Plant fixation of CO2and subsequent incorporation of this carbon into growth rings is the basis for attempts to use tree rings to reconstruct13C/12C ratios (δ13C) of atmospheric carbon dioxide. Accurate δ13C chronologies of atmospheric CO2are important to distinguishing biospheric and fossil fuel CO2inputs to the atmosphere. We have sampled 14 sites in the American Southwest, pooling four cores from each of four trees at most of the sites. Cellulose from pooled 5‐year ring groups was analyzed to develop δ13C chronologies for each of the sites. The 10 pinyon (Pinus edulis, P. monophylla) chronologies, plus two additional pinyon chronologies formed from groups of sites where single trees had been analyzed, were examined in detail. Significant correlations of differences in successive values of ring width indices and δ13C (first differences) were used to “correct” each chronology for climate influences primarily responsible for high‐frequency fluctuation. The master corrected δ13C chronology reveals an overall decrease of 1.2–1.4‰ from 1700 to present, greater than the atmospheric change expected from fossil fuel CO2inputs alone and comparable to that measured in ice cores. The biospheric CO2input to the atmosphere implicated in this chronology is greatest is the nineteenth and early twentieth centuries. High‐frequency fluctuations in the chronologies appear dominantly related to
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