Ecological factors shaping post‐fire resilience in mature black spruce forests of eastern North America

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 km 2 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.