An explanation of 13C/12C variations in tree rings

Variations in the 13C/12C ratio in trees are examined in the light of a simple expression relating the relative isotope composition of plant material, δp13 to δa13, the atmospheric isotope value, ca, the atmospheric CO2 concentration and ci, the internal concentration of CO2 in leaves. The expression gives good agreement with δp13 measurements where independent information on ci exists, such as seasonal growth, growth low in the canopy and in conditions of low humidity. The expression provides possible explanations for two previously unexplained phenomena: the absence of anticipated changes due to fossil fuel-induced changes in δa13, and regional differences in δp13 trends.

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