The sequence of physiological stress thresholds responses to drought in a temperate mixed forest

Predicting the influence of worldwide drought on forests remains constrained by lacking of mechanistic understanding of trait interactions in defining physiological dysfunction under water stress, particularly with respect to the interaction between plant stomatal regulation and hydraulics. In this study, we investigated key stomatal and hydraulic traits of leaves and stems for 17 temperate woody species in northeastern China. We found that the stomatal closure point (the water potential at 88 % loss of maximum stomatal conductance) was positively correlated with the water potentials at leaf turgor loss, stem hydraulic conductivity, and 50 % loss of stem hydraulic conductivity. The 50 % loss of leaf hydraulic conductance of all studied species occurred before 50 % stem embolism, in agreement with the vulnerability segmentation hypothesis. Almost all studied tree species reached complete stomatal closure before 50 % stem embolism, but after the 50 % loss of leaf hydraulic conductance. Collectively, the observed sequence of water potential for key physiological dysfunctions not only facilitates carbon assimilation for as long as possible, but also helps to avoid substantial loss of the hydraulic function in woody stems with high construction costs. The interactions between stomatal regulation and multiple hydraulic traits may scale up to influence the water use of whole plants.