Whole tree hydraulic conductance and water loss regulation in Quercus during drought: Evidence for stomatal control of embolism?

The water relations of 30-year-old Quercus petraea were studied for three consecutive growing seasons. Whole tree specific hydraulic conductances (gl) were computed from sap flow densities (dF)/leaf water potential (Psi(leaf)) relationships. gL was clearly reduced with the development of the drought. The decrease of gL with Psi(predawn) was of an exponential type, ie, high variations of gL were found whereas Psi(predawn) remained high and constant. These early variations of gL were most 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 linear relation was found between dF(midday) and gL. Xylem embolism significantly developed in the petioles and twigs of Q petraea when Psi(midday) became less than -3 MPa. We argue that, because of changes in gL, Q petraea progressively adjusted its water loss throughout the drought development with the effect of maintaining Psi(midday) above the cavitation threshold. It is shown that if no water loss regulation had occurred, considerable tensions would have developed in the crown of these trees with predictable branch mortality due to runaway embolism.