Comparison of nutrient cycling abilities between the invasive Acacia mearnsii and the native Virgilia divaricata trees growing sympatrically in forest margins in South Africa

Australian acacias have significant impacts on the habitats that they invade. For example, they can increase nutrient input and alter natural nutrient cycles. Here we assessed how the invasive Acacia mearnsii and native Virgilia divaricata, both nodulating legume trees with similar growth forms and general ecologies, compared in terms of N and P nutrient content, nutrient resorption and their Biological Nitrogen Fixation (BNF) capacities where they co-occur within forest margins in South Africa. Fresh and senesced leaf samples were collected in sympatric populations and analysed for N and P concentrations. We also measured the δ15N/δ14N isotope ratio, and used it to calculate percentage nitrogen derived from the atmosphere. The two species were very similar in their total nutrient content, but V. divaricata showed strong dependence on available P which determined its dominant source of N. It had a higher use of BNF, an energetically-expensive process, than the invasive species. Acacia mearnsii was also more efficient in N resorption than the native species. Our results highlight nutritional economy differences between these two ecologically similar species and provide further data on reasons for the success of A. mearnsii compared to native tree species.