WATER RELATIONS OF ADULT NORWAY SPRUCE (PICEA-ABIES (L) KARST) UNDER SOIL DROUGHT IN THE VOSGES MOUNTAINS - WHOLE-TREE HYDRAULIC CONDUCTANCE, XYLEM EMBOLISM AND WATER-LOSS REGULATION

Drought-induced changes in whole-tree hydraulic conductances (gL) were monitored throughout a growing season in a 30-year-old stand of Picea abies, gL was derived from concurrent measurements of leaf water pote ntials and sap flux densities through the trunk. Soil water deficits c learly reduced gL, the reduction being most likely located in the soil -root compartment of the soil-plant sap pathway. The decreases in gL d id not result in large decreases in midday leaf water potentials becau se midday sap flux densities were reduced proportionally to gL. We the refore hypothesized that maximum sap flux densities in Picea abies are adjusted under dry conditions according to changes in whole-tree hydr aulic conductances with effect of maintaining midday water potentials above the point of xylem dysfunction caused by water stress-induced ca vitations.