Increased dampness causes vegetation change

The Dutch dune area has dried out at a number of locations as a result of water extraction and drainage of adjacent polder areas. Wildlife managers are searching for favourable locations to restore the natural environment to the original wet dune valleys. Chris Bakker has compiled a number of characteristics that a dune valley must satisfy for a successful restoration project to be carried out.

Chris Bakker investigated dune valleys in Zuid Kenemerland National Park. He discovered that the restoration of plant growth in nutrient-poor, wet dune valleys set off a chain reaction with respect to changes in the quantity of dead plants, responses of individual plants and the species composition of the vegetation.

Changes in the water level were found to have a direct and major impact on the ecosystems. First of all plants submerged by water for the first time die. Then the increased water level influences the plant growth in three different ways: Firstly a high water level is highly favourable for the germination and establishment of plants from wet dune valleys. Secondly at some locations seepage water that is rich in chalk and iron returns to the surface. As a result of this the nutrient phosphate is stored in chemical compounds that cannot be accessed by the majority of plants. Therefore only typical dune plants, which use phosphate very sparingly, can grow at these locations. Finally the water can easily transport seeds from other populations, thus facilitating the spread of species.

The conditions for germination and establishment are relatively favourable for all plants following the development of open areas as a result of flooding or if the top layer in wet dune valleys is removed. It seems to be a question of ’first come first served’. Therefore the order in which plant species arrive in the opened up areas and the sizes of their populations, can strongly influence the effect of restoration projects on the species composition.

Dried up dunes

The Dutch dune area is particularly important for water extraction and is of international importance from the viewpoint of nature conservation. The water extraction has resulted in a considerable drying out with negative consequences for the wildlife value. At present the water extraction in a number of dune areas is being strongly reduced, which in combination with heavy winter rainfall has resulted in the increased dampness of the dune valleys.

Therefore the results of Bakker’s research are being used to select favourable locations for the restoration of wet dune valleys. A favourable location must satisfy a number of characteristics. Prior to the increased dampness a low overgrowth must be present. There must be a water connection with populations of the target species and the valley must flood for a part of the year. Finally after the increase in dampness, there must be an increase in the amount of seepage water that reaches the surface and is rich in iron and chalk. If all of these characteristics are satisfied there is a maximum chance of the original characteristic plants returning in the nutrient-poor, wet dune valleys.

Chris Bakker’s research was sponsored by Technology Foundation STW.