Positive feedback may drive invasion by Euonymus fortunei

Background/Question/Methods

Euonymus fortunei, an exotic vine commonly known as purple wintercreeper, is an emerging invader of eastern deciduous forests.  This popular horticultural plant forms dense carpets over woodlands, where it suppresses the growth of native plants.  Previous studies have not tested mechanisms of Euonymus invasion, but one possible mechanism is positive plant-soil feedback.  Positive feedback occurs when a plant “conditions” the soil in a way that promotes its own growth without promoting growth of heterospecific plants.  Previous work from our lab demonstrated that the soil microbial composition associated with Euonymus differs from that associated with a co-occurring native groundcover, Asarum canadense. These results are consistent with a role for soil feedback in the dynamics of these species.  

We conducted an experiment to detect feedback between Euonymus and Asarum from three invaded sites. We conditioned replicate pots of live woodland soil with either Euonymus or Asarum for sixteen weeks.  We used conditioned soils to inoculate monocultures and pairwise mixtures of Euonymus and Asarum in a full factorial design (two inocula x three planting types).  After eight weeks of growth, we harvested above and belowground biomass and measured percent growth for each plant.  

Results/Conclusions

We found significant evidence for net positive plant-soil feedback between Euonymus and Asarum.  Euonymus grew significantly better in soil conditioned by a conspecific than in soil conditioned by Asarum (82% growth in own soil, 34% growth in Asarum-conditioned soil; p=0.002).  Asarum growth was not significantly different in soil conditioned by conspecifics vs. Euonymus (p=0.99). These results held regardless of whether Euonymus and Asarum grew in monoculture or in pairwise competition.  This study suggests a role for positive plant-soil feedback as a driving factor behind the invasion of Euonymus fortunei into temperate woodlands.  Future work should be conducted to detect 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.