The photosynthetic rate dominates the seasonal variation of tree intrinsic water-use efficiency in humid East Asian Monsoon region

Intrinsic water-use efficiency (iWUE) in trees, defined as the ratio of photosynthetic rate (A) to stomatal conductance (gs), is a key indicator characterizing the carbon-water balance in trees. Previous studies have suggested that seasonal variation in iWUE is primarily controlled by gs in arid regions; however, the main driving factors in humid regions remain unclear. Therefore, this study utilized 8 years of high-resolution tree-ring δ13C and δ18O data from P. massoniana (coniferous) and S. tzumu (broadleaf) in the humid East Asian monsoon region to reconstruct the seasonal dynamics of iWUE. We estimated iWUE based on δ13C and derived the leaf water δ18O enrichment (Δ18Olw) from δ18O to represent gs, thereby distinguishing the relative contributions of A and gs to variations in iWUE. Both species exhibited synchronous seasonal iWUE patterns: decreasing from spring to summer before autumn recovery. Dual-isotope analysis revealed that iWUE seasonal variations were primarily driven by fluctuations in A, contrasting with gs-dominated mechanisms in arid regions. Summer iWUE declines resulted from reduced A, constrained by relatively low CO2 and high temperatures. This study reveals an A-dominated regulatory mechanism of iWUE in humid regions, providing a theoretical basis for more accurate predictions of forest carbon-water coupling under varying moisture conditions.