Numerical classification of oak forests on loess in Hungary, Croatia and Serbia

Oak forests on loess are floristically one of the richest types of broadleaved forests and are among the most threatened types of natural habitats in the Carpathian Basin. They are classified in several communities in Hungary, Croatia and Serbia. These syntaxa are distinguished mostly on the basis of traditional phytosociological methods without comparison across a larger geographical scale. The recognition of some of these local syntaxa in the field, therefore, can be difficult, and the application of their names to communities in other areas may be questionable. The goal of this study was to develop an international typology for oak forests on loess based on a numerical analysis. A data set of 437 phytosociological relevés (stands of 12 associations from three countries) was stratified and 270 relevés were analysed using multivariate statistical methods. Six types were distinguished: Primula vulgaris type (xero-mesic to mesic sub-Mediterranean closed-canopy oak forests); Ruscus aculeatus type (xeric to xero-mesic sub-Mediterranean type); Vinca herbacea type (xeric continental open-canopy woodlands); Pulmonaria mollis type (xeric to mesic continental closed-canopy forests); Corydalis cava type (mesic closed-canopy oak forests in nutrient-rich habitats); and Stellaria media type (xeric to mesic oak forests in nutrient-rich habitats). The vegetation types identified are related to syntaxa traditionally recognized by phytosociologists. Our analysis did not support the distinction of some associations with local distributions. The geographical distributions of the two main forest types exhibited a gradient-like pattern in a north-east–south-west direction. The dry continental forest steppe woodland is mainly distributed in the north-eastern part of Hungary, whereas the xero-mesic sub-Mediterranean forests are restricted to the southwestern and southern part of our study area. This pattern corresponds to a climatic gradient from the North Hungarian Mts to north-eastern Croatia and northern Serbia.