FOR 43

UNIVERSITY OF FLORIDA

Cooperative Extension Service

Institute of Food and Agricultural Sciences

Mangroves-Florida's Coastal Trees

______________________________________

Beverly E. Law and Nancy P. Arny*

   Mangrove forests are found in tropical and subtropical tidelands throughout the world. In Florida, mangroves occur in coastal areas from St. Augustine on the Atlantic southward and up the Gulf Coast to Cedar Key. Killing frosts keep mangroves from growing any farther north. Mangrove forest communities increase in size and importance towards the southern tip of Florida. The Ten Thousand Islands of southwest Florida have the highest concentration of mangroves in the state.

    The mangrove ecological community includes four tree species collectively called mangroves. Scientifically, they are only distantly related and are grouped more by ecological function. The mangrove community is valued for its protection and stabilization of lowlying coastal lands and its importance in estuarine and coastal fishery food chains. Mangroves serve as feeding, breeding, and nursery grounds for a great variety of fish, shellfish, birds, and other wildlife.

The Mangrove Habitat

    Mangroves are found on offshore islands and in tidal estuaries where freshwater from the land mixes with saltwater. Each mangrove species has a different level of salt tolerance, which in part determines its location in tidal zones. Mangroves are highly dependent on freshwater runoff to maintain an optimum salinity balance.

[Map of Mangrove Ranges]

Along the coast, mangrove-fringed estuaries appear to be a maze of connected islands surrounded by twisting water channels. Soils associated with mangrove communities are peat or fine sand underlain by sand or clays that become saturated with this brackish water. Mud flats are exposed as tides recede, drawing hundreds of white ibis and other wading birds to the abundance of crustaceans and other marine life. The Rookery Bay Sanctuary near Marco Island is a prime example of mangrove habitat and is a bird watchers paradise.

Mangroves are an integral part of estuarine food chains, producing eight metric tons per hectare (3.6 tons/acre) of leaf litter per year. Leaves dropped from the trees are decomposed by fungi and other microorganisms which add to the nutritive value of the litter. As the leaves decompose into detritus and are flushed into the estuary, they become food for a wide variety of marine life. The food chain continues with these organisms serving as a valuable food source for economically important shrimp, crabs, and fish. Anestimated 75 percent of the game fish and 90 percent of the commercial species in south Florida depend on the mangrove system.

A common myth about mangroves is the belief that they build land. This is true only under certain conditions. Mangroves typically follow local patterns of erosion and accumulation of soil, although they do stabilize soil following the colonization of newly formed shoals and embankments.

Removal of these trees and the destruction of mangrove wetland habitats by waterfront development endangers the natural systems of Florida's coastal zone. Even activities in nearby uplands may cause changes in water runoff patterns which eventually alter the plant composition of the community. This, in turn, affects us through coastal erosion and reduction of food at the base of the chain supporting commercial fisheries.

The following descriptions of Florida's four main mangrove species do not rank them in order of importance. In some cases red mangroves and the fringe forest type they are associated with have been given preference over the other species and their forest types for protection from destruction. There is no scientific evidence to justify this action.

Red Mangrove -(Rhizophora mangle L.)

Red mangroves can be found from Daytona Beach and Cedar Key southward. They grow closer to the water than the other mangroves and are usually flooded at high tide. Red mangroves can root on intertidal surfaces such as oyster beds and sand bars forming "mangrove islands". They are easily identified by the tall arching roots, called prop-roots, originating from trunks and branches. These roots are adapted to supply air to the underground roots, as well as adding to the stability of the trees.

[Drawing of Red Mangrove Stem]

[Drawing of Red Mangrove With Prop Roots]

New trees establish themselves further away from the original trees by a well adapted reproductive process. Seeds sprout into torpedo-shaped seedlings while still attached to the parent tree. Sprouts then drop to the ground or into the water. When a seedling lands in the water it may either take root among the other mangrove roots or float with the current until it drifts on to suitable ground. Seedlings remain viable for long periods of time and are able to become established after floating as long as 12 months. Seed germination on the tree is a feature common to both red and black mangroves. It undoubtedly gives the seedlings a better chance of survival in a harsh environment.

In Florida, red mangroves are shrubs or small trees, averaging 20 feet in height. Leaves are opposite, eggshaped to elliptical, leathery and shiny dark green above and paler underneath. Leaf edges are smooth and somewhat thickened. Clusters of 3 to 4 pale yellow flowers can be seen in the spring.

Black Mangrove -Avicennia germinans (L.) L.) (syn: A. nitida)

Black mangroves grow closer to the shore where they are reached only by high tides. At the northern edge of their range, St. Augustine and Cedar Key, these trees are small and shrub-like. Larger trees up to 50 feet tall with a branch spread of 35 feet have been located further south around Sanibel-Captiva Islands.Black mangroves can be easily identified by the numerous pencil-like breathing tubes, called pneumatophores, which grow vertically from the mud to just above the highest sustained water level. Like the prop roots of the red mangrove, these provide air to the underground and underwater roots.

