Generally dominated by species of broad-leaved evergreen trees and shrubs, maritime hammocks are climax communities influenced heavily by salt spray. Soils are predominantly composed of either sand or peat. Sandy soils are more common along forested dune ridges, while peat is more common among interdune swales and wetlands (Bellis 1995). Chapman (1976) described the progress of sandy soil formation and dune stabilization through four stages: embryo dunes, yellow dunes, gray dunes, and mature, vegetated dunes. Embryo dunes are formed by newly deposited sands accreting on beaches. Over time, sea oats and other coastal plants may colonize the dune and increase its stability. Once this occurs, the dune is called a yellow dune. Gray dunes are characterized by the presence of woody species and shrubs. At this stage, a soil microfauna, consisting largely of mycorrhizae has developed, and organic material from dead leaves and stems begins to accumulate in the substrata. Should these gray dunes remain stable over long periods of time, the climax community of a maritime forest develops. Mature vegetated dunes are characterized by distinct soil profiles: an upper horizon consisting of leaf litter and twigs; a deeper, ashy white horizon that results from leeching of organic materials deeper into the soil; and beneath this, a tan or orange horizon which receives substances leeched from above.

Many factors influence whether particular species will be successful colonizers of the maritime forest. Strong winds, low nutrients, unpredictable supplies of freshwater, erosion, sand-blasting, storm exposure, sand migration, and overwash from the ocean during storm events, are all major influences; however, tolerance to salt spray has been found to be the principal factor that controls vegetative cover in maritime forests (Oosting and Billings 1942, Boyce 1954, Proffitt 1977, Seneca and Broome 1981). Trees closest to the ocean are subject to onshore winds carrying sand and salt spray, which acts not only to prune terminal buds in the canopy top, but also encourages growth of lateral buds, producing over time, the familiar windswept shape of maritime forest canopies. Streamlining of the canopy profile assists growth of maritime forests in several ways. First, the windswept profile of the maritime forest canopy helps to deflect winds up and over the forest, preventing trees from being uprooted during intense storms. Second, dense canopies provide shelter to understory plants and protect the understory from large temperature fluctuations, reducing warming of the soil during the day, and preventing heat loss at night. Third, because trees on the windward edges of the forest show increased growth in their lateral buds, they are somewhat denser overall than more interior trees. As winds blow across the dense canopy, salt spray is deposited. Interior trees are thus protected from the effects of salt spray by the windward trees. This feature allows trees in the interior forest to assume characteristic heights and growth patterns resembling those of mainland forests.

Fire is also considered an "organizer" of forest cover patterns on barrier islands in Florida (Bellis 1995), and has long been a traditional agricultural tool for maintaining open areas, improving grazing lands, and eliminating pest species. Fire characteristics differ between oak-dominated hammocks, and pine-dominated hammocks. In oak forests, a dense evergreen canopy is usually coupled with a sparse, shade-tolerant understory and a somewhat moist litter layer. In pine forests, dense understory vegetation is coupled with a tall, sparse canopy, and significantly drier soils. Thus, fires in pine forests are likely to have a large fuel source close to the ground, resulting in the increased likelihood of intense crown fires. Conversely, oak forests have less fuel at ground level due to a sparsely grown understory. When fire occurs, oak forests tend to smolder close to the ground, consequently making intense crown fires more rare. An examination of fire temperatures in pine vs. oak forests illustrates these characteristics. A study by Williamson and Black (1981) documented that during a fire, air temperatures from the seedling zone to approximately 0.5 m above the soil in pine forests averaged 290° C, while oak forests averaged 175° C. This is significant because pines are often considered to be inferior long-term competitors to oaks. However, Williamson and Black (1981) concluded that maximum temperatures during fires in mixed forests were high enough to eliminate oaks from an area entirely. Thus, even though pines may be inferior competitors to oaks, they may gain competitive advantage over oaks in areas where fires occur.

Maritime forests also have distinctive hydrological features that affect a barrier island’s natural communities, as well as help determine whether development can be sustained. Rainfall is generally the only source of fresh water on barrier islands, and the maritime forest community acts as the primary watershed. Precipitation entering the watershed is rapidly drawn deep into a freshwater lens, which floats above the denser salt water in the permeable sediments beneath barrier islands. A counter-flow is established at the area of contact between fresh and salt water, allowing freshwater at the periphery of the lens to seep upward to the surface and into the ocean or lagoon. Hydrological models show that under ideal conditions, the freshwater lens on a barrier island contains approximately 40 meters of freshwater for each meter of free water table above mean sea level (Ward 1975). Water in the lens is generally fairly low in salts (Proffitt 1977), in spite of the fact that salt spray is a major ecological influence. However, excessive pumping of freshwater from the lens for residential and commercial purposes can lead to loss of the hydrostatic head in the freshwater lens, which could, in turn, increase the rate of salt water intrusion into surface waters on the island (Ward 1975, Winner 1975, 1979; Bellis 1995).

