Mesophyll conductance in land surface models: effects on photosynthesis and transpiration.

The CO transfer conductance within plant leaves (mesophyll conductance, g ) is currently not considered explicitly in most land surface models (LSMs), but instead treated implicitly as an intrinsic property of the photosynthetic machinery. Here, we review approaches to overcome this model deficiency by explicitly accounting for g , which comprises the re-adjustment of photosynthetic parameters and a model describing the variation of g in dependence of environmental conditions. An explicit representation of g causes changes in the response of photosynthesis to environmental factors, foremost leaf temperature, and ambient CO concentration, which are most pronounced when g is small. These changes in leaf-level photosynthesis translate into a stronger climate and CO response of gross primary productivity (GPP) and transpiration at the global scale. The results from two independent studies show consistent latitudinal patterns of these effects with biggest differences in GPP in the boreal zone (up to ~15%). Transpiration and evapotranspiration show spatially similar, but attenuated, changes compared with GPP. These changes are indirect effects of g caused by the assumed strong coupling between stomatal conductance and photosynthesis in current LSMs. Key uncertainties in these simulations are the variation of g with light and the robustness of its temperature response across plant types and growth conditions. Future research activities focusing on the response of g to environmental factors and its relation to other plant traits have the potential to improve the representation of photosynthesis in LSMs and to better understand its present and future role in the Earth system.

© 2019 The Authors The Plant Journal © 2019 John Wiley & Sons Ltd.