Results of tests and measurements assessing spatial aggregation of diploid and tetraploid plants of Centaurea stoebe and vegetation-cover densities of microsites in which these plants were recorded in three mixed-ploidy sites surveyed in the years 2009 and 2020. The numbers of flow cytometrically analysed plants for each of the two cytotypes (2x/4x) are given in parentheses following the site-year codes. Mantel test—correlation coefficient values (r_m). Distance—mean pairwise distance computed separately for diploid and tetraploid plants per site and year with results of spatial aggregation test—SAT (see Materials and Methods). Vegetation density—mean vegetation-cover density of 20 × 20 cm plots around each of 2x and 4x plants (see Materials and Methods), ↑ denotes a significant increase, while ↓ denotes a significant decrease (Wilcoxon test) in vegetation-cover density per site and ploidy in 2020 when compared to 2009. Vegetation density 2x versus 4x—signed-rank sum T values (Wilcoxon test) corresponding to differences in vegetation-cover densities between 2x and 4x plants at a given site and year.