Thermophilous oak forests of the steppe and forest-steppe zones of Ukraine and Western Russia

We present a formal classification of thermophilous oak forests of the steppe and forest-steppe zones of Ukraine and Russia. Using 45 sources (synoptic tables; some from Central and Western Europe were also included for comparative purposes), we classified the data using cluster analyses, followed by post-classification tools aimed at formal identification of the optimal number of clusters and fidelity-based table sorting. Db-RDA ordination and a CART were used to identify the lead putative climatic drivers of the vegetation patterns. Of the six clusters identified by our classification procedures, two clusters are interpreted here as new alliances (Betonico-Quercion, Scutellario-Quercion). Some new associations classified into these alliances were also either validated or described as new. We further show that the Quercion petraeae is of heterogenous nature and the position of the units previously classified as the Potentillo albae-Quercion should be re-evaluated. NMDS was used to analyse the patterns of the phytocoenologic elements (diagnostic species of relevant syntaxonomic classes) in the six clusters. This analysis revealed that the classification of the Ukrainian and Russian thermophilous oak forests into the Quercetea pubescentis class is untenable and remains open to further scrutiny.