Hydraulic redistribution of soil water by roots of two desert riparian phreatophytes in northwest China's extremely arid region

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. 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. 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. 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.