Dunes
On virtually any barrier island, wind and sand combine to create sand dunes. Dunes play a vital role in protecting coastlines and property from storms, high winds and saltwater intrusion.
Photo credit: R. Pastor
Dunes also act as sand reservoirs, which are important for replenishing coastlines after tropical storms, hurricanes, intense wave action, or other erosional events.
The process of dune formation begins with the transport of sand landward. This happens in three ways: saltation, surface creep, or suspension.
Saltation occurs when winds blow medium-sized sand grains up the slope of a beach. Surface creep occurs when larger grains are rolled along the beach as they collide with smaller wind-blown particles during the saltation process.
The most common transport process is suspension. Wind picks up small sand grains and brings them landward in onshore breezes. When plants, driftwood and other obstructions impede wind and causes the airflow to lose momentum, suspended grains fall out of the air on the slip face, or lee side of the obstruction, where they accumulate.
Over time, sand builds up behind obstructions, creating a series of long, elevated spits of sand, called wind shadows. These grow at right angles to the shoreline, and as they present an ever-larger barrier to the wind, sand accumulates more rapidly. Plants colonize these stabilized areas, and their roots further anchor the sand and fortify the dune structure. As plants continue to colonize the upper beach, wind shadows join together to form dunes, which lie parallel to the shoreline.
Within dune systems, which resemble a series of low peaks and valleys, the first dune above the intertidal zone is called the primary dune, or foredune. This is the area of active colonization by plants, and the area most affected by waves and heavy winds.
Landward over the crest of the foredune lies the swale: a low, somewhat wet area separating primary dunes from secondary dunes. In swales, winds can scour the sand nearly down to the water table, and plant communities may consist of more freshwater species that show some salinity tolerance. It is in the shelter of swales that scrub communities and maritime forests first become established.
Many dune systems also feature secondary dunes. These dunes form when severe storms breach primary dunes and deposit sand further inland. Deposition of sand onto secondary dunes also occurs as winds blow fine-grained sand inland over the primary dune. Due to their relative stability over time, and because they are generally protected by primary dunes, secondary dunes support a significantly broader variety of vegetation than primary dunes.
Vegetation colonizing the upper beach and foredune must be well-adapted to periodic disturbance, and generally consists of grassy, salt-adapted species. Growth of these colonizing species must keep pace with the rate of sand build-up along the foredune, which can be rapid.
Beyond the pioneering zone in the shelter of swales and secondary dunes, plants are generally more protected from the effects of salt spray, seawater and sand burial. The resulting communities are more diverse than on adjacent beaches.
When established dunes remain stable over time, plants’ cycles of growth, reproduction and leaf shedding slowly enriches the sandy soil with decaying plant matter. As this humus accumulates, soils become richer and hold more water. This allows other types of vegetation to take root, and begins the process of succession, where shrubs and trees replace the pioneering vines and herbaceous species.
On the foredune, beach pioneers include railroad vine and shoreline sea purslane. South of Cape Hatteras, sea oats are the principal dune colonizer; this coarse grass grows up to 6 feet tall and spreads laterally via rhizomes. Along with sea oats, two other dune-building species, bitter panic grass and beach cordgrass, are stimulated to grow upward by burial in sand.
Subsequent lateral growth in these plants allows for the construction and stabilization of a continuous dune ridge.
The dune crest is the area where shrubby and woody species begin to replace herbaceous vines and grasses. Common herbaceous plants of the dune crest include sea ox-eye daisy, beach sunflower, firewheel, and annual phlox. Also common on dune crests are several woody species including sea grape, saw palmetto, and the invasive Brazilian pepper.
Many of the woody species growing on dune crests are low-growing and shrubby, while inland the same species can demonstrate a more robust growth habit. Dry, low-nutrient soils, frequent high winds and salt spray conspire to stunt dune-situated individuals. Salt spray kills the tender terminal buds of many trees and shrubs on contact, resulting in the salt-pruned, windswept tree canopies of Florida's dune communities.
Swales between dunes gain an increased measure of protection from winds and salt spray as the dune system builds over time. Swales can support freshwater plants, though most plants that grow in swales have some degrees of salinity tolerance as well. Stands of sea grape, saw palmetto, and the Brazilian pepper are common woody species on dune crests and in swales.
Backdunes and secondary dunes generally support a wider variety of vegetation than foredunes. The same species that grow as low shrubs or stunted trees on dune crests do grow in backdune areas as well, but in these more protected locales they are often able to attain full height. Saw palmetto, cabbage palm, live oak, and prickly pear cactus are all common inhabitants of backdunes and secondary dunes.
A number of rodents, some of which are becoming increasingly rare, utilize dune habitats. The threatened southeastern beach mouse can be found in scattered populations from Cape Canaveral to Sebastian Inlet. Other rodents that inhabit dunes include the cotton mouse, cotton rat and rice rat, as well as eastern cottontail rabbit and the marsh rabbit. Several other mammals such as gray foxes, raccoons, feral pigs and feral cats also use dunes for feeding.
Many species of shorebirds utilize dunes for feeding; and several species also nest in dune habitats. Among the nesting species are the willet, American oystercatcher, and Wilson's plover, which prefer nest sites in dune areas with sparse grass or herbaceous cover. The laughing gull, Caspian tern, and the gull-billed tern also nest in dunes but prefer areas with somewhat more dense coverage.
Reptiles are also common inhabitants of dunes. Several species of anoles and snakes are common, including green anole, Eastern coachwhip snakes and Florida rough green snakes. Gopher tortoises, while not plentiful, can often be observed in stable backdune areas.
In spite of the stabilizing ability of dune plants, dunes are highly susceptible to human impacts. Vehicles traversing beaches, as well as heavy foot traffic, damage vegetation by shifting sand and roots, thus destabilizing the dune community. Coastal development can also impact the natural process of dune replenishment by adversely influencing natural erosion patterns.
