COMPARATIVE STUDIES IN SELECTED SPECIES OF EUCALYPTUS USED IN REHABILITATION OF THE NORTHERN JARRAH FOREST, WESTERN-AUSTRALIA .1. PATTERNS OF XYLEM PRESSURE POTENTIAL AND DIFFUSIVE RESISTANCE OF LEAVES

Land use which reduces tree canopy density and the impact of Phytophthora cinnamomi are believed to be altering the hydrological balance of parts of the northern jarrah forest, Western Australia. In the drier eastern zones of the forest, replacement plant communities must maintain the soil-salt-water balance to prevent significant increases in salinization of streams in water supply catchments. Daily and seasonal patterns of the diffusive resistance of leaves and xylem pressure potential were determined for the major natural dominant of the region, E. marginata and 5 other spp. of Eucalyptus used in rehabilitation. Three types of daily and seasonal patterns were observed. E. marginata and E. calophylla exhibited little stomatal control of water loss, and leaf resistances remained low throughout the study period (type 1). E. maculata, E. resinifera and E. saligna exhibited marked stomatal regulation during summer days when xylem pressure potentials fell below -2.0 MPa [megapascal] (type 2). E. wandoo (type 3) also controlled water loss but developed xylem pressure potentials far lower than all other species tested (< -3.0 MPa). Although none of the species tested replicated the summer stomatal resistance and xylem pressure potential patterns of E. marginata, total annual water use should be examined before selecting the most appropriate species to rehabilitate disturbed sites in the eastern zones of the northern jarrah forest region.