Abstract
During the night, plant water loss can occur either through the roots, as hydraulic redistribution (HR), or through the leaves via the stoma, as nocturnal transpiration (E(n)), which was methodologically difficult to separate from stem refilling (R(e)). While HR and E(n) have been reported across a range of species, ecosystem, and climate zone, there is little understanding on the interactions between E(n) and/or R(e) and HR. As water movement at night occurs via gradients of water potential, it is expected that during periods of high atmospheric vapor pressure deficit (VPD), water loss via E(n) will override water loss via HR. To test this hypothesis, sap flow in stems and roots of Populus euphratica Oliv. trees, growing in a riparian zone in a hyperarid climate, was measured once in a year. Nocturnal stem sap flow was separated into E(n) and R(e) using the "forecasted refilling" method. Substantial nocturnal sap flow (38% of 24-hr flux on average) was observed and positively correlated with VPD; however, the strength of the correlation was lower (R(2) = .55) than diurnal sap flow (E(d)) (R(2) = .72), suggesting that nocturnal stem sap flow was attributed to both water loss through the canopy and replenishment of water in stem tissues. Partitioning of nocturnal sap flow shows that R(e) constituted approximately 80%, and E(n) ~20%, of nocturnal sap flow. The amount of root sap flow attributed to redistribution was negatively related to E(d) (R(2) = .69) and the amount of acropetally sap flow in stems, R(e) (R(2) = .41) and E(n) (R(2) = .14). It was suggested that the magnitude of HR is more strongly depressed by R(e) that was recharge to the water loss via E(d) than by E(n). It was consistent with whole-tree water balance theory, that the nighttime upward sap flow to xylem, stem refilling and transpiration, may depress hydraulic redistribution of roots.