Photosynthetic capacity of Eucalyptus globulus is higher when grown in mixture with Acacia mearnsii

Mixed species plantations of Eucalyptus and N₂-fixing species can be significantly more productive than monocultures. The aim of this study was to determine whether the improved growth resulted from increases in photosynthesis, light absorption and light-use efficiency, in addition to previously measured increases in leaf area, water-use efficiency and higher ratios of annual above-ground net primary production per unit of total annual below-ground carbon allocation in 1:1 mixtures near Cann River, Victoria, Australia. Light-saturated photosynthetic rate (A _max), electron transport (J), stomatal conductance (g _s) and foliar nitrogen concentrations were higher for Eucalyptus globulus trees growing in mixtures than those in monocultures. Similar increases in maximum rates of carboxylation (V _cmax), Rubisco, chlorophyll, and phosphorus concentrations were not significant. In contrast, A _max, V _cmax and J did not vary between mixtures and monocultures for A. mearnsii, whose growth was negligible by age 15 years. Mixtures also absorbed 24 and 41% more light than E. globulus and A. mearnsii., respectively, and were 38 and 154% more light-use efficient in the mixtures compared to monocultures. The increased nutrient availability in mixtures appeared to increase productivity of E. globulus by increasing the photosynthetic capacity of the foliage, as well as the leaf area, light absorption and light-use efficiency of the canopy.