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快。冬季气候变化对生态系统的影响由于对积雪深度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，许多北方硬木林的土壤在冬季通常保持不冻。缺雪可能导致土壤冻结，这对森林生态系统是一个严重的干扰，可能会杀死树根和微生物，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，研究人员将使用一个小尺度的方法来评估三个方面的影响，在雪深度的变化对土壤冻结和碳和氮的循环在北方硬木林在哈伯德布鲁克长期生态研究网站（HBR）在新罕布什尔州。首先，这项研究将解决在一个温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氮的损失，氮是一种重要的植物营养素，也是水和空气污染的来源，来自HBR的北方硬木林。在一个温暖的世界里，土壤冻结的程度和影响的不确定性根源于我们对两个“临界点”发生在经历气候变化的森林景观中的理解的局限性;积雪太浅，无法隔离土壤，空气温度太高，无法冻结土壤。这种不确定性将通过测量和模拟整个~3000公顷哈伯德布鲁克实验森林（HBEF）的积雪深度和土壤气候来解决，其中包括预测美国东北部未来50-100年的气候变化范围。土壤温度，湿度和霜冻，以及氮损失到水和空气的测量将在20个实验现场进行，这些现场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，拟议的研究将解决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冬季碳流的变化，测试碳流是冬季微生物活动的关键综合调节剂的假设。为了验证这一假设，同位素标记的碳和氮的运动将被跟踪糖枫碎屑进入和通过土壤生态系统在六个密集的研究地点，代表了在HBEF冬季气候的变化的全部范围。第三，拟议的研究将解决，如果土壤冻结改变水文控制生态系统氮处理在融雪和随后的出口氮接收沃茨。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。土壤冻结对输出到接收沃茨的影响将通过分析氮输出和溶质来测试，这些溶质作为冬季气候/土壤冻结不同的小流域水文流动路径的天然示踪剂。该项目将包括探索美国东北部冬季气候变化对娱乐，木材采伐，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。该项目将通过一个“科学链接”计划，将生态系统研究人员与相关利益攸关方和利益集团联系起来，包括伐木工和护林员、滑雪场经营者、枫糖生产商、娱乐性雪上摩托车用户、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。还将开展两项试点教育活动：为大学生开设冬季实地课程，为初中和高中教师编写教学指南。最后，这项研究将很好地融入正在进行的HBR研究，对气候变化的影响进行建模，促进当前气候变化建模研究的扩展，以描述未来气候变化情景下土壤冻结的水文和地球化学响应。

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.