Mixed-species plantations are widely recognized for enhancing understory biodiversity compared to monospecific stands; however, most evidence comes from conifer-deciduous mixtures. Thus, it remains unclear whether intraspecific mixtures, such as polyclonal plantations, can harbor greater understory vegetation diversity than monospecific plantations. This study evaluated how monoclonal (one clone) and polyclonal (four clones) hybrid poplar (Populus spp.) plantations influence the taxonomic and functional diversity and composition of understory vegetation. We selected four hybrid poplar clones planted in monoclonal or polyclonal blocks across three sites of contrasting environmental conditions and land-use histories in northwestern Québec, Canada. We assessed understory vegetation (vascular plants, bryophytes, and lichens) diversity and composition and quantified vascular plant functional diversity. Polyclonal plantations supported higher vascular plant functional diversity than monoclonal plantations at the sites established on abandoned agricultural land. In contrast, functional diversity did not differ between plantation types at the previously forested site, likely due to strong environmental filtering and legacy effects of prior forest cover. Community functional composition was mainly driven by land-use history and latitude: the forest site was associated with more competitive, late-successional strategies, whereas abandoned farmlands favored more acquisitive and spatially expansive traits, and conservative traits increased toward higher latitudes. Overall, plantation type and site conditions jointly shaped understory community composition. These results underscore the ecological relevance of clonal mixing for plantation management and biodiversity conservation, while highlighting the importance of site context in modulating the effects of plantation design.