| Summary: | Unprecedented modern rates of warming are expected to advance boreal forest into Arctic tundra, thereby reducing albedo, altering C-cycling, and changing global climate, yet the patterns and processes of this biome shift remain unclear. We describe the 20th century colonization of an Arctic basin by a widespread boreal conifer, Picea glauca, 40 km north of the nearest established treelines. The population approximately doubled each decade, with radial growth in main stems increasing exponentially and correlating positively to July air temperature. Juvenile height and adult lateral growth were 90% faster than at established treelines. This climate-forced range expansion, cast in the context of invasion theory, informs forecast models of vegetation change with the ecological conditions driving this biome shift. While surpassing temperature thresholds is a necessary condition for boreal forest advance, our empirical results indicate high soil nutrient availability, deep snow, and winter winds facilitate long-distance dispersal and promote recruitment.
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