| Summary: | Warming environmental conditions are often credited with increasing Arctic shrub growth and altering abundance and distribution, yet it is unclear if tundra shrub expansion will continue into future decades. Water availability may begin to limit Arctic shrub growth if increasing air temperatures create drier soil conditions due to increased evapotranspiration and permafrost-thaw-induced soil drainage. However, few studies have effectively considered how dominant tundra shrub species respond to variations in both temperature and moisture. To better understand the key effects of temperature variation and soil moisture on two dominant circumpolar deciduous shrubs, we studied shrub growth along a natural landscape gradient in West Greenland. We found that the growth forms of both grey willow (Salix glauca) and dwarf birch (Betula nana) were sensitive to variations in air temperature and soil moisture. For both species, increases in air temperature were positively correlated to greater shrub volume, with much of the additional canopy volume due to increased woody biomass. Leaf biomass was best predicted by edaphic features including extractable ammonium, which was positively related to soil moisture, and bulk density. Warmer soils generally tended to be drier, suggesting that ongoing warming in the area would lead to significant water limitation. Our findings suggest that drier soil conditions are likely limiting foliar production by two circumpolar dominant shrubs, Betula nana and Salix glauca, which could have wide-ranging, biome level consequences about ongoing and predicted shrub growth and expansion.
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