In arid and semi-arid regions, nocturnal water use in trees plays a critical role in regulating forest hydrology and maintaining plant water equilibrium. However, the physiological partitioning of nocturnal sap flow (NSF) into nocturnal transpiration (En) and stem recharge (Re), as well as their environmental regulation, remains inadequately characterized. This study aimed to investigate the characteristics and regulatory mechanisms of NSF in Populus simonii Carr. and Pinus sylvestris var. mongolica Litv., two dominant afforestation species in the Three North Shelterbelt region of northern China. Using thermal dissipation probes (TDP), we determined that NSF accounted for 23.97 % and 20.08 % of the total daily sap flow (Q) in P. simonii and P. sylvestris, respectively. The average nocturnal flow velocities (Vn) were 2.71x10-degrees and 1.51x10-degrees cm & sdot;s-1, respectively. Leaf gas exchange measurements confirmed a low, yet non-negligible, nocturnal transpiration (Tr: 0.04-0.12 mmol H2O & sdot;m-2 & sdot;s-1), and stomatal conductance (Gs: 0.06-0.15 mmol H2O & sdot;m-2 & sdot;s-1), supported by quantitative stomatal imaging, demonstrating partial nocturnal aperture (18.6 % in P. simonii, 12.4 % in P. sylvestris). Path analysis revealed that VPD and SWC jointly regulated NSF. A VPD-based exponential decay model estimated that Re accounted for over 79 % of Qn in both species, signifying that stem recharge is the dominant component of nocturnal water movement. Our findings suggest that NSF is an active physiological process characterized by species-specific traits and environmental sensitivity. Accurate quantification and modeling of En and Re are essential for improving forest water-use strategies in the context of climate change.