Enhanced water use efficiency in a mixed Eucalyptus globulus and Acacia mearnsii plantation.

Abstract Significant increases in aboveground biomass production have been observed when Eucalyptus is planted with a nitrogen-fixing species due to increased nutrient availability and more efficient use of light. Eucalyptus and Acacia are among the most popular globally planted genera with the area of Eucalyptus plantations alone expanding to over 19 Mha over the past two decades. Despite this, little is known about how nutrition and light availability in mixed-species tree plantations influence water use and water use efficiency (WUE). This study examined to what extent water use and WUE have been influenced by increased resource availability and growth in mixed-species plantations. Monocultures of Eucalyptus globulus Labill. and Acacia mearnsii de Wildeman and 1:1 mixtures of these species were planted. Growth and transpiration were measured between ages 14 and 15 years. Aboveground biomass increment (Mg ha −1 ) was significantly higher in mixtures ( E. globulus ; 4.8 +  A. mearnsii ; 0.9) than E. globulus (3.3) or A. mearnsii monocultures (1.6). Annual transpiration (mm) measured using the heat pulse technique was also higher in mixtures ( E. globulus ; 285 +  A. mearnsii ; 134) than in E. globulus (358) and A. mearnsii (217) monocultures. Mixtures exhibited higher WUE than monocultures due to significant increases in the WUE of E. globulus in mixtures (1.69 kg aboveground biomass per cubic metre water transpired) compared to monocultures (0.94). The differences in WUE appear to result from increases in canopy photosynthetic capacity and above- to belowground carbon allocation in mixtures compared to monocultures. Although further studies are required and operational issues need to be resolved, the results of this study suggest that mixed eucalypt–acacia plantations may be used in water-limited environments to produce a given amount of wood with less water than eucalypt monocultures. Alternatively, because mixtures can be more productive and use more water per unit land area (but use it more efficiently), they could be utilized in recharge zones where rising water tables and salinity result from the replacement of vegetation (fast growing trees) that uses higher quantities of water with vegetation (shallow rooted annual crops) that use lower quantities of water.