The invasion of an annual exotic plant species affects the above- and belowground plant diversity in deciduous forests to a different extent

Invasive plant species can significantly affect native biodiversity and ecosystem functioning. Even though the majority of ecosystems have more than 50% of the plant biomass belowground, most studies investigating the effects of invasive species on plant diversity focus on the aboveground vegetation. DNA-based methods allow the determination of belowground plant structures. Using these techniques, we examined potential effects of the annual invasive plant Impatiens glandulifera on both the above- and belowground plant species richness and composition in mixed deciduous forests in Northwestern Switzerland. We established 24 plots in three forest areas invaded by I. glandulifera and in three adjacent forest areas, which were not yet invaded. In each plot, we determined plant species richness and abundance in the aboveground vegetation, and collected soil samples at depths of 0-10_cm and 11-20_cm to determine belowground plant species richness. We extracted DNA from fine roots in the soil samples and applied the FAFLP technique (fluorescent amplified fragment length polymorphism) for two different regions of the chloroplast DNA (trnL-trnF intergenic spacer and P6 loop). We established a reference library for all plant species occurring in the study areas to identify the species present in mixed-root samples. Our results showed that I. glandulifera caused shifts in both the above- and belowground plant species composition. Plant species richness was reduced by 30% aboveground in invaded plots, but not belowground in the same plots. Many geophytes and woody species were found belowground but not aboveground in invaded plots. Root biomass was reduced by 35-55% in invaded plots, most probably due to allelopathic compounds released by the invasive plant into the soil. Our field survey shows that above- and belowground plant communities respond differently to the invasion of an annual plant species, and that the invasive species can negatively affect forest ecosystem functions, by reducing root biomass and altering plant species richness and composition.