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

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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.