ECOLOGY AND BIOGEOGRAPHY OF HIGHLY INVASI VE PLANTS IN EUROPE - GIANTKNOTWEEDS FROM JAPAN (FALLOPIA-JAPONICA AND FALLOPIA-SACHALINENSIS)

The alien perennial knotweed (Fallopia japonica) as well as its close parent Fallopia sachalinensis frequently invade European plains. Vario us degrees in invasion are however observed in the floodplain forests of the Rhine-Meuse basin, which are here interpreted with the help of the genetical, ecophysiological and biogeographical characteristics of both species. Fallopia japonica and F. sachalinensis may easily hybri dize. Hybrids are viable thanks to polyploidy and vegetative reproduct ion. Their gigantic sizes, their large leaves, their big rhizomes, the rapid growth of stems and roots in spring allow a very efficient phot osynthesis and a good nutrient stocking. Giant Fallopia species are li ght, nutrient and water demanding species. They become highly invasive (especially F. japonica) when soils are acidic, well-drained, inundat ion waters are eutrophic and when river functioning is badly altered b y human activities (embankment, deforestation). These conditions are f requent in Europe plains surrounded by granitic mountains and densely populated. Fallopia japonica invades deforested river edges, pioneer s oftwood forests (Salix or Populus species) or plantations of cultivate d poplars. But when subnatural hardwood forests (Quercus, Ulmus or Fra xinus species) are still present along the river, Fallopia is only pre sent by small scattered bushes within gaps or along the paths and clea r-cuttings. In calcareous floodplains, giant Fallopia species are scat tered along deforested river edges and their expansion is slowed-down by competition with native species. In situations of constant stress ( lack of nutrient or water, for example along roads, dykes), Fallopia s p. can survive but will never become invasive. The highly efficient bi ological strategies developed by giant Fallopia species (essentially c ompetitors according the terminology of Grime 1979), the lack of natur al enemies in Europe, the constant degradations of natural habitats (i n which native species should be competitive) explain that Fallopia is in constant expansion from sea level to about 1 200 m altitude, in sp ite of local controlling methods. Frequent mowing are not efficient be cause Fallopia species can easily grow again and again after superfici al destruction. Chemical actions destroy aerial stems but often do not reach rhizomes. The best way to decrease (but not eliminate) the knot weed invasion should be the renaturation of the alluvial floodplain an d the availability of biological control agents.