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.

Boreal forest soils store more carbon than the aboveground vegetation and play a critical role in the global carbon cycle and ecosystem function. Despite their importance, the long-term impacts of forest management on soil organic carbon (SOC) remain unclear, especially in boreal forests with contrasting climatic conditions. This study assesses the effects of commercial thinning on SOC stocks (20-year post-treatment) in black spruce stands across two boreal regions in Québec: the warmer, drier Abitibi and the colder, wetter Cote-Nord, accounting also spatial variability from skidding strips. We measured SOC stocks at three soil depths (forest floor, mineral layer: 0–15 and 15–30 cm) in both thinned and unmanaged stands, using bulk density and SOC concentration. Soil pH, cation exchange capacity (CEC), and macronutrients were also conducted to characterize soil fertility. Thinning had no effect on total SOC stocks of the whole profile in either region, but horizon-specific differences were detected. A decrease in forest floor SOC stocks was observed in Côte-Nord, whereas SOC stocks in Abitibi remained similar. These contrasting outcomes reflect regional differences in soil fertility (higher CEC, potassium in Abitibi) and climatic conditions which may have influenced vegetation regrowth and, consequently, SOC stocks’ recovery over time. Although SOC stocks decreased in skidding strips, their small proportion within the stand minimized their effect on total SOC stocks at the plot level. Overall, the study emphasizes the need to consider climatic and soil context, as well as limited spatial extent of skidding strips, in evaluating silvicultural impacts on boreal carbon sequestration.