Environmental impact of Fallopia

Overview

Results

The team

Related information

So what's the problem?

Japanese knotweed (Fallopia japonica, also known as Reynoutria japonica and Polygonum cuspidatum) and giant knotweed (Fallopia sachalinensis, also known as Reynoutria sachalinensis) are rhizomatous perennial plants native to eastern Asia and were introduced into Europe as ornamental and fodder plants. Today, both species as well as hybrids between the two (Fallopia × bohemica) are considered to be among the most serious exotic weeds in Europe, particularly along riverbanks. All three taxa are presumed to spread primarily by vegetative means.

What is this project doing?

In March 2004, a project started at CABI’s centre in Switzerland to assess the ecological and economic impact of exotic invasive knotweeds in Europe. The research focusses on various aspects of Fallopia taxa in Europe (genetics, phenotypic plasticity, seed production), on the effect of climate change on the likelihood of sexual reproduction, and on the impact of Fallopia spp. on below- and above-ground communities and energy flow in natural habitats.

A long-term impact experiment testing three commonly applied non-chemical control methods was initiated in May 2005 at a field site in Belfort, France. The control methods selected were two different cutting regimes (a single cut or six cuts at monthly intervals during knotweed growing season) and a combination of cutting and covering the ground with vegetable fibre matting.

Results so far

Findings suggest that habitats invaded by knotweeds support lower numbers of plant species and lower overall abundance and richness of invertebrates, compared to native grassland and bush habitats. It also suggests that there is a link between the replacement of native plant species by exotic Fallopia species and the reduction in overall invertebrate abundance and morphospecies richnesss (a measure for diversity). Moreover, biomass of invertebrates sampled in grassland and bush-dominated habitats was almost twice as high as that in Fallopia-invaded habitats. Large-scale invasion by exotic Fallopia species is therefore likely to seriously affect biodiversity and reduce the quality of riparian ecosystems for amphibians, reptiles, birds and mammals whose diets are largely composed of arthropods.

Feeding damage on exotic Fallopia spp. was lower than on native plant species. The lowest levels of damage were consistently recorded on F. japonica.

Studies on the impact of Fallopia spp. on pollinators showed that during Fallopia flowering, in September, equal numbers of pollinators were recorded in traps at Fallopia flower level and in native vegetation. In summer, however, pollinators were more abundant in native vegetation. We found that the seed set of the native plant Silene dioica was strongly reduced in Fallopia stands.

Results of a field survey showed significant variation in seed production among different regions in Germany, Belgium and Switzerland. According to the literature, the hybrid F. x bohemica is more abundant in natural habitats in Europe than its parent species. Results of an experiment set up in 2007 indicate that biomass produced in one growing season was higher for F. x bohemica than for its two parent species.

In addition, current management recommendations are being experimentally evaluated in terms of their effect on knotweed biomass reduction, and on the recovery of plant and invertebrate assemblages.

Results of the evaluation of different mechanical Fallopia management methods showed that cutting exotic Fallopia patches repeatedly during the growing season is the most effective of the treatments. Fallopia biomass was so strongly reduced by this treatment that management was switched from cutting to manually pulling out the remaining shoots twice during the growing season from 2007 onwards. In 2008, after three seasons, the number of native plant species and the abundance of invertebrate herbivores were significantly increased in plots where Fallopia was repeatedly cut, compared to control plots. We are planning to continue with the experiment until knotweed is eradicated on the experimental plots.

Published results:

Krebs, C., Gerber, E., Matthies, D., Schaffner, U. (2011) Herbivore resistance of invasive Fallopia species and their hybrids Oecologia 167(4):1041-52.

Murrell, C., Gerber, E., Krebs, C., Parepa, M., Schaffner, U., Bossdorf, O. (2011). Invasive knotweed impacts native plants through allelopathy. American Journal of Botany 98(1): 1–6.

Krebs, C., Mahy, G., Matthies, D., Schaffner, U., Tiébré, M-S., Bisoux, J-P. 2010. Taxa distribution and RAPD markers indicate different origin and regional differentiation of hybrids in the invasive Fallopia complex in central-western Europe. Plant Biology 12, 215–223.

Samuel Bouille, Pierre-André Frossard, Laurent Gogniat, Noël Buchwalder, Natalia Guazzone, Urs Schaffner and Esther Gerber. 2009. Inventaire de l’impatiente glanduleuse et des renouées exotiques dans la vallée du Doubs (Jura). Botanica Helvetica, 119: 138–141.

Gerber, E., Krebs, C., Murrell, C., Moretti, M., Rocklin, R., Schaffner, U., 2008. Exotic invasive knotweeds (Fallopia spp.) negatively affect native plant and invertebrate assemblages in European riparian habitats. Biological conservation 141, 646-654.

Gerber, E., Jaegle, B., Heinrich, F., Schaffner, U., 2008. Impact des renouées exotiques envahissantes (Fallopia spp.) sur les pollinisateurs, ainsi que sur la reproduction d'une espèce végétale indigène. Actes de la Société jurassienne d'Emulation.

Project Executive