SUMMARYTo explain the decline of Hippopha~ scrub in the vegetation succession in the dunes of The Netherlands, the growth and nodulation of Hippopha~ plants grown in pots, using soil from an early stage (site AH) and a post-optimum stage (site HP), were investigated. In HP-soil nodulation, yield, and the nitrogen and phosphorus content of test plants were always lower and the number of necrotic nodules and the dry matter content were always higher than in AH-soil, even after inoculation with crushed nodules and the addition of a nutrient solution. Plants in HP-soil also had darker roots, less root hairs, a higher number of short lateral roots and a higher percentage of dead roots than those in AH-soil. These characteristics of adverse growth conditions disappeared upon ignition or gammairradiation of HP-soil.Possible explanations of these results are discussed. The degeneration of Hippophai5 scrub cannot be ascribed to the age of the plants, the absence of sutticient infective endophyte particles or to abiotic factors such as unfavourable physical (particle size) or chemical soil conditions but is caused by biotic factors. No indications were obtained that plant-pathogenic fungi and bacteria are involved. HP-soil in contrast to AH-soil, however, contained large numbers of the nematode Longidorus sp., a species known to cause root deformations. The conclusion was that this nematode is one of the biotic factors involved in the degeneration of the Hippophai~ scrub. This degeneration is due to a restriction of the root system resulting in a low phosphate uptake, a low nodulation capacity and, as a consequence, a low nitrogen content. The results demonstrate that biotic soil factors are important in influencing succession in higher plant communities.
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