Pulsatilla patens has a circumpolar distribution from Eurasia to North America. This species occurs in the central and central-eastern parts of Europe, where is a rare and threatened species. In Poland, the largest number of sites is found in the north-east. A study on populations of P. patens was carried out in 2009–2010, at two wildlife refuges of the NATURA 2000 (N2000) network situated in North-Eastern Poland. Local names are following: the “Sejneńskie Lake District” (SLD) and the “Grasslands in the Military Training Grounds in Orzysz” (GMO). The current population structure of P. patens was estimated including the number, density and structure of different life cycle stages. Seventy-nine research plots of 2 m × 2 m were established at sites: dry heath, xeric sand calcareous grassland and pine forest. In each plot, all individuals were recorded and classified into life cycle stages: juvenile, vegetative and flowering individuals. The studies were carried out twice a year – in spring, during the full bloom of Easter pasque flower and in summer, in time of its fructification. The bioindication method was applied in order to determine the climatic and soil conditions of the habitats. Relationships were estimated between these population characteristics and habitat-related features such as the cover of phanerogams, cryptogams, litter, and bare soil. The population found on the GMO, was composed of 316 individuals which gives the density 6.4 ind. per plot while the one in SLD consisted of 202 individuals which gives the density 6.7 ind. per plot. The age structure of the populations proved that these were stable populations. This was evidenced by a large share of vegetative individuals – in both populations they prevailed (ca 64%). The highest share of juvenile individuals was noticed at the sites characterized by a moderate cover of cryptogams (21–35%). The analysis of correlations showed that on the non-forest habitats – dry heath and xeric sand calcareous grassland, the total number of individuals and number of vegetative individuals were negatively correlated with the cover of phanerogams and the cover of cryptogams. It was recorded negative impact of dense moss layer on the number of juvenile individuals. Moreover, the cover of litter had negative influence on the total number of individuals, number of juvenile and vegetative individuals. In contrast, the total number of individuals and number of juvenile individuals at that sites was positively affected by the cover of bare soil. A positive correlation was found between the number of vegetative individuals and phanerogams, and also between the number of juvenile individuals and the cover of bare soil at forest habitats. Negative impact on total number of individuals and the number of flowering individuals is exerted by the cover of cryptogams. Considering the ecological indicators set for the plant communities in which Pulsatilla patens appeared, it should be concluded that these were the habitats characterized by moderate light and moderately warm climate condition. Thecommunities with Pulsatilla patens grew on dry and fresh soils, oligotrophic and mesotrophic ones, composed of sand and argillaceous clay. The pH ranging was from moderately acidic to neutral. The analysis of the age structure of the examined populations, the latter can be said to be stable. It can therefore be assumed that the Eastern pasque flower has met the favorable habitat conditions at the analyzed sites, therefore it is justifiable to claim that these sites will exist in the future and will constitute an important part of the resources of this species in Poland.
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