Assessing the impact of internal conductance to CO2 in a land-surface scheme: Measurement and modelling of photosynthesis in Populus nigra

Abstract: Vegetation plays a key role in both the global carbon and water cycles. Therefore, the representation of leaf-level fluxes of carbon and water in process-based land-surface schemes is central to accurately predicting these surface exchanges on a larger scale. Leaf-level models of photosynthesis used in such schemes are commonly based on the equations of , which were founded on the assumption that differences in the drawdown of CO2 from sub-stomatal cavities (c i ) to the site of carboxylation inside chloroplasts (c c ) were negligible. Recent research, however, indicates an important role for this additional internal pathway of CO2 transfer (g i ) in photosynthesis. This work therefore combined fieldwork and modelling to assess the impact of g i on estimation of key photosynthetic parameters, and on the accuracy of simulated photosynthesis (A net ) and stomatal conductance (g s ) in a coupled model of leaf-level A net and g s embedded in a land-surface scheme. It was shown that, in a fast growing poplar genotype (Populus nigra), the photosynthetic parameter V max was sensitive to g i . Determination of V max under the assumption of finite g i led to estimates of V max in well-watered trees that were, on average, 52% higher than values calculated on a c i basis. Drought induced declines in all key photosynthetic parameters measured were observed (V max , J max and g i ), in addition to a two-fold increase in photosynthetic biochemical capacity upon re-watering. Reasons for this and the implications for land-surface modelling are discussed. It was shown that inclusion of a constant (non-water stressed) internal conductance to CO2 in a coupled model of leaf-level A net and g s did not improve the accuracy of these simulated fluxes. It was concluded that, for application within a land-surface scheme, currently, accurate calibration of V max potentially has a greater impact on simulated A net and g s than the inclusion of additional, fine-scale leaf-level processes such as g i . [Copyright &y& Elsevier]