Implications of accounting for mesophyll conductance, temperature acclimation, and coordination in a land surface model photosynthesis scheme

An accurate representation of photosynthesis in land surface models is critical for assessing the capacity of the terrestrial biosphere to take up carbon in a changing climate. We present results from an improved physiology scheme embedded in the land surface model CABLE, which extends the standard Farquhar et al. 1980 representation of photosynthesis by three mechanisms: 1) mesophyll conductance, which reduces the CO₂ concentration available for photosynthesis, 2) photosynthetic acclimation to temperature, which adjusts the temperature response of biochemical rates depending on growth temperature, and 3) photosynthetic coordination, which regulates the within-canopy distribution of nitrogen to achieve a more balanced limitation of canopy photosynthesis by light- and Rubisco limited rates. Results from site-level simulations for a diverse set of Australian ecosystems ranging from temperate forests to tropical savannahs show that these model improvements lead to changing responses of simulated photosynthesis to environmental drivers at leaf and canopy scales. Most striking are significant changes in the photosynthetic CO₂ sensitivity with temperature, which are consistent across ecosystems. Compared to land surface models ignoring these mechanisms, the new model version simulates a CO₂ response of photosynthesis that varies less with temperature, which translates into a stronger carbon uptake by vegetation in cooler climates but a weaker response in tropical regions.