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ECOLOGICAL study of the role of soil microorganisms in vegetation succession has focused mainly on organisms affecting plant nutrition, such as mycorrhiza and nitrogen-fixing bacteria. But, soil-borne diseases are involved in the degeneration of Ammophila arenaria (Marram grass) and Hippophae rhamnoides (Sea buckthorn), two plant species that dominate the coastal foredunes of Europe and are widely planted for sand stabilization. We have used reciprocal transplantation and report here that soil-borne diseases may contribute to the succession of foredune plant species. In pot experiments, plant species that succeed A. arenaria were tolerant of the soil-borne diseases of this species. Plant species that were grown in soils from both previous and later succession stages were reduced most in soils from the later stages. During foredune succession, therefore, plants disappear from sites where the soil has become colonized with specific growth-depressing microorganisms. The soil-borne diseases must have considerable importance for the outcome of interspecific competition and may be involved in patterns of clonal growth. The different sensitivities of plant species for the soil-borne pathogens could be an evolutionary response to selection pressures of the succession stage to which a species is confined by the combined effect of local abiotic and biotic environmental factors.

Ammophila arenaria (marram grass) is the most important plant species for sand stabilization in European coastal foredunes. Stand degeneration due to poor supply of wind-blown sand enhances the susceptibility for wind erosion when successional species do not become established. Replanted A. arenaria often fails to become established. In the present study we examined whether management practices such as mowing, fertilizing, burning or below-ground cutting of plants may be applied to re-establish the vigour of A. arenaria. Field experiments performed at exposed sites, where naturally succeeding plant species are not supposed to become established due to salt spray, showed that none of the applied methods resulted into enhanced tiller density of A. arenaria. Thus, further studies are necessary to solve this type of management problem. At the leeward side of foredunes, the successional species Festuca rubra ssp. arenaria and Elymus athericus could be established successfully as both pre-grown seedlings and planted bundles of cuttings (all further experiments were fertilized). When planted in spring, cuttings of successional plant species established less than those planted in early winter. Water repellency of the sand surface is supposed to be the main cause for this. Pre-grown seedlings were less susceptible for the season of planting. Direct sowing was not effective. Rabbit browsing had to be omitted to obtain successful establishment. When living rhizomes of successional plant species were still present in the soil profile, fencing alone turned out to be effective to re-establish vegetation at initially bare sites