The impacts of invasive African olive on native Australian legumes via altered soil conditions do not persist as legacy effects

Background and aims: Invasive plants often alter soil abiotic and biotic conditions which can benefit their own growth while harming native species. The impacts on native species may persist as legacy effects after the invasive species has been controlled and removed. This study focused on the Critically Endangered Cumberland Plain Woodland (CPW) vegetation in Australia, where we examined the soil impacts, and their associated legacy effects, associated with the invasion of African olive (Olea europaea subsp. cuspidata).

Methods: Seedlings of O. europaea subsp. cuspidata and two native legume species, Acacia implexa and Indigofera australis, were grown in different sterilised and unsterilised soils: uninvaded CPW soil, restored CPW soil where O. europaea subsp. cuspidata was removed approximately 20 years ago, and soil from sites still under O. europaea subsp. cuspidata invasion. We characterised nitrogen-fixing rhizobia in the root nodules of seedlings of the two legumes using next-generation sequencing (NGS) barcoding. 

Results: Olea europaea subsp. cuspidata did not appear to condition the soil to favour its own growth and grew best in uninvaded CPW soil. The performance of both native legume species, however, was negatively impacted when grown in sterilised and unsterilised invaded soils, relative to their growth in CPW and restored soils. The soils from invaded sites affected the associations between both legume species and their rhizobium mutualists. Nodulation was higher in CPW and restored soils than in invaded soils, indicating that the availability of rhizobia was negatively impacted by O. europaea subsp. cuspidata. This was confirmed by a negative link between nodulation and the abundance of rhizobia that were characteristic of invaded soils. 

Conclusion: Our findings demonstrate that O. europaea subsp. cuspidata invasion affects the availability of microbial mutualists for native legumes in the CPW. The soil conditions created by O. europaea subsp. cuspidata do not benefit its own performance and these impacts do not persist as legacy effects 20 years after the removal of the invader.