HYDRAULIC REDISTRIBUTION: NEWLY RECOGNIZED SMALL CYCLE WITHIN THE SOIL-PLANT-ATMOSPHERE CONTINUUM

Abstract

Water uptake is generally accepted as the most important function of plant roots, while the fact that roots can also release water to soil and its ecological consequences and significance to ecosystem function are not well understood. Studies in the past 20 years confirmed the existence of hydraulic redistribution (HR), which describes the passive movement of water from moist soil to dry soil via roots when transpiration ceases (usually at night). The direction of HR can be upward, downward or lateral in soil along water potential gradients. Because this newly recognized small cycle of soil-root-soil exists within the Soil-Plant-Atmosphere Continuum (SPAC), the mechanism of dynamic movement and storage in the soil-root system has interested many functional ecologists. Continuous measurement of soil water potentials or contents and root sap flow and the use of stable isotopes techniques have been extensively used in HR research. HR acts as a good feedback mechanism between plant water status and soil water dynamics. When deep soil water is plentiful, HR could enhance the efficiency of water uptake via roots, helping plants to make full use of water to improve transpiration and photosynthesis rates and hence maintain physiological function and hydraulic conductivity of the SPAC system. When rainfall comes after the dry season, more precipitation could be transferred to deep soil as an effective storage because of HR, thus increasing availability of rainfall to plant growth. HR was significant for water flux in some ecosystems, for example arid-semiarid sandlands and grasslands, as well as seasonal dry forests. Experiments on HR in Chinese ecosystems are expected or are already underway. On the other hand, integrating HR into ecosystem and hydrology models to study the water relationships between neighboring plants and predict the dynamics of ecosystems with HR is also important. The implications of HR to designing agro-forestry, vegetation recovery, calculating ecological water demand and water conservation in agriculture also deserve more attention.