A number of compensatory mechanisms may contribute to the regulation of water stress as the influences of gravity and path length increase with tree height. We tested for two such mechanisms – adjustment of branch hydraulic properties and leaf area/sapwood area ratio – in the upper crown of the tallest angiosperm, Eucalyptus regnans, in trees of 60 and 90 meters height growing at Kinglake National Park in Victoria, Australia. We found no difference in sapwood hydraulic conductivity (p = 0.95), leaf specific conductivity (p = 0.17) or xylem vulnerability to cavitation (P50 Tall = -3.58 MPa, P50 short= -3.98 MPa, p =0.50) in five trees of each height class. Leaf area/sapwood area ratio of upper crown branches did not differ between height classes (p = 0.98). Related studies with these same trees indicated similar diurnal patterns of water potential in 60 and 90 meter trees, yet lower diurnal leaf conductance in the taller individuals. Based on our xylem vulnerability curves and the in situ minimum water potentials, trees of both height classes experience no more than 32% loss of conductivity under normal conditions. We conclude that in E. regnans strong regulation of transpiration at the leaf level may obviate the need for adjustment of hydraulic properties and maintain xylem pressures well above values causing extensive xylem cavitation.
