Physiological regulation of productivity and water use in Eucalyptus:: a review

Eucalyptus species in Australia occur naturally across a wide range of environmental conditions. In native forests, the leaves are typically thick, tough and long-lived with low specific leaf area, and low leaf concentrations of nitrogen and phosphorus. Potential productivity is high but maximum rates are rarely achieved because of limitations due to drought and nutrient availability. In contrast, when Eucalyptus species are grown in conditions with adequate supplies of water and nutrients, or in managed plantations at favourable sites, light-use efficiency and productivity are high. We review the physiological basis for the observed rates of growth and water use in Eucalyptus forests. Our aim is to demonstrate that a process-based approach provides a robust framework to predict productivity and water use by incorporating differences between species, the effects of climate and options for silvicultural management. Concern about decreases in water yield from catchments where Eucalyptus plantations have been established has prompted interest in estimating transpiration rates using a range of techniques. The consequence of the marked sensitivity of decreasing stomatal conductance to increasing air saturation deficit, D, is that, in well-watered conditions, transpiration from Eucalyptus forests can be explained largely by leaf area index and D. Measurements of photosynthesis for many Eucalyptus species over a wide range of conditions have confirmed the potential for high rates of carbon uptake. Measurements of maximum rates of photosynthesis, A(max), maximum rate of carboxylation, V-cmax, and the maximum rate of electron transport, J(max), for Eucalyptus trees are high in relation to other broad-leaved tree species, but actual rates of photosynthesis are often much lower because of water and nutrient limitations. This results in a wide variation in light-use efficiency ranging from 0.7 to 2.7 g (dry matter) MJ(-1) (intercepted photosynthetically active radiation between 400 and 700 nm). Several mechanisms for drought avoidance are identified, including low values and large seasonal dynamic changes in leaf area index, near-vertical arrangement of leaves, high stomatal sensitivity to air saturation deficit, deep rooting ability and osmotic manipulation to maintain turgor in leaves. Further evidence from measurements of carbon isotope fractionation at sites along rainfall gradients and estimation of the relationships between leaf area, sapwood cross-sectional area and conductance demonstrate homeostatic adjustment of hydraulic properties in relation to growing conditions. Validation of models for native and managed Eucalyptus stands confirm their usefulness as a management tool to predict productivity and water use in relation to species, climate and silvicultural options. (C) 2003 Elsevier B.V. All rights reserved.