Maritime forests occur along the entire Atlantic coast of the United States, interrupted by natural features such as inlets and bays, and human activities including coastal development and agriculture. Adjacent maritime forests tend to be vegetatively similar to one another, but overall vegetation profiles change with latitude.
Florida, which has the longest coastline in the contiguous United States, has approximately 468,000 acres of barrier islands. Maritime forests occupy the highest, most stable areas of these islands, atop stable, vegetated dunes. The present location and extent of today's maritime forests were established approximately 5,000 years ago.
Many factors influence whether particular species will be successful colonizers of the maritime forest. Soil composition, salt spray, fire, hydrology and human activity are all major influencers of hammock plant community composition.
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.
Mature vegetated dunes feature distinct soil profiles. The upper horizon consists of leaf litter and twigs. A deeper, ashy white horizon results from leeching of organic materials deeper into the soil. Beneath this is a tan or orange horizon which receives substances leeched from above.
Tolerance to salt spray is considered the principal factor that controls vegetative cover in maritime forests. 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 considered an "organizer" of forest cover patterns on barrier islands in Florida. It has long been a traditional agricultural tool for maintaining open areas, improving grazing lands, and eliminating pest species.
Fire characteristics differ depending on canopy species composition.
In oak forests, a dense evergreen canopy is usually coupled with a sparse, shade-tolerant understory and a somewhat moist litter layer. With less fuel at ground level, when fire does occur, it tends to smolder close to the ground, making crown fires infrequent.
By contrast, in pine forests, dense understory vegetation is coupled with a tall, sparse canopy, and significantly drier soils. 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.
Though pines are considered to be inferior long-term competitors to oaks, maximum fire temperatures in mixed forests are high enough to eliminate oaks from an area entirely. Fire may allow pines to gain a competitive advantage to oaks in areas where fires occur.
Maritime forests have distinctive hydrological features that affect a barrier island's natural communities, and help determine whether human 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. The volume of water in these lenses can be substantial: hydrological models have shown that the freshwater lens on a barrier island can contain approximately 40 meters of freshwater for each meter of free water table above mean sea level.
At the area of contact between fresh and salt water, freshwater at the edges of the lens seeps upward to the surface, and into the overlying ocean or lagoon. Water in the lens is generally fairly low in salts, in spite of the fact that salt spray is a major ecological influence.
However, excessive pumping of freshwater from the lens reservoir for residential and commercial purposes can lead to loss of the hydrostatic head in the freshwater lens. In turn, this could increase the rate of salt water intrusion into surface waters on the island.
In addition to water management, habitat fragmentation and road construction are two other major impacts of human activities on maritime hammock barrier islands in Florida.
Because maritime forests occur on the most stable areas of barrier islands, they are highly desirable building sites. Clearing lots for houses involves disturbing or destroying most, if not all, the natural vegetative cover to make space for buildings, parking areas, drainage fields, and septic systems. Following construction, dominant landscape plants include grass lawns and ornamental shrubs, many of which are exotic.
The fragmentation of forest hammocks allows non-native and weedy species to expand more rapidly, and increases competitive pressure on native plant and animal species by compressing them into smaller and smaller disconnected islands of habitat.
Road construction has several surprising impacts as well. To permit easy access to beaches, usually 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, with other roads built laterally for access to developments and residences. While the roads themselves minimally impact existing forests, the openings they create in the forest canopy exposes the forest interior to increased salt penetration, threatening growth patterns and species composition.
Florida’s maritime hammocks fall into three 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 live oak and sabal palms. Southern mixed hardwood forests are dominated by Southern magnolia, American holly, flowering dogwood, pignut hickory and American beech. Tropical forests are characterized by both evergreen and deciduous species such as mastic, Eugenia, wild tamarind and 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, birds of prey, small mammals and larger mammals such as river otters and wild boar. Reptiles include soft-shelled turtles, gopher tortoises, a variety of snakes, as well skinks and lizards which prey on the abundant insect, frog, and small mammal population.