Regeneration and colonization abilities of aquatic plant fragments: effect of disturbance seasonality

The regeneration (regrowth into viable plants or production of propagules, such as turions or buds) and colonization (development of roots and establishment in the sediment) of fragments of six aquatic plant species (Elodea canadensis Michaux, Hippuris vulgaris L., Luronium natans (L.) Rafin., Potamogeton pusillus L., Ranunculus trichophyllus Chaix, Sparganium emersum Rehmann) occurring in habitats frequently disturbed by floods, were investigated through laboratory experiments conducted in two seasons, and compared to the recolonization patterns depicted after field experiments. Hypothesis was that differences observed between recolonization patterns after spring and autumn flood disturbances should be related to differences in recolonization (via rooting) and regeneration (via propagules) abilities of species fragments. In May and in August, five types of fragments were collected from the plants. Their development and/or rooting abilities were recorded over 10 weeks in the greenhouse. Fragments from E. canadensis and H. vulgaris had higher regeneration and lower colonization abilities in spring and conversely in autumn. Fragments from R. trichophyllus and S. emersum had high colonization and low regeneration abilities during both seasons. Fragments from L. natans developed new buds in spring, whereas root development occurred only in autumn. Fragments from P. pusillus never rooted into the sediment, but developed turions in autumn. Differences between recolonization patterns observed in the field at the two seasons can most often be related to differences in regeneration and colonization abilities of species fragments. Species that colonize disturbed areas rapidly whenever the disturbance by flooding occurs have at least one type of vegetative fragment with a high colonization potential; this is called the 'always-ready strategy' which appears to be an adaptation of aquatic plants to the unpredictability of flood disturbances.