Burning coal seams in southern Utah: a natural system for studies of plant responses to elevated CO 2 J. R. EHLERINGER, D. R. SANDQUIST AND S. L. PHILIPS Stable Isotope Ratio Facility for Environmental Research. Department of Biology, University of Utah, Salt Lake City, Utah 84112, USA. SUMMARY In the Burning Hills and Smoky Mountains of southern Utah (USA), coal deposits exposed to the surface have been ignited by lightning and have been burning for periods of years to over a century. We examined one of these sites, where the below-ground combustion of this low-sulfur coal releases gases to the atmosphere from vents above the burning scam. The surrounding vegetation is cold-desert shrub, typical of the region and consisted of both C 3 and C 4 perennial species. Additionally, at least one weedy C 4 species had invaded disturbed locations immediately adjacent to the active vent area. Atmospheric CO 2 concentrations in the vicinity of the vents fluctuated significantly, however, CO 2 concentrations measured approximately 500 m from the most active vents were 7 ppm elevated above ambient concentrations measured at a control site 10 km from the burning vents. CO 2 concentrations at sites nearer the vents, but still with natural, undisturbed vegetation, were elevated 65 ppm above ambient background values. At vegetated sites nearest the vents, CO 2 concentrations were elevated by an average of 542 ppm above ambient values. The continuous distribution of C 4 vegetation along the CO 2 concentration gradient provides a means of estimating the long-term integrated CO 2 concentrations at each location. Using the carbon isotope ratio of the C 4 vegetation (Atriplex confertifolia and Salsola iberica) to estimate the atmospheric CO 2 concentration, we observed that the ratio of intercellular to atmospheric CO 2 concentrations of Co vegetation decreased in response to elevated CO 2 concentrations. This decreased ratio for Gutierrezia sarothrae (C 3 ) was sufficient to result in a predicted doubling of water-usc efficiency.