Based on kindly provided information by the Scottish Natural Heritage
General description
Vegetation encourages dune growth by trapping and stabilising blown sand. Transplanting marram grass (Ammophila arenaria) to the face of eroded dunes will enhance the natural development of yellow dunes above the limit of direct wave attack. Sand couchgrass (Elymus farctus) or lyme grass (Leymus arenarius) can be transplanted to encourage the growth of new foredunes along the toe of existing dunes, as these species are tolerant to occasional inundation by seawater. Planting grasses from seed can be undertaken but will not normally be successful in the very active foredune environment.
These natural dune grasses act to reduce wind speeds across the surface, thereby trapping and holding sand. They grow both vertically and horizontally as the sand accumulates. Marram grass is particularly effective as it positively thrives on growing dunes, and is perhaps the easiest to transplant.
Function
Transplanting vegetation will not prevent erosion, but it will accelerate natural recovery after storm damage creating a reservoir of sand within the foredunes that will make the dunes better able to withstand the next period of erosion. Additional works are often necessary to increase the potential for success. Thatching, fencing and beach recycling will assist in the accretion of sand, will provide minor protection from waves and will reduce damage due to trampling. Once grasses are well established they may well become self-sustaining, although any storm erosion damage will need to be rapidly made good.
Transplanting can also be used to enhance the appearance and effectiveness of built erosion defences. Rock, timber or gabion structures can provide a fixed defence line but are incongruous along a natural dune coast: partial burial of these structures using recycled sand, followed by transplanting, will create a more natural dune appearance if conditions are favourable. However transplanting to inappropriate locations can be a waste of resources, as illustrated in the photograph below.
Methods
The subject of dune vegetation transplanting is covered in detail within various publications. It is common practice in the management of wind erosion and recreational pressures and has been undertaken at many sites. Essentially, vigorous plants are pulled or dug up from a suitable borrow area, and are replanted on an eroded face, where they will be immediately effective in trapping blown sand. Where stock, such as sheep or cattle, can gain access to the replanted site these may cause considerable damage through grazing and trampling of the transplanted grasses. Growth and stabilisation will often only be realised once these have been excluded, for instance through fence construction. Additional works such as re-profiling of steep or unstable faces, fencing or thatching should be done in advance to avoid damaging the transplants. Without these works, transplanting is unlikely to be successful unless natural recovery is well underway. The basic principles of transplanting are illustrated above.
Marram grass is tolerant of salt spray but not immersion, and should be planted above the expected run-up limit of storm waves. Lyme grass can tolerate occasional inundation and can be transplanted down to the beach dune interface. In practice there is little point in trying to extend the vegetation cover too far as wave damage may quickly remove the lowest transplants before they have a chance to take root.
Transplanting should not be seen as a one-off operation. Regular maintenance is required to replace unhealthy plants, apply fertilisers, prevent trampling and extend cover to adjacent areas. Work can be undertaken at any time of year, but early spring is generally considered to be the optimum to avoid frosts, storm erosion, drought or trampling. Even assuming no storm erosion, it is likely that it will take two or three years before transplants begin to thrive and spread.
For small schemes transplants may be taken locally, ensuring that the borrow area is not denuded to the extent that blow outs may form. Larger schemes require a commercial supply or a managed nursery area within the dune system. Forward planning is required to ensure that sufficient seedlings and/or mature plants are available for transplanting. Sowing dune grass seeds is not a practical solution to marine erosion, but can be applied successfully to more stable backshore dunes, if considered necessary.
Costs for transplanting are dependent on labour, sources of transplants, extent of works, the need for ongoing management and the cost of ancillary works to help stabilise the dune face. Small schemes implemented by volunteer labour using local transplants may cost almost nothing, while extensive schemes using commercial nursery transplants and contracted labour may cost up to £20,000/km, plus ongoing management costs. Cost assessment should allow for the possibility that the transplant scheme may be substantially damaged by the first storm event following the works.
Impacts
Transplanting and management of appropriate dune grasses to the dune face will have no damaging impact on the natural environment of the receiving area, but can be harmful to the borrow area. Over harvesting of transplants from any area can give rise to increased local erosion. This may be most significant for sand couch grass, as the borrow area will necessarily be a foredune susceptible to wave over-washing and wind erosion. Construction of fences or thatching will disrupt public use of the beach, so provision must be made for controlled access.
Best practice and environmental opportunities
New vegetation cover to an eroding dune face will encourage recovery without damaging the ecological and geomorphological interests or the aesthetic appeal of the shoreline. The approach is potentially self-sustaining if the grasses become well established.
Transplanting and associated dune fencing can improve the appearance and environmental acceptability of built defences that might otherwise detract from the dune landscape.