Sufficient conditions for rapid range expansion of a boreal conifer

Unprecedented modern rates of warming are expected to advance boreal forest into Arctic tundra(1), thereby reducing albedo(2-4), altering carbon cycling(4) and further changing climate(1-4), yet the patterns and processes ofthis biome shift remain unclear'. Climate warming, required for previous boreal advances(6-17), is not sufficient by itself for modern range expansion of conifers forming forest-tundra ecotones(5,12,15,17-20). No high-latitude population of conifers, the dominant North American Arctic treeline taxon, has previously been documented(5) advancing at rates following the last glacial maximum (LGM)(6-8). Here we describe a population of white spruce (Piceaglauca) advancing at post-LGM rates(7) 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(1,6-17), 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.

Automated summary

Through studying a population of white spruce in the Arctic region, it has been discovered that they are expanding at unprecedented rates in areas far from established treelines. This expansion is facilitated by surpassing temperature thresholds, winter winds for long-distance dispersal, deeper snowpack, and increased soil nutrient availability. These observations provide important insights for forecast modeling and understanding the conversion of biomes.