Towards improved simulations of carbon and water fluxes: evaluating competing stomatal optimisation hypotheses

Abstract:

Novel formulations of canopy-level gas-exchange regulation, that optimise stomatal conductance with respect to photosynthetic and hydraulic functions, have emerged as avenues to improve land-surface model (LSM) simulations of carbon and water fluxes. In the past five years, at least ten novel alternative formulations for coupling carbon-water fluxes through leaves have been proposed, but independent and systematic evaluations of how they impact predictions of these surface fluxes are lacking. This motivates an inter-model comparison to understand how different mechanistic assumptions across optimal models affect plant behaviour. Here, we embed eight competing optimality approaches within a single LSM framework and ask how, and under what conditions, their predicted behaviours differ from one another and from three empirical stomatal conductance models. Model behaviour was evaluated against leaf-level photosynthetic and hydraulic field/experimental data of species spanning a mean annual precipitation range of 360 - 1620 mm, and originating from Australia, France, and the US. We also conducted a series of idealised model experiments to isolate model-specific surface flux responses and sensitivities to (i) different rates of soil moisture dry-downs, (ii) high vapour pressure deficit, and (iii) elevated CO₂. Finally, we assessed the suitability of the models for use at the global scale, depending on the number of model-specific parameters and on easiness of calibration.