Résumé
Exotic species threat biodiversity worldwide making invasions by non-native organisms to one of the major risk. Climate change including higher temperature is likely to increase the invasive range of many species. This is confirmed by numerous environmental envelope models (EEM) which predict a further expansion of many alien species. However, propagule pressure is only rarely taken into account in such models. Moreover, current distribution of invasive species on which most EEM are based, may not reflect the species physiological limits, but only current propagule dispersion. In this study, experimental planting of rhizomes of Bohemian knotweed (Fallopia x bohemica) along an altitudinal gradient in two contrasting habitats in the French Alps is used to examine the roles of climate, propagules pressure, disturbance and neighbouring plants on the performance of a plant highly invasive in the lowland. While it seems obvious that climate is a major constraint for the establishment of non-native species, the key question is whether the current distribution of Fallopia is limited the above mentioned factors or the results of several interactions between these factors. At three different altitudinal levels (montane [M, 1100 m], sub-alpine [S, 1950 m], and alpine [A, 2550 m]), a grassland (G) and a scree (S) frequently disturbed by hang slides were selected resulting in a total of six experimental plots where Fallopia rhizomes were randomly assigned and planted in the middle of 15 hard plastic trays per plot. Soil from the study plots was previously filled in the trays which were then put back in the dug hole and buried to the brim. Rhizomes were planted on 02/07/2007 and presence, height, diameter and productivity (height x diameter) of each plant were assessed regularly until 27/10/2007 when all above-ground parts of Fallopia died back.