Modeling 13 C discrimination in tree rings

Annual variations from 1877 to 1995 in tree‐ring α‐cellulose 13 C/ 12 C isotopic ratios for four subarctic Pinus sylvestris trees were determined, and, in conjunction with a recent record of atmospheric 13 CO 2 / 12 CO 2 ratios, the historical pattern of photosynthetic isotope discrimination, Δ 13 C, was evaluated. Year‐to‐year variability in Δ 13 C has been as much as 1.5‰ with the period 1900–1920 showing an extended period of unusually high photosynthetic discriminations. The summers during these years were, on average, unusually cold. Since 1920 a long term trend of increasing Δ 13 C of ∼0.016‰yr −1 is inferred. We compared measured Δ 13 C with those predicted on the basis of the theoretical relationship between Δ 13 C and the ratio of substomatal to ambient CO 2 concentration, C i /C a using mechanistic equations for chloroplast biochemistry coupled with a stomatal conductance model. Two variations of a nonlinear optimal‐regulation stomatal conductance model were compared. Although both models were based on the assumption that stomata serve to minimize the average transpiration rate for a given average rate of CO 2 assimilation, one version of the model incorporated reductions in stomatal conductance in response to recent increases in atmospheric CO 2 concentrations and the other did not. The CO 2 sensitive stomatal model failed to describe the long‐term increase in 13 C discrimination, especially after 1950. The insensitive model gave good agreement, suggesting that an observed increase in subarctic Pinus sylvestris Δ 13 C since 1920 is attributable to recent increases in atmospheric CO 2 concentrations with subsequent increases in the ratio of substomatal to ambient CO 2 concentrations. The model was also capable of accounting for high frequency (year‐to‐year) variations in Δ 13 C, these differences being attributable to year‐to‐year fluctuations in the average leaf‐to‐air vapor pressure difference affecting stomatal conductance and hence C i /C a .