Collaborative Research: Impact of Permafrost Degradation on Carbon and Water in Boreal Ecosystems

北方森林包含了全球陆地上以植被和土壤有机质形式储存的大约三分之一的碳。然而，由于永久冻土的广泛退化，这些碳的命运尚不确定，而永久冻土在隔绝土壤碳方面起着关键作用。如果像IPCC（2001）预测的那样，阿拉斯加的气候再变暖5到8摄氏度，那么几乎所有的永久冻土都可能从这个生物群落中消失，导致北方生态系统的水和碳平衡发生巨大变化。冻土退化（热岩溶）对地表水和碳的影响是高度不确定的，因为地形、地形、植被、火情和冻土特征的空间变异性。虽然实地研究已经开始认识到热岩溶在碳积累和随后的甲烷排放中的重要性，但目前的建模方法仍然假设一个相当均匀的土壤景观，其中融化均匀地降低了永久冻土的水位并使土壤干燥。这一假设可能适用于受永久冻土层影响的高地地区（占阿拉斯加北方景观的23%），但不适用于热岩溶蓄水的低地地区（占景观的41%）。此外，永久冻土的退化对高原和低地景观碳输出的影响非常不同。在高地景观中，永久冻土的丧失增加了排水，从而消除或改变了地表径流的季节性。相比之下，低地景观的热岩溶将水蓄水到孤立的湿地中，从而破坏了排水，增加了储存能力，从而减少了径流，增加了溶解有机碳在孤立湿地中的停留时间。因此，目前的建模方法忽略了永久冻土影响景观中水和碳的不同方式。为了解决这些问题，本项目将产生一种新的方法来模拟北方森林系统，其研究任务旨在：(1)评估气候变化和火灾对永久冻土稳定性的相互影响；(2)量化永冻土退化的不同模式如何通过影响寒带系统的微地形、排水和土壤热机制，在景观上启动不同的解冻机制；(3)确定排水或池塘系统等各种解冻机制如何影响生物量和土壤中的碳损失或积累；(4)描述流域溶解有机碳输出的特征，以努力识别永久冻土退化引起的各种解冻机制。使用重复设计，我们将研究解冻历史的年龄序列，以捕捉自解冻以来碳和水随时间的变化。我们将描述每个解冻状态的温度、湿度、地下水位，以参数化每个解冻状态的物理条件，并将在一组独特的地点测试基于解冻水的化学指纹和微量气体通量的模型结果。更广泛的影响：现实的空间生物地球化学模型必须量化永久冻土退化导致的水和碳在整个景观中的再分配。本项目开发的基于过程的生物地球化学和空间明确的永久冻土模型将解决北方地区气候、火灾、永久冻土、碳和水之间的相互作用。这个提议将把建模者和实地科学家联合起来。该项目将用于通过普渡大学和阿拉斯加大学费尔班克斯分校的研究生教育培养新一代生态系统科学科学家。将通过参加政策会议和讲习班来进行公众宣传。项目结果将通过科学会议和出版物进行交流，并将通过普渡大学气候变化研究中心的通讯和年度报告进行分发。除了对全球变化研究界的贡献外，冻土退化对生态系统碳和水循环的影响的知识对联邦土地上的火灾和栖息地管理至关重要。

The Boreal Forest contains about 1/3 of all global terrestrial carbon stored as vegetation and soil organic matter. The fate of this carbon, however, is uncertain because of the widespread degradation of permafrost, which plays a key role in sequestering soil carbon. If the climate warms another 5 to 8 C in Alaska, as predicted by the IPCC (2001), nearly all of the permafrost could be eliminated from this biome, causing dramatic changes in the water and carbon balance of boreal ecosystems.The effects of permafrost degradation (thermokarst) on surface water and carbon is highly uncertain because of the spatial variability in terrain, topography, vegetation, fire regime, and permafrost characteristics. While field studies have begun to recognize the importance of thermokarst in carbon accumulation and subsequent methane emissions, current modeling approaches still assume a fairly homogenous soil landscape where thawing uniformly lowers the permafrost table and dries the soils. This assumption probably holds up well for permafrost-affected upland areas (23% of boreal landscape in Alaska), but is not valid for lowlands areas (41% of landscape), where thermokarst impounds water. Furthermore, the degradation of permafrost affects the export of carbon very differently in upland and lowland landscapes. In upland landscapes the loss of permafrost increases drainage, which eliminates or alters the seasonality of surface runoff. In contrast, thermokarst in lowland landscapes impounds water into isolated wetlands, thereby disrupting drainage and increasing storage capacity that in turn reduces runoff and increases the residence time of dissolve organic carbon in isolated wetlands. Thus, current modeling approaches neglect the varying ways in which permafrost affects water and carbon on the landscape. To address these issues, this project will generate a new approach to modeling boreal forest systems by using research tasks designed to (1) assess interactive effects of climate change and fire on permafrost stability; (2) quantify how the varying modes of permafrost degradation initiate various thaw regimes on the landscape by affecting the microtopography, drainage, and soil thermal regimes of boreal systems; (3) determine how various thaw regimes such as drained or ponded systems affect carbon loss or accumulation in biomass and soils, and (4) characterize the export of dissolved organic carbon from watersheds in an effort to fingerprint the various thaw regimes induced by permafrost degradation. Using a replicated design, we will study age sequences of thaw history to capture changes in carbon and water over time since thaw. We will characterize temperature, moisture, water table of each thaw regime to parameterize the physical conditions of each thaw regime and will test model results based on the chemical finger print of thaw-water and on trace gas flux in one unique set of sites. Broader Impacts: Realistic spatial biogeochemistry models must quantify the redistribution of water and carbon across the landscape that results from permafrost degradation. Process-based biogeochemistry and spatially-explicit permafrost models developed in this project will address interactions among climate, fire, permafrost, carbon and water for the boreal region. This proposal will unite modelers with field scientists. The project will be used to train a new generation of scientists in ecosystem sciences through graduate education at Purdue University and University of Alaska at Fairbanks. Public outreach will be achieved by participating in policy meetings and workshops. Project results will be communicated through scientific meetings and publications and will be distributed through Newsletters and Annual Reports of the Purdue Climate Change Research Center. In addition to the contributions to the global change research community, the knowledge of impact of permafrost degradation on ecosystem carbon and water cycling is critical to the management of fires and habitats on federal lands.