Mycorrhizal legacies of plant invasion - not just a story of doom and gloom.

Plant invasions alter ecosystem functions and incur costs in the billions of dollars. While severe invasions reduce native plant diversity, relatively little is known about shifts in belowground microbial communities that could affect further invasions and create soil legacies that impair restoration success. Some recent work supports the reduced mutualism hypothesis and show that some invasive plants either do not form a root symbiosis with arbuscular mycorrhizal fungi (AMF) or have a low mycorrhizal dependency. However, knapweed (Centaurea stoebe) and leafy spurge (Euphorbia esula) are highly colonized by AMF and may gain increased growth and competiveness from the symbiosis, suggesting that broad generalizations among invasive plants should be avoided. Sampling from six locations that each harbored sites dominated by knapweed, leafy spurge, cheatgrass (Bromus tectorum), and native plants, we used 454 sequencing to show that knapweed and leafy spurge invasions have a greater AMF abundance and taxa richness relative to native remnant plant communities, whereas cheatgrass invasions harbor fungal communities of lower abundance and richness. Because AMF communities grouped according to plant community, invasion mediated-shifts appear predictable, and the lower evenness and greater phylogenetic dispersal of AMF taxa within cheatgrass invasions suggests a stronger fungal competition within this plant relative to the others. Whether or not the greater AMF richness in knapweed and leafy spurge invasions is due to a co-invasion of AMF taxa, or a loss of fungal taxa in native communities, is uncertain but could have functional consequences and suggests that native plant communities may not be islands of diversity to colonize restored areas. Overall, we show great differences in belowground responses among invasive plants, which supports the recent plea to move away from the native/non-native dichotomy and instead focus on individual plants and their traits regardless of their origin.