Balsam poplar (Populus balsamifera L.) communities on the Arctic Slope of Alaska
Breen, Amy L.
Trees are absent from the Arctic Slope of Alaska except for isolated stands of balsam poplar (Populus balsamifera L., Salicaceae) disjunct by over 100 km from the boreal forest south of the Brooks Range. Here, I describe balsam poplar plant communities on the Arctic Slope and interior Alaska and Yukon. I established 32 relevés, used the Braun-Blanquét approach to classify balsam poplar communities, and applied nonmetric multidimensional scaling (NMDS) to identify the ecological gradients underlying community differentiation. The ordination revealed a clear differentiation between arctic and boreal communities. Ecological gradients, reflected by ordination axes, correspond to a complex productivity gradient and a complex gradient in slope angle and aspect. A new order and alliance are described, Populetalia balsamiferae and Eurybio-Populion balsamiferae, respectively. Within the alliance, two new associations are described: (1) Salici alaxensis-Populetum balsamiferae (arctic communities) with three variants (typical variant in riparian areas, var. Androsace chamaejasme on south-facing slopes and var. Cystopteris montana associated with perennial springs), and (2) Rosoacicularis-Populetum balsamiferae (boreal communities). In all communities, species richness is driven by herbaceous and woody species, which make up 85 % of the total species. Species richness of lichens and mosses is low throughout the communities, most likely because of annual flooding in riparian sites and shading by the balsam poplar overstory. A comprehensive baseline map documenting the current distribution of extralimital stands of balsam poplar is presented that significantly expands upon our previous knowledge of this species' northern distribution. A strong link between summer warmth index (SWI), the sum of mean monthly temperatures > 0° C, and the presence of balsam poplar is observed for the Arctic Slope (SWI > 25 for ∼80 % of the stands). The alteration of temperature regimes resulting from global climate change will likely lead to an increase in the abundance and distribution of balsam poplar in Arctic Alaska.