Plant-soil-foliage feedbacks on seed germination and seedling growth of an invasive plant Ageratina adenophora

Some exotic plants become invasive because they partially release from soil-borne enemies and thus benefit from the positive plant-soil feedbacks (PSFs) in the introduced range. However, these reports that have focused only on PSFs may exaggerate the invader’s competitiveness. Here, we conducted a series of plant-soil-foliage feedbacks (PSFFs) experiments in a greenhouse to determine the consequences of above- and belowground feedback on the early growth stages of an invasive plant, Ageratina adenophora. Surprisingly, most feedbacks from aboveground (mature leaves and leaf litter) had negative effects on A. adenophora’s seed germination and seedling growth. With increased invasion time, a more diverse fungal community (e.g., Didymella) or/and more virulent fungi (e.g., Fusarium) developed aboveground and partially contributed to this negative feedback, particularly in leaf litter. Interestingly, the adverse effects can be weakened by microbes from the rhizosphere soil, particularly those from sites with 80 years of invasion, indicating that the pathogenic microbes aboveground, as well as the counteraction of beneficial microbes belowground against pathogenic microbes aboveground, were enhanced with increased invasion time. These novel findings emphasize the important role of leaf feedback in the evaluation of plant invasiveness, and its commonness and significance remain to be explored in other invasive systems.