Performance and responses to competition in two congeneric annual species: does seed heteromorphism matter?

Variations in seed characteristics observed in heteromorphic species may affect various stages of their life cycles, e.g. seed dormancy, germination characteristics or even adult plant performance. Highly specialised seed morphs - described as colonisers and maintainers - exhibit a trade-off between colonisation capacity and competitive traits. The performance of distinct seed morph progeny under competitive conditions, and especially in multi-species arrangements, had previously not been given much attention. In this study, we compared performance and response to competition among distinct seed morph progenies in two congeneric, co-occurring species: the invasive Bidens frondosa and the non-invasive Bidens tripartita. We hypothesised that maintainer seed morphs of both species would perform better under increased plant densities and within inter-morphic mixtures, while coloniser morphs would show stronger responses to increased densities and perform relatively poorly in inter-morphic mixtures. We conducted a growth trial and a greenhouse experiment, which revealed that seed morph progeny differed significantly in plant height when grown without competition, while under competitive conditions such differences became less apparent. The observed pattern was more strongly pronounced in B. frondosa, which showed a general predominance in stature and biomass over its non-invasive congener. Although seed morphs performed equally well under competitive conditions, increased plant height and more rapid germination can favour the maintainer seed morph on sites where vegetation is already present.