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).北极林线（Arctic treeline）的位置是调控陆地表面能量收支、生态系统-大气碳循环、野生动物栖息地以及当地社区生计资源可获取性的关键因子。主流假说认为，林线位置由生长季的气温决定。由于树木高于苔原植被（tundra vegetation），且其冠层延伸至更暖的边界层上方，因此树木组织的温度低于苔原植被。据推测，这种低温环境会限制细胞分裂与生长，使得幼苗无法成长为成熟树木。然而，我们的前期研究发现，阿拉斯加北极林线附近的气温较此前认知更高，且气温对土壤养分有效性的间接效应可能是树木生长的重要调控因素。我们提出假说：林线处的低温土壤（尤其在冬季）会抑制微生物活性与养分有效性，以至于树木仅能勉强存活与生长。本项目在阿拉斯加西北部的阿加沙肖克河（Agashashok River）周边开展，通过连续六个冬季（2016/2017—2021/2022）布设雪栅栏（snowfences），在三个土壤湿度与苔原植被类型各异的林线样地中，分别对每个样地内的8棵树开展积雪深度人工增雪实验。本数据集包含对照样地与布设1.5米高雪栅栏的处理样地中，林线白云杉（treeline white spruce）的枝条初生生长年度手动测量数据。三个林线研究样地的土壤湿度与林下植被类型各不相同，分别为湿生型（wet sedge tundra，湿薹草苔原）、中湿型（tussock tundra，丛状苔原）与旱生型（dry heath tundra，干石楠苔原），每个样地均设置8个雪栅栏处理样方与8个对照样方。