Tree-level stomatal regulation is more closely related to xylem hydraulic traits than to leaf photosynthetic traits across diverse tree species

Understanding stomatal regulatory mechanisms across various woody species is helpful for explaining their adaptations to diverse environmental conditions. Stomatal opening and closing are driven by the requirements for maintaining water transport integrity and carbon uptake; however, distinguishing which factor plays a dominant role in the regulation of tree-level stomatal behavior has seldom been explored. To address this knowledge gap, we investigated differences in tree-level iso/anisohydric stomatal regulation among various tree species (61 and 59 species, at daily and hourly timescales, respectively) across diverse biomes, and analyzed the association of tree-level stomatal regulation with xylem hydraulic and leaf photosynthetic traits. Xylem hydraulic traits were closely related to tree-level stomatal regulation, whereas leaf photosynthetic traits showed non-significant correlations. Reduction in tree-level stomatal conductance with the same vapor pressure deficit increment (ranging from 0.6 to 2 kPa; Lcond), representing the degree of iso/anisohydry, was positively correlated to xylem pressure inducing 50% loss of hydraulic conductivity and vessel diameter, but negatively correlated to the hydraulic safety margin. The associations between xylem hydraulic traits and Lcond revealed that tree species with greater xylem hydraulic efficiency were more likely to adopt an avoidance strategy for tree-level stomatal regulation, whereas tolerance strategy occurred in species with a stronger hydraulic safety system. Furthermore, Lcond was positively correlated to mean annual precipitation and temperature, suggesting that species inhabiting humid and warm regions rely upon isohydric stomatal regulation. Moreover, Lcond displayed a phylogenetic signal, suggesting that variation in Lcond has been influenced by evolutionary history. Overall, tree-level stomatal regulation is more closely related to xylem hydraulic traits than to leaf photosynthetic traits, and maintaining water transport integrity rather than fulfilling requirements for carbon uptake is the major factor impacting tree-level stomatal regulation.