Successional development of saltmarsh in two managed realignment areas in SE England, and prospects for saltmarsh restoration

Abstract

The primary successions of saltmarsh vegetation in 2 managed realignment sites in the Blackwater estuary (SE England), Tollesbury and Abbotts Hall, began with domination by the opportunistic annual species Salicornia europaea and Suaeda maritima. These species were gradually replaced by perennial species at the higher elevations, and a vertical zonation of dominant species was established in the order Salicornia europaea/Spartina anglica, Puccinellia maritima, Atriplex portulacoides and Elytrigia atherica, from low to high saltmarsh. Suaeda maritima became rarer and patchily distributed. Ordination analysis confirmed that after 12 yr the new saltmarsh at Tollesbury was similar to the adjacent ancient saltmarsh. At Abbotts Hall the broadly similar successional sequence took only 5 yr. These were not facilitated successions-plants increasing the elevation of the sediment-because little sediment had accreted. Instead these were characteristic of tolerancetype successions, with the early opportunistic species having no apparent facilitative or inhibitive effect on the perennial species that arrived later and outcompeted them. If land at the appropriate elevation is provided by coastal realignment, saltmarsh develops to resemble the vegetation structure of ancient saltmarshes rapidly, with positive prospects for saltmarsh restoration. However, the erosion of saltmarsh vegetation that developed in historical realignment sites indicates that this benefit may be relatively short-term. Hence, long-term saltmarsh creation must also account for the processes causing saltmarsh erosion.

The introduction of coastal infrastructure as a driver of change in marine environments

“…This involves dismantling or breaching shore defences and eliminating them or moving them inland, preferably taking advantage of natural topographic contours to reduce the cost of engineering to the standards required for alleviating flooding risks (French 2008; Townend 2008). Although this alternative to further armouring of shorelines is promising, there are concerns for its long‐term performance (French 2008; Hughes, Fletcher & Hardy 2009) and it is only feasible where there is the possibility of the shoreline being relocated inland. Subsidence of sites caused by shoreline armouring has, in some cases, prevented the achievement of restoration targets (Hulme 2005).…”

Is saltmarsh restoration success constrained by matching natural environments or altered succession? A test using niche models

“…Natural saltmarshes develop by facilitated succession, where initial submergence‐tolerant colonists increase sedimentation, raising the level of the marsh surface and facilitating the growth of less tolerant species (Bertness & Shumway, ; Castellanos, Figueroa, & Davy, ). A wider range of conditions may be available in restored marshes and so facilitated succession may not be needed to create suitable conditions for high‐marsh species (Hughes, Fletcher, & Hardy, ). This would reduce the importance of competition, as species could colonize bare sediment wherever conditions are suitable; while under facilitated succession, species must invade existing lower marsh vegetation.…”

Remote sensing of geomorphological and ecological change in response to saltmarsh managed realignment, The Wash, UK

“…The Tollesbury site is up to 1 m lower than the fronting marsh (Reading et al, 2008). Thus, only 29% of the site was vegetated 6 years after breaching due to the low initial site elevation (Garbutt et al, 2006;Hughes et al, 2009), compared to 88% intertidal vegetation coverage at Freiston after 4 years. Marshes in San Francisco Bay, California reclaimed in the late 1800s are up to 3 m lower than the surrounding marsh; over 13 sites there was a strong inverse relationship between surface elevation and the time required for vegetation to colonize 50% of a site (Williams and Orr, 2002).…”

“…Sites elsewhere in NW Europe have only shown a shift to mid-marsh communities after 6 years of renewed tidal inundation (Barkowski et al, 2009). At Abbotts Hall and Tollesbury realignment sites, SE UK, saltmarsh composition in the realignment site was similar to the surrounding reference marsh after 5 and 12 years respectively (Hughes et al, 2009), though only 32% of the regional species pool had established at Tollesbury within 8 years (Garbutt et al, 2006). Thus, more than 5 years is needed to assess realignment trajectory (Dixon et al, 2008).…”