Radiation and Drought Impact Residual Leaf Conductance in Two Oak Species With Implications for Water Use Models

Stomatal closure is one of the earliest responses to water stress but residual water losses may continue through the cuticle and incomplete stomatal closure. Residual conductance (g(res)) plays a large role in determining time to mortality but we currently do not understand how do drought and shade interact to alter g(res) because the underlying drivers are largely unknown. Furthermore, g(res) may play an important role in models of water use, but the exact form in which g(res) should be incorporated into modeling schemes is currently being discussed. Here we report the results of a study where two different oak species were experimentally subjected to highly contrasting levels of drought (resulting in 0, 50 and 80% losses of hydraulic conductivity) and radiation (photosynthetic photon flux density at 1,500 μmol m(–2) s(–1) or 35–45 μmol m(–2) s(–1)). We observed that the effects of radiation and drought were interactive and species-specific and g(res) correlated positively with concentrations of leaf non-structural carbohydrates and negatively with leaf nitrogen. We observed that different forms of measuring g(res), based on either nocturnal conductance under high atmospheric water demand or on the water mass loss of detached leaves, exerted only a small influence on a model of stomatal conductance and also on a coupled leaf gas exchange model. Our results indicate that, while understanding the drivers of g(res) and the effects of different stressors may be important to better understand mortality, small differences in g(res) across treatments and measurements exert only a minor impact on stomatal models in two closely related species.