SEED BANK DYNAMICS AND COEXISTENCE OF ANNUAL MACROPHYTES IN A TEMPORARY AND VARIABLE HABITAT

The seed bank dynamics of annuals in two Mediterranean temporary marsh es were studied in order to investigate the impact of the disturbance regime on coexistence. The disturbance regime is made up of drought pe riods in the marshes and of predation. The density of diaspores buried in the sediment was measured in 4 consecutive years together with the biomass produced during two growing seasons. The total density of dia spores in the sediment varied markedly from year to year but always re mained at a high level for all species (between 73000 and 800000 per m (2)). These perennial seed banks allowed the populations to be maintai ned despite two successive years without any reproduction. In experime ntal conditions, the seed bank was not depleted after the sediment sam ples had been submerged during 5 successive periods. The majority of t he seedlings emerged as soon as the first period of flooding and tempo rary droughts stimulated the germination of Zannichellia spp. and Ranu nculus baudotii. The decrease in the number of seeds buried in the soi l was essentially due to germination. This was stronger when the seeds were recently produced and situated at the surface of the sediment. T he seed density varied only slightly with the depth of the marsh, exce pt in the case of Ruppia maritima. The seed bank is highly variable on a spatial microscale bur no aggregation pattern could be demonstrated . In the field, the biomass of a population generally appeared not to be limited by the availability of the diaspores, notably because of th e ability to propagate vegetatively which is shown by all species stud ied. Similarly, a high vegetative biomass does not guarantee a high re productive output as the droughts occur in a density-independent way. The success of a species is related to the interaction between its lif e history traits and the environmental conditions met each year. The m acrophyte communities we studied function in non-equilibrium, with pat terns close to those described in lottery models of coexistence. Estab lishment, growth and reproduction fluctuate strongly from one year to another for all species, in a partially asynchronous way. This would e nable competitive exclusion to be indefinitely delayed. The seed bank introduces a storage effect by magnifying the effect of the ''favourab le years'' in comparison with the ''bad years''.