Hydraulic failure and repair are not routine in trees

Abstract

Trees can pull tons of water up to 100 m above ground, whereas even the best engineered suction pumps can manage only 10 m at most. Beyond this height, the pull of gravity exceeds atmospheric pressure. The pressure at the top of the water column then becomes negative, and pumps drain by a process of vaporization called cavitation. So how do plants, particularly tall trees, cope with cavitation? Do they have an astonishingly high resistance to this process or are they routinely exposed to cavitation events and possess remarkable repair capacities? These questions were first asked when the mechanism of sap ascent in trees was discovered in the late nineteenth century (Brown2013),but were not answereduntil reliable methods for measuring cavitation were introduced a century later. Early studies of plant hydraulics suggested that cavitation occurred only in conditions of severe drought (Fig.1a).However, manystudies carried out inthe last decade