Perennial pepperweed invasion on a restored seasonal floodplain

The science of restoration ecology emphasizes the importance of managing, and often manipulating, both physical and biotic factors to facilitate ecosystem recovery. Among many other factors, our investigation of successful riparian floodplain restoration has focused on the positive impact of floodwaters on both vegetation and geomorphology. While we have found passive restoration– in our case the reconnection of riparian floodplains to seasonal floodwaters through intentional levee breaches – to be successful in creating initial opportunities for native riparian plant species establishment, it has also provided the opportunity for the invasion of non-native species. Invasive species, once established, are a threat to the long-term success of our passive floodplain restoration effort. We have both monitored and removed perennial pepperweed (Lepidium latifolium), a highly invasive noxious weed, on the Cosumnes River experimental floodplain (Sacramento County, California) to determine if restoration efforts suffer from and simultaneously promote its invasion. We analyzed the spatiotemporal dynamics of population establishment, spread and disappearances in relation to floodplain topography and proximity to disturbance. We developed scenarios of future conditions in relation to known ecological processes which will help managers predict population spread and prioritize control. Initial results indicate that both inundation and scour from floodwaters, two common processes on floodplains, negatively effect pepperweed population growth and establishment and could be used as tools to restore invaded areas. Vegetation response, one-year post pepperweed removal, shows a significant increase in native species cover and species number; resulting in 42 native species post-removal from 29 species pre-removal in test plots.