Abstract Aim We assessed the relative contribution of ploidy-level variation and adaptive trait diversity to the diversity of taxon realized niches in a plant lineage with several ploidy levels. Location A large number of sites covering a wide range of environmental conditions in France. Methods Our material comprised a collection of populations from several taxa of the ‘fine-leaved fescue’ lineage, namely section Aulaxyper (Festuca rubra s.l.) and subsection Festuca (Festuca ovina s.l.) of the subgenus Festuca. This collection was assessed for several traits related to vegetative growth, investment in seed production, morphology and phenology. Environmental parameters that best differentiate taxon-realized niches were identified by multivariate ordination and logistic regression. Canonical correlation analyses were performed to relate plant traits and ploidy-level variation to these environmental parameters. Taxon differentiation was assessed along the first canonical directions of the plant descriptor (plant-trait and ploidy-level) space. Results The climatic summer water balance, soil texture and land use were identified as the main environmental parameters differentiating taxon realized niches. Canonical correlation analyses revealed associations between plant traits and these environmental parameters, independent of ploidy-level variation. More specifically, the production of long and abundant rhizomes appeared to be an efficient adaptation to poor climatic summer water balance. In contrast, the ploidy level was rather weakly associated with environmental parameters. The marked prevalence of high ploidy levels in the collection suggests that they could have a competitive advantage in the range of environmental conditions investigated. Main conclusions Adaptive trait diversity appears to contribute more than ploidy-level variation to the present diversity of realized niches for fine-leaved fescue taxa, although plant-trait variations were associated to some extent with ploidy-level variation. However, cytotypes with high ploidy levels could have been efficient colonizers and competitors, favouring the expansion of the lineage and leading to their present prevalence. Furthermore, adaptive trait diversification may have been an efficient factor in niche diversification for the present cytotypes with high ploidy levels.