Salt-marsh restoration: evaluating the success of de-embankments in north-west Europe

De-embankment of historically reclaimed salt marshes has become a widespread option for re-creating salt marshes, but to date little information exists on the success of de-embankments. One reason is the absence of pre-defined targets, impeding the measurement of success. In this review, success has been measured as a saturation index, where the presence of target plant species in a restoration site is expressed as a percentage of a regional target species pool. This review is intended to evaluate and compare success of many different sites on an idealistic concept where all regional target species have the potential to establish in a site, but may not actually do so because the site is unsuitable or inaccessible. Factors affecting suitability and accessibility and management options to increase regional species diversity are discussed. The results show that many sites contain less than 50% of the regional target species, especially when sites are smaller than 30 ha. Higher species diversity is observed for sites exceeding 100 ha and for sites with the largest elevational range within mean high water neap to mean high water spring tide. Most sites younger than 20 years contain more target species than older sites. For future de-embankments it is recommended that clear targets are set from the start. This brings along the need for monitoring. Only 37 out of 70 sites with de-embankment were monitored for plant species assemblages. Setting targets will also allow adaptive management of the site. Management options that are likely to result in higher species diversity are the construction and maintenance of drainage structures and the implementation of a grazing or mowing regime.