AbstractThe diversity of planted forest stands of the alien species Quercus rubra L. in Central Europe was studied. The northern red oak is a neophyte that has been cultivated in Europe since the seventeenth century as an ornamental as well as a useful tree. In 38 phytosociological relevés from Slovakia, Poland and the Czech Republic, we recorded a total of 223 vascular plant taxa, of which 10% were alien (with 9 invasive species). The stands were species-poor compared to the most native forests, with an average of 17 species per relevé, and most of the species had a low coverage value (around 5% on average). The representation of endangered species was very low, only three species were recorded. The dense canopy of Q. rubra and large cover of litter consisting mainly of fallen old red oak leaves affected the richness of the undergrowth. The mean cover of the tree layer was 84%, the mean cover of the shrub layer was 9% and the mean cover of the herb layer was 22%. The EuroVegChecklist Expert System assigned the relevés to five classes: Carpino-Fagetea sylvaticae, Alno glutinosae-Populetea albae, Rhamno-Prunetea, Epilobietea angustifolii and Robinietea.