Question: Do species from communities with different flooding dynamics differ in seed buoyancy? Is there a trade-off between seed buoyancy and seed longevity?

Methods: Seeds of 55 freshwater wetland species were collected and related to communities along the hydrological gradient, ranging from constantly flooded reed beds, through tall herb fens and rich fens, to rarely flooded wet meadows. Species were experimentally tested for seed buoyancy over 210 days in standing water and moving water. Seed longevity for each species was calculated.

Results: Community occurrence along the hydrologic gradient was mirrored by seed buoyancy. Seed buoyancy was highest for species of almost permanently inundated reed beds and lowest for species of rarely inundated wet meadows. The seed buoyancy characteristics of species of reed beds were best adapted to hydrological dynamic conditions mimicked by the treatment moving water and were the most independent of the flow rate of the water. A high percentage of the species used had a transient seed bank. Together with high buoyancy in many species, this suggests a trade-off between dispersal capacity and seed persistence.

Conclusions: In freshwater wetlands with rather stable water levels, many species lack a persistent seed bank and depend on dispersal for colonization of new habitats. Seed buoyancy enhances aquatic seed dispersal, and can be of great importance in both vegetation dynamics and restoration. In wetlands that are inundated for a long period during the year and with a water level above soil surface, higher seed buoyancy enhances the possibility for hydrochory.

Nomenclature: van der Meijden (1996) for phanerogams; Schaminée et al. (1995, 1996) and Stortelder et al. (1998) for syntaxa.