How to differentiate nighttime transpiration and water recharge in nocturnal sap flow?

Nocturnal sap flow (Q_n) affect not only forest carbon and water budgets but also their responses to climate change as it consists of two ecohydrological and ecophysiological significant components: nighttime transpiration and water recharge. A vapor pressure deficit (VPD) based sap flow partitioning method has been developed to estimate nighttime transpiration, which is normally quantified through the discretely measured nighttime stomatal conductance, from the widely and continuously measured sap flow. However, given the increasing knowledge of Q_n mechanisms, whether Q_n could be partitioning simply by VPD and whether this method is valid in semi-arid regions remain unclear. We measured sap flow of Pinus tabuliformis and Acer truncatum in a middle-aged and a young monoculture forest stand, respectively, in a semi-arid mountainous area of northern China. We found the influence of VPD on Q_n conditioned by soil moisture. Meanwhile, a considerable impact of wind speed on Q_n was observed. In the stands with relatively dry soils, both increased and decreased soil moisture promoted Q_n, which might be due to enhanced nighttime water recharge for two distinct purposes, i.e., capacitance refilling and avoiding hydraulic failures. For these three environmental factors (i.e., VPD, wind speed, and soil moisture) that have been considered most in previous studies, their total effect explained less than 55% of the Q_n variations. This study highlights that physiological influences of VPD on nighttime stomatal water loss were uncertain. Furthermore, it suggests that there could exist considerable nighttime water loss induced by wind, possible region-specific patterns of nighttime water recharge, and limited concurrent environmental controls on Q_n. Our findings are helpful to improve the VPD-based sap flow partitioning method to differentiate nighttime transpiration and water recharge.