Ehleringer, J.R., D.R. Sandquist, and S.L. Phillips. 1997. Burning coal seams in southern Utah: a natural system for studies of plant responses to elevated CO2, p. 56-68. In A.Raschi, F. Miglietta, R. Tognetti and P.R. van Gardingen, (eds.), Plant Responses to Elevated CO2 Evidence From Natural Springs. Cambridge University Press, London.

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 belowground combustion of this low-sulfur coal releases gases to the atmosphere from vents above the burning seam. The surrounding vegetation is cold-desert shrub, typical of the region and consisted of both C3 and C4 perennial species. Additionally, at least one weedy C4 species had invaded disturbed locations immediately adjacent to the active vent area. Atmospheric CO2 concentrations in the vicinity of the vents fluctuated significantly, however, CO2 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. CO2 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, CO2 concentrations were elevated by an average of 542 ppm above ambient values. The continuous distribution of C4 vegetation along the CO2 concentration gradient provides a mean of estimating the long-term integrated CO2 concentrations at each location. Using the carbon isotope ratio of the C4 vegetation (Atriplex confertifolia and Salsola iberica) to estimate the atmospheric CO2 concentration, we observed that the ratio of intercellular to atmospheric CO2 concentrations of C3 vegetation decreased in response to elevated CO2 concentrations. This decreased ratio for Gutierrezia sarothrae (C3) was sufficient to result in a predicted doubling of water-use efficiency.