Interaction of restored hydrological connectivity and herbicide suppresses dominance of a floodplain invasive species

Abstract

Invasive species present one of the largest threats to the recruitment and persistence of native plant communities. Land managers generally apply two approaches to help control invasive species and restore native plant communities: (1) improve ecosystem processes that preferentially support native species (e.g. reintroduce disturbance regimes) or (2) directly treat non‐native species (e.g. herbicide use). Few utilize both. Over nearly a decade, we tracked the establishment and population growth of Lepidium latifolium (perennial pepperweed) on a hydrologically restored floodplain and adjacent grassland sites, concurrently with a controlled herbicide application experiment. We found that perennial pepperweed stem counts were lower in years with longer duration flood events (p < 0.0001) and after herbicide application (p < 0.0005). In floodplain areas, native species increased in the years following herbicide treatments or after wet water years, while grassland sites were reinvaded by other non‐native species. Our results suggest that long inundation periods can effectively control perennial pepperweed over large areas when used in concert with herbicide control during dry years or in drier areas. Given the natural variability in inundation periods, our study highlights the need for longer‐term (>3–5 years) studies to evaluate the effectiveness of invasive species control programs. Overall, our study emphasized the need for further integration of traditional weed control approaches with restored ecological function to effectively control invasive plants and stimulate native plant recovery.