Abstract: Aims To comprehensively investigate the dynamics of atmospheric water vapor concentration, water vapor isotopes, and evapotranspiration components in the forest ecosystem of the Greater Khingan Mountains across different plant growing seasons and at a diurnal scale. Methods This study conducted high-frequency monitoring of water vapor concentrations and isotopes at different heights using a stable water vapor isotope analyzer, while also determining the δ18O values of plants and soil using vacuum extraction and a liquid water isotope analyzer. Additionally, evapotranspiration components in the Larix gmelinii ecosystem were partitioned and compared across different periods by applying Isotope Steady-State (ISS) and Non-Steady-State (NSS) theories. Important findings During the peak growing season of Larix gmelinii (July~August), atmospheric water vapor concentration and isotopic enrichment were elevated, whereas depletion occurred during the leaf-fall period. Diurnal variations exhibited a complex "V-shaped" cycle with high-low-high fluctuations. On the diurnal scale, the δ¹⁸O of soil evaporation vapor ranged from −27.15‰ to −18.31‰, while that of ecosystem evapotranspiration vapor varied between −15.48‰ and −8.05‰, both demonstrating unimodal trends. Under Isotope Steady-State (ISS) conditions, the δ¹⁸O of plant transpiration vapor spanned −10.83‰ to −5.31‰, contrasting with −12.21‰ to −6.63‰ under Non-Steady-State (NSS) conditions. Minimal divergence between ISS and NSS estimates occurred during 13:00~17:00, where transpiration contributions showed closest alignment. Overall, the transpiration contribution to evapotranspiration was 69.48%~85.08% (ISS) and 76.38%~91.05% (NSS), indicating substantially lower soil evaporation compared to vegetation transpiration, with plant transpiration dominating the forest ecosystem's evapotranspiration.

Key words: stable isotope, water vapor, larix gmelini, evapotranspiration, Non-Steady-State assumption