HYDRAULIC LIFT AND WATER-USE BY PLANTS - IMPLICATIONS FOR WATER-BALANCE, PERFORMANCE AND PLANT-PLANT INTERACTIONS

During drought periods, sugar maple (Acer saccharum) demonstrates ''hy draulic lift''; nocturnal uptake of water by roots from deep soil laye rs that is released from shallow roots into upper soil layers. Using s tandard water relations methods and stable hydrogen isotope analysis o f both source-water and plant-water, I investigated (1) the magnitude and radial extent of hydraulic lift by mature, relatively open-grown t rees, of A. saccharum, (2) the proportion of hydraulically-lifted wate r (HLW) used by shallow-rooted neighbors growing at different distance s from target trees, and (3) the influence that this water source had on stomatal conductance to water vapor (g), water balance and growth o f these neighbors. Soil water potentials (PSI(s) at -20 and -35 cm sho wed a distinct diel fluctuation. Soil pits dug beneath three mature tr ees revealed a distinct hard-pan (e.g. fragipan) layer at a depth of a pproximately 50 cm. Examination of root distributions obtained from so il cores and soil pits revealed that some larger diameter roots (1.9-3 .7 cm) did penetrate the fragipan and were established in the ground w ater table. The presence of the fragipan indicated that the ''rewettin g'' of the upper soil layer during the night could not be explained by capillary rise from the shallow water table; it was the trees that we re taking up ground water and then redepositing it at night into the u pper 35 cm of soil, above the fragipan. The greatest fluctuations in P SI(s) occurred within 2.5 m of trees and only extended out to approxim ately 5 m. Application of a two-end-member linear mixing model which u sed stable hydrogen isotopic data obtained from environmental water so urces and xylem-sap demonstrated that all neighbors used some fraction (3-60%) of HLW supplied by sugar maple trees. Plants that used a high proportion of HLW (e.g. rhizomatous or stoloniferous perennials) main tained significantly higher leaf water potentials and g, and showed gr eater aboveground growth when compared with (i) neighbors that used li ttle or no HLW or (ii) conspecifics found growing at distances greater than about 3 m away from maple trees. Three important conclusions can be drawn from the results of this investigation that have not been de monstrated before: (1) hydraulic lift need not only occur in arid or s emi-arid environments where chronic water deficits prevail, but can be important in relatively mesic environments when subjected to periodic soil water deficits, (2) that plants neighboring trees which conduct hydraulic lift can use a significant proportion of this water source, and (3) that the HLW source can effectively ameliorate the influence o f drought on the performance and growth of neighboring vegetation. The results are also discussed in terms of their influence on plant nutri ent relations (including plant-mycorrhizal associations), the nature o f plant-plant interactions and the water balance of individuals, commu nities and floristic regions.