Unprecedented modern rates of warming are expected to advance boreal forest into Arctic tundra¹, thereby reducing albedo^2,3,4, altering carbon cycling⁴ and further changing climate^1,2,3,4, yet the patterns and processes of this biome shift remain unclear⁵. Climate warming, required for previous boreal advances^{6,7,8,9,10,11,12,13,14,15,16,17}, is not sufficient by itself for modern range expansion of conifers forming forest–tundra ecotones^{5,12,13,14,15,17,18,19,20}. No high-latitude population of conifers, the dominant North American Arctic treeline taxon, has previously been documented⁵ advancing at rates following the last glacial maximum (LGM)^6,7,8. Here we describe a population of white spruce (Picea glauca) advancing at post-LGM rates⁷ across an Arctic basin distant from established treelines and provide evidence of mechanisms sustaining the advance. The population doubles each decade, with exponential radial growth in the main stems of individual trees correlating positively with July air temperature. Lateral branches in adults and terminal leaders in large juveniles grow almost twice as fast as those at established treelines. We conclude that surpassing temperature thresholds^{1,6,7,8,9,10,11,12,13,14,15,16,17}, together with winter winds facilitating long-distance dispersal, deeper snowpack and increased soil nutrient availability promoting recruitment and growth, provides sufficient conditions for boreal forest advance. These observations enable forecast modelling with important insights into the environmental conditions converting tundra into forest.

史无前例的现代变暖速度预计将推动北方森林进入北极苔原¹，从而减少反照率^2,3,4，改变碳循环⁴并进一步改变气候^1,2,3,4，然而这种生物群落转变的模式和过程不清楚⁵。气候变暖，以前的北方进展所需的^{6,7,8,9,10,11,12,13,14,15,16,17}本身并不足以形成森林-苔原交错带^{5 的针叶树的现代范围扩张， 12,13,14,15,17,18,19,20}。以前没有记录到高纬度的针叶树种群，这是北美北极树线的主要分类群^{5以末次盛冰期 (LGM) 6,7,8}之后的速度前进。在这里，我们描述了一群白云杉 ( Picea glauca ) 在远离既定林线的北极盆地以 LGM 后的速率^{7前进，并提供了维持前进的机制的证据。}人口每十年翻一番，个别树木的主茎呈指数径向增长，与 7 月的气温呈正相关。成年的侧枝和大型幼体的末端领导者的生长速度几乎是已建立树线上的两倍。我们得出结论，超过温度阈值^{1,6,7,8,9,10,11,12,13,14,15,16,17}，加上冬季风促进长距离扩散，更深的积雪和增加的土壤养分可用性促进补充和生长，为北方森林的前进提供了充分的条件。这些观察结果使预测模型能够对将苔原转变为森林的环境条件具有重要的洞察力。