Forests worldwide face a growing threat from drought, with their fates depending on the ability of trees to cope with mortality risks by hydraulic failure and carbon starvation. However, a clear picture of how trees adjust relevant functional traits to persistent drought across species and sites is lacking. Here, using data from global forest throughfall reduction experiments, we showed that tree water potential was lowered significantly while embolism resistance was unchanged, leading to a narrower hydraulic safety margin. Meanwhile, hydraulic efficiency and leaf photosynthetic capacity were maintained, despite net photosynthetic rate being constrained by stomatal conductance. Non-structural carbohydrates in different organs were not changed significantly, suggesting that droughted trees were not at a higher risk of carbon starvation. The findings indicate that forest trees cannot shift their resistance to embolism when water potential declines from drought, leading to greater risk of hydraulic failure.

forest drought | throughfall exclusion experiment | functional trait | plant hydraulics | plasticity | gas exchange