For much of the past century, researches on water relations of plant-soil systems mostly focued on the mode and efficiency of plant water use and water movement in soils. Although hydraulic redistribution of soil water by plant roots was first reported as early as the late 1920’s, the importance of the process in regulating soil water and associated ecological processes was not widely recognized by plant physiologists and ecologists until the late 1980’s. The term “hydraulic redistribution", known also as “hydraulic lift" as reported in many studies, refers to the phenomenon of soil water redistribution by plant roots through passive processes of water uptake and release driven by water potential gradient in the root-soil interface. However, hydraulic lift has been used more specifically to describe the transportation of soil water from deep wetter layers to shallow drier layers. Recent measurements of sapflow in taproots and lateral roots of trees have demonstrated that roots can also redistribution water either downward or laterally from moist surface soils to drier bottom soil layers. Because of the bidirectional movement of the transported water, “hydraulic redistribution" has been proposed as a more comprehensive term than “hydraulic lift" to describe the phenomenon.
Hydraulic redistribution usually occurs at night when transpiration has diminished sufficiently, or during periods when transpiration is substantially reduced, to allow the water potential of the roots to exceed that of the drier parts of soil layers. To date, much progress has been made in understanding the role of hydraulic redistribution in maintaining water balance of plant community under drought conditions, and in identifying the internal and external factors that regulate the occurrence and magnitude of hydraulic redistribution. Three techniques have been commonly used in the hydraulic redistribution research: the first involves determination of the direction and rate of sapflow in roots using heat pulse or heat dissipation sapflow measurement sytems; the second is to measure the deuterium content of neighbor species when deuterated water is supply to deep roots of the target trees to trace the movement of water to shallow roots of grasses; and the third is to measure changes in soil water potential and/or soil water content. There have been more than 60 cases of hydraulic redistribution reported for plant species across diverse habitats, and it is expected that the hydraulic redistribution is widespread wherever conditions are conductive to its occurrence. Nevertheless, more efforts are needed in exploring the ecological function and significance of the hydraulic redistribution, especially its significance in ecosystem restoration of degrading ecosystems in arid and semi-arid environments.
In this article we reviewed our state of knowledge on hydraulic redistribution, and examined the significance of the process in water balance of plant-soil systems. Based on our review of literature and synthesis of existing information, we stressed the need for further research to evaluate the contribution of hydraulic redistribution to the water balance of plant community and ecosystem restoration in arid and semi-arid areas or in regions experiencing the stage of frequent droughts. Specific questions that should be addressed in future research may include, but not limited to: (1) How dependent are the plants with shallow roots on hydraulic redistribution by plants with deep roots in arid ecosystems? (2) What are the biological basis and significance of hydraulic redistribution from perspective of plant evolution? (3) What is the significance of hydraulic redistribution to maintaining soil microbial activities? (4) Do the mode and magnitude of hydraulic redistribution differ among different types of vegetation? Moreover, quantitative information on the threshold of soil water potential and/or soil water content for inducing hydraulic redistribution in different ecosystems would be useful in elucidating the plant water relations in arid and semi-arid regions.