Optimal investment in a multi-mutualist system: Tree maintenance of ectomycorrhizal fungal diversity

Background/Question/Methods

Many tree species form mutualistic partnerships with a group of belowground fungi known as ectomycorrhizae. The maintenance of these partnerships depends upon tree payments of photosynthetically fixed carbon to the fungi. In return, the fungi provide nutrients, water, and pathogen defense services to the host tree. Interestingly, an individual tree may host dozens of species of ectomycorrhizae simultaneously, including fungi that appear to be less beneficial than other community members at that time. Empirical evidence suggests that some of this diversity may be explained by niche differences among fungi, with some species better able than others to access particular nutrient pools, provide pathogen defense, and so on. Here, we use optimal control theory in combination with bioeconomic models to examine the importance of temporal variation to the maintenance of fungal diversity. In particular, we ask whether a tree that experiences environmental variation might ”bet hedge,” by investing in a suite of fungi more diverse than its present environmental settings dictate because future conditions might require ready access to other partners.

Results/Conclusions

We find that host tree investment changes as a function of environmental variability. In particular, our results suggest that tree investments should be more diversified when environments are variable. Strikingly, under certain periodic environmental conditions, optimal tree investment strategies actually include investment in fungi that are relatively “worse” partners (i.e., offer lower returns on investment to the host tree), even when these partners would not be maintained in constant environmental conditions. These results support the idea that environmental variability is likely to be an important component in enhancing mycorrhizal diversity in real-world systems.