Water loss and carbon gain are balanced by stomatal control1, a trade-off that has allowed trees to survive and thrive under fluctuating environmental conditions2–4. During periods of lower water availability, stomatal closure prevents excess water loss5. Various strategies of stomatal control have been found among tree species6,7, 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 biology8–10, and often used in predictive water-use modelling11,12. 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.
