Collaborative Research: Soil temperature, mycorrhizal association and tree nutrition as determinants of warming-induced changes in tree growth and abundance in arctic Alaska

近几十年来，北极地区的植被变化十分普遍，对碳储存、地表能量交换和当地野生动物栖息地产生了影响。大多数对导致植被变化的模式和过程的研究都集中在落叶灌木上，这些灌木在北极大部分地区的数量都有所增加。很少有研究考察北极冻土带和北方森林交界处树木丰富度的变化。该项目将利用20世纪50年代和70年代在布鲁克斯山脉收集的历史航空图像，对阿拉斯加北部近几十年来树木丰度的变化进行全面普查。研究人员还将测试营养差异、更冷的土壤和更广泛的永久冻土是否解释了为什么布鲁克斯山脉西部的树木为了应对最近的变暖而增加了生长，而布鲁克斯山脉东部的树木却没有。此外，调查人员还计划在阿拉斯加的农村和城市开展广泛的教育和外联活动。农村的努力将包括访问Kotzebue高中的教室分享研究成果，为Kotzebue高中的学生提供加入实地研究团队的机会，安排接受当地公共广播电台的采访，以及在当地几个村庄的国家公园管理局总部进行公开讲座。在安克雷奇，研究人员计划为安克雷奇的一所公立高中配备进行树木年轮研究所需的设备，培训学生使用标准方法，并帮助他们设计一项研究，将自家后院的白云杉年轮数据与长期气候记录联系起来。森林和冻土带的相对丰富对高纬度地区的地表能量交换、碳循环、野生动物栖息地和生计资源的可获得性具有重要影响。阿拉斯加北方森林的北端形成于布鲁克斯山脉内。东部布鲁克斯山脉是树木年代学家所熟知的地区，因为它是树木生长对气候变暖做出不同反应的震中。分化指的是温度和树木生长之间历史上强烈的正相关关系的恶化。人们认识到，环北极地区普遍存在差异，这削弱了人们对古气候重建的信心，并在植被-气候反馈预测方面造成了不确定性。虽然分歧是一个众所周知的现象，但它对树木丰富度的变化和树线位置的变化的影响仍然未知。最近在布鲁克斯山脉从西向东倾斜的四个分水岭上的发现表明，更寒冷、更受永久冻土影响的土壤限制了树木对土壤养分的获取，这可能是布鲁克斯山脉东部分歧的原因。在这项研究中，研究人员将扩大这项先前工作的空间范围，以检查布鲁克斯山脉分歧的原因和后果。他们将把重复的航空摄影与树木年轮分析相结合，并对25个Brooks Range流域的树木小气候、菌根协会、营养关系和繁殖努力进行详细测量，以产生对气候变化中树木生长控制和树木丰度变化的广泛机械见解。他们将测试这一假设，即对气候变暖和树木丰富度增加的积极生长反应将在西部布鲁克斯山脉和土壤温暖的栖息地盛行，而对气候变暖和停滞的树线的中性生长反应将在东部布鲁克斯山脉和寒冷土壤地区普遍存在。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Vegetation change has been widespread in the Arctic in recent decades, with implications for carbon storage, surface energy exchange, and local wildlife habitat. Most studies of patterns and processes responsible for vegetation change have focused on deciduous shrubs, which have shown increases in abundance throughout much of the Arctic. Fewer studies have examined changes in tree abundance at the interface between arctic tundra and the boreal forest. This project will take advantage of historical aerial images collected throughout the Brooks Range during the 1950s and 1970s to provide a comprehensive census of changes in tree abundance in recent decades in northern Alaska. Investigators will also test hypotheses about whether differences in nutrition, colder soils, and more widespread permafrost explain why trees in the western Brooks Range have increased their growth in response to recent warming, while those in the eastern Brooks Range have not. In addition, the investigators plan extensive education and outreach activities in rural and urban Alaska. Rural efforts will include visiting classrooms in Kotzebue High School to share research results, providing opportunities for Kotzebue High School students to join the research team in the field, arrainging to be interviewed by local public radio stations, and delivering public lectures at National Park Service headquarters in several local villages. In Anchorage, the investigators plan to equip a public high school in Anchorage with supples needed to conduct tree-ring research, train students in standard methods, and help them design a study to link white spruce tree ring data in their backyards to long term climate records. The relative abundance of forest and tundra has important implications for surface energy exchange, carbon cycling, wildlife habitat and the availability of subsistence resources at high latitudes. The northern limit of the boreal forest in Alaska is formed within the Brooks Range. The eastern Brooks Range is an area well known to dendrochronologists as an epicenter of divergent tree growth responses to climate warming. Divergence refers to the deterioration of historically strong positive correlations between temperature and tree growth. The recognition that divergence has been widespread in the circumboreal has undermined confidence in paleoclimate reconstructions and created uncertainty in projections of vegetation-climate feedbacks. While divergence is a well-known phenomenon, its implications for changes in tree abundance and shifts in treeline position remain unknown. Recent findings in four watersheds along a west to east gradient in the Brooks Range suggest colder, more permafrost-affected soils limit tree access to soil nutrients and may be the cause of divergence in the eastern Brooks Range. In this study, investigators will expand the spatial extent of this previous work to examine the causes and consequences of divergence in the Brooks Range. They will combine repeat aerial photography with tree-ring analysis and detailed measurements of tree microclimates, mycorrhizal associations, nutrient relations, and reproductive effort in 25 Brooks Range watersheds to yield broad-scale mechanistic insights into controls on tree growth and changes in tree abundance in a changing climate. They will test the hypothesis that positive growth responses to warming and increased tree abundance will prevail in the western Brooks Range and in habitats with warmer soils, while neutral growth responses to warming and stagnant treelines will be common in the eastern Brooks Range and in areas with cold soils.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.