| In desert ecosystems, the mainly mechanisms of plants delay dehydration to resist droughts is to increase water income via roots and reduce it expenditure via stoma of leaf. To tolerate drought during dry season, desert also riparian plants undergo hydraulic redistribution (HR), the passive movement of water between different soil parts via plant root systems, driven by water potential gradients in the soil-plant interface. Patterns of HR include hydraulic lift (HL), hydraulic descent (HD), lateral redistribution (LR), foliar uptake (FU) and tissue dehydration (TD). Now, HR has been described in approximately120species that involves a wide variety of ecosystems and a wide range of life forms. Apart from its magnitude, the potential ecohydrologic consequences of HR have attracted recent attention. However, the study status in China is started late, single means and the content focus on HL and the rarely reported refer to ecohydrologic consequences of HR. The objectives of this study were (1) to search for the evidence, quantify the magitude and discuss the ecohydrologic consequences of that the roots of two desert riparian plants, Populus euphratica Oliv. and Tamarix ramosissima Ledeb., carry out HR. To demonstrate HR, we present data on patterns of sap flow in the stems or branches and lateral roots of those two desert riparian species and soil volumetric moisture content where these species grow, and the datas of plant water physiology with different scales also been provided.(1) Patterns of hydraulic redistributionWe not only confirm the previous knowledge on HL of P. euphratica, but also the water can transport from moist topsoil to dry subsoil after rain, i.e. HD. In addition, water also moved from lateral moist soil layer to opposite dry soil layer, i.e. LR. Interestingly, we observed bidirectional flow in the lateral roots of P. euphratica during the time of lateral redistribution, and it may be mediated by stem tissues and, by inference, the radial sectoring in the xylem that the stem base seems in connection with the root xylem. The water absorbing from the wet side adjacent to river was transported to the stem; the circumferential movement of sap flow around the heartwood becomes available to the downstream flow. 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 (ii) the identification of primary water sources as groundwater and vadose zone water through stable isotope studies. The reason of HR not occurred in lateral roots of T. ramosissima was potential nocturnal transpiration suppression. Thus, we inferred that HR occurs in T. ramosissima via adventitious roots rather than via lateral roots.(2) Magitudes and controlling factors of hydraulic redistribution For P. euphratica. the magnitude of HR based on nagitive sap flow in lateral roots was ranged from0.16mm d-1to0.26mm d-1with an average of0.21mm d-1. For T. ramosissima, the magnitude of HR based on diurnal fluctuations of soil volumetric moisture content at20cm to60cm depths was ranged ranged from0.14mm d-1to1.02mm d-1with an average of0.48mm d-1during the growing season, which was far less than that in2011. The correlation and stepwise regression analysis demonstrated that apart from the transpiration tension, the climate and soil moisture availability can accounted for the variation of HR. Specifically, HR was significantly positive correlation with vapor pressure deficit, soil temperature and soil moisture content, instead negative correlation with relative humidity.(3) Hydrological effects of hydraulic redistributionThe effect of HR on soil moisture balance is positive. For example, in T. ramosissima stand, approximately71.00%of the water removed daily (7.59mm) from the upper80cm of soil was replaced by nocturnal hydraulic redistribution (5.42mm) during dry season (14to23July). If hydraulic redistribution had not occurred, the soil water content observed at the end of the10-day monitoring period would have been attained about4days sooner. Because of the increase of soil moisture, the transpiration was also increased that indirectly induced by HR. For P. euphratica. hydraulic redistribution increased transpiration ranged from14.60%to113.04%with an average of38.75%during the growing season. For T. ramosissima, hydraulic redistribution increased transpiration ranged from10.46%to53.33%with an average of19.44%. In the same way, the effect of HR on community water balance is positive. In2012, approximately37.00%of evapotranspiration (313.00mm) was replaced by nocturnal hydraulic redistribution (123.00mm) during dry season for P. euphratica stand. Accordingly, approximately19%of evapotranspiration (607.27mm) was replaced by nocturnal hydraulic redistribution (117.20mm) during dry season for T. ramosissima stand. In addition, the difference of evapotranspiration also affects the microclimate among stands, that the temperature and vapor pressure deficit for P. euphratica stand was maximal higher4℃and1.60kPa than T. ramosissima stand, respectively, instead relative humidity for the former maximal lower10%than the latter during the typical days (12to21July).(4) Ecological effect of hydraulic redistributionThe directly ecological effect of hydraulic redistribution on plants were to maintain the effectiveness of fine roots in the shallow soil, that made it can quickly reaponse to precipitation pulse in arid area. HD, on the other hand, can affect overall ecosystem water budgets by increasing deep soil water recharge. Water movement to deep soil layers after a rain event is faster through HR than via infiltration or preferential flow and, in the absence of HR. water recharge of deep soil layers would be very low. And then gas exchange parameters of leaf were affected by increased water absorbing via fine roots in the shallow soil. Finally, HR may also have effects on plant community composition and structure, that further investigation is needed to substantiate this.In contrast to the conventional view on water use strategies of plants, we thought the roots of T. ramosissima can simultaneously absorb plentiful water in deep soil layer and nutrient in shallow soil layer throught HR, which made the leaves of T. ramosissima can keep stoma open and maintain higher transpiration rate and photosynthetic product accumulation than P. euphratica. When soil water is limited, transpiration reduced rapidly to a minimum under the protection of thickly cutin layer and skin, along with descend of the leaf water potential and closure of stoma, which suggest the transform of T. ramosissima from water spender to saver. However, because the growth decreases of leaf or assimilating shoots, the excessive amounts of photosynthetic products was transport to the roots that lead to the faster growth and then more water absorbing in deep soil layer. So given enough time, the proportion of roots and overground part will increase further. Therefore, xerophytes of water-consuming as T. ramosissima is more drought tolerant than water-saving like P. euphratica. it could be the reasons that widely distributed in central Asia and crazy grow along rivers in southwestern United States. |
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