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This study deals with population dynamics of Cirsium rivulare (brook thistle) present in a meadow that undergoes succession as a result of former disuse. The effect of the following in the succession, currently dominant clonal species and necromass on the performance of the brook thistle population (presence, size of clusters and reproduction) was investigated. The regression stage, the Cirsium rivulare population had attained after mowing, was discontinued. Different kinds of influence from the contiguous species on the brook thistle population could be identified. In the case of Filipendula ulmaria and Carex cespitosa , Cirsium rivulare clusters were found to be present on the edge of the dominant patches. In case of Carex acutiformis patches brook thistle ramets did not form clusters but were present among the sedge ramets. No Cirsium rivulare ramets were found in Phragmites australis and willow ( Salix cinerea , S. pentandra and S. aurita ) patches. The highest number of generative brook thistle ramets were observed in a Carex acutiformis patch, whereas the highest number of inflorescences on the shoots of the analysed species were identified in the patch dominated by Filipendula ulmaria . Experimental vegetation gaps were created in patches dominated by various species to determine the effect of necromass on the presence of Cirsium rivulare seedlings. A considerable rise in the number of Cirsium rivulare seedlings was noted in Carex acutiformis and Filipendula ulmaria patches following necromass removal in comparison with plots containing necromass. On the other hand, removing the necromass and foliage did not contribute to an increase in the numbers of brook thistle seedlings in the patch dominated by Cirsium rivulare itself.

Cirsium rivulare is a perennial plant that forms patches consisting of ramets resulting from sexual reproduction by seeds and asexual propagation by rhizome fragmentation. We examined the relationship between the size of patches and genetic differentiation of ramets within and between patches. Ramet genotypes were identified using microsatellites. From among 216 ramets examined in the studied population, 123 had a unique genotype, while 93 were clonal, i.e., their genotype was present in at least two ramets. The frequency of ramets with clonal genotypes was 43% and the frequency of unique genotypes was 57%. Ramets with identical genotypes were dominant in small patches. Large patches consisted of ramets with both unique and clonal genotypes, usually with the predominance of the latter. A molecular variance analysis showed the highest level of variance between ramets and the lowest between patches. Additionally, 21.02% of the total variance was recorded between ramets and within patches. The size of patches was correlated with the number of clonal ramets and the number of unique ramets, but it was not correlated with the clonality index. This population of C. rivulare is currently in a phase of decline from 30 years of vegetation transformation, and there appears to have been an increase in sexual propagation based growth over clonal propagation based growth. Hence, a predominance of ramets with unique genotypes was observed. This can happen as a result of disintegration of large patches and formation of gaps between them. These gaps become convenient places for seed germination and the subsequent development of seedlings.