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Ammophila arenaria is a dominant sand-fixing plant species of the European coastal foredunes. It remains vigorous under regular burial conditions on seaward slopes, but starts to degenerate when sand accumulation diminishes. Several hypotheses have been put forward to explain this degeneration. In this study, we test the hypothesis that upward growth of plants following sand burial enables them to escape harmful soil organisms. Plants in a degenerating field stand of A. arenaria and potted plants grown in sterilized sand (outdoor pot experiment) were buried with sterilized or nonsterilized sand. Burial in both sterilized and nonsterilized sand resulted in stem elongation, increased numbers of living shoots, and increased shoot and root biomass. However, when plants were grown in and buried with sterilized sand, the numbers of shoots were significantly higher than those buried with nonsterilized sand. The new root zone of buried plants was colonized by pathogenic soil organisms (nematodes and fungi) during the same growing season. It is concluded that by upward growth through pathogen-free sand, the plants benefit, at least temporarily, from escaping its pathogens and parasites. [KEYWORDS: Clonal growth; upward growth; migration; soil-borne disease; growth reduction Harmful soil organisms; hippophae-rhamnoides l; growth;breviligulata; succession; ecology]

Ammophila arenaria benefits from regular burial of windblown beach sand as it allows escape from soilborne pathogens (nematodes and fungi). The present study was done to obtain more insight into the timing and order of migration of the soil organisms towards the newly formed roots. Accordingly, plants were grown in non-sterilized root zone soil, buried by 20 cm of beach sand and harvested periodically during one growing season. Plants were buried in November. After 17 weeks, the internodes of the planted seedlings elongated and again 10 weeks later, in May, new roots began to develop. The majority of plant parasitic nematodes remained in the original core until root formation started in the deposited layer. Juveniles of Heterodera spp. successfully infected the roots, formed cysts and produced an offspring in October, resulting in high numbers of larvae at the end of the experiment. In February, at the first harvest of plants, fungi were isolated from soil and buried shoots in the whole deposited layer, although it was still winter. During the growing season a shift in the community of fungi in soil and buried shoots occurred in both the original core and the deposited layer. In both layers, the plant pathogenic fungi were gradually replaced by saprophytes. As all fungal and nematode species were able to colonize the deposited sand layer within one year, we conclude that both fungi and nematodes in the root zone of A. arenaria are very well adapted to sand deposition. Because of the fast migration rate of both pathogenic fungi and parasitic nematodes, the window for A. Arenaria to escape in time from its soil-borne pathogens seems to be narrow. [KEYWORDS: clonal growth; colonisation; soil pathogens; interactions; Heterodera spp.; sand dunes Plant-parasitic nematodes; breviligulata poaceae; morphology; organisms; decline; growth; dunes]