VERTICAL STRUCTURE OF SEED BANKS AND THE IMPACT OF DEPTH OF BURIAL ONRECRUITMENT IN 2 TEMPORARY MARSHES

The structure of the seed bank (including Chara oospores), in relation to depth within the sediment and disturbance, was studied in two Rhon e 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 se diment by animals, the vertical repartition of seeds at site 2 was mor e pronounced than outside the exclosure. In experiment 1, the emergenc e capacity of seeds from different depths and buried under layers of s terile equivalent to those in the field was measured. Depending of the species, 22 to 98% of the seeds germinated from unburied seeds in the top 2 cm. Only 1% of the oospores of Chara (from site 2) at 2 to 4 cm depth in the sediment emerged. In experiment 2, surface seed bank sam ples were placed under 0, 2 or 4 cm sterile sediment depth. The sample s contained numerous recent seeds and the emergence percentage reached 41% (for Ruppia maritima). Only the seeds of Zannichellia spp failed to germinate from a depth of 2 cm or more. The emergence percentage fr om 2 cm depth or more was always lower than at the surface. These expe riments showed that both burial and ageing of seeds decrease germinati on capacity. The majority of the active seeds located at the surface g erminate when the marsh is flooded. Seeds located between 2 and 4 cm c an 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.