Allelopathic vs. autotoxic potential of a grassland weed—evidence from a seed germination experiment

Background: Mechanisms affecting invasiveness of non-indigenous species have received much attention. Few studies have investigated invasions by native plants. Invasive native species such as common ragwort (Jacobaea vulgaris) may become noxious weeds. They challenge farming and nature conservation by outcompeting fodder plants or rare herbs. One mechanism that can result in outcompeting plants is by allelopathy. Aim: We evaluated the potential of J. vulgaris to suppress germination in common and rare grassland species by allelopathy. Methods: In a germination chamber experiment, we exposed 22 species and J. vulgaris itself to ragwort leachate. We controlled for osmotic effects by germination tests in mannitol solution and water. We assessed germination percentage—time —synchrony and radicle length. Results: Leachate reduced germination percentage, germination time, synchrony and radicle length. These effects were similar for J. vulgaris, indicating autotoxicity. Rare species germinated less than common species but were not more sensitive to the phytotoxic effects of J. vulgaris. Restraining effects of the leachate were similar to the mere osmotic effect. Conclusion: Our results question allelopathy as the main driving mechanism behind J. vulgaris gaining dominance. However, the impact of J. vulgaris might depend on the composition of the invaded plant community due to species-specific effects.