The process of dune formation begins with the transport of sand landward via saltation, surface creep, or suspension. Saltation occurs when medium sized grains of sand are transported up the slope of a beach as the result of winds blowing them along the beach surface. Surface creep is the movement of larger sized grains that are rolled along the beach surface due to collisions with bouncing mid-sized sand grains as they are blown up the beach during the saltation process. Perhaps the most common transport process is suspension, in which small sand grains are picked up by winds and blown landward in onshore breezes. This sand is deposited on the upper beach when the flow of wind is impeded by some obstruction (plants, driftwood, flotsam, etc.) that causes the wind to lose speed and momentum. Suspended sand grains then fall out of the air and are deposited on the slip face, or lee side of the obstruction. Over time, sand builds up behind obstructions, creating a series of elongate, elevated spits of sand called wind shadows, which lie at right angles to the shoreline. As they increase in size, wind shadows present an even larger barrier to wind, thus more and more sand accretes quickly. Plants are able to colonize wind shadow areas because they are significantly more stable than other areas on the beach. As plants begin to grow, their roots then assist in further stabilizing and anchoring deposited sands. Later, as more plants colonize the upper beach, wind shadows are joined together laterally to form dunes which lie parallel to the shoreline.

Profile of a coastal dune system showing 1) stable backdune;  
2)  secondary dunes;  3) primary dune;  4) foreshore.  

Depending on local wind and wave patterns, a single dune, or a system of dunes may be created over time. Within dune systems, which resemble a series of low peaks and valleys, the first dune above the intertidal zone is called the primary dune. This is the area of active colonization by plants, and the area most affected by waves and heavy winds. Over the crest of the primary dune is the swale: a low, somewhat wet area that separates primary dunes from secondary dunes. In swales, winds generally 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. In many dune systems, secondary dunes are also observed. 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 to secondary dunes. 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.

Dune plants:
Vegetation colonizing the upper beach and foredune must be well adapted to periodic disturbance, and is generally characterized by the presence of salt-adapted, grassy species. Growth of these colonizing species must keep pace with the rate of sand build-up along the foredune if the plants are to survive. On the foredune, beach pioneers such as railroad vine (Ipomoea pes-caprae) and shoreline sea purslane (Sesuvium portulacastrum) meet the primary species of dune colonizers. South of Cape Hatteras, sea oats (Uniola paniculata), a coarse grass that grows as tall as 6 feet and spreads laterally via rhizomes is the principal dune colonizer (Stalter 1993). Sea oats and 2 other dune-building species, bitter panic grass (Panicum amarum) and beach cordgrass (Spartina patens), have growth patterns in which upward growth is actually stimulated by burial in sand. Subsequent lateral growth in these plants allows for the construction and stabilization of a continuous dune ridge. Other plant species that colonize foredunes must be able to grow at a relatively fast rate to prevent their burial in sand (Wagner 1964; Oertel and Lassen 1976; Myers and Ewel 1990).

The dune crest is the area where herbaceous vines and grasses begin to be replaced by shrubby or woody species. Common herbaceous plants of the dune crest include sea ox-eye daisy (Borrichia frutescens), beach sunflower (Helianthus debilis), firewheel (Gaillardia pulchella), and annual phlox (Phlox drummondii). Also common on dune crests are several woody species including sea grape (Coccoloba uvifera), saw palmetto (Serenoa repens), and the invasive Brazilian pepper (Schinus terebinthifolius). Many of the woody species growing on dune crests are often observed to be low-growing and shrubby, despite their growing as robust shrubs or trees in areas inland of the dunes. Much of the reason for this growth habit is due to the well-drained, low nutrient soils of dunes, as well as to the effects of high winds and salt spray. Though most grasses and vines found on dune crests are well adapted to saline conditions, the tender terminal buds of many trees and shrubs growing on dune crests and in swales are killed upon contact with salt spray, resulting in the salt-pruned, windswept canopies commonly seen in the low, stunted trees of Florida’s dune communities.

Swales located between dunes gain an increased measure of protection from winds and salt spray as the dune system builds over time. Because swales can be scoured down nearly to the water table, they are able to support freshwater plants, though most plants that grow in swales have some degrees of salinity tolerance as well. Stands of sea grape (Coccoloba uvifera), saw palmetto (Serenoa repens), and the invasive Brazilian pepper (Schinus terebinthifolius) are common woody species on dune crests and in swales.

Backdunes and secondary dunes generally support a wider variety of vegetation than do foredunes. Additionally, the same species that grow as low shrubs or stunted trees on dune crests, grow in backdune areas as well; though in these more protected locales, they are often able to attain full height. Saw palmetto (Serenoa repens), cabbage palm (Sabal palmetto), live oak (Quercus virginiana), and prickly pear cactus (Opuntia stricta), are all common inhabitants of backdunes and secondary dunes.

