Plant-specific soil-borne diseases contribute to succession in foredune vegetation

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.