Dune Restoration

Coastal dunes are a natural feature of sandy shorelines and are present in most temperate regions. They are among the most dynamic landforms, shifting with the winds and storm tides. Dunes are part of the near shore environment that change seasonally and with episo dic storms. They serve as reservoirs of sand to re nourish the beach during storms as erosion of sand transports it offshore where it is deposited on sand bars to be returned gradually by the tides. Formation of dunes requires a source of sand, usually carried from the beach by onshore winds, and vegetation to catch sand and stabilize it.

Dunes are stressful environments characterized by blowing sand that abrades vegetation, salt spray, high soil temperatures, low water holding capacity and low nutrients, especially nitrogen (N). Historically, many dunes were deforested for timber then used for grazing. Today, they are under stress from shoreline development and overbuilding of the coastal fringe. Development of the shore often leaves no room for dunes to migrate inland, as occurs when sea level rises. The combination of ever changing environmental conditions and urban encroachment makes coastal dunes a globally endangered ecosystem.

The Dune Community

Dune vegetation consists of distinct plant communities, pioneer, scrub, and forest zones, that occur along a gradi ent of increasing distance from the sea and increasing age (Figure 1). The pioneer zone occurs on the upper beach or foredune area, closest to the sea. Vegetation consists of a few species of grasses, sedges, and forbs that are able to withstand salt spray, sandblast, burial by sand, temperature extremes, drought, episodic flooding with salt water and low nutrient (N) availability (Table 1). Pioneer species include dune initiators and dune builders. Dune initiators are annuals such as sea rocket, Cakile maritima, which is widely distributed throughout the world, sea purslane (Sesuvium spp.), and other species. Dune initiators are important for trapping seeds of dune builders. Dune builders consist of perennials such as American beach grass (Ammophila breviligulata), European beach grass or marram grass (A. armaria), European dunegrass (Elymus arenarius), and American dunegrass (Elymus mollis) in cool climates, and sea oats (Uniola paniculata) and bitter panicum (Panicum amarum) in warm climates. Saltmeadow cordgrass, Spartina patens, also is common in the foredune zone where it is found on low, moist sites where exposure to salt is greater. Once dune initiators and dune builders become established, secondary invaders such as Abrona, Ambrosia, Artemesia, Croton, Carex, Carpobrotus, Euphorbia, Erigeron, Festuca, Fimibristylis, Hydrocotyle, Ipomoea, Lathyrus, Lupinus, Schizachyrium, Solidago, Spartina, and Sporobolus colonize the pioneer zone.

The shrub zone lies immediately behind the foredune zone and consists of secondary dunes and low lying areas, swales, and flats, between them. In addition to pioneer species, the shrub zone is colonized by woody vegetation that stabilizes dunes. The shrub zone receives less salt spray and fresh sand relative to the foredune zone. Nitrogen supply also is low. In the shrub zone, woody shrubs and trees are stunted by salt spray and wind. Along the US Atlantic coast, seashore elder, Iva imbricata, is important in the shrub zone. It is highly adaptable and tolerates saltwater, salt spray, sandblast, and sand accu mulation. Seashore elder grows in foredunes, swales, maritime forests, and upper fringes of the salt marsh.

Figure 1 Vegetation of the coastal dunes from the sea inland to the forest.

Table 1 Environmental constraints on establishment of coastal dune and tidal wetland vegetation

Constraints

Dune

Tidal marsh/mangrove

Physical

Inadequate water/drought

Excess water/anoxia

Salt spray

Salinity

Wind/sand abrasion

Excess wave action

Excessive soil temperature

Excessive soil temperature

Nutritional

Inadequate N, (P)

Inadequate N, (P) Acid sulfate soils

Disturbance

Foot/vehicular traffic

Grazing/herbivory

Grazing/herbivory

Colonization

Excessive tidal energy

Disease

Fungal pathogens, scale insects

See text for an explanation of the various constraints.

See text for an explanation of the various constraints.

Growth of pioneer species is poor in the shrub zone relative to the foredune zone. Shrub species do well though and they are important in stabilizing sand and initiating soil development. The decline of vigor and growth of pioneer species in the shrub zone is thought to be linked to reduced sand accumulation that supplies mineral nutrients, especially phosphorus (P).

The forest zone is the oldest and most stable dune community. It forms only after substantial time passes and soil formation begins. Considerable protection from salt spray and flooding is needed for the forest zone to develop. Once dunes are stabilized and pioneer and shrub communities develop, trees may be planted to accelerate succession. However, trees are not planted in the fore dune or shrub zones near the sea because salt spray inhibits their growth. Also, trees shade out pioneer and shrub species and inhibit regeneration of these species following a severe disturbance such as disease outbreak, insect outbreak, or fire.

Limiting Factors for Establishment

Establishment of dunes requires a supply of sand and vegetation to catch and stabilize it. Sand is supplied by onshore winds that pick it up from the beach and carry it inland where it is intercepted by vegetation. Sand is transported mostly by saltation, bounced along as the impact of moving grains dislodges other grains, and by surface creep. Sand fences often are erected to trap sand and initiate dune building. Fences may be built using wooden pickets, boards, bamboo, reeds, fabric, or other materials such as branches that deflect and slow the wind. Guidelines for using fences include (1) use fences of 40-50% porosity as they are most efficient in trapping sand, (2) install fences parallel to the shoreline, and (3) a single row of fence is suitable at lower wind speed but double fences may be needed at higher wind speed. As the dune builds, continued sand trapping and dune growth is facilitated by installing additional fences atop the original fence as it becomes buried. Other sources of sand for dune building include dredged material pumped onto the beach and sand pushed up by bulldozers.

