Context-dependency of mutualism: Evaluation of responses of annual prairie plants to mycorrhizal fungi

Background/Question/Methods   Mutualists such as pollinators, seed dispersers, and mycorrhizal fungi have been recognized as important in conservation and restoration of their partner plants. However, the benefits that mutualists give may depend on abiotic conditions. In this study, we used seven different species of prairie plants to investigate whether the benefit that mycorrhizal fungi give to plants are affected by water availability. We predicted that mycorrhizal plants would benefit more than non-mycorrhizal plants, but only under water-limited conditions when fungal hyphae can provide water by drawing water from a larger volume of soil. We used five species of annual prairie plants in our experiments because benefits to perennial prairie plants from arbuscular mycorrhizal fungi have been shown in previous research, but the effect of fungi on prairie annuals has not been investigated. We conducted a factorial designed experiment investigating the effect of inoculation by mycorrhizal fungi under conditions of high and low water availability. We explored the context-dependency of benefits to plants in growth, survival, and reproductive success in a greenhouse experiment. Effects of mycorrhizal fungi on annual plants were also compared with effects on a perennial forb, Coreopsis palmata, and a perennial species of grass, Agrostis hyemalis.

Results/Conclusions   The effects of mycorrhizal fungi on plants varied from neutral to beneficial and whether there were context dependent effects depended on the species of plant. The perennial grass, Agrostis hyemalis, and the annual Plantago virginica did not benefit from mycorrhizal fungi under either high or low water availability. Other species only benefited under conditions of low water availability, indicating that the benefits were context-dependent. Our results suggest that mycorrhizal fungi may not be as important to the restoration of native prairie annuals as it is to native perennials.