Water stress, xylem hydraulic failure and tree death

Extreme water stress episodes induce tree mortality, but the mechanistic relationships linking stem embolism and species drought performance remain poorly understood. Barigah et al. study potted juvenile trees of beech (Fagus sylvatica) and poplar (Populus deltoides × P. nigra) and find that the xylem pressure inducing 50 % mortality differs sharply between the species, being 1.75 and 4.5 MPa in poplar and beech, respectively. However, the relationships between tree mortality and the degree of cavitation in the stems are similar, with mortality occurring suddenly when >90 % cavitation has occurred. This is in contrast to the 50 % embolism threshold reported for conifers. The results demonstrate that massive cavitation is probably a causal factor for tree mortality under extreme water stress conditions.