The dune environment is harsh. The sandy soils hold little water and are low in nutrients, especially N. Dune vegetation responds favorably to fertilizer additions, espe cially N with a lesser response to P. Because of its fragile nature, it is essential to keep existing dunes and dune plantings free of vehicular and foot traffic (Table 1). In some regions, grazers, rabbits in Europe, nutria in Louisiana (USA), may graze on dune vegetation, reducing plant cover (Table 1 ). In Europe, where dunes were afforested centuries ago, grazers such as sheep and rabbits are important for maintaining early successional vegetation.

Restoration and Ecosystem Development

Restoration of coastal dunes has been widely used around the world, including Europe, North America, South Africa, New Zealand, and elsewhere to help stabilize beaches and barrier islands.

In Europe, coastal dunes have been used for centuries for low intensity agriculture (e.g., grazing). In the nine teenth century pine trees, Pinus maritima, P. mugo, and other trees were planted along the coast of France for dune stabilization and silviculture. With increasing urban encroachment and fragmentation in the twentieth cen tury, public awareness of the importance of dunes for shoreline defense, habitat for nature and esthetics led to interest in restoring these ecosystems.

In Europe, where trees were planted for silviculture, early dune restoration efforts involved cutting down the trees and shrubs to promote conditions favorable for pioneer vegetation. The first, scientifically based restora tion efforts involved planting European beach grass, American beach grass, and other pioneer vegetation to identify suitable species for use in dune plantings. In the 1960s, experimental plantings were established along the US Atlantic coast. Different sources of sand were tested and different species, including American beach grass, were planted to determine their geographic range and environmental requirements. On the Pacific coast, similar experiments were underway evaluating European beach grass.

Frequently, dunes are established too close to the beach which does not allow for the natural ebb and flow of sand between the dune, beach, and offshore sand bar. Along the US Atlantic coast, it is recommended that dunes be placed at least 100 m from mean high water to avoid dune erosion during storms. Once a site is selected, a sand fence is installed to catch and perennial vegetation is planted to stabilize and hold the sand in place (Figure 2a). Plant stems and leaves increase surface roughness that decreases wind velocity near the ground and interferes with sand movement, and roots that hold sand in place. Over time, a healthy dune community develops (Figure 2b). Vegetation is planted early in the growing season to avoid high soil temperatures that occur later and to give plants the full growing season to colonize the site.

Dune vegetation is N limited and sometimes P limited. N-P-K fertilizer (30-10-0) is applied at the rate of 23 kg ha 1 (50 lb acre 1) 2-3 months after planting to promote plant growth. Do not fertilize at the time of planting as the fertilizer will leach from the soil. Rather, once the root system begins to develop, fertilizer is applied in three applications (69 kg ha 1 total) spread out over the growing season.

Several pioneer species should be planted as multi species plantings generally outperform monoculture

Figure 2 (a) Installation of a sand fence for dune restoration in January 1978. (b) The same site in October 1981.

plantings. Beach grass traps sand and accumulates it faster than other species. Beach grass also migrates faster toward the ocean than other species such as sea oats and bitter panicum, but it is susceptible to disease and insect pro blems (Table 1). Infestations by insects and pathogens are less of a problem in cool temperate climates as opposed to warm climates. In North Carolina (USA) American beach grass is attacked by a scale insect (Eriococcus carolinae) and a fungal pathogen (Marasmus). Woody vegetation, which is common on older dunes, is not planted in the foredune area because it does not grow well. Rather, shrubs and trees will colonize the site as succession proceeds, assum ing environmental conditions are stable (i.e., the sea is not encroaching).

Monitoring studies indicate that multispecies plant ings are most successful in accumulating sand, building dunes, and providing good plant cover. On Ocracoke Island (North Carolina, USA), 10 years after planting a mixture of American beach grass and sea oats, sea oats dominated the zone where sand was no longer

CO 5

1966

-198S (S.9 species m-2, 15 species total)

1966

-198S (S.9 species m-2, 15 species total)

Planted area

Planted area i i r~

0 10 20 30

40 50 60 70 Distance (m)

80 90 100

Figure 3 Change in dune profile characteristics and plant species composition 4, 10, and 20 years after dune restoration along the North Carolina (USA) coast.

accumulating. On the active sand accumulating zone toward the beach, beach grass was dominant. Long term monitoring of restored dunes on the Brittany Coast of France revealed that, after 10 years, fixed (nonmigrating) dunes had been reestablished but plant species diversity was lower than in mature dunes. In North Carolina, 20 years after installing a sand fence and planting with American beach grass, a 3 m high dune developed that had built 30-40 m seaward (Figure 3). After 10 years, species richness averaged three species m 2 with a total of nine species observed on the planted site. After 20 years, species richness doubled to 6.2 species m 2 and 15 species were counted. Over time, American beach grass decreased in importance as it was replaced by later successional vegetation, S. patens, and woody vege tation, red cedar (Juniperus virginiana), wax myrtle (Myrica cerifera), and silverling (Baccharis halimifolia).

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

Get My Free Ebook


Post a comment