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hydraulic redistribution
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.
==Process==
During 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.〔S.S.O. Burgess, M.A. Adams, N.C. Turner, C.R. Beverly, C.K. Ong, A.A.H. Khan, and T.M. Bleby (2001), An improved heat pulse method to measure low and reverse rates of sap flow in woody plants, Tree Physiology, 21, 589-598〕〔J.H. Richards and M.M. Caldwell (1987), Hydraulic lift: Substantial nocturnal water transport between soil layers by ''Artemisia tridentata'' roots, Oecologia, 73, 486-489〕〔D.R. Smart, E. Carlisle, M. Goebel, and B.A. Nunez (2005), Transverse hydraulic redistribution by a grapevine, Plant, Cell and Environment, 28, 157-166〕 This phenomenon has been documented in over sixty plant species spanning a variety of plant types (from herb and grasses to shrubs and trees)〔R.S. Oliveira, T.E. Dawson, S.S.O. Burgess, and D.C. Nepstad (2005), Hydraulic redistribution in three Amazonian trees, Oecologia〕〔T.E. Dawson (1993), Hydraulic lift and water use by plants: implications for water balance, performance and plant-plant interactions, Oecologia, 95, 565-574〕〔M.M. Caldwell (1990), Water parasitism stemming from hydraulic lift: a quantitative test in the field, Israeli Journal of Botany, 39, 395-402〕 and over a range of environmental conditions (from the Kalahari Desert to the Amazon Rainforest).〔〔〔S.S.O. Burgess and T.M. Bleby (2006), Redistribution of soil water by lateral roots mediated by stem tissues, Journal of Experimental Botany, 57:12, 3283-3291〕〔E.D. Schulze, M.M. Caldwell, J. Canadell, H.A. Mooney, R.B. Jackson, D. Parson, R. Scholes, O.E. Sala and P. Trimborn (1998), Downward flux of water through roots (i.e. inverse hydraulic lift) in dry Kalahari sands, Oecologia, 115, 460-462〕
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