Daytime stomatal regulation in mature temperate trees prioritizes stem rehydration at night

Trees remain sufficiently hydrated during drought by closing stomata and reducing canopy conductance (Gc) in response to variations in atmospheric water demand and soil water availability. Thresholds that control the reduction of Gc are proposed to optimize hydraulic safety against carbon assimilation efficiency. However, the link between Gc and the ability of stem tissues to rehydrate at night remains unclear. We investigated whether species-specific Gc 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 Gc reduction was weakly related to the water potentials at which 50% of branch xylem conductivity is lost (P50). Instead, we found a stronger relationship with stem rehydration. Species with a stronger Gc 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.

Automated summary

Trees remain hydrated during drought by reducing canopy conductance and closing stomata. The control of canopy conductance is proposed to optimize hydraulic safety against carbon assimilation efficiency. However, the relationship between canopy conductance and stem rehydration remains unclear. This study investigated the species-specific responses of canopy conductance and found that it is weakly related to water potential, but strongly related to stem rehydration. The findings highlight the importance of stem rehydration in water-use regulation in mature trees.