Temperate forests in northern Iran are one of the most important natural ecosystems with the highest economic value and a key role in the conservation of forest biodiversity in Iran. We studied vascular plant species diversity components (alpha, beta and gamma) of herb and tree layers on 120 plots at six sites in forests managed by single tree selection and forests that have been left unmanaged for more than 30 years in the Hyrcanian forest zone in northern Iran. Generalized linear mixed models (GLMM) were used to analyse the effect of management on stand properties, tree and herb layer local diversity and soil characteristics. Despite the differences in herb layer local α-diversity, forest management did affect neither community composition of the herb layer nor regional γ-diversity. Also, species richness as well as α- and γ-diversity of the tree layer was significantly higher in managed forests, while β-diversity showed no directional response across herb and tree layers. According to constrained ordination analysis, main drivers for explaining variance in plant species composition were soil organic matter, total nitrogen, potassium, phosphorous, pH and forest structural variables (basal area, number of trees per hectare and canopy cover) in herb and tree layers. We found that in Hyrcanian forests single tree selection management promotes environmental conditions that effectively contribute to increased diversity of the herb and tree layers at the local and regional scale compared to unmanaged forests. The higher diversity of herb and tree layers is likely being driven by higher within-stand heterogeneity, improved light availability in managed forests due to lower stocking density and larger canopy gaps, or also results from topsoil disturbances through wood extraction. This study indicated that single tree selection system has the potential to be used in order to improve biodiversity conservation, and these results can be useful for development of management guidelines and decision-making processes.Because it may not be specific enough to dismiss or prioritize forest management regimes, based only on their impact on plant diversity, further assessments of forest management effects on other crucial factors (such as fauna diversity, structural diversity and forest ecosystems services) are needed.