Ecology and biogeography of highly invasive plants in Europe: giant knotweeds from Japan (Fallopia japonica and F. sachalinensis).
Perennial knotweed (Fallopia japonica [Reynoutria japonica]) and its close parent F. sachalinensis frequently invade European plains. Various degrees of invasion are observed in the floodplain forests of the Rhine-Meuse basin. Hybrids of Fallopia japonica and F. sachalinensis are viable due to polyploidy and vegetative propagation. Gigantic sizes, large leaves, big rhizomes, and rapid growth of stems and roots in spring allow efficient photosynthesis and adequate nutrient supplies. Giant Fallopia species are light-, nutrient- and water-demanding species. They are highly invasive (especially F. japonica) on acid and well-drained soils, when inundation waters are eutrophic, and when river functioning is badly altered by human activities (embankment and deforestation). F. japonica invades deforested river edges, pioneer softwood forests (Salix or Populus spp.), and plantations of cultivated poplars. However, when sub natural hardwood forests (Quercus, Ulmus or Fraxinus spp.) still exist along rivers, Fallopia is only present in small scattered bushes within gaps or along paths and clear-cuttings. In calcareous floodplains, giant Fallopia species are scattered along deforested river edges, and their spread is reduced by competition with native species. In situations of constant stress (lack of nutrients or water, i. e. along roads and dykes) Fallopia spp. can survive, but will never become invasive. The highly efficient biological strategies developed by giant Fallopia species, the lack of natural enemies in Europe, and the constant degradations of natural habitats explain the spread of Fallopia spp. from sea level to about 1200 m altitude, in spite of local control methods. Frequent mowing is not efficient, because of easy regrow after superficial destruction. Chemical control destroys aerial stems, but often does not reach rhizomes. The best ways to decrease (not eliminate) the knotweed invasion are the renaturation of alluvial floodplains and the use of biological control agents.