Keywords:
onshore barrier dune
open dune-grassland
grass encroachment
Nitrogen
mowing
grazing

Location:  The Netherlands. Kraansvlak – Middelduinen – Tilanuspad – Zuidervlak.

Dry coastal dunes
Dry coastal dunes are of great conservation importance in the Netherlands since they harbour almost 70% of the Dutch flora, of which 15% can be found exclusively in the dunes. However, during recent decades this species-rich, open dry dune grasslands have been dominated by tall grasses. Changes in soil chemistry, especially increased N-availability, are assumed to allow grasses to compete more effectively with herbs and cryprogams. In the past, often the atmospheric nitrogen deposition was hold responsible for the higher N-availability. Following carbon, hydrogen and oxygen, nitrogen is the quantitatively most important nutrient for plants and can often be a limiting factor for plant growth. Three aspects of the nitrogen cycle of terrestrial, plant-dominated ecosystems can be distinguished: (1) the plant-internal cycle, (2) the ecosystem-internal cycle and (3) the ecosystem-external cycle. The substrate in sand dunes is extremly low in nitrogen in the beginning of the succession, but with increasing age and a more dense vegetation N-pools increase.

Observations
Four sites were selected in calcareous dry dunes representing vegetation types of the alliance Galio-Koelerion: Kraansvlak, Middelduinen, Tilanuspad and Zuidervlak. In each vegetation type, grass-dominated and open dune grassland plots were selected. The biodiversity was measured with the Shannon-Weaver index. A sequential incubation experiment was carried out to calculate the mineralisation. The above-ground biomass including the litter layer as well as the below-ground biomass was sampled to estimate the amount. Additionally, the N and P-contents of the biomass and of the soil as well as the organic matter and carbon content of the litter layer were determined.

Results
In grass-dominated plots lower biodiversity was found but a higher biomass as well as a higher N-availability by mineralisation and turnover rate. Whereas it is known from other studies that turnover rates and nitrification drops during succession, biomass, mineralisation and turnover rates were positively correlated in this study. Therefore the grass-dominated plots do not represent a natural successional stage of dry dune succession and they do have an elevated nitrogen cycle compared to the open dune grassland plots. In open dune grassland, atmospheric N-input is an important source of N, whereas in grass-dominated plots mineralisation largely exceeds atmospheric N-input. The grass-encroachment therefore cannot be simply due to atmospheric N-deposition. It may have been triggered by atmospheric deposition and subsequently enhanced as a result of enhanced N-mineralisation. The mineralisation rate could increase with increasing N-input like a type of positive feedback, perhaps because of specialised adaptions of grass species.
To restore dry dune grasslands mowing and grazing are suggested. Mowing removes biomass, opens the canopy and thus improves the conditions for small species to compete with grasses. In addition mowing removes the dense litter layer, which may inhibit species to germinate and exposes the ground surface promoting the colonisation of new species. Apart from a higher diversity, mowing also encourages rabbit activity. Also grazing by large herbivores results in a reduction of standing crop and a more open vegetation structure. Mowing and grazing not only cause a changed light regime, but also a decrease in biomass and litter input, and thus have a strong impact on N-cycling. Eventually these measures may lead to a decrease in nitrogen availability.

References

• Gerlach, A. (1993): Biogeochemistry of nitrogen in a coastal dune succession on Spiekeroog (Germany) and the impact of climate. Phytocoenologia 23: 115-127. (31R5Sp93)
• Gerlach, A., E.A. Albers & W. Broedlin (1994): Development of the nitrogen cycle in the soils of a coastal succession. Acta Botanica Neerlandica 43 (3): 189-203. (31R5Sp94).
• Veer, M.A.C. (1997): Nitrogen availability in relation to vegetation changes resulting from grass encroachment in Dutch dry dunes. Journal of Coastal Conservation 3: 41-48. EUCC. Opulus Press Uppsala. Sweden. (KJc97a).