Isoeto-Nanojuncetea species and plant communities in their eastern distribution range (Poland)
Poland constitutes the eastern periphery of the distribution range of some taxa and syntaxa included in the Isoëto-Nanojuncetea class. 25 species and five associations reach the limit of their overall eastern (or north-eastern) or European distribution here. Depending on their type of distribution in Poland the species are divided into four groups. Historical and contemporary floristic data were also analysed. The taxa considered are native to the Polish flora, recently occurring mainly in anthropogenic habitats. The species restricted to the western part of Poland find their secondary habitats on the bottom of fish-ponds. The widest (throughout Poland) distribution type is characteristic of taxa recently occurring in arable fields. For the range expansion of these plants an anthropogenic hypothesis can be presented. A common phenomenon observed in Poland is the reduction of the ecological and sociological amplitude of the taxa. The taxa that occur in many different communities in the centre of their distribution area in the Atlantic climate, become exclusive character species for their associations in Poland. The associations of the Isoëto-Nanojuncetea occurring within the territory of Poland at their distribution limits are floristically similar to those in the centre of the synarea, although they seem to be "restricted" in floristic composition and habitat. In Poland they generally develop in secondary habitats. The actual distribution of the Centunculo-Anthoceretum and the Spergulario-Illecebretum associations seems to be of anthropogenic nature, similar to the anthropogenic spreading of the species characterizing these associations.