Invasive plants decrease arbuscular mycorrhizal fungal diversity and promote generalist fungal partners.

Invasion by exotic plants is a major threat to ecosystem biodiversity globally. Although mutualistic belowground associations can play a significant role in successful invasion, studies have shown mixed results regarding the effects of plant invasion on arbuscular mycorrhizal fungi (AMF). Here, we tested how exotic dominance (i.e. invasion extent) in plant communities impacts arbuscular mycorrhizal fungal community diversity, composition, and generalism at the scale of individual plants and plant communities, and whether these impacts are explained or moderated by plant root traits (root diameter). We characterized root traits and arbuscular mycorrhizal fungal communities in the roots of native and exotic plants growing together in experimental plant communities that ranged in invasion extent (exotic plant dominance from 0% to 100%). Increases in exotic dominance decreased arbuscular mycorrhizal fungal diversity in both individual plant roots and plant communities. Greater relative abundance of generalist AMF was also observed in plant communities with increasing exotic dominance. Although root diameter affected fungal composition, it did not moderate or drive the effect of exotics. Our results highlight the role of invasion extent in understanding how arbuscular mycorrhizal fungal communities respond to exotic plant invasion and the importance of preserving belowground biodiversity.