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

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 Farquhar et al. (1980), which were founded on the assumption that differences in the drawdown of CO2 from sub-stomatal cavities (ci) to the site of carboxylation inside chloroplasts (cc) were negligible. Recent research, however, indicates an important role for this additional internal pathway of CO2 transfer (gi) in photosynthesis. This work therefore combined fieldwork and modelling to assess the impact of gi on estimation of key photosynthetic parameters, and on the accuracy of simulated photosynthesis (Anet) and stomatal conductance (gs) in a coupled model of leaf-level Anet and gs embedded in a land-surface scheme. It was shown that, in a fast growing poplar genotype (Populus nigra), the photosynthetic parameter Vmax was sensitive to gi. Determination of Vmax under the assumption of finite gi led to estimates of Vmax in well-watered trees that were, on average, 52% higher than values calculated on a ci basis. Drought induced declines in all key photosynthetic parameters measured were observed (Vmax, Jmax and gi), 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 Anet and gs did not improve the accuracy of these simulated fluxes. It was concluded that, for application within a land-surface scheme, currently, accurate calibration of Vmax potentially has a greater impact on simulated Anet and gs than the inclusion of additional, fine-scale leaf-level processes such as gi.