Studies on the Status of Unburnt Eucalyptus Woodland at Ocean Grove, Victoria. III. Comparative Water Relations of the Major Tree Species.

Abstract

At Ocean Grove, unburnt Eucalyptus woodland is being replaced by Casuarina scrub. Autecological studies were initiated to study aspects of the mechanism of replacement. C. littoralis seedlings are more drought-resistant than E. ovata seedlings. The drought resistance of C. littoralis is largely due to desiccation avoidance facilitated by efficient reduction of stomatal and cuticular transpiration. C. littoralis seedlings reduced their transpiration rate at high relative water contents and water potentials, and maintained a turgid condition for several days longer than E. ovata seedlings. Although there was a high rate of water loss from detached E. ovata leaves, the induced water saturation deficits were reversible over a wider range of water deficits than in C. littoralis. Hence E. ovata may be more desiccation-tolerant than C. littoralis. The greater desiccation-avoiding capacity of C. littoralis is partly due to its more xeromorphic foliage, its slower growth rates and to differences in tissue water relations. Both adult and juvenile cladodes of C. littoralis exhibited a more rapid decline in shoot water potential with decreasing water content than did E. ovata leaves. In field trials, more Eucalyptus than Casuarina seedlings died during the hot dry summers. Hence the differential survival of E. ovata and C. littoralis seedlings in the field may be partly due to differences in their drought resistance.