Plant colonization after managed realignment: the relative importance of diaspore dispersal.
Deliberate breaching of sea defences is frequently practiced with the aim of restoring salt-marsh vegetation on previously embanked land. However, experience so far has shown that it may take several years before salt-marsh vegetation is fully established, and it is possible that limited diaspore dispersal plays a role in this. In order to ascertain whether salt-marsh development may be constrained by limited diaspore dispersal, we studied the dispersal of salt-marsh species by tidal water in the Blackwater Estuary, southeast England, UK. From October 2001 to the end of March 2002 a total of 38 species, of which 18 were salt-marsh species, was trapped in a restoration site and adjacent marsh. Aster tripolium, Limonium vulgare, Puccinellia maritima, Salicornia spp., Spergularia media and Suaeda maritima were the most abundant salt-marsh species, with more than 3 diaspores m-2 trapped during the study period. For most species, the number of diaspores trapped was representative of their abundance in nearby vegetation. Hence, despite the potential for long-distance transport by tidal water, our results indicate a predominantly local dispersal of salt-marsh species. For the restoration of salt-marsh vegetation after de-embankment, relatively rapid colonization may be expected from pioneer and low-marsh species, provided they are present in a nearby source area and the restoration site is at the appropriate altitude. The establishment of species absent from the adjacent marsh may be dependent on the presence of birds or humans as the main dispersal agents. Breaching of sea defences should preferably take place before or during September, in order to take advantage of the peak in dispersal of salt-marsh species in the first year after breaching.