Within the continental United States, Florida is second only to California in the richness and level of endemism of its indigenous flora. The state has approximately 3,500 species of native and naturalized vascular plants, eight percent of which occur only within its borders (Cox et al. 1994). These species are distributed among ecosystems that range widely in geologic history, some dating to the emergence of Florida from the sea, others coming together in their current form only since humans arrived on the peninsula. The remarkable diversity of plant taxa and habitat types that is the natural heritage of Florida is the product of a complex interaction of environmental and human factors that has unfolded over many generations.
The terrestrial history of Florida began about 25 million years ago when most of the peninsula north of present-day Lake Okeechobee emerged from the sea. Florida was never again completely below water, but its length and width varied greatly over subsequent years. The most rapid and dramatic oscillations occurred during the ca. 2 million years of the Pleistocene epoch, as land successively emerged and was inundated during numerous glacial-interglacial cycles. At the peak of the most recent glacial advance (ca. 20,000 to 13,000 years B.P. [before present]), the peninsula was twice its present size. As the retreating glaciers melted, the extensive mesic forests and scrub communities that had covered the western reaches of the Floridan plateau gradually submerged beneath the rising waters of the Gulf of Mexico. It was during this post glacial Holocene period of rising seas that most of Florida's 7,800 lakes and extensive freshwater wetland systems came into being.
Today, Florida is a long, narrow extension of the North American continent into tropical seas. It stretches 500 miles from the Georgia state line to the tip of the Keys, with no point more than 60 miles from the Gulf of Mexico or the Atlantic Ocean. Bathed by warm Gulfstream currents for much of its length, the peninsula enjoys a warm and humid climate favorable for plant growth. Florida thus provides a climatic and geographic bridge between temperate North America and the Caribbean tropics -- a bridge which has facilitated colonization by tropical plant species hundreds of miles north of the Tropic of Cancer. In the southern part of the state the intermingling of temperate and tropical plants -- many near their range limits -- has created rich and singular species assemblages (Long and Lakela 1977). In contrast, the resident south Florida fauna is relatively depauperate and of predominantly temperate origin. From an ecosystem perspective, several potential functional niches are currently unfilled by this species-poor faunal mix; feedback effects on plant community structure are possi ble, but have not yet been well documented.
Over Florida's Paleozoic basement rock of quartz and sandstone, marine carbonate sediments have accumulated over the course of 200-225 million years. The limestone platform that formed from these sediments may be several hundred or more meters thick. In typical upland settings, the soil capping this massive carbonate bed rock is a thin sand or clay. One exception is along the ancient central spine of the peninsula, where deep sand ridges deposited during recent interglacial periods comprise some of Florida's highest elevations -- ca. 100 meters above sea level. Another exception is the Miami Rock Ridge, extending into the Keys. Here extensive outcrops of limestone are rarely covered by more than a few inches of organic soil.
Most of Florida is flat and low. With the water table close to the surface, minimal differences in topography result in dramatic changes in vegetation character. Extremely xeric and hydric habitats can be juxtaposed over elevational gradients measured in centimeters. Hammocks arise from the surrounding marshes, flatwoods give way to swamps, and sandhills are dotted with bogs. In these settings, small changes in the level of the surficial aquifers -- Floridan in the north, Biscayne in the south -- may profoundly affect vegetation distribution and the fate of whole ecosystems.
The late Pleistocene pollen record illustrates Florida's landscape on the eve of its colonization by native Americans, approximately 10,000 years B.P. At that time the ecosystems of north-central Florida were like those of today -- pineland and sandhill, interlaced with temperate broadleaf forests. Further south, sand pine scrub and xeric oak woodlands cloaked the sandy central ridge of the state. In South Florida, scrub vegetation may have persisted until 5,000 B.P., when rising sea level elevated the water table to the surface, creating cypress swamps and sawgrass marshes, including the modern Everglades.
Thus, when European colonization began in the mid-16th century, interior Florida was a mixture of xeric or mesic pineland and wetlands. Mangrove swamps dominated low-wave-energy shorelines in the subtropical southern half of the state, while Spartina salt-marshes were predominant in the temperate northern half. High-wave-energy shorelines, most prominently on banier islands and the Atlantic coast, were characterized by grassy sand dunes backed by scrub or maritime hammocks. The most complete description of the pre-settlement distribution of Florida vegetation is provided by Davis (1967), which we have included as Figure 1. The mapping units used by Davis are equivalent to 13 major ecosystem types still present in the state. Their descriptions are based in large part on the essays in Ecosystems of Florida (Myers and Ewel 1990).
Pine flatwoods are characterized by a low, flat topography, sandy soils and frequent fire (Abrahamson and Hartnett 1990). Overstory composition may be any of four pine taxa either in pure or mixed stands: longleaf pine (Pinus palustris ), pond pine (P. serotina ), northern slash pine (P. elliottii var. elliottii ) or southern slash pine (P. elliottii var. densa ). The understory is usually dominated by saw palmetto (Serenoa repens ), gallberry (Ilex glabra ), fetterbush (Lyonia lucida ), and wiregrass (Aristida stricta ). Wet prairie, a subtype within pine flatwoods mostly found in the Panhandle, is a graminaceous, seasonally inundated community with scattered longleaf or slash pines. The dominant species include wiregrass and Rhynchospora spp. in wetter areas. There is a high level of endemism.
