Contemporary Terrestrial Biosphere May Be More CO2 Limited than Previously Thought
In plants with C3 photosynthetic pathways, CO2 concentrations drop considerably along leaf mesophyll diffusion pathways from sub-stomatal cavities to chloroplasts where CO2 assimilation occurs. Global carbon cycle models have not explicitly represented this internal drawdown, overestimating CO2 available for carboxylation and underestimating photosynthetic responsiveness to atmospheric CO2. Researchers at Oak Ridge National Laboratory sought to determine how mesophyll diffusion affects the global land CO2 fertilization effect estimated by global carbon models. The team found that current carbon cycle models underestimate by 16% the long-term responsiveness of global terrestrial productivity to CO2 fertilization. This underestimation of CO2 fertilization is caused by an inherent model structural deficiency related to a lack of explicit representation of CO2 diffusion inside leaves, which results in an overestimation of CO2 available at the carboxylation site. The magnitude of CO2 fertilization underestimation matches the long-term positive growth bias in the historical atmospheric CO2 predicted by Earth system models. This finding implies that the contemporary terrestrial biosphere is more CO2 limited than previously thought and will lead to improved understanding and modeling of carbon-climate feedbacks.
Sun, Y., L. Gu, R. E. Dickinson, R. J. Norby, S. G. Pallardy, and F. M. Hoffman. 2014. “Impact of Mesophyll Diffusion on Estimated Global Land CO2 Fertilization,” Proceedings of the National Academy of Sciences (USA) 111(44), 15,774-779. DOI: 10.1073/pnas.1418075111.