Positive feedback between invasive and native plants may contribute to invasive species dominance, although this pattern may not be general for all invasions and has not been well explored in woodland systems. We examined the pairwise feedback relationship between Euonymus fortunei, an emerging invader of North American deciduous forests, and Asarum canadense, a co-occuring native groundcover, to determine whether positive feedback might contribute to Euonymus dominance. In a greenhouse in Bloomington, Indiana, USA, we conditioned live woodland soil via growth with either Euonymus or Asarum, then used conditioned soils to inoculate monocultures and pairwise mixtures of each species. After eight weeks, we harvested plants and measured percent growth. We found evidence for positive plant-soil feedback between Euonymus and Asarum. Euonymus grew better in soil conditioned by conspecifics than in soil conditioned by Asarum, whereas total Asarum growth was not different in soil conditioned by conspecifics vs.Euonymus. These results held whether Euonymus and Asarum grew in monoculture or pairwise competition. This study supports a role for positive plant-soil feedback as a driving factor behind the invasion of Euonymus fortunei. Future work should examine the role of abiotic vs. biotic factors in mediating feedback between Euonymus and Asarum. Furthermore, researchers should examine pairwise feedback between Euonymus and other native species in order to better understand the role of positive feedback in Euonymus invasion. By evaluating the importance of this and other possible invasion mechanisms, we can improve our understanding of invasion ecology while facilitating management of harmful invasive species.
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We would like to thank two anonymous reviewers for helpful comments on our manuscript. Thanks to Stephanie Dickinson of the Indiana Statistical Consulting Center for assistance with statistical analysis, and to Therese Burkhard and Gerald Smith for field and greenhouse assistance. Financial support was provided by an Indiana University Sustainability Research Development Grant.
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