Daytime stomatal regulation in mature temperate trees prioritizes stem rehydration at night
Richard L. Peters, Kathy Steppe, Christoforos Pappas, Roman Zweifel, Flurin Babst, Lars Dietrich, Georg von Arx, Rafael Poyatos, Marina V. Fonti, Patrick Fonti, Charlotte Grossiord, Mana Gharun, Nina Buchmann, David N. Steger, Ansgar Kahmen
Abstract
Summary Trees remain sufficiently hydrated during drought by closing stomata and reducing canopy conductance ( G c ) in response to variations in atmospheric water demand and soil water availability. Thresholds that control the reduction of G c are proposed to optimize hydraulic safety against carbon assimilation efficiency. However, the link between G c and the ability of stem tissues to rehydrate at night remains unclear. We investigated whether species‐specific G c responses aim to prevent branch embolisms, or enable night‐time stem rehydration, which is critical for turgor‐dependent growth. For this, we used a unique combination of concurrent dendrometer, sap flow and leaf water potential measurements and collected branch‐vulnerability curves of six common European tree species. Species‐specific G c reduction was weakly related to the water potentials at which 50% of branch xylem conductivity is lost ( P 50 ). Instead, we found a stronger relationship with stem rehydration. Species with a stronger G c control were less effective at refilling stem‐water storage as the soil dries, which appeared related to their xylem architecture. Our findings highlight the importance of stem rehydration for water‐use regulation in mature trees, which likely relates to the maintenance of adequate stem turgor. We thus conclude that stem rehydration must complement the widely accepted safety–efficiency stomatal control paradigm.