Christina R. Bentrup, Melinda R. McKinney, and George W. Koch. Northern Arizona University
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.