Questions: How successful was the restoration of a salt marsh at a former summer polder on the mainland coast of the Dutch Wadden Sea 10 yr after de-embankment? What were the most important factors determining the level of restoration success?
Location: Noard-Fryslan Butendyks, northwest Netherlands.
Methods: The frequencies of target plant species were recorded before de-embankment and monitored thereafter (1, 2, 3, 4, 6 and 10 yr later) using permanent transects. Vegetation change was monitored using repeated mapping 14 yr before and 1, 7 and 10 yr after de-embankment. A large-scale factorial experiment with 72 sampling plots was set up to determine the effects of distance to a breach point, distance to a creek and grazing treatment on species composition. Abiotic data were also collected from the permanent transects and sampling plots on elevation, soil salinity and redox potential.
Results: Ten years after de-embankment, permanent transect data showed that 78% to 96% of the target species were found at the restoration site. Vegetation mapping, however, showed that the diversity of salt marsh communities was low, with 50% of the site covered by the secondary pioneer marsh community. A multivariate analogue of ANOVA indicated that the most important experimental factor determining species composition was the interaction between distance to the nearest creek and livestock grazing. The combination of proximity to a creek and exclusion from livestock grazing always resulted in development of the high marsh community. In contrast, the combination of being located far from a creek, grazed and situated at low elevation with accompanying high salinity resulted in development of the secondary pioneer marsh community.
Conclusions: Using target species as criteria, restoration success could be claimed 10 yr after de-embankment. However, the diversity of communities in the salt marsh was lower than desired. Variable grazing regimes should be applied to high-elevation areas to prevent dominance by single species of tall grasses and to promote formation of vegetation mosaics. Low-elevation areas need lower grazing pressure. Also, an adequate soil drainage network should be preserved or constructed in low-elevation areas before de-embankment.