Differentiated responses of daytime and nighttime sap flow to soil water deficit in a larch plantation in Northwest China

Tree transpiration contributes to the forest water budget and its environmental responses differ between daytime and nighttime. Elucidating these divergent responses separately from their biophysical controls is crucial for ascertaining the forest-water relationship in changing environments. To investigate the biophysical controls on daytime and nighttime sap flow (Qd and Qn), the sap flux density, meteorological conditions, soil moisture, and canopy leaf area index (LAI) were simultaneously monitored in a Larix principis-rupprechtii plantation in the Liupan Mountains of Northwest China during the growing seasons (June to September) of three hydrological years (2019, 2021, and 2022). The results showed that the main control factors for Qd and Qn varied depending on the timescale. The variation in Qd was mainly controlled by daytime solar radiation (Rd), vapour pressure deficit (VPDd), relative extractable soil water (REWd) and LAI at a daily scale, but only by Rd at a monthly scale and only by VPDd at an interannual scale. The variation in Qn was mainly controlled by the nighttime vapour pressure deficit (VPDn) and relative extractable soil water (REWn), LAI, and Qd at a daily scale but only by VPDn at monthly and interannual scales. The responses of Qd and Qn to soil water deficit were different and varied by timescale. At the daily scale, the effect of VPDd on Qd became weak, while the effect of VPDn on Qn became strong in drier years (2021 and 2022). At the monthly scale, as the main controlling factor of the monthly variation in Qd (Qn), Rd (VPDn) played a decisive role in the wet (dry) months. At the interannual scale, VPD limited Qd but drove Qn, leading to a decrease in Qd and an increase in Qn with increasing soil water deficit. Overall, our findings revealed the different responses of Qd and Qn to biophysical factors and underscored that Qd and Qn should be explored both separately and synchronously; additionally, our results indicated that the proportion of Qn might increase if soil water stress is increased due to future climate change.