Biogeography of Pannonian steppe grasslands

The steppe grasslands of the Pannonian Basin are refuges for many rare species of plants and animals, and significantly contribute to the biodiversity of European landscapes. From a long-term perspective, steppe grasslands have been the dominant vegetation in large parts of Europe for most of the Quaternary. The degree of continuity in the floristic composition of these grasslands, however, remains unclear. It has been hypothesised that a considerable portion of thermophilous steppe species that occur in the Pannonian grasslands today, was restricted to refuge areas near the Black Sea coast during the Last Glacial Maximum and re-immigrated from these refugia into the central Pannonian region during the late glacial interstadials or the early Holocene. On the other hand, it has been hypothesised that the southern slopes of the hills surrounding the Hungarian Plain might have provided refuge areas for some thermophilous steppe species. There is also growing evidence that the distribution of many species is limited not only by environmental factors but also by species-specific seed dispersal and colonisation capacity. For several taxonomic and ecological groups, distance to potential glacial refuge areas has been shown to be a better predictor of regional species richness than the current environmental conditions. As the species occurring in different types of steppe grasslands have different environmental requirements, they might have been subjected to different processes of glacial survival and post-glacial migration. These differences can only be disentangled based on a consistent classification of grasslands types and species groups being at hand. Thus, the two complementary aims of the proposed research are (1) to achieve a consistent classification of the steppe grasslands in the whole Pannonian region using multivariate methods and a supra-national network of plot data; and (2) to identify patterns of regional species diversity, genetic diversity and areas of endemism for the species pool of each main type of steppe grassland, to evaluate the explanatory power of historical versus present-day environmental factors to explain these patterns, and to validate different potential refuge areas based on population genetic analyses of several diagnostic plant species representing the main types of steppe grassland. Beside evidence based on genetic diversity patterns, intraspecific genetic structuring will allow insights into routes of migration connecting present-day occurrences with likely refuge areas. The proposed study will be an important step towards the development of a consistent international typology of European grasslands. It will provide new insights into refuge areas and post-glacial re-colonisation processes of Pannonian steppe species and the role of limited migration abilities in steppe grasslands under a changing climate.

The Pannonian steppe grasslands are among the most species-rich habitats in Europe. In order to better understand their history, and to increase the scientific basis for their effective conservation, we conducted a new classification of the steppe grasslands based on their plant species composition. We compared thousands of local species lists (so-called vegetation relevés) of steppe grasslands from Central, Eastern and South-eastern Europe. As a result, we could show that there are three main types of steppe grasslands, which are distributed not only in the Pannonian region but in all dry areas of Europe: meadow steppes (semi-dry grasslands), grass steppes (or true steppes) and rocky steppes. The Pannonian meadow steppes and grass steppes are similar to the steppes of Eastern Europe, while the rocky steppes rather show floristic connections to southern Europe. Despite these broad-scale similarities, however, the Pannonian steppes have many floristic peculiarities distinguishing them from steppes in other regions. The spatial distribution of the character species of the three main steppe types, as well as the genetic patterns of selected steppe species analysed by us, suggest that all three steppe types have been present in the Pannonian region during the last glacial period and persisted therein until today. Areas with high topographic heterogeneity, like the eastern margin of the Alps, the lower mountain ranges of Hungary and the margin of the Carpathians, harbour significantly more character species than areas with low topographic heterogeneity, such as the Great Hungarian Plain. The current climatic conditions are not able to explain this pattern in a satisfying manner. Thus, we conclude that historic factors are equally important. In areas exhibiting high topographic heterogeneity, steppe species were able to find suitable habitats even under the extremely variable climatic conditions of the past, without being forced to migrate over long geographical distances. Meadow steppes, which are characteristic for forest steppe zones, show notable diversity hotspots at the northwestern margin of the Pannonian region and Transylvania. The characteristic species of this steppe type also have the highest intraspecific diversity values in Transylvania. Meadow steppes exhibit a strikingly similar species composition from eastern Central Europe eastwards as far as southern Siberia. Its persistence in Central Europe might partly be a result of human land use, which prevented a complete post-glacial reforestation. However, natural fires and grazing by nowadays extinct wild mammals likely played an important role, too. The countries in the Pannonian region have a high responsibility for the conservation of these species-rich habitats.