Comparative ecology and coexistence of introduced and native congeneric forest herbs: Impatiens parviflora and I. noli-tangere

Background and aims – Impatiens parviflora DC. is an invasive species which disturbs the natural vegetation composition in many European forests where it is frequently mixed with the native I. noli-tangere L. According to the literature, both species seem to share many biological attributes and have very similar ecological requirements. Extinctions have sometimes been reported for I. noli-tangere, and knowledge is lacking about the possible contribution of I. parviflora to this phenomenon. Methods – In a large forest from central Belgium, we compared soil characteristics (inferred by the Ellenberg's indices) and strategies of the species from the recipient vegetation (according to Grime's system) between areas where (1) only one of the two Impatiens is present, and (2) both Impatiens grow together. In order to refine the knowledge of their ecological behaviour, we modelled the response of both species to these soil and community properties. Results – Contrary to what is reported in the literature, we found that, in our study area, I. parviflora prefers soils of low base and N-status, it shows a preference for compacted soils, and it easily penetrates non-ruderal communities and dense groundlayers. Conclusions – These findings suggest that the autecology of I. parviflora in western and central Europe is not very well known yet and deserves further studies. Our results also highlighted major differences in the ecological requirements of both species. We suggest that the regression of I. noli-tangere observed in many regions is not due to the invasion by I. parviflora, although additional studies are necessary to untangle this question.