Campsites as unexpected hotspots for the unintentional introduction and subsequent naturalization of alien plants in Belgium and the Netherlands

Between 2015 and 2019 field work was carried out in campsites in Belgium and the Netherlands. These proved to harbor an extraordinary rich flora, consisting of (mostly) alien species and (very) rare native species that probably also were introduced, at least for the most part. These species were all introduced inadvertently by tourists, their seeds and / or fruits being attached to tent floors, car mats, shoes and clothing, car tires, etc. Although a number of the species had never been recorded before in the study area, several of them are clearly naturalized and are even relatively widespread. This applies at least to species such as Poa infirma, Soliva sessilis, and Trifolium suffocatum. Other interesting records include several Mediterranean Gnaphalieae (Bombycilaena erecta, Filago congesta, F. gaditana, F. pygmaea), various legumes of the genera Medicago and Trifolium (e.g. Medicago littoralis, M. rigidula, Trifolium nigrescens), Parentucellia latifolia, etc. Mechanisms for the introduction and spread for these species, their preferred habitat, local distribution and frequency, degree of naturalization, and recognition are commented upon. Most of the interesting species are also illustrated and a few distribution maps and a phytosociological relevé are also presented. The survival rates of these southern species are discussed. As a result of a changing climate their chances for a future local naturalization and spread seem realistic. To our knowledge, a similar large-scale study has never been conducted before.