Vestiging van bijzondere plantesoorten in het infiltratiegebied van de Amsterdamse Waterleidingduinen

The ‘Amsterdamse Waterleidingduinen’ is a dune area near Haarlem in the calcareous Dune district. Since 1851 it is the water catchment area of Amsterdam. Due to the lowering of the groundwater level the moist dune slacks have disappeared gradually. Since 1957 infiltration of water from the river Rhine takes place for the water supply of Amsterdam. This water, rich in nutrients, causes among others a ruderal vegetation of high forbs and poor in species instead of the former species-rich dune slack vegetation. Since about 1970 locally a divergent vegetation is developing. In the so-called Groot Zwarteveld not only dune slack species have established, but also a number of species characteristic for dunes poor in lime and nutrients: Empetrum nigrum, Erica tetralix, Dryopteris cristata, Osmunda regalis and some Sphagnum species. The most remarkable species is Sphagnum imbricatum, which has been refound in the Netherlands after a long period of (seeming?) absence. It was its first observation in the Dune district. The species mentioned were not known from this dune area in former times. This vegetation development is a result of the new habitat. Firstly this dune area is superficially decalcified, stimulated by former agricultural management owing to which the humus content of the soil has increased. Secondly a layer of nutrient-poor rainwater has been formed on the nutrient-rich riverwater. This is caused by the very slow water movement in this area (without drains) and also by the small groundwater fluctuations (contrarily to most other places with a rapid water movement through the soil and great fluctuations in the groundwater). An important fact is that the vegetation is mown annually since 1974. A similar vegetation development takes place in other parts of the Amsterdamse Waterleidingduinen where similar habitats have originated.