Successful control of invasive plant species often leads to the question, is restoration necessary?  Leafy spurge (Euphorbia esula) has been successfully controlled by introduced flea beetles (Aphthona spp.) in many areas of the northern Great Plains.  However, previously infested sites do not always return to the desired native plant communities, but rather become havens for other invasive species.  The objective of this study is to understand the legacy of a leafy spurge infestation in both soil and vegetative characteristics.  We conducted greenhouse studies with soils conditioned by three periods of growth of either leafy spurge or a community of native species and measured seedling biomass of native species subsequently planted into these soils.  Soils conditioned by leafy spurge resulted in reduced growth in several species compared with their growth in soils conditioned by native species.  We also established permanent plots at sites in mixed-grass prairie that (1) had never been infested, (2) were previously infested but had returned to native-domination, and (3) were previously infested but were now dominated by other invasive species.  We measured in situ nitrogen mineralization and nitrification rates at these plots during June and August 2007.  Sites that had previously been infested by leafy spurge, regardless of current vegetation, had higher N mineralization and nitrification rates than never-infested sites.  Finally, we compared vegetation types at these plots from a 1980 map with vegetation types described in a 2001 vegetation map of the same area.  Plots that had been classified as warm-season communities in the 1980 map had all been classified as cool-season communities in 2001, regardless of whether or not they had been infested by leafy spurge. 

Results/Conclusions

Although it is clear that leafy spurge has had an effect on soil processes, restoration targets are not clear-cut.  Should we aim for the warm-season communities that existed prior to infestation, even though non-infested sites have shown the same trend in plant community composition?  This is a fundamental question that we must now consider both in terms of management goals and environmental changes that are independent of infestation.  To address this question, we have undertaken extensive experimental plantings of native species within our permanent plots and, with collaborators, are examining not only survival, but also rhizosphere biota and fungal mutualists and pathogens present on roots of these species, in an attempt to tease apart the role of infestation versus environmental change in successful establishment and growth.