Aboveground and belowground interactions in mixed plantations of Eucalyptus globulus and Acacia mearnsii

This study investigated whether increased productivity in mixed plantations of Eucalyptus globulus Labill. and Acacia mearnsii de Wild when compared with monocultures could be explained by niche separation of the fine-root systems. For this purpose fine-root architecture, nutrient concentration, and fine-root distribution were examined in two horizons (0-15 and 15-30 cm) of mixed and pure stands. Investigations were carried out in 6.5-year-old plantations consisting of 100% eucalypts, 75% eucalypts + 25% acacia, 50% eucalypts + 50% acacia, 25% eucalypts + 75% acacia, and 100% acacia. Aboveground the two species interacted synergistically. Stem volume and tree height was highest in the 50:50 mixtures. For acacias, intraspecific competition was stronger than interspecific competition with eucalypts. Fine-root biomass and length density were similar for all species combinations, and there was no synergistic effect. The vertical distribution of fine roots and fine-root architecture were similar for acacias and eucalypts. This indicated that soil exploitation strategies may be similar, which can result in strong competition for soil resources. Fine-root nitrogen concentrations of eucalypts were highest in the 50:50 mixture. Improved productivity of mixtures appears to be a result of both canopy stratification and improved N nutrition of eucalypts through N fixation by acacias.