COLLABORATIVE RESEARCH: WINTER CLIMATE CHANGE IN A NORTHERN HARDWOOD FOREST

在美国东北部，冬季的气候变化比夏季更快。冬季气候变化对生态系统的影响因积雪深度和土壤冻结的影响而变得非常复杂。雪作为土壤的绝缘体很重要，许多北方阔叶林土壤在冬季通常不会结冰。缺雪可能导致土壤冻结，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，并扰乱养分循环过程，导致养分流失到水和空气中。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夏州哈伯德布鲁克长期生态研究遗址(HBR)北部阔叶林中土壤冻结和碳、氮循环的影响的三个方面。首先，这项研究将解决变暖世界中是否会出现更冷的土壤的不确定性，以及这种模式是否会增加HBR北部阔叶林中氮的损失，氮是一种重要的植物养分，也是水和空气污染的来源。在气候变暖的世界中，土壤冻结的程度和影响的不确定性源于我们对气候变化的森林景观中哪里会出现两个“临界点”的理解上的局限性；哪里的积雪太浅而不能隔绝土壤；哪里的气温太热而不能冻结土壤。这种不确定性将通过对整个约3000公顷的哈伯德·布鲁克实验森林(HBEF)的积雪深度和土壤气候进行测量和建模来解决，该森林包含了对美国东北部未来50-100年气候变化的预测范围。将在经历了广泛的长期积雪状况的20个试验田地点测量土壤温度、水分和霜冻以及氮素向水和空气中的损失，以探索围绕土壤冻结事件的关键不确定性及其对氮素循环的影响。其次，拟议的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的过程，以及由此导致的冬季碳流的变化，验证碳流是冬季微生物活动的关键综合调节因素的假设。为了验证这一假设，将在代表HBEF冬季气候变化的六个密集研究地点，追踪糖枫碎屑进入土壤生态系统并通过土壤生态系统的碳和氮的移动。第三，拟议的研究将探讨土壤冻结是否会改变生态系统氮在融雪时的处理以及随后向接收水输出氮的水文控制。将通过向积雪中添加同位素标记的氮并跟踪其进入和通过土壤生态系统的移动来测试融雪动力学。将通过分析氮出口和溶质来测试土壤冻结对出口到接收水域的影响，这些氮出口和溶质是冬季气候/土壤冻结不同的小流域水文水流的天然示踪剂。该项目将包括探索美国东北部冬季气候变化对娱乐、木材收获、生物质能源生产和其他生态系统服务的影响的教育和政策活动。该项目将把生态系统研究人员与相关的利益相关者和利益集团联系起来，包括伐木者和林农、滑雪区经营者、枫糖生产商、休闲雪地摩托用户、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。还将有两项试点教育举措：为本科生开设冬季实地课程，以及为初中和高中教师创建教学指南。最后，这项研究将很好地融入正在进行的关于气候变化影响建模的HBR研究，促进当前气候变化建模研究的扩展，以描绘未来气候变化情景下对土壤冻结的水文和生物地球化学反应。

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.