Germination and colonisation dynamics of Nuphar lutea (L) Sm in a former river channel

Some aspects of the germination and colonisation dynamics of Nuphar lutea (L.) Sm. were investigated in a former river channel of the River Rhone (France) from 1991 to 1995. The colonisation by Nuphar lutea of bare areas of 1 m(2) was a very slow process. The seed production of the species in the study site was at least 600 seeds m(-2) but no seedlings were observed in the site for the 5 years of the experiment. The absence of seedlings could not be explained by unfavourable environmental conditions. The specific weight of the seeds (1.24) and that of the sediment (1.26) are not significantly different. However, the sediment is very loose and easily resuspended in water, so seeds may sink more than a few centimetres into it. Therefore, the absence of seedlings is probably due to the fact that the seeds sink into the sediment to a depth from which seedling emergence is not possible. The dynamics of the Nuphar population in the study site is due only to vegetative propagation by extension of the rhizome system.