Modeling 13 C discrimination in tree rings

• Published in: Global Biogeochemical Cycles
• Main Authors: Berninger, F., Sonninen, E., Aalto, T., Lloyd, J.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: 2000
• Subjects: Subarctic
• Online Access: http://hdl.handle.net/11858/00-001M-0000-000E-CC48-8; http://hdl.handle.net/11858/00-001M-0000-000E-CC47-A

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 (CO 2 )-C-13/(CO 2 )-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 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 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 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 Delta(13)C, these differences being attributable to year-to-year fluctuations in the average leaf-to-air vapor pressure difference affecting stomatal ...