|Cable, W. - UNIVERSITY OF WYOMING|
|Hultine, K. - UNIVERSITY OF UTAH|
Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 20, 2007
Publication Date: February 29, 2008
Citation: Scott, R.L., Cable, W.L., Hultine, K.R. 2008. The ecohydrologic significance of hydraulic redistribution in a semiarid savanna. Water Resources Research. Vol. 44, W02440, doi:10.1029/2007WR006149. Interpretive Summary: Plant roots capture, store and transport soil water, and in doing so alter ecosystem water, energy, and nutrient balance. One potentially important process facilitated by roots is the redistribution of water from moist to dry soil layers. This scientific paper investigates the movement of water in the roots of mesquite trees in a semiarid rangeland. It was found that when the surface soil layers were wet, mesquite trees moved water from the surface and downward in their taproot even during the winter when the canopy was leaf-less. When the surface was dry, mesquites brought deep moisture up to the surface in their taproot, and this moisture was either used to support transpiration or, at night, it was sent out laterally into the near surface soil. These results directly show that mesquites can move significant amounts of water both upward and downward in the soil. Redistribution has important ecological and hydrological consequences, and so, this process should be accounted for in many plant/soil models.
Technical Abstract: Recent studies have illuminated the process of hydraulic redistribution, defined as the movement of soil moisture via plant root systems, but the long-term ecohydrologic significance of this process is poorly understood. We investigated hydraulic redistribution (HR) by Prosopis velutina Woot. (velvet mesquite) in an upland savanna ecosystem over a two-year period. Our goal was to quantify patterns of HR by mesquite roots and assess how this affects tree water use and productivity. We used the heat ratio method to monitor bi-directional sap flow, an analog of HR, in both lateral and tap roots. Additionally, we monitored soil water content and whole ecosystem carbon dioxide and water exchange using eddy covariance. Mesquite roots transported large amounts of water throughout the year, even during periods of canopy dormancy. Dormant season precipitation (November – March) was often taken up by shallow lateral roots and transferred downwards in the soil profile by deeper lateral and tap roots. This trend also occurred when trees were active and moisture from summer rainfall was plant available in the upper soil layers. As the upper soil layers dried, sap flow moving toward the canopy in the lateral roots diminished and water use from deeper soils increased via the taproots. The relationship between root sap flow and above-canopy fluxes suggested that deeper "stored" water from HR allowed the trees to transpire more in the spring that followed a winter with significant downward redistribution. Patterns of lateral and tap root sap flow also implied that redistribution may extend the growing season of the trees after summer rains have ended and surface soils are dry, thus allowing the trees to photosynthesize through periods of seasonal drought. The large hydrologic magnitude and the ecological effects of HR we studied, along with mounting evidence of this process occurring in many other ecosystems, indicates that this process should be accounted for in many ecohydrologic modeling efforts.