Behind The Dune
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Sculpted by winds and strengthened by vegetation, sand dunes are a spectacular feature that are not only beautiful but crucial to the island's health. These dunes offer the first line of defense against wave action from storms.
Some of Fire Island's primary dunes east of Watch Hill are as high as 40 feet. It can take decades for dunes this size to develop. Dunes form gradually over time, providing shelter from wind and salt spray for plants that grow on their leeward side.
Dune vegetation, like American beachgress, is important for the formation and stabilization of dune complexes on barrier islands. American beachgrass helps build dunes over time by trapping windblown sand. Plant parts above and below ground restrict sand movement and help secure the dune. You can help protect dune vegetation and, in turn, the dunes by keeping off the dunes.The natural dune building process in time will result in a predictable continuum of plant inhabitants from beach to bay: salt spray-tolerant, sandy-soil grasses and succulents thrive nearest the edge of the sea; less salt-tolerant shrubs inhabit the sunny swale on the leeward side of the dunes; and, shade-loving woody plants and herbs thrive in the cover and rich soils of the maritime forest protected by the dune.
Behind some of the primary dunes lies a series of crescent-shaped secondary dunes, with a low interdunal swale habitat in between. The primary dune ridge (foredunes) lies adjacent to the shoreline. Secondary dune fields may lie further inland. Dunes may form anywhere that aeolian processes (wind transportation) occur.
Picturesque though they are, coastal sand dunes serve a more important purpose than beauty. Dunes act as flexible barriers to ocean storm surges and waves, protect low-lying backshore areas, and help preserve the integrity of low barrier islands. In addition, they provide a habitat for many animals, including migratory birds. With proper planning and management, their functions may be enhanced. This publication discusses the features of coastal dunes and construction of new dunes using vegetation.
Coastal barrier dunes are formed by wave and wind action. In North Carolina, waves bring sand to shore from the adjacent inner continental shelf, and it is transported landward by onshore winds. Obstacles, such as driftwood, a sand fence, or vegetation, reduce wind speed, causing sand to accumulate. As sand accumulates, plants adapted to the beach environment emerge, stabilizing the surface and promoting further dune formation. In the absence of stabilizing vegetation, blowing sand may drift into large "live" dunes that move back and forth with the wind, such as Jockey's Ridge near Kill Devil Hills.
Dunes formed as a result of establishing vegetation act as flexible barriers to storm tides and waves and serve as sand reservoirs for beach nourishment. During storms, sand erodes from the beach-dune system and redeposits as shallow sandbars offshore. In a stable beach-dune system, the sand moved offshore during storms is returned during calm weather. Thus, the dunes, beach, and near-shore sandbars act as a dynamic, integrated unit, often referred to a the beach-dune system.
Although dunes serve as temporary protective barriers during storm tides of short duration, they are not effective against persistent beach recession caused by rising sea level, migrating inlets, or changing shoreline dynamics. Therefore, they cannot be considered permanent structures that will "hold off the ocean."
The value of dunes and their fragile nature are often misunderstood or not appreciated. Excessive use often upsets the natural balance, damaging the vegetation and deteriorating the dune system. One of the earliest uses of dunes in North Carolina, which resulted in considerable damage, was overgrazing by livestock, including cattle, horses, and sheep. Today, shoreline development and pounding of dunes by feet and vehicles pose serious threats to dune vegetation and dune stability. Intensive beach use increases the need to restore, construct, protect, and manage dunes.
Dune sands are readily moved and shaped by wind and water action. Consequently, disturbed dunes revert rapidly to unstable conditions, regardless of their stage of development at the time of the disturbance. As a result, dune restoration usually begins with the establishment of pioneer plants.
Selection of plant species is of paramount importance when restoring vegetation in bare areas of existing dunes. Dune plants must be able to survive sand blasting, sand burial, salt spray, saltwater flooding, heat, drought, and a limited nutrient supply. Only a few plant species can tolerate these stresses.
Perennial grasses are the primary stabilizers of frontal dune systems along the Atlantic and Gulf Coasts. The North Carolina coast is a transition zone between the northern-dominant American beachgrass and the southern-dominant sea oats. Bitter panicum or "running beachgrass" is also an important grass on frontal dunes in North Carolina. A woody shrub, seashore elder, grows well on frontal dunes and may be transplanted to add diversity to the dune system. A fourth grass, saltmeadow cordgrass, is not a true dune grass, but often traps sand to initiate dune growth. Saltmeadow cordgrass grows well in low, moist areas such as sand flats and high salt marshes. It is more salt- and flood-tolerant than the dune grasses. Planting a combination of several of these species can enhance the beach-dune system's diversity and long-term viability.
