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

在考虑21世纪世纪气候变化时，一个关键的挑战和关切是确定和避免生物圈碳循环中的正反馈（这可能导致“临界点”），在这种情况下，生物圈的某些部分通过成为大气中温室气体的主要排放者来应对气候变化。高纬度冻土带是特别令人关注的地区，因为它们拥有大量的永久冻土碳储量-特别是西伯利亚东北部和北美西北部的Yedoma土壤-并且也是大气甲烷的重要来源。虽然这些地区现在主要是潮湿的灌木和苔藓为主的苔原和森林苔原植被，但有证据表明，在整个更新世冰川和间冰期，该地区主要是高产草原（“猛犸草原”），这是地球上最广泛的陆地生物群落，尽管气温很低，但仍然维持着高的动物生物量。SA Zimov（1995年，2012年）提出，猛犸草原是由大量的大型食草动物创造和维持的（例如野牛、马、犀牛、猛犸象），而正是这些巨型动物的灭绝，最有可能是由于人类狩猎种群在更新世晚期和全新世早期扩散到北极造成的，这导致了猛犸草原的崩溃，取而代之的是目前生产力低下的潮湿苔原植被。此外，他还提出，将一个拥有马和野牛等多种进食策略的巨型动物食草动物协会引入北极，可能会使这一失落的寒冷高纬度生态系统重生。这将稳定土壤碳储量，并作为一种机制来分散永久冻土地区碳循环正反馈的威胁。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冻土带转变为草原的潜力。虽然该试验成功地启动了植被从湿苔原和森林苔原向开阔的、以草为主的景观的转变，但迄今为止尚未进行详细和系统的监测，以检验SA Zimov生态系统-气候假设的核心组成部分。这些概述了这种生态系统的转变将如何影响地表辐射和水收支、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，有和没有巨型动物重新野化。我们将使用通量测量塔和土壤CO2排放测量，结合土壤和大气条件以及能量平衡的详细测量，并使用无人机地图缩放，测量从生态系统到大气的碳和水的净通量。对实地生态系统和小气候过程的详细观测将为两个地球系统模式（BNU-ESM和CAS-ESM）中系统的关键方面提供参数化，从而可以探索高纬度生物群落变化对行星气候和地球化学过程的不同可能情景的潜在影响。这项工作将为北极大部分地区现在、过去和未来潜在的生态系统和气候动态提供独特的机制见解

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