PARENT SESSION
Oral Session #69: Elevated CO₂ and Global Change. Presiding: L. Gough.
Thursday, August 9, 2001. 1:00 PM to 4:45 PM. Hall of Ideas H.

Physiological responses of trees from the Rancho La Brea tar pits throughout glacial/inter-glacial cycles.

Ward, Joy¹, Harris, John ², Cerling, Thure¹, Ehleringer, James¹, ^{1 2}

ABSTRACT- The Rancho La Brea tar pits and tunnel excavations in Los Angeles provide an excellent source of preserved wood ranging from 13,000 to 49,000 years of age. This wood provides a direct means of evaluating past physiological responses of trees to changing levels of atmospheric CO₂ throughout glacial/inter-glacial cycles during the late Pleistocene. CO₂ concentrations were approximately 180 ppm during glacial periods and increased to 270 ppm during inter-glacial periods. Past studies with current C3 genotypes have indicated that low CO₂ during glacial periods may have reduced growth by as much as 90% relative to inter-glacial periods, and thus low CO₂ may have been a major stress on C3 species. To test whether plants had the capacity to adjust to changing CO₂ concentrations, we extracted alpha-cellulose from ancient wood samples and measured stable carbon isotope ratios. We assumed a 1.5 per mil offset between cellulose and leaves, and calculated ci/ca (internal leaf CO₂ concentration versus ambient) values for several time periods (13, 14, 15, 23, 27, 29, 38, 49k years before present). We found that ci/ca values were generally similar in response to changing atmospheric CO₂ levels and ranged between 0.43 and 0.58. This suggests that plants maintained a near constant set-point for CO₂ uptake and water loss, even during periods when CO₂ levels may have been the lowest during the evolution of land plants.

KEY WORDS: low carbon dioxide, La Brea tar pits, stable carbon isotopes, Pleistocene