Temporal dynamics of plant-soil feedback and root-associated fungal communities over 100years of invasion by a non-native plant

Pathogens can accumulate on invasive plants over time, which could lead to population declines. The time required for these dynamics to occur is unknown and seldom addressed. Furthermore, no study has assessed plant-soil feedback while characterizing plant pathogen and mutualist root fungal communities in the context of invasion time. We used a plant-soil feedback study and 454 pyrosequencing to investigate pathogen accumulation over 100years on a highly invasive plant in eastern North America that shows localized declines, Vincetoxicum rossicum (Apocynaceae). We collected soil from five sites representing each of four invasion periods of V.rossicum across Ontario, Canada (old, c.100years; intermediate, 50-60years; young, < 12years; and uninvaded), and grew V.rossicum in these soils in a glasshouse study. Our hypothesis was that plants grown in soils invaded for longer periods of time would experience less positive feedbacks compared to those grown in more recently invaded or uninvaded soils. We collected roots of V.rossicum from the invasion periods and performed 454 pyrosequencing targeting fungi. We hypothesized that the abundance and richness of fungi that are known plant pathogens would be higher in roots from older invasions compared to more recent invasions. Contrasting with our hypothesis, V.rossicum experienced overall growth promotion due to soil biota, regardless of invasion period. Vincetoxicum rossicum roots were colonized by a large number of fungal taxa, including many known plant pathogens or mutualistic arbuscular mycorrhizal fungi. However, we found no evidence of pathogen accumulation in older invaded sites in terms of species composition, richness or abundance.Synthesis. Our consistent results in the glasshouse and the field highlight the strength of combining high-throughput sequencing data with plant-soil feedback experiments. We showed that the roots of Vincetoxicum rossicum (Apocynaceae) were colonized by many fungal taxa, but found no evidence for changes in plant growth or accumulation of fungal pathogens with longer invasion time. High pathogen loads may not lead to concurrent declines in invasive plants. Plant invasions, as demonstrated by V.rossicum, may be unpredictable in their ability to accumulate pathogens capable of leading to population declines.