Plant hydraulics explain distinct stomatal responses to hot versus dry vapor pressure deficit

Vapor pressure deficit (VPD) is rising exponentially as the globe warms, while relative humidity (RH) is comparatively stable. Stomatal responses to VPD are typically studied by manipulating RH, not air temperature, creating uncertainties for future plant productivity. We tested how air temperature and RH impact the stomatal slope parameter (g1), a proxy for water-use efficiency, and whether three stomatal conductance models capture the observed effects of temperature and RH on g1, which were often positive. Only the hydraulics-based Generalized Stomatal Optimization (GSO) model correctly predicted the observed positive RH-g1 trend. Although all models predicted the observed positive temperature-g1 trends, only the GSO model captured its large magnitude as well as its interspecific variation due to differences in hydraulic traits. Our results show that dry VPD (driven by low RH) leads to hydraulic stress that increases water-use efficiency and closes stomata quickly. In contrast, hot VPD (driven by high air temperature) can lead to decreased water-use efficiency if efficient soil-to-leaf hydraulic transport is maintained and thus slower stomatal closure.