Home Preface Synopsis History of the Study Regional System Ecosystems - Freshwater and Terrestrial - Coastal   -  Sandy beaches   >  Mangroves & salt marshes      - Nutrient cycles & Destruction   -  Estuaries & bays   -  FL. reef tract - Man-dominated Hydrologic Systems Final Word References Appendices PDF version

Mangrove forests and associated salt marshes form a crescent-shaped region around the south tip of Florida. This region ranges in width from a thin fringe of trees along the rivers and behind coastal dunes to mangrove flats and thickets that spread as much as 24 km (15 mi) inland. Mangrove forests achieve their greatest development along the deep estuaries behind and north of Cape Sable, where they penetrate the freshwater marshes along the Shark River and its 38 tributaries (fig. 15).

In addition to fringing the coastline, mangroves form small to moderate-sized islands. The mangroves of the Ten Thousand Islands area form an intricate network. Instead of being built upon sediment mounds, they stand on oyster bars that grow perpendicular to the tidal flows and thus give the islands their characteristic shape (Craighead, 1971). In contrast, mangrove islands within Florida Bay are generally round or elliptical and are of two types. One is completely covered with mangroves; the other is an atoll-like rim of mangroves enclosing a depression. The second type has probably been derived from tree islands that existed in freshwater swamps when sea level was lower.

Mangrove trees on the mainland develop into large stands along tidal riverbanks, where the sediment is deepest, and form extensive forests, called mangrove flats, on the numerous islands and inland tidal flats created by these rivers. Even though elevations along the riverbanks are generally higher than in the center of the flats, breaks in the banks allow frequent flooding into the flats. Because mangrove trees typically decrease in size toward the center of the island flats, these depressions are probably ponds that are being filled from the periphery. This pattern of growth, plus evidence of deep mangrove peat beds hundreds of yards offshore, suggests that south Florida's coastline is gradually being flooded and that the mangrove forests are encroaching on the freshwater marshes (Craighead, 1971).

Larger areas of mangroves, the scrub mangroves, exist inland of the shoreline and mangrove flats. The trees are uniformly 1.2 to 1.8 m (4 to 6 ft) tall and form nearly impenetrable masses. They are stunted perhaps because a thick fibrous peat substrate prevents roots from penetrating more than a few feet. Another type of scrub mangrove grows where the underlying sediment is mud rock. These are rarely more than 1.2 m (4 ft) tall and of low density.

The mangrove zone in south Florida is mixed with ponds, prairies, mounds, and islands of differing vegetation. Craighead (1971) separates the saline mangrove zone into 13 principal plant associations: beaches and mangrove shores; batis marshes; buttonwood or madeira hammocks; saline buttonwood islands and strands; shell mounds; marl or coastal prairies; Cape Sable hammocks; Cape Sable prairies; ponds; mangrove flats; scrub mangrove; Juncus marshes; and riverbanks. Almost all this plant diversity is formed or maintained in some way by tidal inundation and hurricanes.

FIGURE 15. Mangrove coastline of southwest Florida. [larger image]

In 1960 hurricanes destroyed large sections of mature mangrove forests along Florida's southwest coast. Although trees were uprooted or defoliated, they showed signs of some recovery after a few weeks. Shortly after, however, the forests died, and the trees were attacked by wood-boring beetles. It appears that most of the forest was killed by a smothering layer of marl driven inland by hurricane tides. Seeds carried inland with the marl eventually sprouted to form batis prairies, marshes, buttonwood hammocks, and clusters of new mangrove seedlings. In time, mangroves and other trees will grow over the prairies and marshes.

Hurricanes have deposited debris and mud along an old shoreline in south Florida and thus created a low embankment (sometimes called the "Buttonwood Embankment"), which extends along the shoreline of Florida Bay to Flamingo and then continues to the North and Watson Rivers. Except where it is broken by rivers or has been destroyed by fires, the embankment forms a natural levee that impounds freshwater and separates the saline mangrove zone from the freshwater uplands.

Plants that grow in the saline mangrove zone are adapted to varying degrees of tidal inundation. The black mangrove and the white mangrove require periodic saltwater inundation. The red mangrove, which is the dominant mangrove species, can grow in salt, brackish, or freshwater, but seedlings must be covered by water until they are 3 years old. Because of the red mangrove's tolerance to freshwater, it extends farthest inland; however, it seems to be most susceptible to storm damage. Perhaps for that reason, mature mangrove forests and shorelines are dominated by large old black mangrove trees. Another common tree in the mangrove zone, the buttonwood, prefers higher and drier ground. It grows along riverbanks, embankments, old tree islands, and marl prairies where layers of hurricane mud have raised the ground elevation.

In their earliest stages, the marl prairies contain salt-tolerant ground covers like saltwort, glasswort, sea blite, sea purslane, samphire, waterhemp, sea daisy, and the cordgrasses. Many marshes and ponds in the prairies and mangrove flats contain freshwater during the rainy season. Such marsh plants as black rush, fringe rush, sawgrass, and cordgrass grow there because they tolerate changes in salinity.