Relationship between growth and xylem hydraulic characteristics of clones of Eucalyptus spp. at contrasting sites.

To assess relationships between hydraulic characteristics and growth efficiency (expressed as trunk volume increment per unit leaf area per year), we measured xylem specific conductivity, leaf specific conductivity and the vulnerability of xylem to cavitation in branches of 7-8-year-old trees of four closely related Eucalyptus clones growing in plantations on mesic and xeric sites. Growth was influenced more by site than by clone. Hydraulic conductivities were higher in trees on mesic sites than in trees on xeric sites, but within a site, conductivity did not differ among clones. Huber values (functional xylem cross-sectional area:leaf area) were similar across clones and between sites. Vulnerability of xylem to cavitation differed among clones but not between sites. We conclude that, within these genetically related clones, hydraulic conductivity is determined predominantly by environmental conditions, whereas vulnerability to cavitation is more closely associated with genotype. The clone most vulnerable to cavitation showed considerable aboveground die-back during a severe drought and there was a positive correlation between growth efficiency and maximum xylem specific conductivity.