Stable carbon isotope measurements in tree rings have been explored as potential indicators of past climate1,2, and as proxy data for reconstructions of the history of 13C/12C variations of atmospheric carbon dioxide as related to the global carbon cycle3-6. Results of these studies are often conflicting, perhaps partly due to the limited style of sampling natural systems with large inherent spatial variability. We have examined stable carbon isotopic trends in tree rings among several radii of the same tree (Pinus edulis Engelm.) and among several trees from the same site, to determine the extent of this variability. The circumferential range in δ13C values of cellulose is ∼1-1.5‰, whereas among individuals it is ∼2-3‰. Tests to determine how well various combinations of cores fit the trend of an individual tree or of the site as a whole indicate that pooling four cores from four trees accurately represent site δ13C trends and absolute values.
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