Summer and winter embolism in oak: Impact on water relations

In this paper we review vulnerability to embolism caused by drought and freezing in six species of oak (Quercus). The xylem pressure potential that induces 50% loss hydraulic conductivity ranges from -2.5 to -6.0 MPa for the species reviewed and correlates with other measures of drought tolerance in oaks. The probability of vessel dysfunction increases with vessel size for both drought- and freezing-induced embolism. The impact of embolism is a reduction in hydraulic conductivity in the vascular system. We conclude that embolism plays little role in the drought tolerance of oaks since drought-induced embolism occurs at more negative water potentials than are known to cause damage (eg, reduced growth) or mortality. Nevertheless, vulnerability to embolisms probably explains species distributions between wet and dry sites or may explain the evolution of stomatal physiology. Oaks seem to operate close to the point of xylem dysfunction, but oaks protect against embolism by stomatal regulation which keeps water potentials above that causing 'run-away' embolism. In conclusion, vulnerability to summer embolisms correlates with other measures of drought tolerance of oak species but significant summer embolisms are generally avoided. On the other hand, frost-induced embolism may explain species distributions in cold climates.