New insights into the history of Central European steppe grasslands

Spatial patterns of species and haplotypes suggest long-term continuity of steppes in eastern Central Europe.

Above: A species-rich meadow steppe near Cristuru Secuiesc (Hungarian: Székelykeresztúr), Harghita County, Transylvania, Romania. Photographed on 29 May 2017 by Wolfgang Willner..

The steppe grasslands of eastern Central Europe have attracted botanists and vegetation scientists for more than two centuries. Not only are they exceptionally species rich, but they also show a great regional diversity. I have been working on these steppe grasslands for many years, and I’m still amazed by the geographical variability of their species composition. However, this variability seems at odds with the often heard opinion that the Central European steppes are, for the most part, human-made habitats created by extensive land-use after clearing the original forests. If that was true, where did all the species come from? And why are the steppes not uniform across large areas, like other anthropogenic vegetation types? Recent paleobotanical studies provide evidence for at least local persistence of steppe-like vegetation in eastern Central Europe throughout the Holocene. Perhaps, the steppe communities of this region are not so anthropic after all?

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Willner, W., Moser, D., Plenk, K., Aćić, S., Demina, O. N., Höhn, M., Kuzemko, A., Roleček, J., Vassilev, K., Vynokurov, D., & Kropf, M. (2021). Long-term continuity of steppe grasslands in eastern Central Europe: Evidence from species distribution patterns and chloroplast haplotypes. Journal of Biogeography, 48, 3104–3117. 

We know that large parts of Europe were covered by steppes during the last glacial period. Unfortunately, paleobotanical studies don’t tell us anything about the species composition of the glacial steppes. In any case, the last glacial maximum was not the main problem for the steppe grasslands in eastern Central Europe. The main problem was the mid-Holocene forest optimum. Forests are the natural enemies of steppes, and the period just before the arrival of neolithic farmers offered optimal climatic conditions for forest growth all over Central Europe. However, as mentioned above, steppes might have been able to survive the ‘mid-Holocene bottleneck’ in some refugia. That’s where our current study steps in.

In a previous paper, I showed that the number of European beech forest specialists is inversely correlated with the distance from the glacial refugia of beech. The best explanation for this finding is that many species do not fill their potential geographical range due to a migration lag, that is, they are still expanding from their glacial refugia. I wondered if the same method could be used to shed light on the history of Central European steppe grasslands. However, there were several problems with this idea: First, the mid-Holocene bottleneck might have erased signals of earlier migrations. Second, there was no agreement on the position of potential refugia for European steppe species. One region that had frequently been suggested as a refugial area for the more thermophilous steppe species is the Black Sea coast, in particular the Dobruja region. Other potential refugia mentioned in the literature were the Vojvodina region in Serbia, Transylvania in Romania, western Podolia in Ukraine, and the lower mountain ranges in Hungary. Finally, one of our own studies suggested the northwestern Pannonian region, including the eastern margin of the Alps, as a potential refugium. All these areas are characterised by a high topographic heterogeneity and mostly calcareous substrates, providing potentially suitable habitats for steppe species throughout the whole glacial-interglacial cycle.

The third problem was the ecological heterogeneity of steppe grasslands. The term “steppe” denotes a variety of grassland types, which are quite distinct in their habitat, physiognomy and floristic composition. In our study area, three main types can be distinguished: meadow steppes (which are dominated by broad-leaved grasses and mesophilous herbs, often occurring in a mosaic with forest patches), grass steppes (representing the typical steppe grasslands) and rocky steppes (relatively open grasslands on shallow rocky soils, often with a high portion of dwarf shrubs). As each steppe type might have responded differently to past climatic oscillations, we studied them separately. We tested all combinations of potential refugia and found that, in addition to the current climate and topographic heterogeneity, geographical distance to the Hungarian Central Range is the most important factor in explaining the number of habitat specialists in a region, followed by the northwestern margin of the Pannonian Basin, western Podolia and Transylvania.

To get deeper insights into the history of steppe species, we sampled 3–4 habitat specialists of each steppe type and investigated their intraspecific genetic diversity, using chloroplast DNA. Several species showed a strong geographical differentiation, suggesting migration waves from multiple refugia with only limited subsequent genetic intermixture. The degree of genetic differentiation is clearly inconsistent with the scenario of a late-Holocene immigration of steppe species from areas outside the Pannonian Basin. Most species must have been present in the region since at least the early Holocene, probably much longer, highlighting the importance of the lower mountain ranges surrounding the Pannonian Basin as long-term refugia for European steppe species.

On the basis of these findings, we can confidently say that the steppes of eastern Central Europe are not man-made habitats, although millennia of extensive land-use (mainly grazing) have certainly increased their geographical range. However, land-use has drastically changed since the 19th century. Many steppe grasslands have been converted into arable land or otherwise destroyed. With the large herbivores of the early Holocene extinct, the remaining steppes are dependent on human management. Our study will provide conservationists with additional arguments why these species-rich and unique ecosystems should be preserved.

Written by:
Wolfgang Willner, PhD, Department of Botany and Biodiversity Research, University of Vienna, Austria

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Published by jbiogeography

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