Many plants rely on mutually-beneficial interactions with below-ground fungi called mycorrhizae that gather nutrients, supply water, and defend against pests in exchange for energy provided by the plant. An individual plant may host dozens of different species of mycorrhizae simultaneously, including species that appear to be of little to no benefit, or may be harmful. This project proposes the idea of metabolic bet-hedging to account for the diverse array of mycorrhizae maintained by individual trees. Because trees experience marked seasonal and interannual environmental variation, their nutrient requirements also vary over time, potentially altering the relative value of each fungal partner. As human investors maintain a portfolio of stocks to compensate for economic variation, trees may maintain a range of ectomycorrhizae to respond to changing environments. This project tests the metabolic bet hedging hypothesis using a combination of laboratory experiments and mathematical models.
Results from this study will be of direct value to the management of Douglas fir, the tree species targeted in this study, which is both a commercially important forestry species and an aggressive invader in some foreign locales. Both undergraduate and high school students will participate in the project, receiving valuable training in the scientific method.
Trees are a critical part of many terrestrial ecosystems, providing both physical structure and chemical energy through the process of photosynthesis. In order to conduct photosynthesis efficiently, many trees rely on mutually-beneficial interactions with below-ground fungi called mycorrhizae, which gather nutrients, supply water, and defend against pests in exchange for energy provided by the tree. One group of these fungi, the ectomycorrhizae, is particularly diverse, having a large number of species, each with its own suite of metabolic abilities which determine its value as a trading partner to its host tree. Intriguingly, an individual tree may host dozens of different species of ectomycorrhizae simultaneously, including species which appear to be of little to no benefit, or may even be harmful to the tree. Presumably, rather than limiting its interactions to the subset of most useful fungi, the tree incurs fitness costs (i.e. wastes energy that could be otherwise used for growth and reproduction) by supporting a larger fungal community. Why hasn't natural selection produced a tree that can perfectly control its suite of fungal partners, so that it maintains only those that are most beneficial to it at any given time? One possible explanation is metabolic bet hedging. Trees are long-lived organisms that experience seasonal and interannual environmental variation. Therefore, their nutrient requirements also vary over time, potentially altering the relative value of each fungal partner. So, much as human investors maintain a portfolio of stocks to compensate for economic variation, trees may passively maintain a range of ectomycorrhizae in response to changing environments. This project tested the metabolic bet hedging hypothesis using mathematical approaches that applied economic models to determine optimum tree investment portfolios. These models confirmed that, when the environment is variable (for example, because of seasonal patterns in rainfall or nutrient availability), trees invest in a greater diversity of fungi than they would in constant environments. In addition, this project explored how ectomycorrhizal communities change over time as a function of environmental conditions. Fungal communities were established on Douglas fir (Pseudotsuga menziesii, an important commercial species in the Western United States) host seedlings, and then monitored as the seedlings were exposed to changes in soil and neighbor conditions. The effects of neighboring fungal communities on the development of the ectomycorrhizal community, which can be quite strong in some environmental contexts, highlight the ways that fungal diversity can be maintained or lost over time. These results have implications for management decisions on the maintenance and control of Douglas fir forests.