Vegetation diversity of salt-rich grasslands in Southeast Europe

Question How does the plant species composition of Pontic-Pannonian salt-rich habitats vary on a large geographical scale? Do the floristic differences between Pannonia and the Balkans correspond to the current phytosociological classification? Location Pannonia (Hungary, Slovakia, Austria, Czech Republic, Croatia, Serbia, Romania) and the Balkans (Bulgaria, Macedonia, Greece). Methods Two thousand four hundred and thirty-seven releves from halophytic and sub-halophytic habitats were classified using a modified TWINSPAN. The crispness of classification was checked. DCA and CCA with climate data as explanatory variables were applied. Results The classification was best interpreted at the level of 15 clusters. The vegetation changed along the salinity gradient from sub-halophytic grasslands (including Trifolion resupinati alliance of the Molinio-Arrhenatheretalia class and Beckmannion eruciformis and Festucion pseudovinae p. p. alliances of the Festuco-Puccinellietea class) and reed beds (Bolboschoenion maritimi p. p. alliance; the Phragmito-Magnocaricetea class), through steppe and wet inland halophytic vegetation (Festucion pseudovinae p. p., Puccinellion limosae, Pucinellion convolutae, Bolboschoenion maritimi p. p. and Juncion gerardii of the Festuco-Puccinellietea class) towards the extreme halophytic vegetation of the Thero-Salicornietea, Crypsietea and Juncetea maritimi classes. This gradient was longer in the Balkan region, where it spanned from the sub-mediterranean salt-rich grasslands to the extremely halophytic vegetation at the Black Sea coast. The second most important gradient coincided with the water regime. Some vegetation types appeared to be confined to either the Pannonian or the Balkan region (especially within dry sub-halophytic and steppe halophytic grasslands), while others were distributed across the entire study area. The above-mentioned pattern did not always correspond with current classification systems. Conclusions Variation in salt-rich vegetation predominantly follows the salinity and water regime gradients. Geographical variation, generally coinciding with climatic and historical effects, is also important, especially in drier salt-rich habitats. Our large-scale analysis of the floristic variation of salt-rich habitats might be useful for the unification of classification systems that differ substantially between the countries involved. In addition, the analysis may be useful for adjustment of a classification system in the poorly explored Balkan region, where particular vegetation types were identified with, or delimited from, Central European vegetation types without detailed comparative analysis until now.