Chunyan Zhao

Water use sources of desert riparian Populus euphratica forests

Desert riparian forests are the main body of natural oases in the lower reaches of inland rivers; its growth and distribution are closely related to water use sources. However, how does the desert riparian forest obtains a stable water source and which water sources it uses to effectively avoid or overcome water stress to survive? This paper describes an analysis of the water sources, using the stable oxygen isotope technique and the linear mixed model of the isotopic values and of desert riparian Populus euphratica forests growing at sites with different groundwater depths and conditions. The results showed that the main water source of Populus euphratica changes from water in a single soil layer or groundwater to deep subsoil water and groundwater as the depth of groundwater increases. This appears to be an adaptive selection to arid and water-deficient conditions and is a primary reason for the long-term survival of P. euphratica in the desert riparian forest of an extremely arid region. Water contributions from the various soil layers and from groundwater differed and the desert riparian P. euphratica forests in different habitats had dissimilar water use strategies.

Evidences and magnitude of nighttime transpiration derived from Populus euphratica in the extreme arid region of China

Extensive research has found that nighttime transpiration (En) is positively correlated to the vapour pressure deficit (VPD), that suggested En was highest during the night under high temperatures and low humidity along with high soil water availability, typically for the riparian forest in the extreme arid region of China. This study used the heat ratio method to measure sap velocity (Vs) for mature and saplings Populus euphratica Oliv., and then En was conservatively calculated as total nocturnal sap flow (Fs, the product of Vs and sapwood area As) between 01:00 to 06:00. A gas exchange system was used to measure the leaf transpiration rate (Tr) and stomatal conductance (gs) of saplings. For mature trees, nighttime Vs was extensive and logarithmic correlated to VPD (similar to daytime). For saplings, gs and Tr was extensive in different months, and also a strong logarithmic relationship was found between Vs and VPD for both daytime and nighttime periods. Both of stem sap flow and leaf gas exchange suggusted the occurrence of En, whether mature or sapling trees. En contribution to daily transpiration (Ed) was high just as expected for P. euphratica, which was confirmed by proportional En to Ed (En/Ed) means taken in 2012 (24.99%) and 2013 (34.08%). Compared to mature trees, En/Ed of saplings in 2013 was lower with means of 12.06%, that supported further by the shorter duration times and less Tr,n (16.64%) and gs,n (26.45%) of leaf, suggesting that En magnitude is associated to individual the tree size, that effect to stored water of individual trees, although this hypothesis requires further research.

Nighttime sap flow and its driving forces for Populus euphratica in a desert riparian forest, Northwest China

Nighttime sap flow is a potentially important factor that affects whole-plant water balance and water-use efficiency (WUE). Its functions include predawn disequilibrium between plant and soil water potentials as well as between the increments of oxygen supply and nutrient uptake. However, main factors that drive nighttime sap flow remain unclear, and researches related to the relationship between nighttime sap flow velocity and environmental factors are limited. Accordingly, we investigated the variations in the nighttime sap flow of Populus euphratica in a desert riparian forest of an extremely arid region, Northwest China. Results indicated that P. euphratica sap flow occurred throughout the night during the growing season because of the partial stomata opening. Nighttime sap flow for the P. euphratica forest accounted for 31%–47% of its daily sap flow during the growing season. The high value of nighttime sap flow could be the result of high stomatal conductance and could have significant implications for water budgets. Throughout the whole growing season, nighttime sap flow velocity of P. euphratica was positively correlated with the vapor pressure deficit (VPD), air temperature and soil water content. We found that VPD and soil water content were the main driving factors for nighttime sap flow of P. euphratica.

Depressed hydraulic redistribution of roots more by stem refilling than by nocturnal transpiration for Populus euphratica Oliv. in situ measurement

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.