WATER RELATIONS OF ADULT NORWAY SPRUCE (PICEA-ABIES (L) KARST) UNDER SOIL DROUGHT IN THE VOSGES MOUNTAINS - WATER POTENTIAL, STOMATAL CONDUCTANCE AND TRANSPIRATION

The effects of soil water depletion on sap flow, twig water potential, stomatal and canopy conductance were analysed in 2 plots of a 30-year-old stand of Norway spruce. One was subjected to an imposed drought; the other was watered by irrigation. Predawn water potential in trees from the dry plot decreased to -1.2 MPa. In the watered plot, a low between-tree variability of sap flux density was observed, with maximum values of 1.2-1.9 dm(3).dm(-2).h(-1), corresponding to about 0.5 mm.h(-1). In the dry plot, sap flux density showed a higher variability, and decreased during the summer to a minimum midday value of 0.05 dm(3).dm(-2).h(-1). Tree transpiration and stomatal conductance showed a strong reduction in association with drought development, during which the predawn water potential decreased from -0.4 to -0.6 MPa. Canopy conductance was calculated from the reverse of the Penman-Monteith equation assuming that vapour flux over the stand was equal to the estimated stand sap flow. Effects of climatic factors and drought on canopy conductance variations were taken into account in a multi-variable transpiration model.