Phytosociological analysis of white oak (Quercus pubescens s.l.) woodlands and related successional stages. Spatial patterns and their drivers

The woodlands dominated by white oak (Q. pubescens s.l.) in south and south-eastern Europe are a habitat of high conservation concern that is undergoing intense fragmentation as a result of human settlements and agriculture. Owing to this high degree of fragmentation and the taxonomic complexity of the subgenus Quercus, an in-depth analysis of white oak woodland communities is still lacking. The aims of this study were i) to identify and describe the spatial patterns of white oak woodlands and related successional stages in central Italy, and ii) to explore the drivers of compositional variation within these plant communities. We collected in central Italy 337 original phytosociological relevés of Quercus virgiliana and Q. pubescens s. str. woodlands and of the communities representing their successional stages. Samples were distributed within ecologically homogeneous areas (i.e. land units), which were specifically defined and mapped for this study on the basis of climate, lithology and land morphology. Vegetation data were analysed through cluster and constrained ordination analyses using a set of natural and anthropogenic explanatory variables. We identified five land units (consisting of numerous polygons), characterized by a specific set of abiotic constraints, and with a specific series of successional stages resulting in 8 vegetation classes, 9 orders, 10 alliances and 17 community types. The diverse types of white oak woodland and of the related stages are differentiated mainly along a steep climatic gradient derived from the combined effect of altitude and continentality. Indeed, the most striking compositional differences were observed between the Mediterranean and Submediterranean communities of the subcoastal area, and the communities in the temperate inner Apennines.