Vulnerability of xylem to embolism in relation to leaf water potential and stomatal conductance inFagus sylvaticaf.purpureaandPopulus balsamifera

The vulnerability of xylem vessels to water stress-induced cavitation was studied by measuring hydraulic conductivity and ultrasound acoustic emissions (AEs) in Fagus sylvatica L. f. purpurea (Ait.) Schneid. and Populus balsamifera L. The occurrence of xylem embolism in summer was investigated in relation to leaf water potential and stomatal conductance. Populus was extremely vulnerable to cavitation, losing functional vessels due to embolism at water potentials lower than −0.7 MPa. Fagus experienced embolism when water potential fell below −1.9 MPa. Midday water potentials often approached these threshold values. When evaporative demand increased rapidly on sunny days, water loss became limited by low stomatal conductance. Thus water potentials fell only slightly below the threshold values inducing cavitation. Despite the differences in vulnerability, both species tolerated a similar embolism rate of about 10% in the summer. There was no embolism reversal during prolonged periods of rain. AEs were predictive of loss in hydraulic conductivity, indicating that AEs were mainly confined to vessels. Finally, vessel length distribution, vessel diameter (tangential axis), vessel density, and vessel wall thickness had been determined for both species investigated. Populus had longer and wider vessels than Fagus, whereas vessel wall thickness was similar in both species.