Biotic and abiotic soil factors in the succession of sea buckthorn, Hippophae rhamnoides L. in coastal sand dunes

Decline and succession in dune scrub vegetation of sea buckthorn, Hippophaë rhamnoides L. in the Netherlands was previously found to be stimulated by biotic soil factors. In the present study, the role of plant-parasitic nematodes and root fungi was investigated. A field survey showed that during the ageing of H. rhamnoides vegetation there was a change in the plant-feeding nematode fauna. Certain nematode species, such as Tylenchorhynchus microphasmis Loof, were present in different stages and their population increased from stage to stage when expressed per unit of root length. The fraction of root length infected by VA-mycorrhizal fungi increased, as did soil concentrations of nitrogen and organic matter. Pot experiments with selective inhibition of groups of soil organisms indicated that soil fungi could be harmful to H . rhamnoides in the presence of nematodes, although fungi were not harmful on their own. The impact of various densities of T . microphasmis, either alone or in combination with the nitrogen-fixing actinorhizal symbiont Frankia or the fungus Cylindrocarpon destructans Zinnsm. Scholten, was assessed in pot experiments with controlled soil moisture conditions. Results showed inhibitory effects of the nematode on growth and on the acquisition of certain nutrients (especially phosphate). Although root nodulation and nitrogen fixation by Frankia were reduced, this did not appear to be the primary cause of growth reduction. No synergic interaction was found between T . microphasmis and C. destructans in their effect on growth of H . rhamnoides. The results of this study support the hypothesis that the increase in the concentration of soil nitrogen in the field stimulates nematode activity which in its turn impedes the acquisition of phosphate by H. rhamnoides. This reduces the competitive ability of this shrub and gives way to other plant species in the vegetation which are less sensitive to root damage.