The effects of decreased management on plant-species distribution patterns in a salt marsh nature reserve in the Wadden Sea.

摘要

To restore natural salt-marsh habitats, maintenance of the artificial drainage system was discontinued and cattle grazing was reduced in man-made salt marshes in the Dollard estuary, the Netherlands. We studied the vegetation development in these marshes shortly after these marshes became a nature reserve, and again 8-9 years later. Cattle distribution showed a gradient of intensive use close to the landward seawall to a low or zero use more seawards. Elymus repens was mainly found at the highest elevations and in the landward parts of the marsh, and lost much of its dominance. Conversely, Aster tripolium increased landwards, and showed a shift to higher marsh elevations. These changes are explained by the interaction between increased soil waterlogging in the marsh, due to the neglect of the drainage system, and cattle grazing. Phragmites australis, Spartina anglica and Scirpus maritimus were mainly found in the seaward parts of the marsh. Spartina anglica and Scirpus maritimus decreased strongly, partly through competitive replacement by Phragmites australis and partly due to herbivory by greylag geese (Anser anser). Bare soil increased in depressions due to increased waterlogging in the marsh combined with livestock grazing and goose grubbing. Phragmites australis probably forms the climax vegetation in the brackish Dollard marshes. Its increase in the seaward parts is irreversible in practice, but will benefit reed-bed breeding birds. On the landward side, the combination of neglect of thedrainage system with moderate grazing may be effective in maintaining young successional stages suitable for a wider range of halophytic plants, and for breeding redshank and grazing waterfowl.