Jan Barkowski, Dr Holger Freund (ICBM)

All of the East Frisian Islands are typically characterised by their more or less extensive salt marshes on the tidal flats. Since the colonisation of the islands by humans, these have been used primarily for grazing. The appearance of the salt marshes changed due to the construction of ditches and channels for drainage and land reclamation, but the natural flooding dynamics remained. In order to protect the grazing cattle from the summer storm tides, parts of the salt marshes on some islands were enclosed by relatively flat, so-called summer dykes. On Langeoog, an approx. 5.5 km long summer dyke (Fig. 1 green line) was built in 1934/35, enclosing a salt marsh area of approx. 218 ha (Fig. 1).

Dyke line summer polder
Fig. 1: Old and new dyke line in the Langeoog summer polder area (from Steffens 2003)

The construction of this summer dyke changed the natural flooding dynamics and had a decisive influence on the species composition and vegetation structure of the salt marsh. The natural vegetation zonation, which normally runs along a decreasing salinity gradient from the mudflats to the higher salt marsh zones, was cancelled out. Over the decades, species and plant communities that are best adapted to life in the upper part of the salt marsh have been able to expand their habitat towards the mudflats. These changes have been documented on Langeoog by older vegetation mapping and permanent quadrat surveys.

To compensate for the damage to the ecosystem caused by the laying of the Europipe I and II natural gas pipelines, it was decided to dismantle the summer dyke on Langeoog and construct a new summer dyke at the rear of the salt marsh (Fig. 1, red line). This measure will return the old summer polder on Langeoog to the influence of normal tides, storm and spring tides. Two Diplom theses were carried out with the aim of documenting the salt marsh vegetation as a reference mapping at association level before the dyke was rebuilt and storing it in a GIS.

Rebuilt dyke
Fig. 2: Rebuilt dyke in the area of the former summer dyke on Langeoog (photo J. Barkowski 2003)

Furthermore, soil profiles were taken at some points and analysed horizontally for their pollen, diaspore and diatom content. The aim of these investigations was to check whether it is possible to reconstruct the shifts in vegetation known from the vegetation mapping using palaeoecological methods. Furthermore, the analysed surface samples serve as a reference for the growth of diatoms before the summer dyke was removed, as dynamic processes will not only take place within the vegetation cover in the coming years, but drastic changes will also occur in the composition of the microalgae populations.

Soil profile summer polder
Fig. 3: Extraction of a soil profile in the Langeoog summer polder (photo H. Freund 2002). The humus layers of the former salt marsh vegetation stand out clearly in dark colours against the lighter sand subsoil

The results of the mapping show that the area of the lower salt marsh and the middle salt marsh in the western part of Langeoog's summer polder has decreased significantly since the dyke was built in 1935/36 as a result of the decreasing flooding dynamics. Although the proportion of the Bothnian rush meadow (Juncetum gerardii) has been reduced with the simultaneous expansion of units of the upper salt marsh, the Bothnian rush community is still the most common community within the salt marsh vegetation today. The upper salt marsh is predominantly composed of stands of the Festuca rubra community.

Change_Salt marsh zones
Fig. 4: Change in the salt marsh zones in the eastern part of the Langeoog summer polder from 1949 to 2002 (Steffens 2003).

In the eastern part of the study area, the proportion of vegetation units belonging to the lower salt marsh zone, here the Andel grassland (Puccinellietum maritimae), was limited to a small area in the vicinity of the large tidal channel even before the dyke was built (Fig. 4, dark green area). The rest of the salt marsh was dominated by stands of Bothnian rush meadow (Juncetum gerardii). A significant increase in stands of upper salt marsh (especially Atriplici-Agropyretum pungentis) at the expense of the proportion of middle and lower salt marsh vegetation since the construction of the dyke is due both to the decreasing saltwater influence after the dyke was built and locally to grazing abandonment.

The palaeoecological studies (pollen, macrorest and diatom analysis) and permanent quadrat studies from 1936, 1948 and 2000 show that the construction of the summer dyke has had a direct impact on vegetation development, particularly in the area of the western summer polder. This can be seen above all in the fact that the normally slow succession to units of higher salt marsh zones was significantly accelerated by the reduced saltwater influence. As the salt marsh areas in the eastern part of the summer polder are naturally somewhat higher, such an accelerated process of succession can only be traced for the profile sites in the eastern area for locations close to the dyke. Areas far from the dyke even show a continuous development.

Literature:

Barkowski, J. (2003): Vegetation and palaeoecological investigations in the summer polder on Langeoog - western part. - Unpublished Diplom thesis, Institute of Geobotany, University of Hanover, 123 p.

Freund, H., J. Petersen & R. Pott (2002): Investigations on recent and subfossil salt marsh vegetation of the East Frisian barrier islands in the southern North Sea (Germany). - Phytocoenologia 33 (2-3): 349-375.

Steffens, M. (2003): Vegetation and palaeoecological investigations in the summer polder on Langeoog - eastern part. - Unpublished Diplom thesis, Institute of Geobotany, University of Hanover, 123 p.

Contact: holger.freund@icbm.de