Collaborative Research: Metabolic Bet-Hedging as a mechanism for the maintenance of functional diversity in tree-ectomycorrhizal mutualisms

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This project will test how interactions between trees and soil fungi help trees to adapt to changing environments. In temperate forests, trees rely on partnerships, or symbioses, with belowground fungi that help them to obtain the nutrients they need for growth. These symbioses can be incredibly diverse. For example, Douglas-fir is a commercially valuable tree that can have dozens of different fungal species on its root system at the same time. This project tests the idea that Douglas-fir trees maintain this fungal diversity as a way of adapting to variation in the environment. In other words, much like human investors might maintain a portfolio of stocks, fir trees might maintain a portfolio of fungi to “bet-hedge” against unpredictable changes in their environment. Having many fungal partners at the same time might guarantee that at least some of them are able to provide nutrients to the trees even when the environment is unfavorable. To test this hypothesis, the project uses experiments in greenhouses, mathematical models, and field surveys of fungal diversity. In addition to improving knowledge of how beneficial interactions between species affect natural biodiversity, this project will also provide valuable data on Douglas-fir, a commercially and ecologically important tree in the western United States. The results will help scientists to predict how Douglas-fir and its fungal partners will respond to warmer, drier future climate conditions. The project also provides training to the next generation of American scientists, including fieldwork experiences for local high school students.The Douglas-fir ectomycorrhizal symbiosis meets the bet-hedging hypothesis’s assumptions of (1) long-lived partners that must tolerate variable environments, (2) trait variation across mutualist partners, and (3) evidence of variation in partner quality. This project advances eco-evolutionary theory for mutualisms by combining greenhouse experiments that measure tree investments in fungal partners of different quality, mathematical models that predict portfolio diversity based on environmental variation, and fieldwork to quantify fungal diversity across a gradient of environmental conditions. Collectively, these approaches will allow researchers to (1) quantify optimal tree investment strategies in temporally fluctuating environments, (2) test for evidence of bet-hedging across the distribution of coastal Douglas-fir, and (3) link fungal diversity to host tree performance. The project supports three pre-tenure faculty members, provides training opportunities for postdocs, technicians, and student researchers, and supports course-based research experiences and scientific training for college transfer students and teenage volunteers.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.