Evaporation dominates growing season evapotranspiration: A case study from a boreal larch forest with synchronized water supply and energy demand in China

Understanding the relative contributions of transpiration (T) and evaporation (E) to evapotranspiration (ET) is critical for evaluating water use efficiency, ecosystem productivity, and soil–plant–atmosphere interactions in a changing environment. However, such partitioning and its responses to dry, normal, and wet conditions, as well as the controlling factors at multiple temporal scales, remain poorly understood in China's boreal forests, characterized by synchronization of water supply and energy demand. In this study, we used 8 years of ET data from the growing season (GS; May–September) collected via the eddy-covariance system and applied the underlying water use efficiency (uWUE) method to estimate T and E in a boreal larch forest in China. Our results revealed that E was the dominant component of ET. Specifically, T accounted for 0.44 of ET (T/ET), whereas E contributed to 0.56 of ET (E/ET) over the study period. The response of T/ET to dry conditions during the leaf defoliation stage (LDS) was more pronounced than during the leaf expansion stage (LES). Despite an increase in T/ET (reaching 0.49) during the dry season compared to the normal season (0.42), E was still the dominant contributor to ET. Furthermore, E/ET was significantly controlled by vapor pressure deficit (VPD) across daily to GS scales. Interestingly, soil water content (SWC) was not a controlling factor for regulating E/ET, indicating that atmospheric forces strongly constrained the variability of E/ET in this boreal forest. These findings highlight that E should be given greater attention in boreal forests than before. Our study suggests that effective management strategies for improving water use efficiency in such forest ecosystems are urgently needed.