Beyond effective water management, there are a variety of other development considerations regarding maritime forest communities, with habitat fragmentation perhaps being the largest issue. Because maritime forests occur on the most stable areas of barrier islands, they are attractive building sites. Clearing lots for houses involves disturbing or destroying most, if not all, the natural vegetative cover to make space for homes, parking areas, drainage fields, and septic systems. Following construction, native vegetation is often replaced by lawns and ornamental shrubs, many of which are exotic. Another issue regarding the development of barrier islands is road construction. Generally, at least one main road is constructed along the entire length of a barrier island, above the dune ridge at the perimeter of maritime forests, to permit easy access to beaches. Other roads are built laterally to the trunk road for access to developments and private residences. While roads themselves may minimally impact existing forests, they do threaten their growth patterns and species composition because opening the forest canopy allows increased salt penetration to the forest interior. Several studies have confirmed that road building on barrier islands affects salt transport patterns into the interior of maritime forests (Eaton 1979, Seneca and Broome 1981). In these studies, floristic composition, tree viability and canopy height remained nearly constant along the ocean-side perimeter of maritime forests where newly established roads had been constructed. However, along the bay-side of the forests, it was observed that significant die back due to increased salt penetration occurred. Over the 4 years of the study, 57% of the original above-ground vegetation died within 2.5 to 3 m from the bayside edge of the forests, with the most severely affected areas showing complete elimination of the forest canopy. However, die-back in trees was observed to have ceased after approximately 27 months, with 43% of trees able to recover to some degree, showing signs of basal sprouting, stump sprouting, and sprouting from underground stems and roots. At the end of the 4 year study, a new but somewhat lower canopy had begun to develop.

The vegetative composition of maritime forests is diverse, and depends heavily on prevailing physical conditions. Greller (1980) mapped the distribution of maritime forests in Florida, and determined that the upland broad-leaved forests of barrier islands fell into 3 major types: temperate broad-leaved forest, also known as evergreen forest; southern mixed hardwood forest; and tropical forest. Temperate broad-leaved forests are dominated by Quercus virginiana (live oak), and Sabal palmetto (sabal palm) communities. Southern mixed hardwood forests are dominated by Magnolia grandiflora (Southern magnolia), Ilex opaca (American holly), Cornus florida (flowering dogwood), Carya glabra (pignut hickory), and Fagus grandiflora (American beech). Tropical forests are dominated by both evergreen and deciduous species such as Mastichodendron foetidissimum (mastic), Eugenia spp. (stoppers), Lysiloma latisiliqua (wild tamarind), and Bersera simaruba (gumbo limbo).

Many different animal species inhabit Florida’s barrier island communities. In maritime hammocks, insects, small mammals, reptiles and birds dominate the fauna. Common inhabitants include wading birds such as great blue herons (Ardea herodias), great egrets (Casmerodius albus), snowy egrets (Egretta thula), little blue herons (Egretta caerulea), tricolored herons (Egretta tricolor), night herons (Nycticorax spp.), brown pelicans (Pelicanus occidentalis), various ducks, warblers, and others. Birds of prey such as red-shouldered hawks (Buteo lineatus), Cooper’s hawks (Accipiter cooperii), sharp-shinned hawks (Accipiter striatus), and bald eagles (Haliaetus leucocephalus), also utilize hammocks for feeding, roosting and nesting. Small mammals such as eastern cottontails (Sylvilagus palustris), mice (Mus spp.), Norway rats (Rattus norvegicus); and larger mammals such as river otters (Lontra canadensis), and wild boar (Sus scrofa), may thrive in hammock habitats. Reptiles include softshelled turtles (Frionyx ferox), gopher tortoises (Gopherus polyphemus), cottonmouth snakes (Agkistodon piscivorus), southern black racers (Coluber constrictor priapus), Atlantic saltmarsh snakes (Nerodia spp.), eastern diamondback rattlesnakes (Crotalus adamantus), indigo snakes (Drymarchon corais couperi), as well as a variety of skinks and lizards which prey on the abundant insect, frog, and small mammal population.

Click a highlighted link to read more about individual species:

             ¹ Found throughout the IRL
             ² Most common in Northern IRL, Volusia through Brevard Counties
             ³ Most common in Central/Southern IRL
             ⁴ Found from Cape Canaveral to Ft. Pierce Inlet; to south is replaced with
                tropical shrubs and trees

Further Reading:

Art, H., F.H. Bormann, G.K. Voigt, and G.M. Woodwell. 1974. Barrier island forest
     ecosystem: role of meteorological inputs. Science 184:60 – 62.

Bagur, J.D. 1978. Barrier islands of the Atlantic and Gulf coasts of the united States: an
     annotated bibliography. U.S. Fish and Wildlife Service FWS/OBS – 77/56. 215 pp.

Barrick, W.E. 1973. Salt tolerant plants for Florida landscapes. Proceedings of the Florida
     State Horticultural Society 91:82 – 84.