American beachgrass is a cool-season dune grass native to the North, Mid-Atlantic, and Great Lakes coasts. North Carolina is at the southern end of its natural range. American beachgrass is a vigorous, upright grass that grows in dense clumps and is capable of rapid lateral spread by runners. Hence, it is widely used for initial stilling of blowing sand. It is easily recognizable by its dense, cylindrical spikes or seed heads (Figure 1). Several characteristics make American beachgrass suitable for dune building and stabilization in North Carolina:
After establishment, American beachgrass will grow through as much as 4 feet of sand accumulation during one growing season. Although it grows quickly where sand accumulates on the seaward dune edge, it tends to die out behind the the dune crest after only a few years. This die-out is caused by climatic effects, fungal disease, and insects. Dead patches of beachgrass should be replaced with sea oats, bitter panicum, or seashore elder. To further increase the diversity and stability of the beach-dune system, sea oats and bitter panicum should be included in beachgrass plantings.
Because American beachgrass is a cool-season grass, the best planting dates are November through March. Plant small areas by hand using a dibble to open a hole 1 inch in diameter and 8 to 10 inches deep. Pack sand firmly around the plants after they are placed in the holes. Space the plants 18 to 24 inches apart at the crest of the dune and increase the spacing to 2 feet and then to 3 feet for several rows on each side of the crest.
Sea oats are a warm-season grass, native to coastal dunes from the Virginia Capes to Mexico. The plant's striking appearance, especially in bloom or fruiting, has made legal protection necessary in some areas to avoid excessive harvest (Figure 2). Due to an extensive root system, sea oats persist both seaward of the dune and behind the dune crest. Other characteristics include:
Planting stock may be acquired as seedlings from commercial producers, or, with permission on private property, transplants can be dug from existing stands. The best transplants are 1- to 3-year-old seedlings often found in small clusters seaward of the frontal dune.
Although sea oats provide the best long-term stability, the grass does not spread as rapidly as American beachgrass, and its slow lateral spread results in steep dune slopes. It should be planted in conjunction with American beachgrass or bitter panicum because of their more rapid spread and lower cost per plant.
Bitter panicum or "running beachgrass" is a warm-season grass found on dunes from New England to Mexico. It is useful for inclusion in American beachgrass and sea oats plantings to increase plant diversity. A wide variety of species is available, with variable stem sizes and growth characteristics (Figure 3). Although bitter panicum grows and multiplies relatively well in field nurseries, it is not as widely available as American beachgrass. Commercial availability is limited, and plants may have to be obtained by thinning local stands.
Seashore elder is a low-growing, woody shrub adapted to frontal dunes (Figure 4). It spreads both by seed and vegetatively, as roots develop on stems buried by sand. It adds diversity to the landscape and is useful for planting where American beachgrass has died out.
Saltmeadow cordgrass is useful for planting in low areas subject to flooding during storms. Its range includes all of the Atlantic and Gulf Coasts of the United States. Saltmeadow cordgrass often initiates new dunes on low flats that may later become occupied by plants better adapted to dry conditions.
Dune sand is relatively low in plant nutrients, and dune plants are adapted to this condition. However, small amounts of nutrients, primarily nitrogen and phosphorus, supplied by commercial fertilizers, are useful for promoting rapid establishment of transplants and encouraging existing vegetation. Fertilizer may also be needed for periodic maintenance. Other plant nutrients are normally supplied from soil or salt spray.
Fertilizers should be not be used indiscriminately in the beach and dune area. Excess amounts may damage the development of diverse and stable plant communities and increase the occurrence of plant diseases. Excess nitrogen also may leach through beach sand and pollute groundwater.
Suggested rates and timing for dune fertilization are listed in Table 1 and Table 2. During the first growing season, fertilizer should supply nitrogen at the rate of 150 to 200 pounds N per acre and phosphorus at the rate of 50 to 60 pounds P2O5 per acre, which converts to about 3.5 to 4.5 pounds N per 1,000 square feet and 1.0 to 1.5 pounds P2O5 per 1,000 square feet. This should be split into three equal applications. 59ce067264
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