Keywords:
onshore barrier dunes
blow out
acidification
eutrophication

Location:  The Netherlands, North Sea, Haarlem

The Midden Heerenduin, Haarlem
The study area Midden Heerenduin is located in the coastal dunes near Haarlem which belongs to the so-called ‘Younger Dune Landscape’ (developed after 1100 AD) bordering the western Dutch coast. This part is located in a lime rich zone, the original sand containing 7-9 % calcium carbonate. As the study area is situated at a distance to the coast of 1.5 km and because there are several high dune ridges closer to the coast, the area has no function in respect to the safety of the hinterland.
In a non-influenced dune-system, deflation features in the form of blow outs occur. However, nowadays active blowouts are very rare in the Dutch coastal dunes because of several management practices directed to stabilization. Furthermore, the blow out development is reduced mainly because of the increased input of nutrients by atmospheric deposition (eutrophication). This is made responsible for the encroachment of grasses, shrubs and mosses. Another problem connected with the increased air pollution is the acidification of dune soils, which affects also the vegetation.

Management
The experiment forms part of a programme to test measures aimed at mitigating the effects of air pollution on natural landscapes. It is sponsored by the Dutch government and carried out by the Landscape and Environmental Research Group of the University of Amsterdam.
In the study area four small-sized blow outs (1 to 5 m²) and 12 medium-sized blow outs (5 to 50 m²) were reactivated, which had been artificially stabilized with branches cut from trees in the surroundings several years ago. The remains of the branches along with the vegetation, that had established since then, had to be removed. Furthermore, all the soil material, sand mixed with organic matter, was removed, because the organic matter is cohesive and can not easily be eroded. In the blowouts the removed soil layer was between 30 to 50 cm thick. After reactivation the blowouts were monitored over the period of 1991-1994.

Results
Most of the small reactivated blow outs stabilized spontaneously, whereas the reactivation of the medium-sized ones was successful. The blow outs remained active and grew slowly in size and depth despite the inputs of nitrogen by atmospheric deposition. In general the blow out area increased slowly of approximately 5 % per year. Most of the sand was deposited close to the blow outs, the area which received more than 1 cm of sand from the blowouts in three years was up to six times of the area covered of the blow outs itself. This calcareous and nutrient-poor sand which buried the vegetation and affected the soil conditions stopped the encroachment of some mosses and grasses. However, for stopping the encroachment of shrubs probably higher accumulation rates are needed.

References

• van Boxel, J.H., P.D. Jungerius, N. Kieffer & N. Hampele (1997): Ecological effects of reactivation of artificially stabilized blowouts in coastal dunes. Journal of Coastal Conservation 3: 57-62. EUCC. Opulus Press Uppsala. Sweden. (KJc99).
• van der Meulen, F. &  E. van der Maarel (1993): Dry coastal ecosystems of the central and southwestern Netherlands. In: van der Maarel (ed.). Ecosystems of the world 2A. pp 271-306. Elsevier. Amsterdam-London-New York-Tokyo. (BN93).