Red alder is a fast-growing hardwood tree that is found from California to Alaska. It plays an important role in lumber production, biofuel, erosion control, and river-shading to protect fish habitat. Like all alders, it forms a symbiosis with nitrogen-fixing bacteria that live in specialized root nodules. These bacteria convert atmospheric nitrogen, normally unavailable to plants, into a form that red alder can use, allowing it to grow in extremely poor soils and enrich these soils over time with nitrogen and organic matter. While the existence of this plant-microbe symbioses has long been recognized, it is virtually unknown which factors determine the variability, outcome and productivity of this interaction. It is also unclear whether these microbes influence their tree host in other ways, for example by producing defensive plant compounds that protect the tree from pests. This project will fill this knowledge gap and will functionally explain why some alder-bacteria partnerships are more productive and resilient than others. In addition to advancing our basic knowledge of plant-microbe relationships, the project will have broader benefits for society. It will support the economic use of alder species in forestry and restoration, and it will serve as a teaching tool to introduce K-12 students to the hidden world of plant-microbe symbioses. High school and university students will receive hands-on training in advanced lab techniques, helping them develop cutting-edge skills