Disentangling Thermal and Hydraulic Drivers of Stomatal Conductance Under Hot Versus Dry Vapour Pressure Deficit Increases Using Water-Use Efficiency

Predicting plant functioning in a warming world requires capturing its divergent responses to hot versus dry vapour pressure deficit (VPD) increases. Stomatal responses to VPD are typically studied without disentangling the effects of RH and temperature, creating uncertainties for future plant functioning. We tested how air temperature and RH independently impact the stomatal slope parameter ((Formula presented.)), a proxy for water-use efficiency, and whether these effects were captured by the Unified Stomatal Optimization, Least-Cost and Generalized Stomatal Optimization (GSO) stomatal conductance models. Using a leaf gas exchange data set, we found that both effects were positive, incentivizing stomatal opening under warming and humidification. Of the three models, only the GSO model correctly predicted the observed positive RH– (Formula presented.) relationship by accounting for a hydraulic constraint on (Formula presented.). The GSO model captured better the large magnitude of the observed positive temperature– (Formula presented.) relationships, by accounting for hydraulic and thermal effects that seek to maintain stable leaf osmotic pressures, and explained its interspecific variation through hydraulic traits. Our results suggest that increasing dry VPD (driven by low RH) creates hydraulic stress that increases water-use efficiency and closes stomata quickly. In contrast, increasing hot VPD (driven by high air temperature) decreases water-use efficiency and slows stomatal closure if soil-to-leaf hydraulic transport is maintained.