Enemy of my enemy: evidence for variable soil biota feedbacks of Vincetoxicum rossicum on native plants.
Interactions between plants and soil biota can be major drivers of plant invasions. The 'Enemy of my enemy' hypothesis (EEH) predicts that native plants are more negatively impacted by enemies (e.g. pathogens) that accumulate and spillover from the invader's rhizosphere than the invaders themselves, thereby facilitating the invasion. We tested EEH using Vincetoxicum rossicum, one of the most invasive plant species in Eastern North America, and a total of eight co-occuring native plant species. Specifically, we predicted that the native species would grow less in presence of soil biota associated with V. rossicum than that from uninvaded soils. Field soils were collected from four V. rossicum invaded and neighboring uninvaded areas for use in two experiments. In Experiment 1, V. rossicum and each of five native plant species were grown in a plant growth chamber for 3 months, either in presence or absence of soil biota from the invaded and uninvaded soils. We found that only one species (Solidago canadensis) supported EEH. In contrast, the legume Desmodium canadense more than doubled its biomass in response to V. rossicum-associated soil biota. In Experiment 2, four native legume species, including D. canadense, were examined for EEH in the same manner as in Experiment 1. Only one species (Lespedeza hirta) supported EEH and, again, V. rossicum-associated soil biota was advantageous to D. canadense. In summary, we found inconsistent support for the EEH between V. rossicum and native co-occurring species in Eastern North America. We conclude that the accumulation of rhizosphere enemies in native plants may not be a major factor in the invasive success of V. rossicum.