Abstract: The xylem embolism vulnerability and refilling ability were measured in one-year-old twigs of three draught-tolerant tree species, which were Ulmus pumila L., Hippophae rhamnoides L., and Corylus heterophylla Fisch., and three types of vulnerability curves for them were also established. The results showed that the water potential threshold for inducing embolism in three species was -0.1-0.3MPa, and the embolism reached its maximum value at the water potential of -2.5-3.5MPa. The xylem embolism vulnerability of three species was in the order of Hippophae> Ulmus> Corylus. The refilling of embolized xylem could occur while the water potential is below zero in all the species. In Hippophae, fully restoring occurred after thirty minutes under the water potential of -0.5MPa and -1.0MPa, but for Ulmus and Corylus, the restoration could reach about 90% in thirty minutes under the water potential of -0.60MPa and -0.34MPa, respectively. Ulmus could restore fully through one night (12 hours) under the water potential of -0.59MPa. The refilling capacity of three species was Hippophae> Ulmus> Corylus . The xylem of the three draught-tolerant species is not only very vulnerable to embolization or very sensible to water stress but also capable of refilling its embolized xylem in a short period of time. Vulnerable and drought-tolerant species had significant xylem embolism in higher water potential, which decreased the hydraulic conductivity so that limited water loss. Drought-tolerant species had more ability to refill its embolized xylem in a short period of time. These were the ways through which trees resist and tolerate drought or water stress.

Key words: multivariate statistical analysis, water quality, spatiotemporal variation, Taoranting Lake of Beijing.