Plants occupying sand dunes and the slacks, or swales, and flats between them. The density and diversity of dune vegetation are greater on coastal dunes than on desert dunes. See also Dune.
A zonation pattern is evident in the vegetation of the coastal dunes (see illustration). A wrack, or debris, line occurs at the upper limit of the beach. Seeds caught in decaying plant material and other debris washed in on the high tides germinate here and trap windblown sand, initiating the formation of a dune. The foredunes, also called the primary dunes, are those closest to the water and lie behind the wrack line. The plants on these dunes, mostly grasses, are tolerant to sea spray, high winds, and sand accretion. Behind the primary dunes are the secondary dunes, sometimes called the dune field. In this more favorable environment, the vegetation is denser and more diverse; the foredunes block sea spray and reduce wind velocity. The dune slacks are the low areas between dunes and are frequently a result of a blowout, an area where sand has been blown away down to where the sand is moist and close to the water table. Plants typical of wetlands often vegetate these areas. Shrub communities also inhabit the dune field and often form dense patches of vegetation. A maritime forest may be found behind the secondary dunes. In coastal barrier beach or island locations, a salt marsh adjacent to a bay or sound may lie behind the forest.
A coastal dune profile.
Although the plant species occupying the sand dunes of the United States vary from coast to coast, their functions and adaptations are essentially the same. Plants growing on sand dunes are adapted to the environment. The plants closest to the sea are usually the most tolerant of salt spray. The plants in the wrack line must be tolerant to salinity, wind, and burial by sand.
The foredune plants must be tolerant of sand burial, sea spray, and a nutrient-poor substrate. By a system of underground stems called rhizomes, they overcome burial by sand and spread throughout the dune with new shoots arising from buds on the rhizomes. The roots and rhizomes of these dune grasses are important in stabilizing the dune sand and preventing wind erosion. The foredune plants participate in dune formation; by slowing the wind, they favor sand deposition. Furthermore, some of these plants have specialized bacterium named Azotobacter associated with their roots, and these bacteria fix atmospheric nitrogen into a form usable by the plant.
The plants in the dune slacks have morphological and physiological adaptations for growth in flooded areas. For example, the sedge American three-square contains large air spaces (aerenchyma) in its stems and roots, which provide an oxygen pathway from the shoots above the water to the oxygen-deprived roots in the flooded soil. When these plants are flooded, they increase their production of the hormone ethylene, which may stimulate the production of aerenchymatous tissue.
