Ecosystems: Mangroves By Cathy Vogelsong Reprinted from The Understory, Aug.-Sep. 2005 <--Red Mangrove The word "mangrove" is not a taxonomic designation like other plant names. Mangrove is a term applied to plants that share a set of adaptations that allow them to survive in salt water, and to the ecosystem/community dominated by these plants. In Florida, mangrove communities are found in brackish water estuary systems along the coast in the southern half of the state. Ninety percent of mangrove acreage in Florida is found in four southern counties--Lee, Collier, Monroe and Dade. The mangrove system is defined by and limited by five characteristics. These are "climate, salt water, water fluctuation, runoff of terrestrial nutrients, and substrate and wave energy," per Odum (Myers and Ewel, Ecosystems of Florida, p. 521): 1) Mangroves are tropical trees, requiring an average annual temperature over 65 degrees Fahrenheit, though they can withstand occasional light freezes. This limits the northernmost distribution of mangrove communities in Florida to Pinellas County on the west coast and Cape Canaveral on the east coast. The more cold-tolerant black mangrove can be found as far as the north Gulf coast, but in stunted form. Periodic hurricane damage also limits the height of mangroves in Florida. 2) Mangrove trees are "facultative halophytes." That means they can grow as well in fresh water as in salt water. But mangroves are outcompeted in freshwater environments. (Among other reasons, mangroves need full sun to survive and are easily overgrown.) Salt excludes competition. 3) Water fluctuation (both tidal and freshwater runoff) carries nutrients to the trees and flushes out mineral accumulations. For example, mangroves cannot live in salterns (or salt barrens). Salterns appear to be good mangrove habitat, but infrequent water fluctuation causes extremely high salinity there. 4) Mangroves have adaptations to allow them to survive in unstable, fine silty anaerobic soils (which also eliminate competition). These would be disrupted by high waves, so mangroves are found in areas of low wave energy. 5) All of these interrelated conditions affect nutrient availability, most of which comes from terrestrial runoff. In Florida three unrelated species are considered to be true mangroves, with special adaptation to high salt and low freshwater availability. The three species often grow in a distinctive zone pattern. This may be due to interspecies competition and special adaptations, or to simple seed predation and tidal sorting of propagules by size. All mangroves have complex mechanisms to either exclude or excrete salt. They have broad shallow root systems with corky anchoring roots that extend no more than one meter deep. Lenticels (pores) and a special tissue called arenchyma allow oxygen to get to the roots, but close at high tie to prevent water from entering. Thick succulent leaves with a waxy cuticle, and sunken stomates, are also adaptations to xeric conditions. Mangrove trees flower all year. Seeds are called propagules because they germinate while still on the parent plant (vivipary). Propagules are dispersed widely by water and remain viable for a long time. The red mangrove (Rhizophora mangle) is the pioneer species, growing closest to open water, from deep water to low intertidal areas. These have extensive prop roots for stability and feeding, and exclude salt by a reverse osmosis process on the root surface. Red mangroves are wind pollinated, and produce 10-12" cigar-shaped propagules. Black mangroves (Avicennia germinans) grow in the upper tidal zone behind red mangroves. They produce pencil-like roots called pneumatophores containing lenticels and arenchyma tissue. Leaves are long and thin, dark on top and pale, downy underneath, and have salt crystallized on top of the blade (part of the excretion process). Black have a limited salt exclusion ability but they have complex salt excreting mechanisms and are more salt-tolerant than red mangroves. (There is ten times more salt in black mangrove sap than in red mangrove sap.) The black mangrove is bee pollinated, and is called the honey mangrove for the high quality of its honey. The seed resembles a fuzzy lima bean. Black mangroves have the ability to resprout from their roots, making them more cold tolerant. White mangroves (Laguncularia racemosa) grow on higher ground behind black mangroves, largely in disturbed patches. They can produce pneumatophores in extreme conditions, but generally rely on lenticels in the lower trunk for air. Their salt-excreting mechanism is poorly understood. White mangrove leaves are rounded at the base and tip, often notched, light green on both sides, and have two distinctive glands at the base of the leaf blade. Flowers are bee pollinated and the white mangrove produces the smallest of mangrove seeds. A fourth species common to the mangrove ecosystem, buttonwood (Conocarpus erecta), is called a mangrove associate. It does not have true mangrove adaptations but is highly salt tolerant. Taxonomically, it is in the same family as white mangrove. Buttonwood is found behind the mangroves in the sand/strand transition zone, with plants such as sea grape and cabbage palm. Mangrove trees stabilize sediment. They are early colonizers of spoil islands, and they protect the shoreline, buffering it against hurricanes. Their leaves, rich in protein and vitamin B, are a major nutrient source in estuaries. Mangroves are closely associated with seagrass beds and many species of phytoplankton. Between Tampa and Key Largo, 74 species of marine algae were found to be epiphytes on mangrove prop roots. All of this makes the mangrove system one of the most prolific "food factories" in the plant kingdom, and a critical wildlife habitat in Florida. One study reported 220 species of fish, 24 species of reptiles and amphibians, 18 mammals (including river otter and black bear), and 181 bird species in mangroves. Economically they are nursery areas for sport and commercial fish, and invertebrates like spiny lobster and shrimp. They are also habitat for many endangered or threatened species--American crocodile, hawksbill and Atlantic ridley sea turtles, manatee and key deer, among others. Extensive bird rookeries are found in mangroves. Mangroves exist in a stressful environment, and are very susceptible to pollution or change. Oil and excessive silt clog lenticels. Dredge and fill operations and dams destroy mangroves. Changes in salinity and flushing allow competition in, and extended flooding drowns mangroves. Two exotic invasives, carrotwood and Brazilian pepper, have become a major threat to mangrove systems--they cannot grow in saltwater, but are highly salt-tolerant and can outcompete mangroves above the tide line. Reference: Ecosystems of Florida, Ronald Myers and John Jewel, editors, Univ. Press of Florida, 1990.