WHOLE TREE HYDRAULIC CONDUCTANCE AND WATER-LOSS REGULATION IN QUERCUSDURING DROUGHT - EVIDENCE FOR STOMATAL CONTROL OF EMBOLISM

The water relations of 30-year-old Quercus petraea were studied for th ree consecutive growing seasons. Whole tree specific hydraulic conduct ances (gl) were computed from sap flow densities (dF)/leaf water poten tial (Psi(leaf)) relationships. gL was clearly reduced with the develo pment of the drought. The decrease of gL with Psi(predawn) was of an e xponential type, ie, high variations of gL were found whereas Psi(pred awn) remained high and constant. These early variations of gL were mos t probably located in the soil-root compartment of the SPAC because no loss of hydraulic conductivity due to xylem embolism was detected in the crown of the trees. Although gL was reduced, Psi(midday) remained nearly constant and above -3 MPa throughout the drought period because dF(midday) was also significantly reduced. As a consequence, a good l inear relation was found between dF(midday) and gL. Xylem embolism sig nificantly developed in the petioles and twigs of Q petraea when Psi(m idday) became less than -3 MPa. We argue that, because of changes in g L, Q petraea progressively adjusted its water loss throughout the drou ght development with the effect of maintaining Psi(midday) above the c avitation threshold. It is shown that if no water loss regulation had occurred, considerable tensions would have developed in the crown of t hese trees with predictable branch mortality due to runaway embolism.