Seed Bank Dynamics and Coexistence of Annual Macrophytes in a Temporary and Variable Habitat

Abstract

The seed bank dynamics of annuals in two Mediterranean temporary marshes were studied in order to investigate the impact of the disturbance regime on coexistence. The disturbance regime is made up of drought periods 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 diaspores in the sediment varied markedly from year to year but always remained at a high level for all species (between 73000 and 800000 per m 2 ). These perennial seed banks allowed the populations to be maintained despite two successive years without any reproduction. In experimental conditions, the seed bank was not depleted after the sediment samples had been submerged during 5 successive periods. The majority of the seedlings emerged as soon as the first period of flooding and temporary droughts stimulated the germination of Zannichellia spp. and Ranunculus baudotii. The decrease in the number of seeds buried in the soil was essentially due to germination. This was stronger when the seeds were recently produced and situated at the surface of the sediment. The seed density varied only slightly with the depth of the marsh, except in the case of Ruppia maritima. The seed bank is highly variable on a spatial microscale but 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 the ability to propagate vegetatively which is shown by all species studied. Similarly, a high vegetative biomass does not guarantee a high reproductive output as the droughts occur in a density-independent way. The success of a species is related to the interaction between its life history traits and the environmental conditions met each year. The macrophyte communities we studied function in non-equilibrium, with patterns close to those described in lottery models of coexistence. Establishment, growth and reproduction fluctuate strongly from one year to another for all species, in a partially asynchronous way. This would enable competitive exclusion to be indefinitely delayed. The seed bank introduces a storage effect by magnifying the effect of the favourable years in comparison with the bad years.