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

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.

The research contained within this proposal will produce a number of societal and economic impacts. Determining the role of N2-fixing trees in tropical biogeochemical cycles, both past and present, will help to identify how their functional role will change in the future and how they will support a carbon sink in tropical forests as climate changes. The new information will support public policy making, inform conservation and restoration efforts, improve quality of life and health and impact economic activity.

Decision makers about public policy will benefit from our improved understanding of whether mechanisms exist to facilitate a tropical forest CO2 sink. The proposed research will help to determine whether we can count on N2-fixing trees to supply the nitrogen needed for tropical forests to grow more and store more carbon in response to rising CO2 levels. If we positively identify this mechanism, then scientists will be able to improve predictions about the size of the terrestrial CO2 sink, which will reduce uncertainty about the environmental and social costs of carbon that humans emit. This will in turn help to reduce the uncertainty about setting a price on carbon. In addition, policy-makers will be better informed about the importance of protecting forests and biodiversity, through policies such as REDD+.

The findings will inform conservation and restoration efforts by providing evidence of the role of N2 fixers in forests, including their role in supporting forests from recovery and response to climate change. Conservationists will make improved decisions about what forests to conserve and about the importance of protecting biodiversity and ensuring the presence of N2 fixers. People restoring degraded ecosystems will select species (e.g., particular species of N2 fixers) for a particular environment based on scientific evidence to improve restoration efforts and the quality of the restored habitat.

Human quality of life and health will also be improved because the findings will help us to understand what the future environment will be like with regards to climate change. This could lead to improved social welfare and national security because there will be less uncertainty about what the future climate will be like and what our options are with regards to protecting our future and reducing potential future conflicts.

Finally, this research will have economic impacts because understanding the basic mechanisms of how N2 fixers function within biogeochemical cycles can help improve agricultural production, particularly in developing countries where agricultural practices are currently sub-optimal. By understanding when and why N2-fixing trees bring in additional nitrogen fertilizer from the atmosphere can inform farmers about improving agricultural practices such as through intercropping with N2-fixing trees or crops, and improving agricultural efficiency and minimizing agricultural damage via fertilizer application by rotating crops and using N2 fixers as an alternative source of nitrogen.