CARBON ISOTOPES OF TREES FROM ARID ENVIRONMENTS AND IMPLICATIONS FOR RECONSTRUCTING ATMOSPHERIC CO2 CONCENTRATION

Four trees from San Dimas, the Santa Monica Mountains and the White Mountains of California, and Sinai Peninsula are studied for carbon isotope ratios. These trees grew in arid environments where vegetation is sparse and canopy effect is minimized. The delta(13)C time series obtained from wood segments of these trees contain high-frequency variations and a long-term decreasing trend. The high-frequency signals can be effectively explained by the variations of precipitation. The low-frequency trend cannot be accounted for only by the delta(13)C variation of atmospheric CO2. The CO2 concentration in the atmosphere also attributed to the progressive depletion in the C-13 contents of these trees. These results indicate a possibility of using delta(13)C of plants as a proxy indicator of the concentration of atmospheric CO2, provided that the delta(13)C value of the open atmosphere can be constrained independently. Plant water-use efficiency is not a simple function of the CO2 concentration of the atmosphere. It can increase, remain constant, or decrease with an increase of the atmospheric CO2 concentration.