Influence of vapour pressure deficit and CO2 on the thermal sensitivity of stomatal function in tropical trees.

Stomatal conductance to water vapour (gs) is typically considered to operate in coordination with photosynthesis (A) to balance carbon gain and water loss in an optimal manner. However, at high temperatures that suppress A, gs has been seen to increase or remain high - reducing the predictive accuracy of some stomatal conductance models. We investigated the temperature sensitivity of leaf-level gas exchange and evaluated a temperature-dependent modification of a widely used stomatal conductance model across a temperature gradient (30 to 40°C) in three tropical tree species (Mallotus philippensis, Ficus congesta and Elaeocarpus grandis) grown and measured under near-ambient (420 ppm) or elevated (820 ppm) CO2, and under either constant or increasing vapour pressure deficit (VPD). Measured A and gs, as well as the model term g1, all exhibited a temperature sensitivity that was impacted by [CO2] and VPD regime. Importantly, under both constant and increasing VPD, g1 increased with leaf temperature. These findings demonstrate that the coupling between A and gs is not static but varies with temperature and environmental context, and that incorporating a representation of this thermal sensitivity provides a tractable improvement to models of plant carbon-water exchange under future climate conditions.