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

Some exotic plants become invasive because they partially release from soil-borne enemies and thus benefit from positive plant-soil feedbacks (PSFs) in the introduced range. However, reports that have focused only on PSFs may exaggerate the invader's competitiveness. Here, we conducted three experiments to characterize plant-soil-foliage feedbacks, including mature leaves (ML), leaf litter (LL), rhizosphere soil (RS) and leaves plus soil (LS), on the early growth stages of the invasive plant Ageratina adenophora. In general, the feedbacks from aboveground (ML, LL) adversely affected A. adenophora by delaying germination time, inhibiting germination rate and reducing seedling growth. The increased invasion history exacerbated the adverse effects of LL and LS feedbacks on seedling growth. These adverse effects were partially contributed by more abundant fungi (e.g. Didymella) or/and more virulent fungi (e.g. Fusarium) developed in the aboveground part of A. adenophora during the invasion. Interestingly, the aboveground adverse effects can be weakened by microbes from RSs. Our novel findings emphasize the important role of aboveground feedbacks in the evaluation of plant invasiveness, and their commonness and significance remain to be explored in other invasive systems.