Hydraulic redistribution by a dominant, warm-desert phreatophyte: seasonal patterns and response to precipitation pulses

1. Hydraulic redistribution may have important consequences for ecosystem water balance where plant root systems span large gradients in soil water potential. To assess seasonal patterns of hydraulic redistribution, we measured the direction and rate of sap flow in tap-roots, lateral roots and main stems of three mature Prosopis velutina Woot. trees occurring on a floodplain terrace in semiarid south-eastern Arizona, USA. Sap-flow measurements on two of the trees were initiated before the end of the winter dormancy period, prior to leaf flush. 2 Despite the absence of crown transpiration during the dormant season, sap flow was detected in lateral roots and tap-roots of P. velutina. Reverse flow (away from the stem) in the lateral root and positive flow (towards the stem) in the tap-root was observed in one tree, indicating the presence of hydraulic lift. Conversely, reverse flow in the tap-root and positive flow in the lateral root was observed in the second tree, indicating hydraulic descent. 3 Hydraulic descent was induced in the roots of the former tree by wetting the rooting zone in the upper 70 cm of the soil surface with 50 mm of irrigation. 4 Patterns and rates of nocturnal sap flow in roots of a third tree measured during the growing season were similar to those observed during the dormant season. Nocturnal reverse flow in the lateral root and positive flow in the tap-root was observed prior to the onset of the summer monsoon. Hydraulic descent commenced immediately following the first large monsoon rain event, and continued after subsequent rain events. After adjusting for differences in sapwood area, maximum diurnal rates of hydraulic descent in the tap-roots of trees instrumented during the dormant season were 73 and 69% of the maximum night-time rate of hydraulic descent observed during the growing season. 5 Despite very limited potential for direct infiltration, volumetric soil moisture content in deep soil layers (1.5-9.5 m) increased 2-8% by the end of the monsoon (late September), indicating that plant roots were redistributing non-trivial amounts of water to deep soil layers. 6 Roots of P. velutina apparently redistribute significant amounts of soil water during the growing season, but also during periods of crown dormancy in winter. 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 and shallow-rooted herbaceous plants.