Mechanisms of xylem recovery from winter embolism in Fagus sylvatica

Hydraulic conductivity in the terminal branches of mature beech trees (Fagu s sylvatica L.) decreased progressively during winter and recovered in the spring. The objective of this study was to determine the mechanisms involve d in recovery. Two periods of recovery were identified. The first recovery of hydraulic conductivity occurred early in the spring, before bud break, a nd was correlated with the occurrence of positive xylem pressure at the bas e of the toe trunk. Active refilling of the embolized vessels caused the re covery. The second recovery of hydraulic conductivity occurred after bud br eak and was correlated with the onset of cambial activity. Formation of new functional vessels, leading to an increase in xylem diameter, was largely responsible for the increase in xylem conductivity. The two mechanisms were complementary: active refilling of embolized vessels occurred mostly in th e root and the trunk, whereas formation of new functional vessels occurred mainly in young terminal shoots.