Effects of flood timing and livestock grazing on exotic annual plants in riverine floodplains

Riverine floodplains around the world are highly prone to invasions by exotic species. While many processes have contributed to floodplain invasions, two theories have received particular attention: (i) replacement of native perennials by exotic annuals following historical livestock grazing and (ii) terrestrialization because of fewer floods following river regulation. We assessed how exclusion of grazers and flooding in different seasons affected established populations of exotic annuals in an 8-year experiment. We hypothesized that effects of season of flooding would vary depending on timing in relation to annual plant life cycles and that outcomes may be affected by interactions between grazing and flooding. Flooding in any season reduced exotic annuals to <2% cover and converted a dryland flora dominated by exotic annuals to a wetland flora dominated by native perennials. However, flood effects may be short lived, as areas flooded in 1 year supported abundant exotics the next. Grazer exclusion and interactions between grazing and flooding did not affect cover of exotic annuals, although grazing depleted native perennials in recently flooded areas. Synthesis and applications. Consistent with the terrestrialization hypothesis, short-duration floods provide a practical method for controlling exotic annual plants where river regulation has reduced the frequency of natural floods. As exotic impacts are greatest in dry periods, when exotics are most abundant, ecosystem benefits will be greatest when flooding leads to a protracted decline in exotic annuals during the dry (non-inundated) years following flood recession. Otherwise, reductions in exotic annuals may be transient and have little positive impact on floodplain ecosystems and processes.