Dune animals:
A number rodents, some of which are becoming increasingly rare, utilize dune habitats. The threatened southeastern beach mouse (Peromyscus polionotus niveiventris) can be found in disjunct populations from Cape Canaveral to Sebastian Inlet. Other rodents that inhabit dunes include the cotton mouse (Peromyscus gossypinus palmarius), cotton rat (Sigmodon hispidus littoralis ), and rice rat (Oryzomys palustris).   Rabbits, including the eastern cottontail rabbit (Sylvilagus floridanus), and the marsh rabbit (Sylvilagus palustris paludicola), are also observed on dunes. Several other mammals such as gray foxes (Urocyon cinereoargenteus), raccoons (Procyon lotor), feral pigs (Sus scrofa), and feral cats (Felis catus) 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 (Catoptrophorus semipalmatus), American oystercatcher (Haematopus palliatus), and Wilson’s plover (Charadrius wilsonia), which prefer nest sites in dune areas with sparse grass or herbaceous cover. The laughing gull (Larus atricilla), Caspian tern (Sterna caspia), and the gull-billed tern (Sterna nilotica) also nest in dunes, but prefer areas with somewhat more dense coverage.

Reptiles are also common inhabitants of dunes. Several species of anoles, among them the green anole (Anolis carolinensis), and the brown anole (Anolis sagrei), are quite common. Gopher tortoises (Gopherus polyphemus), while not plentiful, can often be observed in stable backdune areas. Many different types of snakes also live and feed in dune systems. Eastern diamondback rattlesnakes (Crotalus adamanteus), yellow rat snakes (Elaphe obsoleta quadrivittata), eastern coachwhip snakes (Masticophis flagellum), Florida rough green snakes (Opheodrys aestivus carinatus), and coastal dunes crowned snakes (Tantilla relicta pamlica) all utilize grassy dunes or more woody areas of backdunes as habitat.

Human Impacts:
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.

Select a highlighted link below to learn more about that species:

Further Reading:  

Austin 1998. Classification of plant communities in south Florida. Internet document.
     Available at: www.fau.edu/divdept/science/envsci/communities.htm

Carter, R.W.G., T.G.F. Curtis, and M.J. Sheehy-Skeffington. 1992. Coastal dunes
     geomorphology, ecology and management for conservation. A.A.
     Balkema/Rotterdam/Brookfield.

Florida Natural Areas Inventory, Department of Natural Resources. 1990. Guide to the
     Natural Communities of Florida. Publication. 11pp. Tallahassee, FL.

Komar, P.D. and Moore, J.R., editors. 1983. CRC handbook of coastal processes and
     erosion. CRC Press, Inc. Boca Raton, Florida.

Komar, P.D. 1998. Beach processes and sedimentation, 2^nd edition. Prentice Hall,
     Upper Saddle River, New Jersey.

Myers, R.L. and J.J. Ewel, eds. 1990. Ecosystems of Florida. University of Central
     Florida Press, Orlando, FL. 765 pp.

Oertel, G.F. and M. Lassen. 1976. Developmental sequences in Georgia coastal dunes
     and distribution of dune plants. Bull. GA. Acad. Sci. 34: 35 – 48.

Otvos, E.G. 1981. Barrier island formation through nearshore aggradation –
     stratigraphic and field evidence. Mar. Geol. 43:195-243.

Packham, J.R. and A.J. Willis. 1997. Ecology of dunes, salt marsh and shingle.
     Chapman and Hall, London.

Pethick, J. 1984. An introduction to coastal geomorphology. Edward Arnold, London.

Pilkey, O.H. and M.E. Feld. 1972. Onshore transport of continental shelf sediment:
     Atlantic southeastern United States. In: Swift, D.J.P., D.B. Duane and O.H. Pilkey,
     eds. Shelf Sediment Transport: Process and Pattern. Dowden, Hutchinson, Ross.
     Stroudsburg, PA

Schmalzer, P.A. 1995. Biodiversity of saline and brackish marshes of the Indian River
     Lagoon: historic and current patterns. Bulletin of Marine Science 57(1): 37-48

Schmalzer, P.A., B.W. Duncan, V.L. Larson, S. Boyle, and M. Gimond. 1996.
     Reconstructing historic landscapes of the Indian River Lagoon. Proceedings of
     Eco-Informa ’96. 11:849 – 854. Global Networks for Environmental Information,
     Environmental Research Institute of Michigan (ERIM), Ann Arbor, MI

Stalter, R. 1976. Factors affecting vegetational zonation on coastal dunes, Georgetown
     County, SC. In: R.R. Lewis, and D.P. Cole, eds. 3^rd Proc. Annu. Conf. Restoring
     Coastal Veg. Fla. Hillsborough Comm. Coll., Tampa, FL

Stalter, R. 1993. Dry coastal ecosystems of the eastern United States of America. In:
     Ecosystems of the World. Volume 2. Elsevier Science Publications, New York, NY.

Tyndall, R.W. 1985. Role of seed burial, salt spray, and soil moisture deficit in plant
     distribution on the North Carolina Outer Banks. Ph.D. Thesis, University of
     Maryland, College Park, MD.

Wagner, R.H. 1964. The ecology of Uniola paniculata L. in the dune-strand habitat of
     North Carolina. Ecol. Monogr. 34: 79 – 96.

 Report by:  K. Hill, Smithsonian Marine Station
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