Habitat overlap of the invasive Impatiens parviflora DC with its native congener I. noli-tangere L.

Small balsam, or Impatiens parviflora, is one of the most invasive herbaceous alien species in European temperate forests, and is increasingly found in the same sites as its native congener I. noli-tangere. The aim of this study was to explore the habitat range of both species and determine whether overlap occurs. Vegetation surveys were conducted in 13 Belgian sites. We performed 139 releves according to the following stratification: pure plots of the invasive balsam, pure plots of the native balsam, mixed plots with both species and plots without either of the two species. Soil characteristics were analyzed (pH, organic matter, C/N ratio and cations) and Ellenberg indicator values were used for light and soil moisture. We assessed species affinities for both Impatiens using indicator species analysis. Discriminant analysis was carried out to assign the sample plots to certain vegetation types. Vegetation and soil characteristics indicated that the species have considerable habitat overlap. This overlap is indicated by similar communities among releves and by the failure of discriminant analysis to correctly assign plots. Populations of both Impatiens species are surrounded by an unsuitable habitat that probably limited their spread. In conclusion, we predicted that the alien balsam could spread rapidly with the help of human disturbance and transport, and could compete with the native congener.