Hydraulic traits exert greater limitations on tree-level maximum sap flux density than photosynthetic ability: Global evidence

Transpiration is a key process that couples the land-atmosphere exchange of water and carbon, and its maximum water transport ability affects plant productivity. Functional traits significantly influence the maximum transpiration rate; however, which factor plays the dominant role remains unknown. SAPFLUXNET dataset, which includes sap flux density of diverse species worldwide, provides fundamental data to test the importance of photosynthetic and hydraulic traits on maximum tree-level sap flux density (J_{s_max}). Here, we investigated variations in J_{s_max} of 2194 trees across 129 species using data from the SAPFLUXNET dataset, and analysed the relationship of J_{s_max} with photosynthetic and hydraulic traits. Our results indicated that J_{s_max} was positively correlated with photosynthetic traits at both leaf and tree level. Regarding hydraulic traits, J_{s_max} was positively related to xylem hydraulic conductivity (K_s), leaf-specific hydraulic conductivity (K_l), xylem pressure inducing 50 % loss of hydraulic conductivity (P₅₀), xylem vessel diameter (V_dia), and leaf-to-sapwood area ratio (A_lA_s). Random forest model showed that 87 % of the variability in J_{s_max} can be explained by functional traits, and hydraulic traits (e.g., P₅₀ and sapwood area, A_s) exerted larger effects on J_{s_max} than photosynthetic traits. Moreover, trees with a lower sapwood area or depth could increase their sap flux density to compensate for the reduced whole-tree transpiration. J_{s_max} of the angiosperms was significantly higher than that of the gymnosperms. Mean annual total precipitation (MAP) were positively related to J_{s_max} with a weak correlation coefficient. Furthermore, J_{s_max} showed a significant phylogenetic signal with Blomberg's K below 0.2. Overall, tree species with acquisitive resource economics or more efficient hydraulic systems show higher water transport capacity, and the efficiency of xylem hydraulic system rather than the demand for carbon uptake predominantly determines water transport capacity.