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

<b>Background</b>: 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 (<i>Jacobaea vulgaris</i>) 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. <b>Aim</b>: We evaluated the potential of <i>J. vulgaris</i> to suppress germination in common and rare grassland species by allelopathy. <b>Methods</b>: In a germination chamber experiment, we exposed 22 species and <i>J. vulgaris</i> 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. <b>Results</b>: Leachate reduced germination percentage, germination time, synchrony and radicle length. These effects were similar for <i>J. vulgaris</i>, indicating autotoxicity. Rare species germinated less than common species but were not more sensitive to the phytotoxic effects of <i>J. vulgaris</i>. Restraining effects of the leachate were similar to the mere osmotic effect. <b>Conclusion</b>: Our results question allelopathy as the main driving mechanism behind <i>J. vulgaris</i> gaining dominance. However, the impact of <i>J. vulgaris</i> might depend on the composition of the invaded plant community due to species-specific effects.