A mangrove is all business up front with a gnarly party going on below. However, there's much more to them than meets the eye.
Exactly what is a mangrove?
I first saw mangroves off the coast of Florida and later in Baja California. Traveling in Costa Rica, El Salvador, and Belize provided opportunities for a much closer look at these trees with their tangled masses of roots that wriggle out of the mud.
Mangroves live along the banks and shores of tropical and subtropical oceans, rivers, and estuaries. Some also grow on sand, coral banks and peat. Remarkably tough, these trees and shrubs thrive in water 100 times saltier than most other plants can tolerate. They thrive when flooded by ocean tides that would destroy most trees that can grow in freshwater. Additionally, mangroves also bear the brunt of hurricanes and storms.
They come in all sizes
Eighty species of mangroves have been identified. Sixty of those live exclusively between high and low tide lines in coastal areas. Mangroves once covered three-fourths of the world's tropical coastlines. Just twelve species grow throughout the Americas.
Mangroves vary in size from small bushes to giant trees that can reach 200 feet tall. Within a mangrove forest, different species have particular functions. Those able to handle tidal soakings grow in sheltered bays, open seas, and on fringe islands. Species adapted to drier, less saline soil are found farther inland. Some mangroves even flourish along riverbanks far inland where ocean tides meet freshwater rivers.
(estuary; Chris Engel from Pixabay)
They're ingenious survivors
Since saltwater can kill plants, it's necessary for mangroves to produce their own freshwater. Many do this by filtering out as much as 90% of the salt in seawater as it enters the roots. Some species are able to excrete salt via glands in the leaves, giving these leaves a very briny taste. A third strategy utilized by some mangrove species concentrates salt in older leaves or bark. When they drop leaves or shed the bark, stored salt is eliminated.
Like desert plants, mangroves store freshwater in thick succulent leaves. A waxy coating on some leaves seals in the water and lessens evaporation. Small hairs on the leaves of some species deflect wind and sunlight, lessening water loss through the tiny pores where gases enter and exit during photosynthesis. On some species, these openings are below the leaf's surface and protected from sun and drying winds.
Some mangroves grow roots that stick out of the dense wet ground. These breathing tubes, called pneumatophores, allow mangroves to cope with daily tidal flooding. Unless clogged or submerged too long, they take in oxygen from the air.
Root systems arching over the water are a distinctive feature of many mangrove species. Aerial roots have several forms. Some are stilt roots that branch and loop off the trunk and lower branches. Others on the ground are wide wavy plank roots extending away from the trunk. Aerial roots broaden the base of the tree like a flying buttress and stabilize the shallow root system in the soft sediment. In addition to structural support, aerial roots play an important part in respiration. Oxygen enters the mangrove via thousands of breathing pores found in the bark and roots. When flooded, these lenticels close tightly to keep the plant from drowning.
Unique reproduction and clever leaves
All mangrove trees have two common reproductive adaptations: viviparity and propagule dispersal. They reproduce by flowering. Pollination is by wind and insects. When pollinated, seeds remain attached to the parent tree, and germinate into propagules before detaching and dropping into the water.
The propagules root by turning vertically and planting themselves when they bump into soil. They also float horizontally on top of the water to be dispersed by tides and currents. In some species, the seed will germinate and grow from its own stored supply of nutrients while still attached to the parent plant. Seedlings of some species are dispersed by currents, but others develop a heavy taproot that penetrates mud when the seedling drops, effectively planting itself.
An ocean's nursery
By providing valuable nursery areas for shrimp, crustaceans, mollusks, and fishes, mangroves are critical to commercial and recreational fishing industries. These habitats provide an abundant source of food along with places to hide from predators. Tarpon, snook, gray snapper, jack, red drum, and sheepshead all feed in Florida's mangroves. Fisheries would suffer a dramatic decline without access to healthy mangrove habitats.
This is the term for carbon captured by the world's ocean and coastal ecosystems. By taking in carbon, mangroves provide a natural way to help reduce the impact of atmospheric greenhouse gases.
Sea grasses, mangroves, and salt marshes along coastlines capture and hold carbon, creating carbon sinks. Though much smaller in area than the planet's forests, they sequester carbon at a much faster rate and can continue to do so for millions of years. Most of the carbon taken up by these ecosystems is stored below ground. This means the carbon currently found in coastal soil is often thousands of years old!
It is of the utmost importance to conserve coastal habitats. When these systems are damaged, an enormous amount of carbon is released back into the atmosphere. Protecting and restoring mangroves is an excellent way to help slow climate change. And nature has provided the perfect plants for the job.
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