WATER-USE BY GERALDTON WAX (CHAMELAUCIUM-UNCINATUM SCHAUER) AS MEASURED BY HEAT-BALANCE STEM-FLOW GAUGES

Calibrated heat balance stem flow gauges were used to study the daily water use by Geraldton wax (Chamelaucium uncinatum Schauer), a plant e ndemic to the coastal sand plains north of Perth, Western Australia. S ap flow was recorded for the trunk, primary and secondary branches, an d partially shaded lower east and sun-lit upper west branches in order to assess the influence of climatic variables, stem development, and exposure. Irradiance and leaf area strongly influenced diurnal sap flo w. Sap flow in primary and secondary branches was comparable when expr essed on a unit leaf area basis. Defoliation reduced water use of the secondary branch. Shedding of leaves under water stress, a common phen omenon in Geraldton wax, probably has similar effects. The level of in ternal shading influenced water use in the crown (1.2 and 1.5 kg/day p er m(2) leaf area for branches with east and west exposure respectivel y). The difference in water use was prominent during the afternoon whe n the east branch was shaded. The results demonstrate the value of the heat balance stem flow gauges in measuring shrub water use directly.