| Abstract: | The vulnerability of xylem vessels to water stress-induced cavitationwas studied by measuring hydraulic conductivity and ultrasoundacoustic emissions (AEs) in Fagus sylvatica L. f. purpurea (Ait.)Schneid. and Populus balsamifera L. The occurrence of xylemembolism in summer was investigated in relation to leaf waterpotential and stomatal conductance. Populus was extremely vulnerableto cavitation, losing functional vessels due to embolism atwater potentials lower than –0.7 MPa. Fagus experiencedembolism when water potential fell below –1.9 MPa. Middaywater potentials often approached these threshold values. Whenevaporative demand increased rapidly on sunny days, water lossbecame limited by low stomatal conductance. Thus water potentialsfell only slightly below the threshold values inducing cavitation.Despite the differences in vulnerability, both species tolerateda similar embolism rate of about 10% in the summer. There wasno embolism reversal during prolonged periods of rain. AEs werepredictive of loss in hydraulic conductivity, indicating thatAEs were mainly confined to vessels. Finally, vessel lengthdistribution, vessel diameter (tangential axis), vessel density,and vessel wall thickness had been determined for both speciesinvestigated. Populus had longer and wider vessels than Fagus,whereas vessel wall thickness was similar in both species. Key words: Acoustic emissions, Fagus, Populus, stomatal closure, xylem embolism |
