ARS | Publication request: Hydraulic Redistribution by a Dominant, Warm Desert Phreatophyte: Seasonal Patterns and Response to Precipitation Pulses 1574

Submitted to: Functional Ecology
Publication Acceptance Date: January 2, 2004
Publication Date: August 1, 2004
Citation: Hultine, K., Scott, R.L., Cable, W.L., Williams, D. 2004. Hydraulic Redistribution By A Dominant, Warm Desert Phreatophyte: Seasonal Patterns And Response To Precipitation Pulses. Functional Ecology 18:530-538.

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 passive redistribution of water from moist to dry soil layers. This scientific paper investigates the movement of water in the roots of mesquite trees. It was found that when the surface soil layers were wet, the mesquite moved water from the surface and downward in its taproot. When the surface was dry, mesquite brought groundwater up to the surface in its taproot and out laterally into the near surface soil. These results directly show that mesquite can move significant amounts of water both upward and downward in the soil even during the dormant season.

Technical Abstract: 1. We used the heat ratio method to measure sap flow in taproots, lateral roots, and main stems of three mature Prosopis velutina trees occurring on a floodplain terrace in semi-arid southeastern Arizona, USA. Sap flow measurements on two of the trees were initiated before leaf flush in late spring (i.e. before the end of winter dormancy). 2. Despite the absence of crown transpiration during the dormant season, sap flow was detected in the roots of both trees. In one tree, reverse flow (flow away from the stem) occurred diurnally in the lateral root and was accompanied by positive flow (i.e. flow towards the stem) in the taproot, indicating the presence of hydraulic lift, while in a second tree, there was reverse flow in the taproot and positive flow in the lateral root indicating hydraulic descent. 3. Hydraulic descent was induced in the roots of the former tree by applying 50 mm of irrigation around the base of the tree. 4. Patterns and magnitudes of nocturnal sap flow in roots of a third tree measured during the growing season were similar to that observed during the dormant season. Before the summer monsoon season, there was nocturnal reverse flow in the lateral root and positive flow in the taproot. Hydraulic descent at nighttime initiated immediately after the first large rain event, and continued after subsequent rain events throughout the experiment. Despite the absence of direct infiltration from above, we found evidence that soil moisture increased during the monsoon in the deep soil layers (1-10m), indicating that plant roots were redistributing significant amounts of water to deep soil layers. 5. After adjusting for differences in sapwood area, maximum diurnal rates of hydraulic descent in the taproots of trees instrumented during the dormant season were 73, and 69% of the maximum nighttime rate of hydraulic descent observed during the growing season. 6. Roots of Prosopis velutina apparently can redistribute significant amounts of soil water during periods of crown dormancy. In arid regions, dormant season hydraulic descent may buffer plants from water and nutrient deficits during initial stages of the growing season by transferring soil water derived from winter precipitation to deep soil layers and away from zones of evaporation in surface layers.