DOI: 10.5091/plecevo.205605

Authors: Bram D’hondt, Senne Brutsaert, Wannes Dermout, Axel Neukermans, Jo Packet, Tim Adriaens

Abstract: Background and aims – Crassula helmsii is a highly invasive aquatic plant in Europe, subject to international policy measures (EU Regulation 1143/2014). While its vegetative reproduction is well-documented, the role of sexual reproduction and seed ecology remains poorly understood. This study aimed to determine the germination capacity of C. helmsii seeds and revisit its potential for dormancy and seed bank formation. Material and methods – We conducted in vitro germination trials using plant material collected from ten populations in Belgium. Following cold storage (winter conditions), seeds were sown on agar, and germination was tracked. In a comparative trial, flowers from the same populations were sown on a soil substrate, with germination estimated based on the number of seeds per flower—the only method previously used to study the species. Key results – Crassula helmsii populations exhibited high germination rates, with an estimated overall mean of 68.5%. When excluding seeds that proved non-viable, germination was nearly complete (95.7%). The flower-based trial yielded lower germination (13.8%), consistent with earlier results. This lower emergence is likely real, not an artifact, and suggests environmental constraints that are relevant under more realistic germination conditions. Conclusion – Our study demonstrates that C. helmsii populations in Belgium produce numerous viable seeds that germinate readily. Considering additional evidence from the Crassulaceae family, we hypothesize that C. helmsii is self-compatible, positively photoblastic, exhibits physiological dormancy broken by chilling, and is capable of forming persistent seed banks. The species can thus combine asexual (clonal growth and dispersal) and sexual strategies (seed dispersal, dormancy, and seed bank formation). These insights have important implications for the management of this invasive species.

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DOI: 10.5091/plecevo.104640

Authors: L. Marie Ende, Lukas Hummel, Marianne Lauerer

Abstract: Background and aims – The cup plant (Silphium perfoliatum) is being grown in Germany as a promising new bioenergy crop with an increasing area under cultivation in the last years. Its alien status, its high productivity, and high reproductive potential could carry the risk of this species becoming invasive. The present study investigates the dispersal and persistence of cup plant seeds, to contribute to the assessment of its invasive potential. Material and methods – For this purpose, four experimental studies were conducted in Germany, Central Europe: wind dispersal distance was measured in a field experiment for wind speeds up to 7 m.s-1. The seeds were offered to rodents in different habitats near to a cup plant field. We observed seed persistence and germination over 4 weeks storing in water and over 4 years storing in different soil depths. Key results – Cup plant seeds are dispersed by wind only over a few meters. In the forest, rodents removed 100% of the offered seeds, in open habitats none. Independent of the duration of storage in water, germination rate of the cup plant seeds was constantly high. Most of the seeds already germinated in water in the first two weeks. Stored on the soil surface and at 10 cm soil depth, the seeds germinated already in the first two years. Stored at 30 cm depth, one third of the seeds retained their germination ability over four years. Conclusion – Wind serves as short-distance dispersal vector for cup plant seeds. Rodents remove the seeds, but it is unknown whether they disperse them or just eat them. Water could disperse the seeds, which retain their germination ability, over long distances. The cup plant could therefore spread and possibly become invasive in Central Europe, and therefore measures are suggested to prevent its dispersal and spontaneous settlement.

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