Flooding and geomorphology influence the persistence of the invasive annual herb Noogoora burr ('Xanthium occidentale' Bertol.) in the riparian zone of the dryland Darling River, Australia

Abstract

The relationship between flooding, and the establishment and persistence of exotic species, is not well understood in highly variable dryland rivers. Increased moisture associated with floods is likely to stimulate establishment and growth of exotic plants, but floods may also act as a stress to exotic plants if floods last for weeks to months. This study examined how physical drivers of dryland rivers - flood inundation and geomorphology - influence the persistence of 'Xanthium occidentale' Bertol. in the dryland Darling River, Australia. The distribution of 'X. occidentale' was associated with flood-related moisture subsidy, moderated by channel geomorphology. Dead stalks and burrs on the ground occurred above the 8-m height of the previous flood. Adult and juvenile plants occurred below 8 m corresponding to smaller flood events. Flatter geomorphic units (floodplains and benches) contained more plants and burrs, whereas steeper geomorphic units (banks) did not retain burrs, limiting plant abundance. Flooding is not a stress to 'X. occidentale'. A glasshouse experiment showed that flood durations of up to 40 days had minimal effect on the germination, survival and growth of 'X. occidentale' burrs, seeds or seedlings. Weed management strategies for 'X. occidentale' in dryland rivers could be enhanced by targeting periods following flooding when moisture availability is increased on the flatter geomorphic units in the river channel.