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Abstract

Annual variations from 1877 to 1995 in tree-ring alpha-cellulose C-13/C-12 isotopic ratios for four subarctic Pinus sylvestris trees were determined, and, in conjunction with a recent record of atmospheric (CO₂)-C-13/(CO₂)-C-12 ratios, the historical pattern of photosynthetic isotope discrimination, Delta(13)C, was evaluated. Year-to-year variability in Delta(13)C has been as much as 1.5 parts per thousand 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 Delta(13)C of similar to 0.016 parts per thousand yr(-1) is inferred. We compared measured Delta(13)C with those predicted on the basis of the theoretical relationship between Delta(13)C and the ratio of substomatal to ambient CO₂ 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 CO₂ assimilation, one version of the model incorporated reductions in stomatal conductance in response to recent increases in atmospheric CO₂ concentrations and the other did not. The CO₂ sensitive stomatal model failed to describe the long-term increase in C-13 discrimination, especially after 1950. The insensitive model gave good agreement, suggesting that an observed increase in subarctic Pinus sylvestris Delta(13)C since 1920 is attributable to recent increases in atmospheric CO₂ concentrations with subsequent increases in the ratio of substomatal to ambient CO₂ concentrations. The model was also capable of accounting for high frequency (year-to-year) variations in Delta(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. [References: 59]