Intensifying wildfire activity and climate change can drive rapid forest compositional shifts. In boreal North America, black spruce shapes forest flammability and depends on fire for regeneration. This relationship has helped black spruce maintain its dominance through much of the Holocene. However, with climate change and more frequent and severe fires, shifts away from black spruce dominance to broadleaf or pine species are emerging, with implications for ecosystem functions including carbon sequestration, water and energy fluxes, and wildlife habitat. Here, we predict that such reductions in black spruce after fire may already be widespread given current trends in climate and fire. To test this, we synthesize data from 1,538 field sites across boreal North America to evaluate compositional changes in tree species following 58 recent fires (1989 to 2014). While black spruce was resilient following most fires (62%), loss of resilience was common, and spruce regeneration failed completely in 18% of 1,140 black spruce sites. In contrast, postfire regeneration never failed in forests dominated by jack pine, which also possesses an aerial seed bank, or broad-leaved trees. More complete combustion of the soil organic layer, which often occurs in better-drained landscape positions and in dryer duff, promoted compositional changes throughout boreal North America. Forests in western North America, however, were more vulnerable to change due to greater long-term climate moisture deficits. While we find considerable remaining resilience in black spruce forests, predicted increases in climate moisture deficits and fire activity will erode this resilience, pushing the system toward a tipping point that has not been crossed in several thousand years.

加剧的野火活动和气候变化可以推动森林成分的快速变化。在北美洲，黑云杉塑造了森林的可燃性，并依靠火来再生。这种关系帮助黑云杉在全新世的大部分时间里保持其主导地位。然而，随着气候变化和更频繁和更严重的火灾，正在出现从黑云杉的优势转向阔叶或松树物种，这对生态系统功能产生影响，包括碳固存、水和能量通量以及野生动物栖息地。在这里，我们预测，鉴于当前的气候和火灾趋势，火灾后黑云杉的这种减少可能已经普遍存在。为了测试这一点，我们从 1 合成数据，北美北部的 538 个野外站点，以评估最近发生的 58 次火灾（1989 年至 2014 年）后树种的组成变化。虽然黑云杉在大多数火灾后具有恢复力 (62%)，但恢复力损失很常见，并且在 1,140 个黑云杉站点中，有 18% 的云杉再生完全失败。相比之下，火后再生在以针叶松为主的森林中从未失败，针叶松也拥有空中种子库或阔叶树。土壤有机层的更完全燃烧，通常发生在排水较好的景观位置和干燥的尘埃中，促进了整个北美北部的成分变化。然而，由于长期气候水分不足，北美西部的森林更容易受到变化的影响。虽然我们在黑云杉林中发现了相当大的剩余恢复力，