Understanding the extreme species richness of semi-dry grasslands in east-central Europe: a comparative approach.

East-central European semi-dry grasslands hold several world records in small-scale vascular plant species richness. However, the reasons for the peculiarity of these grasslands are not well known, which hinders a better understanding of the mechanisms of species coexistence. We suggest that one of the reasons for this gap in our knowledge is the lack of basic information on the geographical distribution of these grasslands, their species richness, species composition and site conditions. Here we report new sites of extremely species-rich semi-dry grasslands in the foothills of the Eastern Carpathians in the south-western Ukraine. Using comparison with other sites with extremely species- rich semi-dry grasslands in the peri-Carpathian region and beyond, we show that these grasslands share many important features, including similar species composition, occurrence of species with disjunct distributions, site conditions, landscape context, long history of human influence and traditional management practice. Based on these findings, we suggest that continuity on a large (Pleistocene-Holocene) time scale and regularly practiced management that reduces the asymmetry in competition (typically mowing and absence of fertilizer application) may be the crucial factors responsible for the extremely high local species richness in east-central European semi-dry grasslands. We also suggest that this richness largely originates from an ancient pool of species of Eurasian forest-steppe and hemiboreal forests.