Aims: Extreme flood events can increase the likelihood of riparian plant invasion. Invasions are not instantaneous and understanding impacts of flooding on various stages of the invasion pathway, from the system of delivery through to establishment and spread, is needed for optimised management of riparian ecosystems. This study incorporates multiple assessments exploring the nature of flood-induced invasions in a large subtropical river including propagule dispersal, seed bank dynamics, and the influence of biotic resistance and standing vegetation. Investigating how these components relate to each other in space and time – and to pre- and post-flood conditions – provides further insight into why riparian zones are susceptible to invasion.
Methods: We tested each stage of the invasion pathway (transport, colonisation, establishment, landscape spread) using invasion of the Logan River in southeast Queensland by Lantana camara (Lantana) and Ricinis communis (Ricinus), pre- and post-flood. The potential of propagules to disperse via hydrochory was assessed using a seed buoyancy experiment. Soil seed bank samples were collected to investigate propagule communities pre- and post-flood, and a mesocosm experiment tested how biotic resistance affects establishment. Extant vegetation was surveyed across twelve sites to assess spread in space and time.
Results: Flooding affected invasion pathways into riparian zones. Inundation imported propagules of invading species, supporting a stronger presence of fast-growing, non-woody invasive species from the seed bank. In pre-flood conditions, Lantana was widely established and acted as a nurse plant of non-woody invaders. Riparian inundation diminished the riparian dominance of Lantana, while promoting invasion and establishment of Ricinus.
Conclusions: Our findings suggest that specialised management strategies are needed to allow for variation among plant invaders of riparian zones across the entire invasion pathway, and particularly given the increasing variability and extremity in flood events occurring in many regions due to climate change.