Growth dynamics in a mixed-species plantation of Eucalyptus globulus and Acacia mearnsii

Previous work has shown greater productivity in mixed than in mono-specific stands of Eucalyptus globulus and Acacia mearnsii at age 3 and 6.5 years. To assess how long the synergistic effects of acacias on eucalypts in mixed stands would last, and what future trajectory growth might take, we investigated the growth dynamics of mixed and mono-specific plantations over the first 11 years since establishment. Monocultures of E. globulus (E) and A. mearnsii (A) and mixtures (75E:25A, 50E:50A, 25E:75A) of these species were planted following a species replacement series. At the tree level, eucalypt and acacia heights, diameters, volumes and above-ground biomass were higher in mixtures than in monocultures 3–4 years after planting. Similarly, at the stand level, volumes and above-ground biomass were significantly greater in mixtures than monocultures after 3–4 years. The difference in productivity between mixed plots and mono-specific eucalypt stands increased with time from 3 to 11 years after establishment. Litterfall was higher in the mixed stands than the monocultures, and this led to an increase in N and P cycling through litterfall in stands containing A. mearnsii. The study indicated that above-ground biomass accumulation in E. globulus plantations can be increased by acacia admixture. This can partially be explained by canopy stratification and improved nutrition of eucalypts. Although the biomass production in acacias peaked early, the synergistic effect of the acacias appears to be long lasting as was indicated by the increasing differences between mixed and pure stands.