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Yu T.,Lanzhou University | Yu T.,Chinese Academy of Sciences | Feng Q.,Chinese Academy of Sciences | Feng Q.,Gansu Hydrology and Water Resources Engineering Research Center | And 7 more authors.
Plant and Soil | Year: 2013

Aims: As deep water uptake is limited by the low densities of fine roots at the subsoil layer at high evaporative demand, hydraulic redistribution (HR) is another possible mechanism that could account for the lack of water stress experienced by desert riparian phreatophytes during dry periods. The objectives of this study were to search for evidence that the roots of two desert riparian phreatophytes, Populus euphratica Oliv. and Tamarix ramosissima Ledeb., carry out HR and to investigate the pattern of this phenomenon in the broader scheme of plant water uptake by roots. Methods: To demonstrate HR, we present data on patterns of sap flow in the stems or branches and lateral roots of those two phreatophyte species and soil volumetric moisture content where these species grow. Results: During the dry season, we observed reverse or acropetal flow in the lateral roots of P. euphratica, a pattern consistent with hydraulic lift. With the onset of heavy rains, this pattern reversed, indicating water movement from moist topsoil to dry subsoil, i.e. hydraulic descent. After lateral irrigation by creek, water moved downward to dry subsoil and outward to opposite sides, suggesting lateral hydraulic redistribution via roots at night and during the day, which may be mediated by stem tissues and, by inference, the radial sectoring in the xylem. Although no direct evidence indicated reverse sap flow of lateral roots and associated HR in T. ramosissima, several factors indicate that HR is occurring: (1) diel fluctuations of volumetric moisture content in the upper soil layer and (2) the identification of primary water sources as groundwater and vadose zone water through stable isotope studies. As a result, we inferred that HR occurs in T. ramosissima via adventitious roots with diameters of 2-5 mm and length of 60-100 cm in the upper soil layer, rather than via lateral roots; further investigation is needed to substantiate this. Conclusions: We confirm and extend previous knowledge on HR patterns of P. euphratica and add a new species, T. ramosissima, to the wide list of existing species involved in HR. However, these two desert riparian phreatophytes exhibit remarkable differences in their patterns and pathways of HR that are possibly associated with root architecture. © 2013 Springer Science+Business Media Dordrecht.

Yu T.,Lanzhou University | Yu T.,CAS Lanzhou Cold and Arid Regions Environmental and Engineering Research Institute | Feng Q.,CAS Lanzhou Cold and Arid Regions Environmental and Engineering Research Institute | Feng Q.,Gansu Hydrology and Water Resources Engineering Research Center | And 5 more authors.
Journal of Arid Land | Year: 2013

Tamarix spp. (Saltcedar) is a facultative phreatophyte that can tolerate drought when groundwater is not accessed. In addition to deep water uptake, hydraulic redistribution (HR) is another factor contributing to the drought tolerance of Tamarix spp. In this study, data on soil volumetric moisture content (θ), lateral root sap flow, and relevant climate variables were used to investigate the patterns, magnitude, and controlling factors of HR of soil water by roots of Tamarix ramosissima Ledeb. in an extremely arid land in Northwest China. Results showed evident diurnal fluctuations in θ at the depths of 30 and 50 cm, indicating "hydraulic lift" (HL). θ increased remarkably at 10 and 140 cm but decreased at 30 and 50 cm and slightly changed at 80 cm after rainfall, suggesting a possible "hydraulic descent" (HD). However, no direct evidence was observed in the negative flow of lateral roots, supporting HR (including HL and HD) of T. ramosissima. The HR pathway unlikely occurred via lateral roots; instead, HR possibly occurred through adventitious roots with a diameter of 2-5 mm and a length of 60-100 cm. HR at depths of 20-60 cm ranged from 0.01-1.77 mm/d with an average of 0.43 mm/d, which accounted for an average of 22% of the estimated seasonal total water depletion at 0-160 cm during the growing season. The climate factors, particularly vapor pressure deficit and soil water potential gradient, accounted for at least 33% and 45% of HR variations with depths and years, respectively. In summary, T. ramosissima can be added to the wide list of existing species involved in HR. High levels of HR may represent a considerable fraction of daily soil water depletion and substantially improve plant water status. HR could vary tremendously in terms of years and depths, and this variation could be attributed to climate factors and soil water potential gradient. © 2013 Science Press, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences and Springer-Verlag Berlin Heidelberg.

Si J.,Desert Research Institute | Si J.,Gansu Hydrology and Water Resources Engineering Research Center | Feng Q.,Desert Research Institute | Feng Q.,Gansu Hydrology and Water Resources Engineering Research Center | And 3 more authors.
Shuikexue Jinzhan/Advances in Water Science | Year: 2014

It is generally assumed that transpiration does not occur at night because leaf stomata are opened during the day and closed at night. Nonetheless, there is increasing evidence that the stomata are opening in many species and in a range of habitats at night. Nighttime transpiration rates are typically 5%-15% of daytime rates. This proportion of nighttime transpiration is often highest (30%-60%) in arid desert systems. Control of nighttime transpiration may have considerable complexity, it relates to the different levels control variables. The magnitude of nighttime transpiration with plant genetic factors, it is also affected by abiotic factors, such as vapor pressure difference, wind speed, soil moisture, atmospheric CO2 concentration, soil temperature and so on. Study shows that water vapor pressure deficit is the most important environmental driving factors to nighttime transpiration. Nighttime transpiration has significant implications for water budgets as an inevitable process. It can reduce leaf water potential of plant, contribute to imbalance between predawn leaf water potential and soil water potential, reduce hydraulic redistribution, affect of ecological water balance, enhance nutrient availability, and affect the productivity and growth of plant. ©, 2014, China Water Power Press. All right reserved.

Yu T.-F.,Chinese Academy of Sciences | Yu T.-F.,Gansu Hydrology and Water Resources Engineering Research Center | Feng Q.,Chinese Academy of Sciences | Feng Q.,Gansu Hydrology and Water Resources Engineering Research Center | And 3 more authors.
Beijing Linye Daxue Xuebao/Journal of Beijing Forestry University | Year: 2014

As an important ecological process, it is fundamental for understanding the eco-hydrological consequences of hydraulic redistribution to illuminate its patterns, magnitude and controlling factors. We selected Populus euphratica, which of constructive species located at the lower reaches of Heihe River in northwestern China, to identify patterns and quantize the magnitude and controlling factors of hydraulic redistribution by the heat ratio method. We not only confirmed the previous knowledge on hydraulic lift, in which rates of flow in lateral roots of P. euphratica were negative during the night, but also demonstrated that water can transport from moist topsoil to dry subsoil after rain, i.e. hydraulic descent. In addition, water also moved from lateral moist soil layer to opposite dry soil layer after lateral irrigation, i.e. lateral redistribution. The magnitude of hydraulic redistribution of P. euphratica roots based on negative sap flow in lateral roots ranged from 0.16 to 0.26 kg/day with an average of 0.21 kg/day, which can accounts for 38.75% of transpiration of the next day. The correlation and stepwise regression analysis demonstrated that hydraulic redistribution was significantly positive correlated with vapor pressure deficit, air temperature and soil moisture content, instead negative correlation with relative humidity, in which vapor pressure deficit and soil moisture content were main controlling factors of hydraulic redistribution.

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