SOIL-WATER DYNAMICS IN AN OAK STAND .1. SOIL-MOISTURE, WATER POTENTIALS AND WATER-UPTAKE BY ROOTS

Soil water dynamics under a mixed stand of mature sessile and peduncul ate oaks were studied both under natural conditions and during imposed water shortages in a lysimeter. Root densities of each species were d escribed in situ by counting roots in the trench surrounding the dry p lot. Soil water contents and potentials, and pre-dawn leaf water poten tials (Psi(wp)) were monitored during three successive years. Soil wat er retention characteristics were obtained from field measurements of water potential and water content. The decreasing rooting density with depth was strongly related to soil physical properties. The root syst em was separated into two compartments by a layer with a high clay con tent. The deepest soil compartment was mainly explored by fine roots. Neutron probe measurements allowed the detection of variations in wate r content down to a depth of 2.00 m. The distribution of water uptake among the different soil layers changed when drought increased. Water was extracted from the deepest reservoir, and capillary rises even occ urred after partial water depletion in the upper part of the soil. Sea sonal trends of pre-dawn leaf water potential generally matched those of soil water potential in the wettest rooted zone, which was at - 140 cm. In the upper, dry, horizons, the sharp loss of soil hydraulic con ductivity reduced water transport to roots leading to impossible equil ibrium between roots and soil at pre-dawn. Finally, Psi(wp) presented a low sensitivity to variations of total soil water content between 40 % and 100% of extractable water. Below this threshold, Psi(pw) decreas ed sharply to a minimal value of about -2.0 MPa.