COLLABORATIVE RESEARCH: WINTER CLIMATE CHANGE IN A NORTHERN HARDWOOD FOREST

在美国东北部，冬天的气候变化比夏天更快。冬季气候变化对生态系统的影响由于雪深和土壤冻结的影响而变得非常复杂。雪是土壤的重要绝缘体，许多北方阔叶林的土壤在冬天通常保持不冻结。缺雪会导致土壤冻结，这是对森林生态系统的重大干扰，可能会杀死树根和微生物，破坏养分循环过程，导致营养物质流失到水和空气中。这些损失会降低森林生态系统的生产力，并导致空气和水污染。在这个项目中，研究人员将使用景观尺度的方法来评估雪深变化对新罕布什尔州哈伯德布鲁克长期生态研究基地（HBR）北部阔叶林土壤冻结和碳氮循环的三个方面的影响。首先，该研究将解决在变暖的世界中出现较冷土壤的不确定性，以及这种模式是否会增加HBR北部阔叶林氮的损失，氮是一种重要的植物营养物质，也是水和空气污染的来源。在一个变暖的世界中，土壤冻结的程度和影响的不确定性，根源在于我们对经历气候变化的森林景观中出现两个“临界点”的位置的理解有限；在积雪太浅而无法隔绝土壤的地方，在气温太高而无法冻结土壤的地方。这种不确定性将通过测量和模拟整个约3000公顷哈伯德布鲁克实验森林（HBEF）的雪深和土壤气候来解决，该森林涵盖了未来50-100年美国东北部预测的气候变化范围。测量土壤温度、水分和霜冻以及氮在水和空气中的损失将在20个试验田进行，这些试验田经历了广泛的长期积雪制度，以探索土壤冻结事件及其对氮循环的影响的关键不确定性。其次，本研究将探讨冬季气候变化如何影响微生物和土壤无脊椎动物过程，以及由此导致的冬季碳流变化，验证碳流是冬季微生物活动关键综合调节因子的假设。为了验证这一假设，将在六个密集研究地点追踪同位素标记的碳和氮从糖枫碎屑进入并通过土壤生态系统的运动，这些地点代表了HBEF冬季气候的全部变化范围。第三，研究土壤冻结是否会改变融雪过程中生态系统氮处理的水文控制，以及随后氮向接收水的输出。将通过向积雪中添加同位素标记的氮并追踪其进入和通过土壤生态系统的运动来测试融雪动力学。土壤冻结对接收水域出口的影响将通过分析氮出口和溶质来测试，这些溶质是在冬季气候/土壤冻结不同的小流域水文流动路径的天然示踪剂。该项目将包括教育和政策活动，探索美国东北部冬季气候变化对娱乐、木材采伐、生物质能生产和其他生态系统服务的影响。该项目将通过“科学链接”计划将生态系统研究人员与相关利益相关者和利益团体联系起来，包括伐木工人和林农、滑雪场经营者、枫糖生产商、休闲雪地车用户、以保护为导向的非政府组织、公民科学家以及公共和私人土地管理者。此外，还将开展两项教育试点活动：为本科生开设冬季实地课程，为初中和高中教师制作教学指南。最后，该研究将很好地整合到HBR正在进行的气候变化影响模拟研究中，促进当前气候变化模拟研究的扩展，以描述未来气候变化情景下水文和生物地球化学对土壤冻结的响应。

In the northeastern U.S., climate is changing more rapidly in winter than in summer. The impacts of winter climate change on ecosystems are greatly complicated by effects on snow depth and soil freezing. Snow is important as an insulator of the soil, and many northern hardwood forest soils normally remain unfrozen during the winter. A lack of snow can result in soil freezing, which is a significant disturbance to forest ecosystems, potentially killing tree roots and microorganisms and disrupting nutrient cycling processes leading to losses of nutrients to water and air. These losses can decrease the productivity of the forest ecosystem and lead to air and water pollution. In this project investigators will use a landscape-scale approach to evaluate three aspects of the effects of changes in snow depth on soil freezing and the cycling of carbon and nitrogen in the northern hardwood forest at the Hubbard Brook Long Term Ecological Research site (HBR) in New Hampshire. First, the research will address uncertainty about the occurrence of colder soils in a warmer world, and whether this pattern will increase losses of nitrogen, an important plant nutrient and source of water and air pollution, from the northern hardwood forest at HBR. Uncertainty about the extent and effects of soil freezing in a warmer world is rooted in limitations in our understanding of where two "tipping points" occur across a forest landscape experiencing climate change; where snowpacks are too shallow to insulate the soil, and where air temperatures are too warm to freeze the soil. This uncertainty will be addressed by measuring and modeling snow depth, and soil climate across the entire ~3000 ha Hubbard Brook Experimental Forest (HBEF), which encompasses the range of climate variability that has been predicted for the northeastern U.S. over the next 50-100 years. Measurements of soil temperature, moisture and frost, and nitrogen losses to water and air will be made at 20 experimental field sites that experience a broad range of long-term snowpack regimes to explore critical uncertainties surrounding soil freezing events and their effects on nitrogen cycling. Second, the proposed research will address how winter climate change affects microbial and soil invertebrate processes and resultant changes in carbon flow during winter, testing the hypothesis that carbon flow is the key integrative regulator of winter microbial activity. To test this hypothesis, the movement of isotopically labeled carbon and nitrogen will be traced from sugar maple detritus into and through the soil ecosystem in six intensive study sites that represent the full range of variation in winter climate at HBEF. Third, the proposed research will address if soil freezing alters hydrologic controls of ecosystem nitrogen processing at snowmelt and subsequent export of nitrogen to receiving waters. Snowmelt dynamics will be tested by adding isotopically labeled nitrogen to the snowpack and tracing its movement into and through the soil ecosystem. Effects of soil freezing on export to receiving waters will be tested by analyzing nitrogen export and solutes that serve as natural tracers of hydrologic flowpaths on small watersheds that differ in winter climate/soil freezing.The project will include education and policy activities that explore the effects of winter climate change in the Northeast U.S. on recreation, timber harvesting, biomass energy production, and other ecosystem services. The project will connect ecosystem researchers with relevant stakeholders and interest groups, including loggers and foresters, ski-area operators, maple sugar producers, recreational snowmobile users, conservation-oriented NGOs, citizen scientists, and public and private land managers through a "Science Links" program. There will also be two pilot educational initiatives: a winter field course for undergraduates and creation of a teaching guide for middle and high school teachers. Finally, the research will be well integrated into ongoing HBR research on modeling the effects of climate change, fostering extension of current climate change modeling studies to depict the hydrologic and biogeochemical response to soil freezing under future climate change scenarios.