Passive Reestablishment of Riparian Vegetation Following Removal of Invasive Knotweed (Polygonum)

Abstract Japanese knotweed and congeners are invasive to North America and Europe and spread aggressively along rivers establishing dense monotypic stands, thereby reducing native riparian plant diversity, structure, and function. Noxious weed control programs attempt to eradicate the knotweed with repeated herbicide applications under the assumption that the system will recover to a native assemblage which will inhibit future invasions. However, eradication efficacy studies typically only measure the amount of knotweed reduced, not the reestablished species diversity or plant origins. For a community scale efficacy study, we measured vascular plant species diversity and cover in riparian areas along five rivers in Washington State, 3 to 6 years after Bohemian knotweed was initially treated with herbicide. Plant species composition was compared between riparian sites treated to remove knotweed and reference sites where knotweed was absent. Sites where knotweed had been removed had significantly more exotic species and vegetative cover than reference sites; however, native species richness and cover were greater in reference sites and areas with more overstory vegetation. The native plants observed were primarily shade tolerant and perennial, as opposed to many of the exotics, which were shade-intolerant annuals. In general, reestablishment of native and exotic vegetation was not related to pretreatment knotweed stem count, size of the invaded area, or timing of herbicide application. However, residual native tree cover was negatively correlated with initial knotweed stem count. Monitoring the success of restoration objectives (e.g., native plant reestablishment or increased species diversity) and characterizing associated habitat features following knotweed eradication will help in the development of site-specific protocols for successful plant community scale restoration. Nomenclature: Glyphosate, imazapyr, Bohemian knotweed, Polygonum x bohemicum Zika & Jacobson, Japanese knotweed, Polygonum cuspidatum L. Sieb. & Zucc. POLCU. Management Implications: Foliar herbicide applications successfully reduced the cover of invasive knotweed; however, treatment effects on riparian plant assemblages from passive reestablishment were mixed and depended on site habitat conditions and stream size. Along smaller streams, 2nd to 3rd order, the post-treatment plant assemblages were primarily native in composition, although exotic species were more abundant in the knotweed-treated areas compared to reference (no knotweed) areas. The removal of knotweed cleared sites for colonization by native plants, but also released nontarget exotic invaders. In contrast, riparian areas along the largest stream (4th order) contained more exotic forbs and shrubs than native plants in both the knotweed-treated and reference sites. The larger stream had riparian areas with little canopy cover and extensive gravel bars and wide floodplains that are subject to higher levels of seasonal flooding. These conditions promote exotic propagule movement and survival. Successful reestablishment of native vegetation following knotweed removal along large, complex rivers may require active restoration, such as the control of secondary invasions by other exotic plants or replanting native species as conditions change over time. Noxious weed control programs can benefit from including post-treatment plant community surveys, in addition to measuring the amount of the target weed species reduced, to determine if continued management is needed to achieve restoration objectives.