ABSTRACT Tree water use ( Q ) is a key component of forest water balance. However, the diurnal ( Q d ) and nocturnal ( Q n ) water use vary with tree size in boreal forests and remain insufficiently understood. Therefore, we continuously monitored sap flow of trees across large, medium and small diameter at breast height (DBH) classes in a boreal larch ( Larix gmelinii ) forest in China during the growing season (May–September) from 2021 to 2024. Concurrent measurements of environmental variables were performed to quantify the effects of tree size on Q d and Q n , as well as to elucidate the underlying environmental controls. The results revealed significant tree size effects on both Q d and Q n . Large trees exhibited an earlier onset and later cessation of daily water use in comparison to medium and small trees, accompanied by greater diurnal variability in water use. Both Q d and Q n increased significantly with increasing tree size, whereas the seasonal peak water use of large and medium trees occurred later than that of small trees throughout the growing season. The contribution of Q n to Q ( Q n / Q ) displayed a U‐shaped seasonal pattern across all tree sizes. However, Q n / Q decreased progressively with increasing tree size, with average values of 17.96%, 13.29% and 12.33% for small, medium and large trees, respectively. The environmental controls on Q d , Q n and Q n / Q differed substantially among tree sizes. Q d was regulated by net radiation (Rn), daytime vapour pressure deficit (VPD d ), deep soil water content (SWC 40 ) and daytime air temperature (Ta d ). In contrast, nighttime vapour pressure deficit (VPD n ) emerged as the dominant driver of Q n across all tree sizes, with its relative influence increasing as tree size decreased. Notably, the drivers of Q n / Q differed from those of Q d or Q n , with nighttime wind speed (Ws n ), nighttime air temperature (Ta n ) and shallow soil water content (SWC 10 ) identified as the primary regulating factors. Moreover, the influence of Ws n on Q n / Q declined with decreasing tree size, whereas the effects of Ta n and SWC 10 increased gradually. These findings demonstrate that tree size plays a critical role in shaping the patterns of Q d and Q n and their responses to environmental controls, and provide new insights into the interaction between water use strategies and tree growth in boreal forests under future climate change by explicitly accounting for tree size effects.
Liu et al. (Wed,) studied this question.