Intraspecific AM Fungal Variation Contributes to Plant-Fungal Feedback in a Serpentine Grassland
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AbstractFeedback between plants and arbuscular mycorrhizal (AM) fungi can affect species diversity in plant and fungal communities. Feedback depends on (1) some specificity between plants and fungi and (2) fungi exhibiting specificity either improving (positive feedback) or decreasing (negative feedback) host performance relative to other fungi. Associations between AM fungi and plant species in a serpentine grassland dominated by Andropogon gerardii, Schizachyrium scoparium, Sorghastrum nutans, and Sporobolus heterolepis were examined, and their performance consequences were evaluated. Specificity was determined from AM fungal spore abundance under plants in the field and from trap cultures established by inoculating greenhouse plants with field-collected roots containing fungal material. Seven AM fungal species were unevenly distributed among plant species, with differences in total spore numbers and evenness. In the field, Gigaspora gigantea exhibited specificity to Sporobolus compared to Andropogon and Sorghastrum, and Glomus microcarpum exhibited specificity to Schizachyrium compared to Andropogon and Sporobolus. In trap cultures, Glomus etunicatum exhibited specificity to Andropogon compared to Sorghastrum, and an unidentified species of Glomus exhibited specificity to Sorghastrum compared to Sporobolus. The AM fungal community associated with Smilax rotundifolia, a grassland invader, was not qualitatively different from those of grasses. In the greenhouse, plant and fungal performance was examined as a function of three host plant species (excluding Sporobolus), four AM fungal species, and the same three plant species on which the fungi had been collected in the field. Fungal species affected host plant biomass and fungal performance, measured by AM root length and percentage of colonization. Compared to other fungal species, Gigaspora gigantea increased plant biomass and had a higher percentage of colonization and AM root length on both Andropogon and Schizachyrium. Glomus etunicatum, which exhibits specificity on Andropogon, depressed growth of Andropogon relative to other fungi, implicating negative feedback. Schizachyrium had reduced biomass when inoculated with Glomus microcarpum, its specific fungus. Additionally, fungi originating on a conspecific host depressed plant growth compared to fungi collected from other host species, an effect strongest for Andropogon. Percentage of AM colonization on Andropogon was also greater for fungi originating on Andropogon. The latter two effects serve as another example of negative plant-fungal feedback and could represent intraspecific fungal variation within this community.
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