Distribution and habitat range of diploid, sexual dandelions (Taraxacum section Vulgaria), a Central European flora element in The Netherlands

The cytogeography of Taraxacum section Vulgaria in The Netherlands has been analysed on the basis of 190 population samples, a literature dataset of several hundreds of herbarium specimens and some samples from the upstream Meuse valley near Liege, Belgium. Diploid, sexual specimens have been found to be restricted to some phytogeographical districts; the Fluviatile and the Chalk; the diploids constitute a Central (South) European flora element in The Netherlands. Outside the Fluviatile district, diploids are absent even in grasslands with low agricultural stress; they were studied in official nature reserves. The taxon must be considered ‘very rare’, but may be overlooked. The possible relictual state of the populations, including some consequent, promising, genetic research items is discussed. Vegetation recordings and ecological data showed a positive correlation between the occurrence of the diploids and the degree of xerothermy of the habitat (relatively dry and warm microclimate, main components of the Fluviatile habitat). It is expected that taxonomic difficulties concerning the microsystematics will be a consequence of the genetic exchange (1) between the diploids inter se and (2) between the cytotypes.