The relative importance of environmental vs. biotic resistance of recipient ecological communities remains poorly understood in invasion ecology. Acer negundo, a North American tree, has widely invaded riparian forests throughout Europe at the ecotone between early- (Salix spp. and Populus spp.) and late-successional (Fraxinus spp.) species. However, it is not present in the upper part of the Rhône River, where native Alnus incana occurs at an intermediate position along the successional riparian gradient. Is this absence of the invasive tree due to environmental or biotic resistance of the recipient communities, and in particular due to the presence of Alnus?