Mycorrhizal fungi determine salt-marsh plant zonation depending on nutrient supply

1. Arbuscular mycorrhizal fungi (AMF) can affect nutrient uptake of associated plants and can vary in function from mutualism to parasitism as nutrient availability increases; thus they may interact with nutrient availability to influence plant community structure. 2. We experimentally investigated the hypotheses that AMF can affect the community structure of salt marshes by affecting plant competitive ability. We focused on: Spartina densiflora, that dominates physically benign high marsh habitats and S. alterniflora, which dominates more stressful low marsh habitats. 3. Colonization by AMF increased S. densiflora growth at low nutrient levels, but reduced growth at high nutrient levels. Spartina alterniflora was not colonized by AMF and showed increased growth only with nutrient amendment. Nutrient or fungicide additions resulted in S. alterniflora migrating to higher marsh elevations, displacing S. densiflora. When nutrient and fungicide additions were made together, however, dominance of S. densiflora was maintained in the high marsh. 4. Synthesis. These results show that AMF can affect the competitive ability of plants and can have a large impact on plant community structure. The community impacts of these symbioses may be especially sensitive to human-induced eutrophication, given that nutrient supply can modulate whether AMF positively or negatively affect associated plants.