HYDRAULIC LIFT AND ITS INFLUENCE ON THE WATER-CONTENT OF THE RHIZOSPHERE - AN EXAMPLE FROM SUGAR MAPLE, ACER-SACCHARUM

Hydraulic lift, the transport of water from deep in the soil through p lant root systems into the drier upper soil layers, has been demonstra ted in several woody plant species. Here the volume of water involved in hydraulic lift by a mature sugar maple tree is estimated. Twenty-fo ur intact soil cores were collected from the vicinity of a sugar maple tree at the same positions at which thermocouple psychrometers had be en placed. Desorption measurements were made on the soil cores and the data were fitted to the Campbell relation for soil matric potential p si versus soil water content theta. The psychrometer data were filtere d to obtain the diurnal component contributed by hydraulic lift. The d iurnal component in psi was combined with the Campbell relation for ea ch soil core to obtain the increase in soil water content Delta theta due to hydraulic lift. The additional water contents Delta theta were numerically integrated to obtain a volume of 102 +/- 54 1 of water whi ch was hydraulically lifted each night. The volume of hydraulically li fted water (HLW) is sufficiently great that in ecosystems where hydrau lic lift occurs it should be included in models for calculating the wa ter balance. However, a previous analysis of the stable hydrogen isoto pe composition (delta D) of water in understory plants around trees co nducting hydraulic lift implies a much greater volume of HLW than that calculated from the analysis performed above. To reconcile these diff erences, it is hypothesized that some understory plants preferentially extract HLW due to its higher matric potential and that the proportio n of this water source within the xylem sap of at least some understor y plants that use HLW was so great that the roots of these plants must therefore be in close proximity to the tree roots from which the HLW comes. The results of this study have implications for studies of plan t competition where positive associations may exist as well as for ion uptake, nutrient cycling and the design of agroforestry systems.