Below- and above-ground mutualisms impact two alien brooms in South Africa differently

Abstract

Mutualisms are important determinants of the invasion success of alien plants. If alien plants are specialized in their mutualistic requirements, or need different types of mutualistic partners (e.g., pollinators and seed dispersers), invasion success may be hampered. Many studies only consider one type of mutualistic relationship when examining the interaction ecology of alien plants, but most plants interact with multiple types of mutualists. In this study, we explored the interactions between two alien legumes, the brooms Genista monspessulana and Spartium junceum, and their associated below-ground (nitrogen-fixing rhizobia) and above-ground (pollinators) mutualists in South Africa. We conducted pollinator observations, breeding system experiments and collected root nodules to identify rhizobia via 16S rDNA sequencing. Both plants species were entirely dependent on pollinators for seed production. Genista monspessulana was pollinated largely by Apis mellifera capensis and was not pollen limited, while the larger flowers of S. junceum were only pollinated by the larger Xylocopa bees and was pollen limited. Genista monspessulana was nodulated by four distinct Bradyrhizobium (Alpha-proteobacteria) strains, while S. junceum was nodulated by one Paraburkholderia (Beta-proteobacteria) and two Bradyrhizobium strains. Low pollination rates in S. junceum suggest that its reproduction and natural spread in South Africa may be slow. The wider distribution of S. junceum, compared to G. monspessulana, thus likely reflects differences in historic propagule pressure between the two species. The role of rhizobia in limiting plant performance is less clear and the connection between nodulation rate and plant fitness requires further study.