Traditional measures of plant diversity from phytosociological relevés are used to compare floristical richness and diversity between different oak-hornbeam forest stands of Thiérache (Northern France). Five sets of physiognomically similar forests are distinguished according to the geological substrate: schists and sandstones, loess on schists, loess on calcareous rocks, jurassic limestones, cretaceous marls and clays. Plant species diversity is investigated at two scale levels: a phytocoenotic level and a synusial one which is supposed to partly avoid a possible silviculture-related bias. Two pools of sets may be distinguished: a calcareous and a non-calcareous one. The calcareous shows the highest floristical diversity, and the non-calcareous one the highest phytosociological richness. For each inter-pool pair of sets, plant species diversity is significantly different at the phytocoenotic level as well as at the level of herbaceous or shrubby synusiae, but not at the arborescent one. Through the coppice-with-standards practice, silviculture was the main determining factor for arborescent synusiae while the herbaceous and shrubby synusiae, which support the major part of the diversity, remained strongly influenced by the substrate. It is suggested that silviculture has got a masking effect on plant species diversity within oak-hornbeam managed forests by making uniform tree layers that outshine the understory vegetation diversity Español