Japanese knotweed increases soil erosion on riverbanks

Abstract

For years, Japanese knotweed (Reynoutria japonica) has been suspected of accelerating riverbank erosion, despite a lack of convincing evidence. The stems of this invasive plant die back following the first autumn frosts, leaving the soil unprotected during winter and spring floods. In Québec (Canada), riverbank erosion may also be accentuated by ice during mechanical ice breakups. The objective of this study was to evaluate the influence of knotweed on riverbank erosion along a river invaded by the species, within a context of floods with ice. The elevation along 120 cross‐sectional riverbank profiles, occupied or not by knotweed, was measured before and after the spring flood of 2019. On average, riverbanks occupied by knotweed had nearly 3 cm more soil erosion than riverbanks without knotweed, a statistically significant difference. Stem density also influenced erosion: the higher the density, the greater the soil loss. Certain riverside conditions, such as the slope of the riverbank or being located on an islet, interacted with knotweed, further accentuating erosion. Soil losses measured between November 2018 and May–June 2019 were particularly pronounced, but the spring flood was also exceptional, with a recurrence interval close to 50 years. On the other hand, soil loss from rivers invaded by knotweed can be expected to increase over time, as this invasive species spreads rapidly in riparian habitats.

Effectiveness and cost of a rapid response campaign against Japanese knotweed (Reynoutria japonica) along a Canadian river

“…This is likely because the spring floods were very different, with peak flow much lower in 2020 than in 2019. The lower forces generated by the water and ice flows in 2020 were probably less effective at dislodging rhizome fragments from the soil than those of 2019 (Matte 2020; Matte et al 2022). Moreover, some of the fragments unearthed in 2019 were probably generated in 2018 (Colleran and Goodall 2015).…”

“…In riparian areas, floods and ice flows break the stems and uproot the rhizomes, facilitating their dissemination downstream. A single rhizome or stem fragment containing at least one meristematic node is sufficient to generate a new individual (Colleran and Goodall 2014, 2015; Gowton et al 2016; Lawson et al 2021; Matte et al 2022; van Oorschot et al 2017).…”

Hydrogeomorphological processes and plant invasion. What interactions in the case of Asian knotweeds along the Herault River (France)?

“…Disturbances of both natural and anthropogenic origin have been recognized as facilitators of the process of Asian knotweed invasion (Lavoie, 2017; Matte et al, 2021). At a finer scale, our distribution analysis reveals that the establishment and maintenance of Asian knotweeds at the alluvial‐bar scale are limited by high‐level disturbances related to hydrogeomorphological constraints.…”

“…Preventing their establishment is by far the most cost‐efficient management approach, but requires prior knowledge of the ecological conditions that favor the species' dispersal and establishment with the aim of prioritising the monitoring. Recent research suggests the importance of hydrological regime and hydrogeomorphological processes on the distribution of Asian knotweeds at the stream scale, particularly in their transport and establishment (Matte et al, 2021; Navratil et al, 2021). Therefore, identifying the hydromorphological conditions suitable for their establishment and dispersal appears as an essential line of research.…”

Interacting effects of climate change and invasions by alien plant species on the morphodynamics of temperate rivers

“…Their underground biomass offers relatively weak reinforcement of the underlying substrate in comparison with many of the native species they replace. For example, a high abundance of Fallopia japonica (Japanese knotweed) in North America has been reported to significantly increase bank erosion in river reaches compared to those dominated by trees (Arnold & Toran, 2018; Matte et al, 2021). Both of these IAPS benefit from widespread dispersal of their propagules by flood waters.…”