Abstract Climate-induced fire regime shifts may reduce post-fire recovery and erode the resilience of the boreal forests. The eastern North American boreal zone is often dominated by near-monospecific stands of black spruce (BS, Picea mariana), a tree species with regeneration traits that are adapted to stand-replacing fire. While post-fire vulnerability of immature BS stands has been extensively studied, no study has evaluated post-fire regeneration of mature BS stands and its ecological determinants at the subcontinental scale. This study assessed the fire resilience of mature BS forests and the effects of major environmental drivers on post-fire regeneration. We analysed an extensive network of 536 mature BS-dominated stands that were affected by 21 fires (1995?2016) across a 50,400?km2 boreal landscape in eastern North America. We first quantified post-fire seedling density (seedlings/ha) of BS and co-occurring tree species to determine the proportion of plots affected by low levels of BS regeneration and potential shifts toward jack pine (JP, Pinus banksiana) or broadleaved species. We then analysed the effects of seed bank condition, fire characteristics (severity and seasonality) and seedbed conditions on post-fire BS regeneration. One-third of the plots exhibited low-regeneration levels, particularly in stands that were dense and closed-canopy prior to the fire. A relatively minor proportion (one-fifth) also experienced compositional changes, mainly toward JP. Generalized linear mixed-effect models indicated that higher pre-fire BS basal area and greater post-fire cover of unburned living Sphagnum promoted post-fire BS regeneration, whereas high-severity crown fires and spring fires had negative effects. Synthesis. Based on a dataset of unprecedented size and spatial extent in eastern Canada, our study provides the first robust assessment of the fire resilience of mature BS forests. A third of pre-fire mature closed-canopy stands are expected to transition into open woodlands, a change that could persist for centuries. While mature BS-dominated forests are generally considered more fire-resilient than younger stands, our study raises concerns about their capacity to persist and maintain ecosystem services under the projected climate-driven increases in burned area and severity.

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.