Abstract
This paper was a part of studies conducted within an island population of the ragwort <em>Senecio umbrosus </em>(White Mt, southeastern Poland), a vulnerable element of xerothermic grasslands. Special attention was paid to the effects of expansive grass encroachment vs. grassland burning episodes on spatiotemporal patterns and life-stage structure of individuals in the population. The population traits were investigated nine times from 1990 to 2010, within three permanent patches differing in soil properties, initial floristic composition, grassland cover (particularly the cover of <em>Brachypodium pinnatum</em>), ragwort cover and density, shrub/tree cover influencing light intensity (full light–shadow), and grassland burning (zero–six episodes). There was a drastic decline in ragwort abundance within all the study patches accompanied by a decrease in the population clustering coefficient and a gradual equalization of the spatial distribution of ramets. The abundance was negatively correlated (PCA analysis) with an increase in <em>B. pinnatum </em>cover and positively correlated with the number of burning episodes, which temporarily delimited persistent litter cover and facilitated recruitment of new individuals. The decrease in ramet abundance ranged from 3.8 times (medium-high, moderately shadowed grassland; six cases of burning) to 8.3 times (high, dense, and shadowed grassland; four cases of burning). The patch of low, loose, sunlit, and never-burned grassland with the greatest initial density of ragwort (a 6.8-fold decrease in abundance) has evolved with time into a high and dense grassland with a greater coverage of <em>B. pinnatum </em>and <em>Calamagrostis epigejos</em>, additionally shaded by shrubs and young trees.
Highlights
Xerothermic vegetation forming climax communities in the steppe and forest-steppe zones in South-Eastern Europe and Central Asia has an extra-zonal character in Central Europe
All the 1m2 plots were occupied by S. umbrosus throughout the study period, the proportion of plots characterized by varied density changed distinctly: from the relatively even proportion of plots with 20–50 ramets m−2 in 1990–1995 to only two or three classes in the final study periods, with a dominance of plots with one–five ramets m−2
A drastic decline in the S. umbrosus abundance within all grassland patches was accompanied by a decrease in the clustering coefficient and gradual equalization of the spatial distribution of individuals
Summary
Xerothermic vegetation forming climax communities in the steppe and forest-steppe zones in South-Eastern Europe and Central Asia has an extra-zonal character in Central Europe. Its localities in Poland are, at least partly, relics of one or several migration waves dating back to the late Pleistocene (Hewitt, 1999; Paul, 2012). The occurrence of chalk grasslands originating due to forest cutting is determined by a specific combination of topographic, climatic, and soil factors. They are usually seminatural communities emerging through the long-term management of the areas. Burning, mowing, and seasonal grazing by native ungulates have for long been influencing the structure and functions of grasslands in several regions. The gradual changes in the farming landscape observed in Central Europe throughout the twentieth century, in recent decades, have led to a
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