Does introduced European Phragmites australis experience belowground microbial enemy release in North America?

Escape from native range enemies can give invasive species a competitive edge according to the enemy-release hypothesis. While more commonly associated with predators and herbivores, release from belowground microbial antagonists has been recently demonstrated to benefit invasive plants. Biogeographic variation in dominance and comparisons of soil communities suggest that invasive European Phragmites australis may have also benefitted from belowground enemy release in North America (NA). Here we examine the effects of native range (Europe) versus introduced range (NA) soil communities on European and North American P. australis using a reciprocal inoculation seedling growth experiment. Contrary to the enemy-release hypothesis, we found that North American P. australis was sensitive to soil community origin in that the seedlings grown in European soil communities had higher total biomass than seedlings grown in North American soil communities. This pattern was not observed in the European P. australis seedlings which had similar biomass when grown with North American or European soil communities. Notably, North American P. australis had higher biomass than European P. australis regardless of which soil community it was grown in, suggesting a growth–defense tradeoff. Though the relative abundance of mutualists and pathogens composition did not differ between the two ranges, an indicator analysis revealed that mutualistic fungi and bacteria were key components of European soil communities but not in North American communities. Interestingly, North American soil communities had lower β diversity than European communities suggesting higher levels of community conservation among North American populations. This research represents the first evidence of growth–defense trade-offs in North American P. australis and offers a novel mechanism in understanding the invasion of P. australis in NA.