Uniform regulation of stomatal closure across temperate tree species to sustain nocturnal turgor and growth

Water loss and carbon gain are balanced by stomatal control¹, a trade-off that has allowed trees to survive and thrive under fluctuating environmental conditions^2-4. During periods of lower water availability, stomatal closure prevents excess water loss⁵. Various strategies of stomatal control have been found among tree species^6,7, but the trigger for this behaviour remains elusive. We found a uniform pre-dawn water potential threshold (-1.2 MPa) for stomatal closure across species, which coincided with stem-growth cessation. Meanwhile, midday water potentials at stomatal closure were more variable across species and stomatal control did not follow species-specific thresholds of hydraulic failure, a commonly adopted theory in plant biology^8-10, and often used in predictive water-use modelling^11,12. This indicates that nocturnal rehydration, rather than daytime hydraulic safety is an optimization priority for stomatal closure in trees¹³. We suggest that these processes are critical for forecasting the global carbon cycle dynamics.