Keywords:
onshore barrier dune
dune-grassland
grazing by rabbits
N-availability
fertilization

Contact:  Mr. van der Hagen. Dune Water Company of South Holland (DZH). Postbus 34. 2270 AA Voorburg. The Netherlands. Mr. ten Harkel. Gildehuisdrift 18. 3436 CB Nieuwgein. The Netherlands.

Location:  The Netherlands, North Sea, The Hague, Meijendel

The Meijendel dunes, The Hague
Meijendel is a coastal sand dune system of calcareous origin bordering the North Sea. It is situated north of the city of The Hague and belongs to the Younger Dunes, which are formed on the Older Dunes in the twelth and thirteenth century. The total area covers approximately 2000 ha with an average width of 3200 m and consists of barrier dunes with a zonation from yellow to grey dunes. The area is managed by the Dune Water Company of the province of South Holland (DZH). The Meijendel dune area consists of dry and wet dune valleys and some forests at the innerdune part. Most of the dry dune valleys have vegetation with grasses, herbs and shrubs, the dry dune grassland cover about 25% of the total dune area. However, since the 1950s the species-rich dune grassland, of which several are in the priority list of the EU Habitat Directive, appendix I, has gradually declined.

Observations
A study was initiated in 1988 for a period of six years by the Department of Physical Geography and Soil Science of the University of Amsterdam (FGBL-UvA) in cooperation with the DZH to quantify nutrient pools and fluxes in a dry coastal dune grasslandsystem. Three dry dune grasslandtypes were observed: short species-rich, short species-poor and predominated by tall graminoids. Plots in each vegetation type were investigated under grazing and non-grazing conditions by rabbits both under increased levels by fertilization with ammonium nitrate and ammonium sulfate. The vegetation development and the nutrient fluxes (input as atmospheric bulk deposition, throughflow and output as soil water) as well as the nutrient pools were measured.

Results
The maintenance of species-rich dry dune grasslands strongly depends on the activity of grazing animals, especially under relatively high atmospheric nitrogen deposition levels. Exclusion of rabbits resulted in an increase in the frequency of perennial graminoids and a decrease in the frequency of annual graminoids and herbs. The vegetation change occured mainly within one year after the exclusion of grazing and remained constant. No new plant species established. Even when the rabbit population increases, graminoids remain dominant for a long time as rabbits do not graze on relatively tall grass swards. Non-grazed vegetation has a relatively low protein content in comparison to grazed vegetation and therefore these animals avoid such areas. The dominance of graminoids can be counteracted by introducing large herbivores like horses. However, heavy grazing can also prevent the completion of the life cycle of annual species. After the introduction of large herbivores the amount of some herbs increased immediately and remained high about four years even after stopping the grazing.
The atmospheric bulk deposition of nutrients on the Meijendel dune area is low compared to other regions in the Netherlands. However, additional fertilization with nitrogen had no significant effect on both grazed and non-grazed vegetation. In the species-rich site 70% of the added Nitrogen leached out of the system due to high nitrification rates in the calcareous soil. Conversely, at the species-poor site most of the added nitrogen remained in the system. Fertilization increased the N concentrations in shoots of herbs and graminoids and in mosses but no increase in growth rate was measured. The nitrogen is immobilized in the litter due to relatively low decomposition rates caused by high C/N ratios. It can be concluded that if grazing pressure by rabbits is low in case of high perennial grasses additional management measures like grazing by livestock or mowing are necessary to maintain dry dune grasslands and to preserve their species-richness. This can also facilitate rabbit grazing. Grass encroachment at low grazing intensities endangeres these habitats even at relatively low levels of atmospheric deposition because of high underground pools of nutrients and a low critical deposition level of Nitrogen.

Location of the measurement sites. I = species-rich site, II = species-poor site, III = grass-dominated site

References

• ten Harkel, M. (1998): Nutrient pools and fluxes in dry coastal dune grasslands. Thesis. Amsterdam. (33R4BCD98). (thesis available on request through DZH; address see above).