Sandhills are also known as high pine in contrast to the "low" pine of flatwoods (Myers 1990). They occupy well-drained sand ridges and are sustained by frequent low-intensity fire. Longleaf pine is the single canopy species over a continuous cover of wiregrass and occasional clumps of deciduous oaks (Quercus spp).
Pine rocklands are associated with outcroppings of limestone. They occur almost exclusively on the Miami Rock Ridge and on several islands in the lower Keys (Snyder et al. 1990). The sole canopy species is southern slash pine. Below the pine canopy is a diverse understory of tropical and temperate shrubs, palms and herbs, with many endemic taxa. Pine forests in the eastern third of Big Cypress National Preserve are generally classified as pine rocklands; in fact, they are intermediate in substrate and species composition between pine rocklands and pine flatwoods, and lack the characteristic endemic herbs of true pine rocklands.
Scrub is a xeric ecosystem associated with either coastal or inland sand dunes (Myers 1990). It is almost entirely endemic to Florida. Coastal scrub occurs as a backdune community landward of high-wave-energy shorelines on both the Atlantic and Gulf coasts. Inland scrub occupies the ancient (Pleistocene) sand dunes of the central ridge in Ocala National Forest and the southern third of the Lake Wales Ridge. Both coastal and inland scrub are dominated by xeric evergreen oaks and for Florida rosemary (Ceratiola ericoides ), with or without a sand pine (Pinus clausa ) overstory. The herb layer is typically depauperate, but characterized by many narrowly endemic species amidst a ground cover of lichens and patches of bare sand.
The environment of dry prairies is similar to that of pine flatwoods, but without a pine overstory (Abrahamson and Hartnett 1990). They are essentially open grasslands of wiregrass, lovegrass (Eragrostis spp.) and bluestem (Andropogon spp.), with scattered patches of low shrubs such as saw palmetto, fetterbush, and blueberry (Vaccinium spp.). Much of the extant dry prairie in the state shows evidence of a previous pine overstory.
Scrubby flatwoods occupy ecotones between pine flatwoods and scrub communities. They are essentially mesic flatwoods with a scrub understory, and are characterized by slash pine instead of sand pine with several species of xeric oak and other shrubs (Abrahamson and Hartnett 1990).
In addition to these six pine-associated types, there are three other major upland ecosystems in Florida:
Temperate hardwood forests occur on a variety of sites throughout the upper two-thirds of the state but are especially abundant in the northern third (Platt and Schwartz 1990). Woody plant species richness in this diverse forest type is considered to be the highest in North America, with both deciduous and evergreen species well represented. The dominant trees are species of well-known temperate genera such as oak, hickory (Carya spp.), beech (Fagus spp.), and maple (Acer spp.). In the Panhandle, these forests often contain a pine component and are sometimes distinguished as Mixed Hardwood/Pine Forests.
Tropical hardwood hammocks, also known as rockland hammocks, are associated with outcroppings of limestone (Snyder et al. 1990). They occur only in the southernmost counties of the state where they occupy elevated, rarely inundated and relatively fire-free sites. About 150 species of woody plants are found in these diverse forests. Most are broadleaved evergreen trees of West Indian origin.
Coastal uplands occur on high energy shorelines and on barrier islands of the Gulf and Atlantic coasts (Johnson and Barbour 1990). These communities occupy three zones: pioneer, transition, and stable. The pioneer zone extends from the high tide line to the foredune, is subjected to the greatest stress from storm tides and salt spray, and is characterized by sand-binding grasses like sea-oats (Uniola paniculata ) and panic grass (Panicum spp.). The stable or backdune zone is furthest from the shoreline, is subjected to the least stress, and generally resembles the dominant plant community of the region in which it occurs. Its floristic character therefore varies from scrub to pine flatwood to maritime hammock. The transition zone is a grassy or shrubby region in termediate in location and character between the foredunes and the backdunes.
Swamps are forested wetlands occurring on a variety of substrates and ranging widely in hydrologic regime (Ewel 1990). Swamps may occur in river floodplains or lake margins, or may occupy more or less extensive depressions. Recognized floristic variation includes cypress (Taxodium spp.), bay (species of Persea , Magnolia , or Gordonia ), hardwood, and shrub (Cyrilla racemiflora or Cliftonia monophylla ) swamps.
Marshes are wetlands dominated by herbaceous plants rooted in and generally emergent from shallow water (Kushlan 1990). The Everglades, Florida's best-known marsh, is recognized as an International Biosphere Reserve. Other extensive marshes are associated with the Kissimmee and St. John's Rivers. Marshes are categorized on the basis of their dominant vegetation. These include sawgrass (Cladium jamaicensis ), cattail Typha spp.), and water lily (Nymphaea odorata or Nelumbo lutea ) marshes as well as seasonally inundated prairies dominated by beakrushes (Rhynchospora spp.) or muhly grass (Muhlenbergia capillaris ).
Salt water wetlands
Salt marshes are coastal ecosystems of salt-tolerant herbs occupying low-wave energy intertidal zones (Montague and Wiegert 1990). They are at least occasionally inundated with salt water. Characteristic species include cordgrass (Spartina spp.), needlerush (Juncus roemerianus ), glasswort (Salicornia spp.), and saltwort (Basis maritima ).
Mangroves are low-wave-energy intertidal swamps dominated by three species of woody facultative halophytes: red (Rhizophora mangle ), black (Avicennia gerrninans ), and white (Laguncularia racemosa ) mangrove (Odum and McIvor 1990). Buttonwood (Conocarpus erectus ), a mangrove associate, occurs inland of the true mangroves and is transitional into upland communities.
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