EAGER: Collaborative research: Shifting control from negative plant-microbe feedback to nutrient limitation: predictions from dominant tree traits and ecosystem nutrient economies

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Forest ecosystems are globally important due to their biological diversity and influence on carbon and nutrient cycles. In forests worldwide, fungi and tree roots form a mutually-beneficial relationship that provides food to fungi in return for helping trees gain access to soil nutrients. This relationship is increasingly recognized as a key trait for predicting long-term forest dynamics. Tree species can be divided into two categories based on whether the fungi grow inside the root cells ("arbuscular mycorrhiza") or on root surfaces ("ectomycorrhiza"). This research addresses whether biochemical differences between mycorrhizal tree types have cascading effects on the soil microbial community and other members of the plant community. Specifically, the project will test the novel hypothesis that the type of mycorrhizae formed by the dominant trees in a forest determines whether the trees and other plants are more likely to be limited by nutrients or by disease (pathogens). Ectomycorrhizal trees produce leaf and root tissue that is more difficult to decompose than the tissue from arbuscular mycorrhizal trees, reducing nutrient availability but also the ability of plant pathogens to persist since many pathogens grow on dead plant tissue when between live hosts. Thus, the mycorrhizal type of the dominant tree species is predicted to create a soil environment that either reduces nutrient availability (under ectomycorrhizal trees) or enhances the speed and severity of plant-pathogen interactions (under arbuscular mycorrhizal trees). This project merges ecological theories about drivers of plant populations and communities, providing a powerful general framework that may transform our understanding of how shifts in tree species composition affect future ecosystem dynamics. The project outcomes will also include training the next generation of scientists and a public forest restoration project that will establish plots of differing mycorrhizal tree types, engage volunteers, help educate the public, and contribute to our understanding of forest restoration.The research will involve experimental manipulations and bioassays to test the overarching hypothesis that the relative importance of nutrient limitation and pathogen-mediated negative feedbacks in temperate forests depends on the type of mycorrhizal symbiosis of dominant tree species, which is an indicator for an integrated set of leaf and root traits. Three geographic areas with previously characterized mycorrhizal gradients will be studied. Experiments will involve manipulating nutrient and pathogen abundance to determine the response of adult tree roots and establishing seedlings to the hypothesized limiting factors in different soils. In addition, pathogens in the microbial communities will be characterized through both sequencing and isolation, and prevalent isolates will be used in bioassays to test Koch's postulates and determine their ability to affect plant community assembly.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.