The Influence of C4 Photosynthesis on the Concentration and Isotopic Composition of Atmospheric CO2

EPA Grant Number: U915012
Title: The Influence of C4 Photosynthesis on the Concentration and Isotopic Composition of Atmospheric CO2
Investigators: Still, Christopher J.
Institution: Stanford University
EPA Project Officer: Lee, Sonja
Project Period: January 1, 1996 through January 1, 1999
Project Amount: $102,000
RFA: STAR Graduate Fellowships (1996) RFA Text |  Recipients Lists
Research Category: Academic Fellowships , Ecological Indicators/Assessment/Restoration , Fellowship - Ecology and Ecosystems


The objective of this research project is to examine the response of cloud forest ecosystems to global change and the role that photosynthetic pathways play in the carbon cycle. Fellow researchers, 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 ecosystems services; from serving as watersheds to regulating seasonal water releases. They also are very rich in endemic species; perhaps this is 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.


I developed a global distribution of C3 and C4 plants 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 C3 plants. Therefore, a better understanding of the terrestrial carbon cycle using forward and inverse modeling techniques 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 (approximately 10 percent of the land surface), and C4 gross primary productivity to be 20 percent of global productivity.

At the regional scale, 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 (approximately 60 percent of ecosystem nighttime respiration), while the hotter and drier summer favors the growth of C4 grasses (50-80 percent 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 additional measurements, would have implications for global budgeting techniques that use carbon isotopes in CO2 to partition the net sink between oceans and land. I also have developed the use of the oxygen isotope composition of CO2 as a new and independent constraint on C3/C4 contributions to net carbon exchanges.

Supplemental Keywords:

fellowship, cloud forest, photosynthetic pathways, carbon cycle, atmospheric CO2, carbon dioxide, C4 photosynthesis, endemic species, cloud contact, carbon exchange., RFA, Air, climate change, Air Pollution Effects, Atmosphere

Progress and Final Reports:

  • 1996
  • 1997
  • Final