Marco Island is considered the northernmost of the "10,000 Islands," the greatest concentration of mangroves left in the state. The development of Marco Island and my community, Isles of Capri, wreaked massive damage on the native flora and fauna, and mangroves in particular. Fortunately there are a few places left in both areas where the mangroves either escaped the devastation or have reestablished themselves. And both areas are now nestled within area protected by the Rookery Bay National Estuarine Sanctuary and Everglades National Park.
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| Undisturbed Mangroves Part of Rookery Bay Sanctuary |
A warren of prop roots makes the red mangrove easy to distinguish. These roots stabilize the shallow-rooted trees, and are vital in transporting oxygen. They possess numerous pores, lenticels, which admit oxygen and allow its transport to the feeding roots, and which close when inundated. In a dense growth of red mangroves it can be impossible to tell which roots belong to which trunk.
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| Prop Roots |
The trees not only send down roots from the trunk, but also long roots from large branches. The lenticels and the plant's low rate of transpiration make it possible for the plant to maintain high osmotic pressure in the roots, which allows them to take in water, but exclude the salt by a process of reverse osmosis.
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| Aerial Red Magrove Roots, Mixed Growth, HWY 952 |
Where hurricane action is rare, the trees can grow to 80 feet tall or more, with a girth of around 3 feet, but the frequency of storms in Florida keeps them much shorter and shrubbier. Some mangrove stands can show tall, straight trunks, while other trunks are contorted and can look even animate, tortured or ecstatic.
Red mangroves have shiny, slightly yellow-green leaves arranged opposite each other on the branch. A special characteristic for the species is the long interpetiolar stipule. (Stipules in between the leaf stems, the petioles). At the very tip of the branch in this sketch the stipule has not fallen off yet.
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| Red Mangrove, Leaves, Stipules |
The edges of the leaves are smooth. Undersides are paler than the top sides. They can get up to a good 4 inches long, and an inch or more wide. Leaves are clustered at or near the ends of the branches, in order to harvest as much light as possible. There is not much understory in a red mangrove forest, but seedlings can get established where there are gaps in the foliage cover. They will stay in the seedling stage until enough light is created for them to grow larger.
Flowers are bell-shaped, with 4 waxy yellow sepals partly hiding 4 feathery white petals. The plant can self-pollinate. Pollen is wind-borne, but it is possible that some may be spread by insects.
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Red Mangrove Flowers |
One of the criteria of inclusion in the "mangrove" group is viviparity - live birth. The actual "seed" of mangroves germinates on the plant. The brownish-olive structures below are the fruits, with the propagules, the "pencils," beginning to develop.
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When the propagules are mature they drop from the plants. Many land at the base of the plant, but others will float away, often in "rafts," with the next tide. Incredibly, the propagules can stay alive for a year. At first they float horizontally, but as they become waterlogged the heavier base begins to sink until the pencil is vertical. If it washes up onto a suitable site, it will send out roots and sprout leaves.
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| Red Mangrove Propagule |
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| Propagule with New Roots Developing |
It is impossible to overstate the importance of red mangroves, and mangroves in general, to the ecosystem. Their roots stabilize shell and mud deposits, provide anchoring places for microorganisms and mollusks, and serve as a nursury and shelter for small fish and other vertebrates. They, in turn supply food for wading birds and other predators. Roots, leaves and pencils are consumed by a variety of native animals and insects. The root region is rich in phytoplankton, the base of the marine food chain.
Mangroves absorb a huge amount of wind energy from hurricane winds and storm surges, often at their own cost. One needs only to see the path of mangrove devastation after a storm like Irma in 2017 to see the incredible force they absorb and deflect. Dense mangrove forests also provide places for birds to perch during the day and mangrove islands provide safe rookeries at night.
Mangroves are enormously effective at capturing and storing carbon. When mangrove habitats are destroyed, the environment gets a double whammy - first by the release of massive amounts of stored carbon, and second by the loss of a significant carbon-removing source.
In some regions red mangroves are used medicinally. They have proven antibacterial properties. Leaves and branches contain high levels of tannin, which is what makes the waters of estuaries and tidal rivers tea-colored. Around the world mangrove tannin is used for curing leather.
The very stuctures that allow the red mangroves to survive can also be their undoing. Lenticels can allow the entry of pathogens. If the lenticels are clogged by fine silt, oil or other clinging pollutants the plant will die. Large swaths of mangroves in Everglades National Park were killed by silt deposits by Hurricane Donna in 1960.
The plant also dies if water levels rise enough to cover the lenticels permanently. While mangroves are threatened in their native range by rising sea levels, they also are posing a threat to salt marsh habitats farther north. Previously these vital habitats were protected by the mangroves' intolerance of freezing weather, but warmer winters with fewer freezing epidsodes are allowing mangroves to invade. Red mangrove also is considered invasive in Hawaii, where it was introduced.
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Red Mangrove Leaves and Stipules |
Sources: Frank C. Craighead, Sr. The Trees of South Florida. Vol I. University of Miami Press.
1971.
Ronald L. Myers and John J. Ewel, Eds. Ecosystems of Florida. University of Central Florida Press. 1990.
"Mangroves Against the Storm."
"Why Blue Carbon is REDD Hot."
Beverly J. Rathcke. "Mangroves: Ecology and Reproduction."