Hydraulic redistribution refers to the mechanism by which some vascular plants redistribute soil water. It occurs in vascular plants that commonly have roots in both wet soil and extremely dry soil, especially plants with both taproots that grow vertically down to the water table, and lateral roots that sit close to the surface.
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Process
Hot, dry periods, when the surface soil dries out to the extent that the lateral roots exude whatever water they contain, will result in the death of such lateral roots unless the water is replaced. Similarly, under extreme wet conditions when lateral roots are inundated by flood waters, oxygen deprivation will also lead to root peril. In plants that exhibit hydraulic redistribution, there are xylem pathways from the taproots to the laterals, such that the absence or abundance of water at the laterals creates a pressure potential analogous to that of transpirational pull. In drought conditions, ground water is drawn up through the taproot to the laterals and exuded into the surface soil, replenishing that which was lost. Under flooding conditions, plant roots perform a similar function in the opposite direction. For a visualization of this process, see "Hydraulic Redistribution Cartoon," Dawson Lab, UC Berkeley, CA
Though often referred to as hydraulic lift, movement of water by the plant roots has been shown to occur in any direction. This phenomenon has been documented in over sixty plant species spanning a variety of plant types (from herb and grasses to shrubs and trees) and over a range of environmental conditions (from the Kalahari Desert to the Amazon Rainforest).
Causes
The movement of this water can be explained by the theory of water transport through a plant. This well established theory is called the cohesion-tension theory. In brief, it states that movement of water through the plant depends on having a continuous column of water, from the roots to the leaves. Water is then pulled from the roots to the leaves, through the plant system, by the difference in water potential between the boundary layers of the soil and the atmosphere. Therefore, the driving force for moving water through a plant is the cohesive strength of water molecules and a pressure gradient from the roots to the leaves. This theory can still be applied when the boundary layer to the atmosphere is closed, e.g. when plant stomata are closed or in senesced plants. The pressure gradient is between soil layers with different water potentials; water moves through the roots from wetter to drier soil layers in the same manner as it does when the plant is transpiring.
Significance
The ecological importance of hydraulically redistributed water is becoming better understood as this phenomenon is more carefully examined. Water redistribution by plant roots has been found influencing crop irrigation, where watering schemes leave a harsh heterogeneity in soil moisture. The plant roots have been shown to smooth or homogenize the soil moisture. This sort of smoothing out of soil moisture is important in maintaining plant root health. The redistribution of water from deep moist layers to shallow drier layers by large trees has shown to increase the moisture available in the daytime to meet the transpiration demand.
The implications of hydraulic redistribution seem to have an important influence on plant ecosystems. Whether or not plants redistribute water through the soil layers can affect plant population dynamics, such as the facilitation of neighboring species. The increase in available daytime soil moisture can also offset low transpiration rates due to drought (see also drought rhizogenesis) or alleviate competition for water between competing plant species. Water redistributed to the near surface layers may also influence plant nutrient availability.
Observations and modeling
Due to the ecological significance of hydraulically redistributed water, there is an ongoing effort to continue the categorization of plants exhibiting this behaviour and adapting this physiological process into land-surface models to improve model predictions.
Traditional methods of observating hydraulic redistribution include Deuterium isotope traces, sap flow, and soil moisture. In attempts to characterize the magnitude of the water redistributed, numerous models (both empirically and theoretically based) have been developed.