Facilitating the tropical forest carbon sink: The evolution and function of symbiotic N2 fixation

能够从大气中获取氮肥的树木(氮气固定器)在支持热带森林生长和碳储存方面发挥着关键作用，随着气候变化，这种作用可能会变得越来越重要。氮气固定器是否会在未来帮助热带森林储存更多的二氧化碳--从而抵消人类的二氧化碳排放--可能取决于是什么控制了它们在生态系统中的功能和演变。为了更好地了解固定器在气候变化中的未来作用，我们可以通过研究固定器如何应对过去的重大全球变化来了解，推断固定器为什么演变，以及是什么因素导致它们的多样化和在森林和其他生态系统中的传播。我将使用多种方法来解决这个以前没有提出过的问题：1)在巴拿马热带森林中建立一个大规模、长期的施肥实验，以测试营养物质如何限制森林从干扰中恢复，以及固定氮树木促进碳储存的能力；2)利用亚马逊盆地固定物普及率和生物多样性的最大数据集，分析营养物质如何控制固定物；3)将最近重建的固定物的进化史(系统发生)与全球热带地区固定物的流行率和生物多样性的数据集结合起来，以确定养分如何控制固定物的进化和地理分布；以及，4)将定位器的发展史与古代地质和气候事件的回顾配对，以确定是什么促成了定位器的主要进化事件，导致了定位器的现代分布。这种方法有望引导人们对地球历史上修复者的功能有一个新的理解，并更好地预测他们的角色在未来可能会发生什么变化。新的发现将使政策制定者和公众了解热带森林缓冲二氧化碳排放的可靠性，科学家了解生物多样性和植物功能的重要性，以及从业人员了解如何保护和恢复生态系统。

Trees with the ability to access nitrogen fertilizer from the atmosphere (N2 fixers) fill a critical role in supporting tropical forest growth and carbon storage and may become increasingly important as climate changes. Whether N2 fixers will help tropical forests store additional CO2 in the future-thereby offsetting human CO2 emissions-may depend on what controls their function and evolution in ecosystems. To better understand the future role of fixers as climate changes, we can learn by examining how fixers responded to major past global change, deducing why fixers evolved and what factors caused their diversification and spread across forests and other ecosystems. I will address this previously-unposed question using a combination of approaches: 1) establishing a large-scale, long-term fertilization experiment in Panamanian tropical forests to test how nutrients limit forest recovery from disturbance and the ability of N2-fixing trees to facilitate carbon storage; 2) utilizing the largest dataset of fixer prevalence and biodiversity across the Amazon Basin to analyze how nutrients control fixers; 3) combining a recent reconstruction of the evolutionary history (phylogeny) of fixers with a dataset of fixer prevalence and biodiversity across tropical regions globally to determine how nutrients govern the evolution and geographical spread of fixers; and, 4) pairing the fixer phylogeny with a review of ancient geological and climatic events to determine what precipitated the major evolutionary events that led to the modern-day distribution of fixers. This approach is expected to lead a new understanding of the function of fixers throughout earth's history and a better ability to predict how their role may change into the future. New findings will inform policy-makers and the public about the reliability of tropical forests to buffer our CO2 emissions, scientists about the importance of biodiversity and plant function, and practitioners about how to protect and restore ecosystems.