Bellis, V.J. 1992. Floristic continuity among the maritime forests of the Atlantic coast of the
     United States. Pages 21-29 in: C.A. Cole and F.K. Turner, editors. Barrier island
     Ecology of the Mid-Atlantic Coast: A Symposium. U.S. Department of the Interior,
     National Park Service, Atlanta, GA.

Bellis, V.J. 1995. Ecology of maritime forests of the southern Atlantic coast: a community
     profile. Biological report 30, May 1995. National Biological Service, U.S. Department
     of the Interior. Washington, D.C. 89 pp.

Bellis, V.J. and C.E. Proffitt. 1976. Maritime forests. Pages 22 – 28 in: D. Brower, D.
     Frankenberg, and F. Parker, editors. Ecological Determinants of Coastal Area
     Management: Vol. 2. University of North Carolina Sea Grant Publication
     UNC-GS-76-05. 392 pp.

Bourdeau, P.F. an H.J. Oosting. 1959. The maritime live oak forest in North Carolina.
     Ecology 40:148-152.

Boyce, S.G. 1954. The salt spray community. Ecological Monographs 24:29-68.

Cockfield, B.A. J.B. Tormey, and D.M. Forsythe. 1980. Barrier island maritime forest.
     American Birds 34: 29.

Cowardin, L.M., V. Carter, F.C. Golt, and E.T. LaRoe. 1979. Classification of wetlands
     and deep water habitats of the United States. U.S. Fish and Wildlife Service
     FWS/OBS-79-31. 103 pp.

Davison, K. and S.P. Bratton. 1986. The vegetation history of Canaveral National
     Seashore, Florida. University of Georgia Institute of Ecology. Cooperative Park Studies
     Unit Technical Report 22. 75 pp.

Doutt, J.K. 1941. Wind pruning and salt spray as a factor in ecology. Ecology 22:195-196.

Eaton, T.E. 1979. Natural and artificially altered patterns of salt spray across a forested
     barrier island. Atmospheric Environment 13:705-709.

Gehlhausen, S. and M.G. Harper. 1998. Management of maritime communities for
     threatened and endangered species. U.S. Army Corps of Engineers, Construction
     Engineering Research Laboratories Technical Report 98/79, May 1998.

Greller, A.M. 1980. Correlation of some climate statistics with distribution of broadleaved
     forest zones in Florida, USA. Bulletin of the Torrey Botanical Club 107: 189-219.

Johnson, A.F. and M.G. Barbour. 1990. Dunes and maritime forests. Pages 429-480 in:
     R.L. Myers and J.J. Ewell, editors. Ecosystems of Florida. University Press of Florida,
     Gainesville, FL. 765 pp.

Oosting, H.J. 1945. Tolerance to salt spray of plants of coastal dunes. Ecology 26:85-89.

Oosting, H.J. 1954. Ecological processes and vegetation of the maritime strand in the
     southeastern United States. Botanical Review 20:226-262.

Oosting, H.J. and W.D. Billings. 1942. Factors affecting vegetation zonation on coastal
     dunes. Ecology 23:131-142.

Proffitt, C.E. 1977. Atmospheric inputs and flux of chloride, calcium and magnesium in a
     maritime forest on Bogue Bank, NC. M.A. Thesis, East Carolina University, Greenville,
     NC. 123 pp.

Seneca, E.D. and S.W. Broome. 1981. The effect of highway construction on maritime
     vegetation in North Carolina. A research report submitted to the North Carolina
     Department of Transportation, Division of Highways, Raleigh, NC. 73 pp.

Simon, D.M. 1986. Fire effects in coastal habitats of east central Florida. University of
     Georgia Institute of Ecology. Cooperative Park Studies Unit Technical Report 27. 
     140 pp.

Ward, R.C. 1975. Principles of hydrology. McGraw-Hill Ltd., Maidenhead, Berkshire,
     England. 367 pp.

Wells, B.W. 1942. Ecological problems of the Southeastern United States coastal plain.
     Botanical Review 8:533-561.

Williamson, R.B. and E.M. Black. 1981. High temperature of forest fires under pines as a
     selective advantage over oaks. Nature 293:643-644.

Winner, M.D. Jr. 1975. Groundwater resources of the Cape Hatteras National Seashore,
     North Carolina. U.S. Geological survey Atlas HA-540, Reston, VA. 2 maps.

Winner, M.D. Jr. 1979. Freshwater availability of an offshore barrier island. U.S.
     Geological Survey Professional Paper 1150, U.S. Geological Survey. 117 pp.

Zucchino, L.R. 1990. A guide to protecting maritime forests through planning and design.
     North Carolina Department of Environment, Health, and Natural Resources, Division of
     Coastal Management. 24 pp.

Report by:  K. Hill, Smithsonian Marine Station
Submit additional information, photos or comments to:
irl_webmaster@si.edu
Page last updated: July 16,  2002