Florencia A. Yannelli1, Jan-Hendrik Keet2 and Johannes J. Le Roux1,3
1Centre for Invasion Biology, Department of Botany and Zoology, Stellenbosch University, Matieland 7602, South Africa
2Department of Botany and Zoology, Stellenbosch University, Matieland 7602, South Africa
3Department of Biological Sciences, Macquarie University, Sydney, 2109, Australia
Invasive Australian acacias pose a major threat to the species-rich fynbos vegetation of the Cape Floristic Region (CFR) of South Africa. In these highly diverse ecosystems, the belowground soil microbial community is closely related to aboveground plant community diversity and structure. When invasive acacias become dominant in these habitats, they change the composition of plant communities as well as their associated soil microbial communities. Such alterations could lead to plant-soil feedbacks, which likely persist even beyond the removal of the Acacia plants (so-called legacy effects) and can have a strong influence on the recovery of native plant communities. Here, we evaluate the impact that invasive acacias have on soil physico-chemical properties and soil microbial communities in the CFR. We seek to disentangle the legacy effects of Acacia invasions on the recovery of native vegetation following clearing by comparing neighbouring areas where pristine fynbos, Acacia saligna-invaded, and previously-cleared sites were present. We conducted vegetation surveys, sampled soil chemical composition and overall soil bacterial communities. In all areas, we also collected leaves from the widespread native species Phylica cephalantha and the invasive A. saligna to analyze the impact of nitrogen fixation by the latter on native and invasive species. We found that invasive Australian acacias change the diversity and the structure of above-ground plant communities, soil bacteria, and some soil chemical conditions, but these alterations were site specific. Our results showed that the impact of acacia-driven nitrogen fixation can often persist in native and invasive species even after sites have been cleared. Furthermore, even when acacias are removed from the system, our field study shows that legacy effects driven by soil conditions hinder the full recovery of the native vegetation.