Ectomycorrhizal community recovery following exotic species invasion and restoration in an oak woodland

Background/Question/Methods Invasive plants can have a multitude of impacts on plant communities through their direct and indirect effects on soil chemistry, nutrient pools and fluxes, and ecosystem function.  Considerably less is known about invasion-associated changes in belowground microbial communities, and the subsequent plant-soil feedbacks during the restoration of native systems.  We conducted a multi-year field study to examine the effects of removing an invasive species, Rhamnus cathartica (European buckthorn), on soil N and P fertility and the temporal sequence of ectomycorrhizal (ECM) community recovery in Quercus macrocarpa (bur oak) woodlands. That buckthorn invasions can increase soil nitrogen (N) levels and increasing soil N fertility has negative effects on ECM communities is well recognized.  A key issue is whether these negative effects can be reversed by restoration.  We tested the predictions that removing buckthorn will reduce soil fertility and increase the abundance of ECM, and the extent of ECM recovery will correspond to reductions in soil fertility.

Results/Conclusions We found no significant difference in soil pH or N fertility (total %N, NO₃, NH₄) between invaded and cleared stands; soil N levels in both treatments showed similar seasonal and inter-annual variations.  Conversely, buckthorn removal resulted in a significant decrease in soil P levels.   Buckthorn removal also resulted in significant increases in the abundance of ECM root tips and species richness over time.  Although there was no significant difference in ECM species richness between invaded and cleared stands in the first year, we detected greater mid-season ECM richness (up to 30% higher) in cleared versus invaded stands in the second and third years.  These findings provide partial support for our predictions.  More notably, they highlight the importance of invasive plants in regulating both soil fertility and ECM communities, and implicate soil P levels as a mechanism by which invasive plants can modify the ECM community.