Catastrophic shifts in tree-microbial symbioses: the causes, consequences, and warning signs of environmental collapse in the global forest system

地球上生命的分布是由气候决定的。从地球赤道到极地冰盖向北或向南移动，生态系统的顺序跨越了潮湿的热带雨林、干燥的沙漠、季节性绿色的温带森林和广阔的常绿森林和冻结的冻土带。这些生态系统中的每个生态系统的生命都适应了气候带来的挑战，从必须生存的极端高温和寒冷，到必须开发的资源的定期循环。随着地球气候以前所未有的速度变化，这种规则的生态系统序列正在大规模重组。作为NERC的一名独立研究员，我提议研究气候变暖是如何在几十年和几个世纪里推动世界上最大的森林地区发生革命性变化的，其速度之快足以见证个人生命的规模。我的研究表明，横跨地球北部大森林的数千亿棵树生活在气候临界点的边缘。当其他生态系统类型与其环境展开拉锯战时，就会出现转折点，每个生态系统都会以一种允许其生存的方式改变当地的条件。这场拉锯战可能会在相互交战的生态系统之间造成不稳定的边界，在那里，微小的气候变化可能会迅速且不可逆转地决定赢家。这些生态系统状态变化的专业术语是灾难性的生态系统变化；总的来说，它们代表了地球对气候变化最戏剧性的反应。我的研究发现的引爆点是巨大的，从北美延伸到欧亚大陆，面积大约是欧洲的两倍。它将以树木为主的森林分隔开来，这些树木与不同的真菌群(称为菌根)形成共生伙伴关系。超过99%的树木只与两种主要菌根中的一种共生，以获得它们生长和繁殖所需的土壤养分。在加拿大和欧亚大陆辽阔的北方森林中，外生菌根在转折点较冷的一侧占主导地位，营养循环较慢；而在温带和热带森林中，丛枝菌根在营养循环较快的温暖一侧占主导地位。此外，这两个群体似乎在营养循环过程中陷入了拉锯战，外生菌根和丛枝菌根分别减缓和加快了营养循环，以努力改变他们喜欢的环境。气候变化正在打破这场全球拉锯战的平衡，气候变暖，加快营养循环，并使外生菌根树和它们的真菌伙伴超过了它们不能再茁壮成长的临界点。如果全球森林遵循参与这种拉锯战的其他生态系统设定的模式，变化发生的速度可能比之前想象的要快得多，发生在数十年和数百年的过程中，而不是数千年。我的目标是准确预测气候变化对这场大规模的生态拉锯战的影响，导致全球森林变化的速度。具体地说，我将使用包含3100多万个森林普查地块的全球数据集来建立一个数学模型，该模型可以用来确定林木共生环境灾难性转变的速度和后果。这些模型将被用来在灾难性的共生变化发生之前进行预测，识别生态系统即将崩溃的警告信号，并预测由此产生的生态系统功能的损失(和潜在收益)。由此产生的研究还将有助于确定是否可以通过减少温室气体排放来避免这些生态系统灾难。我们的目标是提前了解我们森林不断变化的命运，以便我们能够学习如何与它们共处。

The distribution of life on earth is shaped by climate. Moving north or south from the earth's equator to its polar caps, the sequence of ecosystems spans moist rainforests, dry deserts, seasonally green temperate forests and the vast evergreen forests and frozen tundra. Life in each of these ecosystems is adapted to the challenges that climate imposes, from the extremes of heat and cold that must be survived to the regular cycles of resources that must be exploited. As the earth's climate is changing at unprecedented speed, this regular sequence of ecosystems is being massively reorganised. As a NERC Independent Research Fellow, I propose to study how climate warming is driving transformative changes in the world's largest forested region over decades and centuries, fast enough to witness over the scale of individual human lifetimes.My research indicates that hundreds of billions of trees that span the earth's great northern forests are living on the edge of a climatic tipping point. Tipping points occur when alternative ecosystem types engage in a tug-of-war with their environments, with each ecosystem altering local conditions in a way that allows it to survive. This tug-of-war can create an unstable frontier between warring ecosystems, where small climate changes can rapidly and irreversibly determine the winner. The technical term for these changes in ecosystem state is catastrophic ecosystem shifts; collectively, they represent the most dramatic planetary response to climate change.The tipping point my research has unmasked is massive, stretching from North America clear across Eurasia, encompassing an area roughly twice the size of Europe. It separates forests that are dominated by trees that form symbiotic partnership with different groups of fungi (called mycorrhizas). Over 99% of trees associate exclusively with one of two primary groups of mycorrhiza to acquire the soil nutrients they require to grow and reproduce. Ectomycorrhizas dominate on the colder side of the tipping point where nutrient cycling is slow, in the vast boreal forests of Canada and Eurasia, whereas arbuscular mycorrhizas dominate on the warmer side where nutrient cycling is fast, in temperate and tropical forests. Moreover, these two groups appear locked in a tug of war on the process of nutrient cycling, with ecto- and arbuscular mycorrhizas slowing down and speeding it up, respectively, in an effort to transform their environments to their liking. Climate change is tipping the balance in this global tug-of-war, warming climates, speeding up nutrient cycling, and sending ectomycorrhizal trees and and their fungal partners beyond the tipping point where they can no longer thrive. If global forests follow the model set by other ecosystems engaged in such a tug of war, the transformations could occur much more rapidly than previously thought, occurring over the course of decades and centuries rather than thousands of years.My objective is predict exactly how fast global forests will change as a result of climate change acting on this massive, ecological tug of war. Specifically, I will use global datasets comprising over 31 million forest census plots to build a mathematical model that can be used to determine the speed and consequences of catastrophic shifts in forest tree symbioses. These models will be used to predict catastrophic symbiotic shifts before they happen, identify warning signs of imminent ecosystem collapse, and forecast the resulting losses (and potential gains) in resulting ecosystem function. The resulting research will also help identify whether these ecosystem catastrophe's can be averted by reducing greenhouse gas emissions. The goal is to gain advance knowledge about the changing fates of our forests so that we can learn how to live them with them.