[Drawing of black mangrove with pneumatophores]

Leaves of the black mangrove are opposite, 2 to 4 inches long, ¾ to 1½ inches wide, tapering at the base, shiny above, hairy underneath, and sometimes slightly rolled along the edges. The tops and underside of the leaves are often coated with salt crystals eliminated by the tree. Black mangroves bloom heavily in June and July with white flowers.

The "germinans" portion of the scientific name refers to the habit of seed germination of the tree. Black mangroves have green, lima bean-shaped seeds, which are split when they germinate on the parent tree. Seedling establishment is limited to areas above water level at low tide.

[Drawing of Black mangrove stem]

During black mangrove bloom, beekeepers set up their hives to collect the nectar for production of "mangrove honey". This honey is of very high quality. Considerable quantities were made in the United States until about 1895. Hurricane destruction of the best forests decreased honey production. In recent years there has been renewed interest in this product.

[Drawing of Bee on black mangrove blossom]

White Mangrove -(Laguncularia racemosa (L.) Gaertn. f)

White mangroves range from Volusia County and Levy County southward. They are easily differentiated from other mangroves by succulent leaves which are rounded at the base and tip and smooth underneath. Two glands at the base of each leaf excrete sugar and are called nectaries. Some insects feed on the sugar. The fruit is small, dry, leathery and ribbed. It contains a dark red seed, and is buoyant allowing it to float to new growing sites.

[Drawing of White mangrove stem]

The white mangrove may have pneumatophores and/or prop roots depending on habitat conditions. Most plants have neither. It is fast growing in fertile habitats.Its flowers are greenish-white and produce good honey.

Buttonwood -(Conocarpus erectus L.)

This mangrove is in the same family as the white mangrove but is often considered only an associate of mangroves. It grows along the coast from Cedar Key and Brevard County southward. It is shrubby along the shore, but takes on a tree form further inland.

The "Button" part of the name comes from the button-like appearance of the dense, rounded flower heads that grow in a branched cluster, and the purplish-green, round, cone-like fruit. The other three mangroves have leaves located opposite one another. Button mangrove leaves are alternate, leathery, pointed at the tips, have smooth edges, and two glands at the base of each leaf.

[Drawing of Buttonwood stem]

Suggested Reading

The following publications were used in preparation of this fact sheet and are excellent sources for further reading. Growing Native, Nature Guide to Florida, and "26 Ecological Communities of Florida" group Floridas subtropical plants by natural communities, providing descriptions and illustrations of plants, their habitats, and interrelationships. "Coastal Plantsof Florida - A Key to Good Land Management" has color photos of each plant accompanied by shore descriptions. Trees of North America and Guide to Southern Trees are scientific guides for identification of trees. "Mangrove Misconceptions and Regulatory Guidelines" exposes misconceptions of relative importance of mangroves and provides guidelines for making resource decisions. Mangroves of the world are described in Mangrove Vegetation. Rhizophora mangle L. and Avicennia germinans (L.) L. detail the habitat, life history, and uses of the respective species. "Mangroves: their value and perpetuation"covers the mangrove habitat and values.

References

1. Chapman, V.J. Mangrove Vegetation. Strauss and Cramer GmBH, Germany, 1976.

2. Division of Forestry. "Coastal Plants of Florida - A Key to Good Land Management." 1979.

3. Elias, T.S. (1980) Trees of North America. Outdoor Life/Nature Books, New York, 1980.

4. Gore, R. "The Tree Nobody Liked." National Geographic v. 151(5), 1977. National          Geographic Society, Washington, D.C.

5. Harrar, E.S. and J.G. Harrar. Guide to Southern Trees. Dover Publications, Inc., New York, 1962.

6. Jimenez, J.A. and A.E. Lugo. (1985) "Rhizophora mangle L. - Red Mangrove."    SO-ITF-SM-2. U.S. Government Printing Office, Washington, D.C., 1985.

7. Jimenez, J.A. and A.E. Lugo. "Avicennia germinans (L.) L. - Black Mangrove."  SO-ITFSM-4. U.S. Government Printing Office, Washington, D.C., 1985.

8. Rabkin, R., and Rabkin, J. Nature Guide to Florida. Banyan Books, Miami, 1978.

9. Snedaker, S.C. "Mangroves: Their value and perpetuation." Nature and Resources. v. 15(3):   6- 13. Unesco: Paris, France, 1978.

10. Snedaker, S.C. and M.S. Brown. "Mangrove Misconceptions and Regulatory Guidelines."  Proc. U.S. Fish and Wildlife Service Workshop on Coastal Ecosystems of the S.E. U.S., U.S.  Fish and Wildlife Service, Washington, D.C., 1981. p. 61-67.

11. United States Department of Agriculture Soil Conservation Service, "26 Ecological  Communities of Florida." 1985.

12. Workman, R.W. (1980) Growing Native. Sanibel-Captiva Conservation Foundation, Inc.,  Sanibel, 1980.

* Beverly E. Law is a former Extension Assistant and Nancy P. Arny is Associate Professor in Natural Resources Education, School of Forest Resources and Conservation Institute of Food and Agricultural Sciences (IFAS), University of Florida, Gainesville, FL 32611.