Collaborative Research: Hydrologic and Permafrost Changes Due to Tree Expansion into Tundra

在过去的几十年里，北极一直在加速变暖，与此同时，树木和灌木已经侵入了阿拉斯加、加拿大和俄罗斯极地地区的冻土带。这些植被的变化反过来又有可能影响区域水文和气候。这个合作项目的目标是结合研究水、能源和碳循环的方法，以了解森林和冻土带（包括永久冻土）之间的相互作用。研究人员将在西伯利亚北极地区的森林-苔原边界研究树木扩张或侵占对土壤和空气之间水分流动的影响。这些方法将包括收集和分析实地数据、使用遥感和建模。研究人员将与俄罗斯的科学家和学生合作，美国研究生将接触跨学科研究，在俄罗斯进行实地工作，并有机会开展国际合作。各种外联活动将向公众介绍这项研究的结果，更广泛地说，是关于北极在全球环境变化中的作用。这些活动包括在自然历史博物馆（MI）、奥顿地质博物馆（OH）、草原之火博物馆（KS）开展博物馆展览；有亚马尔土著人民参加的公共研究研讨会（俄罗斯）；以及在社交媒体和新闻媒体上展示这项研究的纪录片。该项目的目的是在野外观测项目和西伯利亚西部平原地区的尺度上，研究与树木向冻土带扩张有关的水文和永久冻土的变化。树木侵蚀过程改变了陆地与大气之间热交换的季节性；增加了地表蒸散量；3)随着土壤季节融化深度的增加，导致地下变暖。这项研究建立在对俄罗斯乌拉尔极地和亚马尔半岛南部落叶松和云杉入侵地区50年的观察基础上。研究人员将使用一种结合综合野外观测数据和遥感分析的新理论来揭示冻土带和森林土地覆盖之间热交换的差异。将确定成对的野外监测地块，其中一个地块有落叶松和云杉的扩张，另一个地块没有被侵占。实地观测将包括以下测量：微气象学、雪况和辐射通量；树级SAP流；雪水当量；土壤温度、水分和热通量；活性层厚度；土壤成分。现场数据将为一个模型提供信息，以得出地表能量收支和热通量。从1980年代至今的遥感数据将把从这些实地观测推断出的当地趋势与西西伯利亚平原的区域特征联系起来。观测到的热通量和模拟的热通量将用于综合物理模型，以研究植被侵蚀在局部和区域尺度上对永久冻土动态的影响，不确定性分析将评估项目结果的可靠性。

The Arctic has been warming at an accelerating rate over the last several decades, and at the same time, trees and shrubs have been invading into tundra across the polar regions of Alaska, Canada, and Russia. These shifts in vegetation may, in turn, have the potential to influence regional hydrology and climate. The goal of this collaborative project is to combine approaches to study water, energy, and carbon cycling to understand inteactions between forest and tundra, including permafrost. The investigators will examine the effects of tree expansion, or encroachment, on movement of water between earth and air in an understudied forest-tundra boundary in the Siberian Arctic. The methods will include collection and analysis of field data, use of remote sensing, and modeling. The investigators will collaborate with scientists and students in Russia, and U.S. graduate students will be exposed to interdisciplinary research, perform field work in Russia, and have the opportunity to develop international collaborations. Various outreach activities will inform the public about results of this research and more generally, about the role of the Arctic in global environmental change. These activities include development of museum exhibits at the Museum of Natural History (MI), Orton Geological Museum (OH), Museum of Prairiefire (KS); public research seminars engaging indigenous people of Yamal (Russia); and documentaries showcasing this study in social and news media.The objective of this project is to study changes in hydrology and permafrost associated with tree expansion into tundra areas at the scales of both the field observational program and the regional Western Siberian Plain. The underlying hypotheses are that the tree encroachment process 1) alters the seasonality of heat exchange between the land and the atmosphere, 2) increases surface evapotranspiration, and 3) leads to warming of the subsurface with a progressive increase of thickness of soil seasonal thaw depth. The research builds on fifty years of observations in the areas with documented larch and spruce encroachment in the Polar Urals and southern Yamal Peninsula of Russia. The investigators will use a novel theory combined with comprehensive field observational data and remote sensing analyses to reveal the differences in heat exchange between the tundra and forest land covers. Pairs of field monitoring plots will be identified, with expansion of larch and spruce in one plot and no encroachment in the second of the pair. Field observations will include measurements of: micrometeorology, snow conditions, and radiative fluxes; tree-scale sap flows; snow water equivalent; soil temperature, moisture, and heat fluxes; active layer thickness; and soil composition. Field data will inform a model to derive surface energy budgets and heat fluxes. Remote sensing data from 1980s to present will connect local trends inferred from these field observations with regional characertistics of the Western Siberian Plain. Observed and modeled heat fluxes will be used in a comprehensive physical model to study the impact of vegetation encroachment on permafrost dynamics at both local and regional scales, and analysis of uncertainty will assess the reliability of the project findings.