Résumé
The importance of facilitative processes among plants tend to increase along elevational gradients. Although in most cases it impacts positively ecosystem properties, several examples show that it may facilitate invasion processes by exotics too. The Bohemian knotweed is one of the most agressive exotic plants encountered in Western Europe. Despite strong physiological adaptations of belowground organs to low temperature in its native area, it has not invaded mountain areas yet. We tested experimentally the hypothesis of an invasion once propagules are available, at three elevations in the French Alps : mountain belt (MO, 1100m), subalpine belt (SU, 1950m), and alpine belt (AL, 2550m). From June 2007 to August 2008 we observed at each elevation the performance of individuals under two treatments: without neighbouring species in a naturally disturbed area (scree, S) and among neighbouring species (grassland, G). After one growing season, germination and survival of Fallopia was effective in all habitats except for AL-S. After winter, survival reached its best performance in SU-G and MO-G, while there was no survival at al. Fallopia seems able to colonize both, grasslands and screes, in the montane and subalpine zones in the Alps in all conditions. Propagule availability is certainly the limiting factor of Fallopia's current distribution and colonization may be triggered by human-induced activities making propagules available. For likely physiological reasons (low minimal temperature), the settlement of Fallopia may remain difficult in the Alpine zone. However, our results show that its survival would be highly facilitated by the presence of neighbouring species providing organic soil and a protection against low temperature. In conclusion, Fallopia is clearly able to colonize mountain areas, and it would benefit of the presence of native plants to enlarge its distribution at high elevation.