Many systems experience invasion by multiple invasive plant species, spurring interest in understanding how invasive species interact with other invaders. Plant-soil feedbacks have been implicated in single species invasions, yet little studied for co-occurring invasive species. Enemy escape dynamics might lead to neutral feedback among invasive species (assuming all species escape host-specific enemies) while novel weapons dynamics might lead to positive feedback dynamics (if species are harmed by one another’s allelochemicals). Nutrient dynamics might be expected to lead to neutral feedback (given a nitrogen-fixing invasive that promotes all invasives) or negative feedback (if invasive species are niche-partitioned in nutrient use). Cirsium arvense (a forb), Lotus corniculatus (a nitrogen-fixer), and Phalaris arundinacea (a grass) commonly co-invade tallgrass prairie. We employed a two-phase pairwise plant-soil feedback experiment to investigate plant-soil feedbacks among these species. In Phase I, live prairie soil was trained for 20 weeks by monocultures of each invasive species. In Phase II, seedlings of each invasive species were individually-grown in each type of Phase I-trained soil. After growing for 13 weeks, aboveground biomass was harvested and weighed by species. Interaction coefficients were calculated to determine the sign and magnitude of pairwise feedback for each pair of species.
Results/Conclusions
The interaction coefficients for all species combinations were not significantly different from zero, indicating neutral plant-soil feedback dynamics. These results are consistent with enemy escape dynamics (where all three invasive species have escaped host-specific enemies) or to nutrient dynamics mediated by facilitative effects of the nitrogen-fixing invasive. Further investigation to tease apart the relative contributions of biotic vs. abiotic factors to feedback dynamics among co-occurring invasive species is warranted, and could help elucidate invasion dynamics and promote more effective invasive management strategies.