The influence of drought on the relationship between leaf and conducting sapwood area in Eucalyptus globulus and Eucalyptus nitens

Development of the relationship between leaf area (A(l)) and sapwood area (A(s)) was investigated in two important hardwoods, Eucalyptus globulus (Labill) and E. nitens (Deane and Maiden) Maiden, growing in an experimental plantation established in a low rainfall zone (approx. 515 mm year(-1)) of Tasmania. The experiment compared irrigated controls and a rainfed treatment which was subjected to cyclical summer droughts from age 1 to 6 years old. Leaf area and sapwood area were determined by destructive sampling at ages 2, 3 and 6 years old. There was no effect of stand age on A(l):A(s) when sapwood area was measured at crown break. At age 3 years old A(l):A(s) was significantly greater in the rainfed than the irrigated trees. It was concluded that this difference was due to earlier canopy closure in the irrigated gated trees, When the plantation was 6 years old A(l):A(s) was significantly greater in the irrigated than the rainfed treatment. An analysis based on an equation which links A(l)/A(s) with transpiration and volumetric flow rate (Whitehead et al. 1984) was used to infer a positive correlation between stem hydraulic conductivity (k(h)) and water availability. Independent of water availability E, globulus maintained a higher A(l):A(s) than E. nitens at all ages.