Stomatal conductance modeling: a novel leaf-scale mechanistic approach tailored for large-scale applications

Understanding and predicting plant responses in current and future climates is vital due to tight land-atmosphere interactions from the local to global scales. More specifically, understanding how stomata respond to environmental stressors (e.g., lack of water, increasing CO2, extreme temperatures) is necessary, as they regulate water and carbon exchanges across the soil-plant-atmosphere continuum. Although a wealth of stomatal models exists, they still have limitations emerging, for example, by the underlying empirical assumptions or optimization-based hypotheses. These models are strongly hinged on observations, thus, hampering their scope in different environmental conditions. Indeed, the difficulty of balancing between the complexity of ecophysiological processes and the parsimony required in land surface and earth system models presents a challenge. Here, to address this, a novel mechanistic model is proposed that resolves basic ecophysiological functions while still being parsimonious and agile so it can be seamlessly integrated into larger-scale modelling frameworks. The model resolves guard and epidermal cells turgor, leaf hydraulic paths and gas diffusion through stomata, mesophyll conductance, and carbon assimilation (Farquhar model of photosynthesis) and includes explicitly the roles of both ABA and CO₂ in regulating stomatal aperture. Published data on plant responses to different environmental variables, e.g., VPD, water stress, light, CO₂, and temperature, were combined to parametrize and test the model capabilities in a range of different conditions. In addition, a global leaf gas exchange database was utilized as a further confirmation of the model skill to represent a wide range of stomatal responses under different environmental conditions.