Nighttime water-use strategies of typical fruit tree species along a precipitation gradient in China’s Loess Plateau region

Abstract Understanding the factors and processes of tree water use at night is critical for sustainable fruit production and ecological protection within the context of increasing global climate extremes. A long-term experiment was set up in China’s Loess Plateau region on rainfed fruit trees—jujube (grown under arid, semi-arid conditions) and apple (grown under semi-humid, drought-prone conditions). Data were collected under both wet and dry conditions and then analyzed for total sap flow (Q), daytime sap flow (Qd), nighttime sap flow (Qn) and the related components of nighttime canopy transpiration (QTn) and nighttime water recharge (QRn). The results showed that the percentage fraction of Qn to Q was 27.6% for jujube and 20.9% for apple. For jujube, QTn/Qn was 67.5%, which was higher than that of apple (56.9%), a species that was under relatively humid conditions. At annual scale, higher annual precipitation (P) resulted in higher Qd but lower Qn. At the daily scale, the components of Qn were positively correlated with leaf area index (LAI) but negatively correlated with solar radiation (Rs) and vapor pressure deficit (VPDn) for jujube at Mizhi Station. Under low LAI/Rs conditions, Qn components of jujube trees had negative correlation with soil water content (SWC). The components of Qn are positively correlated with SWC for apple at Luochuan Station. Under adequate SWC, QRn increased with increasing Qd for apples. Structural equation modeling suggested that the main drivers of nighttime water use were similar for the two fruit trees, but with stronger direct effect of LAI on Qn for jujube. Moreover, Rs mainly affected Qn/Q and QTn/Q through an indirect pathway in jujube, while both its direct and indirect effects were strong and almost equivalent in apple. The findings are critical for the management of fruit trees in ecological environments under worsening environmental conditions.