WATER TRANSFER IN A MATURE OAK STAND (QUERCUS-PETRAEA) - SEASONAL EVOLUTION AND EFFECTS OF A SEVERE DROUGHT

The reactions of sessile oak (Quercus petraea (Mattuschka) Liebl.) to drought were studied under natural conditions in a 32-year-old stand n ear Nancy (northeastern France) during the summers of 1989 (strongly r ain deficient) and 1990. A plot of five trees was subjected to imposed water shortage, while a group of irrigated trees was used as a contro l. Measurements of xylem sap flows and water potential enabled the com putation of plot transpiration, canopy conductance, and specific hydra ulic conductance in the soil-tree continuum. Stomatal conductance was measured directly with a porometer. Specific hydraulic conductance of our oaks was of the same order of magnitude as that reported for other species. It decreased significantly during spring because of a time l ag between cambial growth and leaf area expansion. Measured transpirat ion was close to potential evapotranspiration, except during days with high vapor pressure deficits, which promoted stomatal closure in the absence of soil water deficits. Imposed drought caused predawn leaf wa ter potentials to reach values as low as -2.0 MPa and a progressive de cline in hydraulic conductance, which was probably attributable to mod ifications in hydraulic properties at the soil-root interface. This gr adual decline in conductance was attributed to their deep rooting (1.4 0 m). This study revealed that Q. petraea may be considered as drought tolerant because of adaptations like deep rooting, efficient and safe xylem sap transport, maintenance of significant stomatal conductance, and significant transpiration, even during strong drought stress.