Hydraulic lift in Populus euphratica Oliv. from the desert riparian vegetation of the Tarim River Basin

In semi-arid and arid regions, such as the Tarim River Basin, not all plants have deep root systems. However, plants with shallow roots can also grow normally, just as those with deep roots can, and can absorb soil water from deep soils under long-term drought stress. This has been attributed to the adaptive strategies of drought-tolerant plants. One of these adaptive strategies is in the leaf structure. Another, more important, factor is the water sharing that occurs between deep-rooted and shallow-rooted plants, whereby deep-rooted plants supply a certain amount of water to shallow rooted plants through hydraulic lift. Specifically, hydraulic lift is the passive water transfer process that the plant absorbs deep soil moisture by deep root systems, then releases water to shallow soil via shallow plant root because of the existence of water gradient in plant topsoil and subsoil.

In the Tarim River Basin, the desert riparian forest vegetation is under high-temperature and aridity stress. However, the vegetation can grow continuously because of deep rooting that can reach groundwater, which can thus redistribute water into the upper soil profile. Field experimental results of Dr. Hao Xingming show that sap flow velocity in the tap root of P. euphratica had a distinctive diurnal variability. The sap flow velocity reached a peak value at noon, and it decreased rapidly at afternoon, and this low velocity was maintained throughout the night. The tap root sap velocity of P. euphratica is positive during the day and night. However, a reverse sap flow was observed in the lateral roots during the night. The findings demonstrate the direct hydraulic lift characteristics and ecological effects that occur in the desert riparian forest in extremely arid regions.

According to the observation, the water content from the hydraulic lift of P. euphratica at night accounted for 10% daily transpiration water consumption at the soil depth of 60-120 cm, especially in the range of 4 m away from the trunk. In the section of the lower reaches of Tarim River, whose underground water table is below 4 m, the herbaceous plants with shallow roots can accompany with P. euphratica might be related to the hydraulic lift effect of P. euphratica. Dr. Hao Xingming said: the assessment of ecological effects is our current research work, and we can’t give a complete conclusion for this phenomenon of P. euphratica.