Staci Warrington1, 2, Allan G. Ellis2 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, NSW, 2109, Australia
Australian acacias (genus Acacia Mill.) are problematic invasive species in many parts of the world. One of the attributes thought to promote the invasion success of acacias is their mutualistic interactions with nitrogen-fixing bacteria, known as rhizobia. Although rhizobia are cosmopolitan, several researchers have shown that invasive acacias associate with strains of the rhizobial genus Bradyrhizobium, which originate from Australia. This suggests that these Australian Bradyrhizobium strains were co-introduced to acacia-invaded regions. One such region is South Africa.
Here we report on greenhouse experiments that included two invasive acacias (A. saligna and A. cyclops) and two native South African legumes (Psoralea pinnata and P. affinis) grown under various soil, rhizobia, and leaf litter treatments. We investigated how the presence of co-introduced Australian bradyrhizobia may impact acacia performance (growth kinetics and biological nitrogen fixation [BNF] efficiency) at three stages of invasion: introduction, establishment and post-establishment. We found that, under introduction, i.e. when plants were grown in South African soils, acacias inoculated with Australian bradyrhizobia grew faster, accumulated more biomass, and had higher rates of BNF as opposed to those that relied solely on the microbes present within the untreated soils. Similar results were found for the establishment stage. Acacias exposed to Australian bradyrhizobia and acacia leaf litter had higher growth and competitive performances compared to acacias not exposed to these treatments. Lastly, to investigate whether this reliance on Australian bradyrhizobia continued into the post-establishment stage where acacia soils are sufficiently chemically altered to facilitate their growth, acacias were grown in sterilized soils collected from dense invasive acacia populations and either inoculated with Australian bradyrhizobia or not. We found that growth performances were similar between the two treatments, thus demonstrating that the reliance on exotic bradyrhizobia for successful growth/establishment of seedlings in post-establishment acacia soils diminishes due to the facilitatory changes in soil chemistry, such as increases in nitrogen. Overall, we provide evidence that co-invasion between Australian acacias and Australian Bradyrhizobium strains enhance invasion success.