Temporal dynamics and environmental drivers of nocturnal sap flow in a Populus popularis plantation in an arid area with shallow groundwater

Emerging evidence demonstrates that nocturnal sap flow (Q_n) is a nonnegligible component among many tree species, and depends on the climate and water conditions. We investigated Q_n dynamics for Populus popularis, a main tree species in China’s farmland shelterbelts, in arid climate and shallow groundwater environments. Tree water use of the P. popularis stand was quantified using sap flow measurements during the growing seasons of 2018–2021. Leaf area index (LAI), nighttime vapor pressure deficit (VPD_n), nighttime wind speed at 2 m height (u_2n), soil water content in the 0–30 cm soil layer (SWC_{0–30 cm}), and groundwater table depth (GTD) were involved in this study. Results indicated that mean daily Q_n and daily proportion of Q_n to sap flow were 0.11–0.26 kg m^-2 d^-1 and 6 %–9 % in four years, respectively. The mean daily proportion of Q_n to sap flow was higher in May and September than in other months. Compared with clear nights under similar VPD_n and GTD conditions, rainy nights prompted a significant increase in normalized nocturnal sap flow (Q_n/LAI, derived by dividing Q_n by LAI) after midnight as stem refilling accelerated. SWC_{0–30 cm} and GTD were the primary factors determining Q_n/LAI. Q_n/LAI could be depressed by VPD_n under low SWC_{0–30 cm}. Rising u_2n promoted slightly Q_n/LAI over four years. The increase in GTD induced a reduction in daytime sap flow, thereby elevating Q_n/LAI over four years, except for 2018. Q_n in P. popularis stand could be used for stem water recharge under physiological regulation. Our results reveal species-specific water use strategies, thereby understanding nocturnal water uptake of plantations and refining silvicultural techniques in similar climatic zones worldwide.