemistry, C4 plants respond very differently to light, temperature, and carbon dioxide than do C3 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 C4 vegetation to be 18 million km2 (~10% of the land surface), and C4 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 C4/C3 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 the cooler and wetter spring, C3 grasses and forbs predominate (~60% of ecosystem nighttime respiration), while the hotter and drier summer favors the growth of C4 grasses (50-80% of ecosystem nighttime respiration). However, measurements of photosynthesis early in the growing season suggest higher C4 percentages than the nighttime respiration approach. This disagreement between the two approaches might result from a disequilibrium between the isotopic composition of photosynthesis and respiration. This new disequilibrium, if confirmed by further measurements, would have implications for global budgeting techniques that use carbon isotopes in CO2 to partition the net sink between oceans and land. I have also developed the use of the oxygen isotope composition of CO2 as a new and independent constraint on C3/C4 contributions to net carbon exchanges.

cstill@geog.ucsb.edu