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

• Temporal dynamics of nocturnal sap flow (Qn) in Populus popularis were determined. • The increase in nocturnal sap flow normalized by leaf area index (Qn/LAI) after midnight on rainy days was linked to stem refilling. • Soil water content (0–30 cm depth) and groundwater level jointly regulated Qn/LAI. • A negative effect of nighttime vapor pressure deficit on Qn/LAI emerged under low soil water availability. 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 2 n ), 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 2 n 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.