Disentangling the relative contributions of atmospheric demand for water and soil water availability on the stomatal limitation of photosynthesis

Plants open and close their stomata in response to changes in the environment, so they can absorb the CO₂ they need to grow, while also avoid drying out. Since the activities of leaf stomata determine the exchanges of carbon and water between the vegetation and the atmosphere, it is crucial to incorporate their responses to environmental pressure into the vegetation models predicting carbon and water fluxes on broad spatial and temporal scales. The least-cost optimality theory proposes a simple way to predict leaf behaviour, in particular changes in the ratio of leaf internal (c_i) to ambient (c_a) partial pressure of CO₂, from four environmental variables, i.e. c_a, growing-season temperature (T_g), atmospheric vapour pressure deficit (D_g), and atmospheric pressure (as indexed by elevation, z). However, even though the theory considers the effect of atmospheric demand for water on c_i/c_a, it does not predict how dry soils with reduced soil water availability further influence c_i/c_a. Recent research has shown that independent of the individual effects of T_g, D_g, c_a and z on c_i/c_a, the model tends to underestimate c_i/c_a values at high soil moisture and to overestimate c_i/c_a values at low soil moisture. Here, we will try to disentangle the relative contribution of D_g and soil moisture on changes in c_i/c_a and test a new implementation of soil moisture effect in the framework of the least-cost hypothesis. To achieve this goal, we will use stable carbon isotopes measurements in leaves and in tree rings at sites with different soil water availability and different evaporative demand. We will then incorporate the improved model based on the least-cost hypothesis into the UK vegetation model JULES and investigate leaf stomatal responses to recent environmental changes across regions.