Treeline Snowfence White Spruce Branch Primary Growth, Agashashok River, Northwest Alaska, 2017-2022

The position of the Arctic treeline is an important regulator of land surface energy budgets, ecosystem-atmosphere carbon cycling, wildlife habitat and availability of subsistence resources to local communities. The prevailing hypothesis states that treeline position is determined by air temperature during the growing season. Because trees are taller than tundra vegetation and their canopy extends above the warmer boundary layer, their tissues are colder than tundra vegetation. These colder conditions are hypothesized to limit cell division and growth, such that seedlings are unable to grow into trees. However, our early work revealed that air temperature is warmer than previously thought near the Arctic treeline in Alaska and the indirect effects of temperature on soil nutrient availability may be important determinants of tree growth. We hypothesized that cold soils at treeline, particularly during winter, limit microbial activity and nutrient availability to the point where trees are barely able to survive and grow. In this project, near the Agashashok River in northwest Alaska, we used snowfences over six winters (2016/2017-2021/2022) to experimentally increase snow depth around eight trees at each of three treelines that varied in soil moisture and tundra vegetation. This dataset includes annual manual measurements of branch primary growth of treeline white spruce in control plots and those treated with 1.5 m tall snowfences. There were eight snowfence and eight control plots at each of three treeline study sites that varied in soil moisture and understory vegetation: hydric (wet sedge tundra), mesic (tussock tundra) and xeric (dry heath tundra).