Can megafauna shift the carbon and surface radiation budgets of the Arctic?

在考虑21世纪的气候变化时，一个关键的挑战和关切是识别和避免生物圈碳循环中的正反馈（这可能导致“临界点”），生物圈的某些部分通过成为向大气排放温室气体的主要排放者来响应气候变化。高纬度苔原是特别值得关注的地区，因为它们拥有大量的永久冻土碳储备——尤其是西伯利亚东北部和北美西北部的Yedoma土壤——也是大气甲烷的重要来源。尽管这些地区现在主要是湿灌木和苔藓为主的冻土带和森林冻土带植被，但有证据表明，在整个更新世冰川期和间冰期，该地区主要是高产草地（“猛犸象草原”），这是地球上最广泛的陆地生物群系，尽管气温寒冷，但仍维持着较高的动物生物量。SA Zimov（1995, 2012）提出猛犸象草原是由大量的大型食草动物（如野牛、马、犀牛、猛犸象）创造和维持的，这些巨型动物的灭绝，很可能是由于人类狩猎种群在晚更新世和全新世早期进入北极造成的，导致猛犸象草原的崩溃，取而代之的是目前低生产力的湿苔原植被。此外，他还提出，将具有多种喂养策略的大型食草动物（如马和野牛）引入北极，可能会导致这个已经消失的寒冷高纬度生态系统的重生。这将稳定土壤碳储量，并作为扩散永久冻土区碳循环正反馈威胁的机制。1996年，SA Zimov在西伯利亚东北部建立了“更新世公园”，以证明在北极引入巨型动物的可行性及其将生态系统状态从苔原转变为草原的潜力。虽然实验成功地启动了植被从湿苔原和森林苔原向开阔、草地为主的景观的转变，但迄今为止，还没有实施详细和系统的监测来测试SA Zimov生态系统-气候假设的核心组成部分。这些概述了这样的生态系统转变将如何影响陆地表面辐射和水收支，土壤和地表温度和湿度，以及净碳平衡。在这里，我们建议与Sergey和Nikita Zimov密切合作，对上述假设进行第一次详细的评估，使用最先进的技术来评估陆地表面的碳、水和辐射预算，以及没有大型动物的野生化。我们将利用通量测量塔和土壤CO2外排测量来测量生态系统向大气的碳和水的净通量，再加上土壤和大气条件以及能量平衡的详细测量，并使用基于无人机的地图进行缩放。对野外生态系统和小气候过程的详细观测将在两个地球系统模型（BNU-ESM和CAS-ESM）中提供系统关键方面的参数化，从而探索高纬度生物群落转移的不同可能情景对行星气候和生物地球化学过程的潜在影响。这项工作将为北极大部分地区的现在、过去和潜在的未来生态系统和气候动力学提供独特的机械见解

One of the key challenges and concerns when considering 21st century climate change is the identification and avoidance of positive feedbacks (which may lead to "tipping points") in the biosphere carbon cycle, where parts of the biosphere respond to climate change by becoming major emitters of greenhouse gases to the atmosphere. High latitude tundras are particular regions of concern, as they hold substantial reserves of permafrost carbon -especially the Yedoma soils of northeast Siberia and north-western North America- and are also substantial sources of atmospheric methane. Although these regions are now dominated by wet shrub- and moss-dominated tundra and forest-tundra vegetation, there is evidence that throughout Pleistocene glacials and interglacials the region was dominated by highly productive grasslands ("the mammoth steppe"), the most extensive land biome on Earth, which supported high animal biomass despite the cold temperatures. SA Zimov (1995, 2012) proposed that the mammoth steppe was created and maintained by the abundance of large herbivores (e.g. bison, horses, rhinoceros, mammoths), and that it was the extinction of these megafauna, most likely caused by the spread of human hunting populations into the Arctic in the Late Pleistocene and early Holocene, that led to the collapse of the mammoth steppe and its replacement by the current low productivity wet tundra vegetation. Moreover, he proposed that the introduction of a guild of megafauna herbivores with diverse feeding strategies such as horses and bison into the Arctic could lead to the rebirth of this lost cold high-latitude ecosystem. This would stabilise soil carbon reserves and act as mechanism to diffuse the threat of a carbon cycle positive feedback in the permafrost regions. In 1996, SA Zimov established the "Pleistocene Park" in northeast Siberia to demonstrate the feasibility of megafaunal introduction in the Arctic and its potential to shift ecosystem states from tundra to grassland. While the experiment has succeeded in initiating a vegetation shift from wet tundra and forest-tundra to open, grass-dominated landscapes, to date no detailed and systematic monitoring has been implemented to test the core components of SA Zimov ecosystem-climate hypothesis. These outline how such an ecosystem shift would affect land surface radiation and water budgets, soil and surface temperature and moisture, and net carbon balance. Here, we propose to work closely with Sergey and Nikita Zimov to conduct the first detailed evaluation of the above hypotheses, using state-of-the art techniques to assess the carbon, water and radiation budgets of the land surface with and without megafaunal rewilding. We will measure the net flux of carbon and water from the ecosystem to the atmosphere using flux measurement towers and soil CO2 efflux measurements, coupled with detailed measurements of soil and atmospheric conditions and energy balance, and scaled using drone-based maps. Detailed observation of ecosystem and microclimate processes in the field will provide parametrisation of key aspects of the system in two Earth System Models (BNU-ESM & CAS-ESM), allowing exploration of the potential impacts of different possible scenarios of high-latitude biome shift on planetary climate and biogeochemical processes. This work would provide unique mechanistic insights into the present, past and potential future ecosystem and climate dynamics of large parts of the Arctic