Residence time and functional traits shape conspecific plant–soil feedback but not inferred coexistence patterns of alien and native species

Plant–soil feedback (PSF) describes changes in a plant's performance when growing in soil previously occupied by another plant, often driven by shifts in the soil biota. PSF is frequently negative when plants grow in soil conditioned by individuals of their own species (conspecific PSF). If for a given species pair, both perform worse in their own relative to in each other's soils, such negative net pairwise PSF may contribute to species coexistence. PSF likely influences long‐term invasion success of alien plants: While recently introduced species may benefit from a release of soil antagonists, over time such may accumulate, resulting in increasingly negative PSF. We investigated the role of soil antagonists (mostly pathogens) and mutualists in shaping PSF across species from an early stage of invasion up to species that have resided in Germany for thousands of years. We considered plant functional traits as potential factors influencing the magnitude of PSF. In a two‐phase PSF experiment, we tested 30 alien and native Asteraceae and three native species against each other. We tested performance (seedling establishment, biomass, seed production) of ca. 1000 individuals growing in conspecific, heterospecific and control soils. Recently introduced species produced more biomass in conspecific than in control soils, but this effect decreased with increasing residence time. Functional traits explained the strength and direction of conspecific PSF. However, neither residence time nor most traits explained net pairwise PSF. Synthesis: Our results support the hypothesis that aliens benefit from enemy release early in the invasion process, but this advantage diminishes over time. Plant performance generally improved in conditioned soils, suggesting benefits from mutualists, although there was considerable variance among species. The effect of soil conditioning was influenced by species' functional traits related to resource acquisition and defence/collaboration strategies. By contrast, net pairwise PSF was not clearly related to residence time or functional traits, indicating limited explanatory power of these predictors for variation in inferred coexistence patterns of alien‐native species. Although our findings provide insights into the long‐term dynamics of plant invasions, predicting PSF remains challenging but important for understanding and managing threats of invasions to biodiversity.