Thermophilous oak forests in Slovakia: vegetation classification and an expert system

Deciduous thermophilous oak forests (phytosociological class Quercetea pubescentis) are among the most species-rich and most endangered forest communities in central Europe. Thanks to the varied topography, bedrock, biogeographical influences and rather well-preserved semi-natural and near-natural forest vegetation, Slovakia harbours a diverse heritage of thermophilous oak forests. Here we present a revised and unified syntaxonomic classification of Slovak thermophilous oak forests. Although several national classification systems were proposed previously, the new system has the advantage that it is based on the results of numerical analysis of a large dataset of 1131 relevés and reflects modern syntaxonomic concepts. A representative dataset, compiled from the Slovak Vegetation Database and unpublished sources, was selected based on the relative representation of diagnostic species of the Quercetea pubescentis class in the relevés. The final classification was produced using beta-flexible clustering and modified TWINSPAN in JUICE software. Resulting clusters were assigned to phytosociological alliances and associations, whose nomenclature was checked for compliance with the International Code of Phytosociological Nomenclature. An expert system including formal definitions based on the Cocktail algorithm and a dichotomous identification key were created and may be used for the assignment of new relevés to the defined associations. Eleven associations were distinguished and classified to three alliances. The alliance Quercion pubescenti-petraeae includes the associations Lithospermo purpurocaerulei-Quercetum pubescentis, Seslerio albicantis-Quercetum pubescentis, Fraxino orni-Quercetum pubescentis and Euphorbio-Quercetum. The alliance Aceri tatarici-Quercion includes the associations Quercetum pubescenti-roboris and Convallario-Quercetum roboris. The alliance Quercion petraeae includes the associations Carici fritschii-Quercetum roboris, Sorbo torminalis-Quercetum, Genisto pilosae-Quercetum petraeae, Melico pictae-Quercetum roboris and Quercetum petraeo-cerridis. Short descriptions of associations, summarizing information on their species composition, ecology and distribution, are provided and discussed. Detrended Correspondence Analysis revealed a complex gradient structure, separating (i) open oak forests on carbonate bedrock from (ii) oak forests on dry and acidic sites, (iii) oak forests on mesic, moderately base- and nutrient-rich sites and (iv) oak forests on mesic, base- and nutrient-rich sites. Ecological preferences of syntaxa were assessed using ecological indicator values and bioclimatic variables. Conservation value and threats to Slovak thermophilous oak forests are discussed.