Influence of vapour pressure deficit and CO ₂ on the thermal sensitivity of stomatal function in tropical trees

Summary Stomatal conductance to water vapour ( g s ) 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 , g s 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) CO 2 , and under either constant or increasing vapour pressure deficit (VPD). Measured A and g s , as well as the model term g 1 , all exhibited a temperature sensitivity that was impacted by [CO 2 ] and VPD regime. Importantly, under both constant and increasing VPD, g 1 increased with leaf temperature. These findings demonstrate that the coupling between A and g s 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.