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 Mo untains of California, and Sinai Peninsula are studied for carbon isot ope ratios. These trees grew in arid environments where vegetation is sparse and canopy effect is minimized. The delta(13)C time series obta ined from wood segments of these trees contain high-frequency variatio ns and a long-term decreasing trend. The high-frequency signals can be effectively explained by the variations of precipitation. The low-fre quency trend cannot be accounted for only by the delta(13)C variation of atmospheric CO2. The CO2 concentration in the atmosphere also attri buted to the progressive depletion in the C-13 contents of these trees . These results indicate a possibility of using delta(13)C of plants a s a proxy indicator of the concentration of atmospheric CO2, provided that the delta(13)C value of the open atmosphere can be constrained in dependently. Plant water-use efficiency is not a simple function of th e CO2 concentration of the atmosphere. It can increase, remain constan t, or decrease with an increase of the atmospheric CO2 concentration.