Water loss after stomatal closure: Quantifying leaf minimum conductance and minimal water use in nine temperate European tree species during a severe drought

Abstract

Residual canopy transpiration (Emin-canop) is a key physiological trait that determines trees' survival time under drought after stomatal closure and after trees have limited access to soil water. Emin-canop mainly depends on leaf minimum conductance (gmin) and vapor pressure deficit. Here we determined the seasonal variation of gmin and how gmin is related to interspecies variation in leaf cuticular and stomatal traits for nine European tree species in a mature forest. In addition, we determined the species-specific temperature responses of gmin. With this newly obtained insight, we calculated Emin-canop for the nine species for one day at our research site during the 2022 central European hot drought. Our results show that at ambient temperatures gmin ranged from 0.8 to 4.8 mmol m s across the nine species and was stable in most species throughout the growing season. The interspecies variation of gmin was associated with leaf cuticular and stomatal traits. Additionally, gmin exhibited strong temperature responses and increased, depending on species, by a factor of two to four in the range of 25-50 oc. For the-2 studied species at the site, during a single hot drought day, Emin-canop standardized by tree size (stem basal area) ranged from 2.0 to 36.7 L m and non-standardized Emin-canop for adult trees ranged from 0.3 to 5.3 L. Emin-canop also exhibited species-specific rapid increases under hotter temperatures. Our results suggest that trees, depending on species, need reasonable amounts of water during a drought, even when stomates are fully closed. Species differences in gmin and ultimately Emin-canop can, together with other traits, affect the ability of a tree to keep its tissue hydrated during a drought and is likely to contribute to species-specific differences in drought vulnerability.