Vertical structure of seed banks and the impact of depth of burial on recruitment in two temporary marshes.
Résumé
The structure of the seed bank (including Chara oospores), in relation to depth within the sediment and
disturbance, was studied in two Rh6ne delta temporary marshes for two years. The seeds of all species
were concentrated in the top 2 cm of sediment with very low numbers beeing found below 4 cm. When
an exclosure eliminated disturbances of the sediment by animals, the vertical repartition of seeds at site
2 was more pronounced than outside the exclosure.
In experiment 1, the emergence capacity of seeds from different depths and buried under layers of
sterile equivalent to those in the field was measured. Depending of the species, 22 to 98 ~o of the seeds
germinated from unburied seeds in the top 2 cm. Only 1 ~o of the oospores of Chara (from site 2) at 2
to 4 cm depth in the sediment emerged.
In experiment 2, surface seed bank samples were placed under 0, 2 or 4 cm sterile sediment depth.
The samples contained numerous recent seeds and the emergence percentage reached 41 ~o (for Ruppia
maritima). Only the seeds of Zannichellia spp failed to germinate from a depth of 2 cm or more. The
emergence percentage from 2 cm depth or more was always lower than at the surface. These experiments
showed that both burial and ageing of seeds decrease germination capacity.
The majority of the active seeds located at the surface germinate when the marsh is flooded. Seeds
located between 2 and 4 cm can be brought back to the surface by disturbances and play the role of a
reserve involved in maintenance of populations that go without seed production for one or some years.