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

近几十年来，北极地区的植被变化很普遍，对碳储存、地表能量交换和当地野生动物栖息地都有影响。大多数关于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。 很少有研究调查北极苔原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年代在整个布鲁克斯山脉收集的历史航空图像，对北方阿拉斯加近几十年来树木丰富度的变化进行全面普查。调查人员还将测试关于营养差异、较冷的土壤和更广泛的永久冻土层是否解释了为什么西部布鲁克斯山脉的树木在最近的变暖中增加了生长，而东部布鲁克斯山脉的树木却没有。此外，调查人员计划在阿拉斯加的农村和城市开展广泛的教育和宣传活动。农村的努力将包括访问教室在Kotzebue高中分享研究成果，为Kotzebue高中的学生提供机会，加入实地研究团队，传讯接受当地公共广播电台采访，并提供在国家公园服务总部在几个当地村庄的公开讲座。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用品，培训学生使用标准方法，并帮助他们设计一项研究，将后院的白色云杉年轮数据与长期气候记录联系起来。森林和苔原的相对丰富对高纬度地区的地表能量交换、碳循环、野生动物生境和生存资源的供应具有重要影响。阿拉斯加北部森林的北方边界形成于布鲁克斯山脉。东部布鲁克斯山脉是树木年代学家熟知的一个地区，它是树木对气候变暖的不同生长反应的中心。分歧是指温度和树木生长之间历史上强正相关关系的恶化。认识到，分歧已广泛的环北破坏了古气候重建的信心，并创造了不确定性的预测植被气候反馈。虽然分歧是一个众所周知的现象，其影响的变化，树木丰富和树木线位置的变化仍然未知。最近的研究结果表明，在四个流域沿着西向东梯度的布鲁克斯范围更冷，更多的冻土影响的土壤限制树木获得土壤养分，并可能是在东部布鲁克斯范围的分歧的原因。 在这项研究中，研究人员将扩大空间范围的这一先前的工作，以检查的原因和后果的分歧，在布鲁克斯范围。他们将结合联合收割机重复航空摄影与树木年轮分析和详细测量树的小气候，菌根协会，营养关系，并在25个布鲁克斯范围流域的繁殖努力，以产生大规模的机械见解控制树木生长和树木丰富度的变化，在不断变化的气候。他们将检验一个假设，即对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，而在布鲁克斯山脉东部和土壤寒冷的地区，对变暖和停滞的树木线的中性生长反应将是常见的。该奖项反映了NSF的法定使命，并被认为值得通过利用基金会的智力价值进行评估来支持和更广泛的影响审查标准。

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.