Legacies of plant invasions - not always a story of doom and gloom

Background/Question/Methods

Invasions by non-native plants can cost billions of dollars, alter ecosystem functions, and reduce native plant diversity, but relatively little is known about associated changes in belowground microbial communities that could influence further invasion and restoration success.  It has been suggested that invasive plants either do not form root symbioses with arbuscular mycorrhizal fungi (AMF) or have a low mycorrhizal dependency, yet AMF increase growth and competitiveness of the two strong invaders leafy spurge (Euphorbia esula) and knapweed (Centaurea stoebe).  We used 454-sequencing to document shifts in AMF communities with plant invasions by sampling from six locations that each harbored sites dominated by the exotics, cheatgrass (Bromus tectorum), knapweed, or spurge, or sites with remnant native grasses and forbs. To assess if observed shifts were due to the invasive plants, we sampled co-occurring native plants and leafy spurge, and to determine how AMF taxa were distributed within plant communities, we sampled individual native grasses and forbs as well as knapweed and spurge from within one location. 

Results/Conclusions

We show significant and consistent differences in AMF communities among plant communities, with cheatgrass and spurge communities being the most divergent. Compared to the native communities, knapweed and spurge invasions had a higher AMF abundance and almost twice as many taxa, whereas cheatgrass invasions harbored a similar AMF richness but a lower abundance.  Cheatgrass also had the most phylogenetically dispersed AMF community, which could be indicative of competition. AMF communities in co-occurring spurge and native plants differed and clustered within their respective communities, which strongly suggests that the shifts observed were driven by the invasive plants and were not a result of pre-invasion conditions. Individual forbs harbored significantly more AMF taxa than grasses, which indicates that the reduced AMF richness in remnant native communities was not due to a co-invasion of AMF and knapweed and spurge, but rather a loss of taxa in the grass-dominated native communities; a loss likely due to the reduction of forbs as a result of the prevalent use of fob-specific herbicides. Overall, our results show that severe invasions of mycotrophic plants can support rich belowground microbial communities, which questions the assumed relationship between above and belowground diversity.  We also show disparate changes in belowground responses to three successful invaders, suggesting that broad generalizations among invasive plants should be avoided in favor of a more trait-based approach.