Quercus robur, Q. frainetto, Q. cerris and Q. petraea that predominate in forests in the Thrace region, a bridge between the Balkans and Anatolia, were sampled, elaborated and classified. The ecological conditions were estimated by bio-indicator values. Oak forests can be divided into four groups: Q. robur-Fraxinus angustifolia forests thriving in floodplains, Q. petraea forests found at higher altitudes, Q. frainetto-Carpinus orientalis forests appearing in the
warmest and driest sites and Q. frainetto forests on more humid sites. It was established that the most important topographic factor is altitude, while slope and aspect are of minor importance.
Keywords: ?iodiversity, ecology, floodplain, phytogeography, thermophilous forest, vegetation.