Seasonal changes in hydraulic conductance, xylem embolism and leaf area inEucalyptus tetrodonta and Eucalyptus miniata saplings in a north Australian savanna

Eucalypt saplings in north Australian savannas commonly die back, sometimes to ground level, during the 5 months of the long dry season, Wafer potenti als are lower in saplings than large trees during the dry season, and we hy pothesized that low water potentials may lead to high levels of xylem embol ism and consequent death of branches and whole shoots, As the dry season pr ogressed, hydraulic conductance of terminal branches decreased by 50% in Eu calyptus tetrodonta but not in Eucalyptus miniata saplings, Hydraulic condu ctance per leaf area decreased seasonally by 34% in E. tetrodonta branches, These decreases may be associated with the loss of leaves recorded from E. tetrodonta but not E. miniata branches, We modelled the effect of sequenti al loss of parallel resistors, representing petioles on a branch, This show ed there is a non-linear decrease in flow as basal resistors are lost, whic h can lead to a decrease in mean how per resistor due to increased mean pat h-length, Thus the observed loss of basal leaves, together with xylem embol ism, probably contributed to the seasonal loss of hydraulic conductance in E. tetrodonta saplings. Loss of hydraulic conductance due to xylem embolism was generally low (< 15%) in both species, and the seasonal increase in em bolism could not fully account for the decline in hydraulic conductance of E. tetrodonta branches. There was little evidence that branch and shoot dea th was caused by these levels of embolism, Developing an embolism vulnerabi lity curve for species with long vessels is problematic and this issue is d iscussed.