Floods reduce the prevalence of exotic plant species within the riparian zone: evidence from natural floods

Questions Does flooding reduce the prevalence of exotic plant species and promote native riparian plant species? Location Field study, two regulated lowland rivers, SE Australia. Methods We used a large flood in spring to test predictions of vegetation response to flooding. We predicted that the species richness of exotic taxa (mostly terrestrial annual/biennial grasses and forbs in the study systems) would be reduced by flooding, while the richness of native riparian species adapted to fluvial disturbance would increase. We surveyed the riparian vegetation at three sites on each of two regulated rivers in the summers before and after a large flood in early spring that inundated all of the sites on one river, and one site on the other river; the remaining sites were not flooded. Overall, the study was a natural experiment with a before–after control–impact design. Results As predicted, flooding dramatically reduced the richness of exotic taxa, particularly of terrestrial grasses. However, some invasive flood‐tolerant exotic taxa, such as blackberries and willows, were promoted by the floods. Flooding reduced the cover of most native taxa, but did not affect overall native taxon richness. In contrast, the richness and cover of native summer annuals increased following the flood. Conclusions Our study provides evidence to support the use of managed floods to reduce the prevalence of terrestrial exotic taxa within the riparian zones of regulated rivers. The frequency and intensity of flow pulses necessary to keep terrestrial exotics at low levels requires further research.