What Are Mangroves And Why Are They Important

By D.C. Demetre •  Updated: 09/23/22 •  9 min read

A mangrove is a tree or shrub that grows in brackish or salty water along the coast. Mangrove trees are classified into about 80 different species in 17 different families, and the term ‘mangrove’ can also be applied to any coastal vegetation area rich in the species.

Mangroves are found all over the world in the tropics and subtropics, as well as in some temperate coastal regions. They are most common between latitudes of 30° N and 30° S, with the largest mangrove area occurring within 5° of the equator.

Mangroves are the only species of tree that are capable of tolerating saltwater. They handle salt levels that would otherwise be toxic by exuding excess salt through glands in their leaves.

Because of the low oxygen content of their environment, along with its high salinity, and frequent tidal flooding, mangrove plants evolved a number of physiological adaptations. One of the most noticeable adaptations of many mangrove species is their root systems that arch high over the water.

Why Are Mangroves Important?

Avicennia marina - white mangrove

White mangrove (Avicennia marina) Credit: Ton Rulkens CC-BY

Mangrove forests are important for several reasons.

Mangroves play an important part in supporting rich biodiversity. A variety of fish, birds, insects, and reptiles make their home in mangroves. Mangroves also serve as a nursery, feeding ground, and habitat for many other species.

That includes humans, too. Mangroves provide food, wood for construction and fires, income from fishing and tourism, and support for the mental and spiritual well-being of millions of people who live nearby.

Because of their hardiness and resistance to rot and insects, the trees are a dependable source of wood for building as well as for fuel. The leaves of mangrove trees are frequently used as animal feed, and locals collect plant extracts for their medicinal properties.

For many mangrove species, oxygen in the soil is very limited to nonexistent. As a result, the mangrove root system needs to absorb oxygen from the air. Mangroves have special above-ground roots called pneumatophores for this purpose.

The tissues of these roots can receive oxygen through the many pores that they have. Some plants have breathing roots called buttress roots, which also support the tree’s structure mechanically.

These root tangles help combat global warming by removing and storing carbon dioxide from the atmosphere. According to a 2011 study, the majority of other forests cannot store carbon as effectively as coastal mangroves. Mangrove forests were found to contain up to four times more carbon per hectare than other tropical rainforests.

Where Are Mangroves Found?

Mangrove total area change (%)

Mangrove total area change (%). Data published by United Nations Sustainable Development Goals – United Nations (2022-07-07)

Mangroves are found in tropical and subtropical coastal areas around the world. According to the FAO’s 2020 Global Forest Resources Assessment, more than 40 percent of the global area of mangroves is in four countries: Indonesia (19 percent of the global total), Brazil (9 percent), Nigeria (7 percent) and Mexico (6 percent). Indonesia also is home to 55 of the world’s roughly 60 to 110 mangrove species.

South Florida, southern Japan, South Africa, New Zealand, and Victoria, Australia are just a few subtropical latitudes where outlier mangroves can be found. These deviations arise from either unbroken coastlines and island chains or propagules from mangrove-rich regions floating on warm ocean currents.

Temperature is a key factor in determining where mangroves grow. They cannot survive the colder temperatures of northern temperate regions. A ten degree change in temperature over a brief period of time is enough stress to harm the plant, and even a few hours of freezing temperatures can be fatal to some mangrove species.

Mangroves are now encroaching on temperate wetlands like salt marshes and expanding their ranges farther from the equator, however, as a result of climate change’s increased sea level and temperature.

What Endangers Mangrove Forests?

Mangrove forests are threatened by a number of factors, including deforestation, pollution and climate change. Deforestation is a major problem, as it results in the loss of mangrove habitats.

The rapidly growing shrimp aquaculture sector is one of the biggest threats to mangrove forests worldwide. To make room for man-made ponds that are densely stocked with shrimp, hundreds of thousands of acres of lush wetlands have been cleared.

Channels are dug by shrimp farmers to supply the ponds with large amounts of freshwater and salt water. The natural water flow that supports the health of the nearby mangroves as well as other ecosystems farther inland and offshore is altered by these water diversions. Water rerouting can kill trees by cutting off their access to freshwater, and also can prevent their seeds from being dispersed via seawater.

Agriculture is another threat. To make room for rice paddies, rubber trees, palm oil plantations, and other types of farming, tens of thousands of acres of mangrove forest have been cleared.

Fertilizers and other chemicals are frequently used by farmers, and runoff that includes these pollutants enters water supplies. Despite their toughness, mangroves can only withstand a certain amount of agricultural and industrial pollution before they perish.

Mangrove Forest Ecosystems

Dappled sunlight through mangrove trees

Mangrove trees near Umgeni River in Durban South Africa. Credit: Rudantu CC-BY

A large part of the mangrove forest’s capacity to store so much carbon can be attributed to the rich, deep organic soils in which it grows. Incoming tidal waters are slowed down by the intricate root systems of the mangrove forest.

In mangrove forest ecosystems, the roots anchor the plants in the sediment below, allowing organic and inorganic material to settle into the sediment surface. Low oxygen levels slow the rate of decay, which causes the majority of the carbon to accumulate in the soil.

According to Ian Hendy, a researcher at the University of Portsmouth in England who completed his doctoral dissertation on the mangrove habitat of Wakatobi in eastern Indonesia, mangrove roots act as a filter by “stabilising the fine sediment” that runs off from land, preventing it from smothering sensitive tropical coastal coral and seagrass habitat. In fact, Hendy and other researchers frequently refer to the three habitats — mangroves, coral reefs, and seagrass beds — as the “holy trinity” of tropical marine environments.

“Snapper, moray eels, trigger fish, even octopuses – you go into mangroves and you find them in these pieces of burrowed out, waterlogged, old wood. That wood is their nursery habitat,”

said Hendy.

The babies of some fish spend months sheltered in holes in underwater mangrove roots burrowed through by marine worms. Hendy’s 2013 study “Dartfish use teredinid tunnels in fallen mangrove wood as a low-tide refuge”  found that the fish took to the holes at night and low tides.

“During high tide free-swimming dartfish were observed to favour areas of Rhizophora roots over open creeks. In aquaria, fish swam actively during the day, but took refuge in teredinid tunnels at night. Sampling of wood at low tide and direct observations at high tide indicate that a substantial proportion of the dartfish population takes refuge in wood during low tide,”

the authors wrote.

Do Mangroves Have Fruit?

Generally, mangroves produce floating fruits or seeds. For plants that spend at least a portion of their lives in water, this stands to reason. As the fruit or seeds fall down, they are carried away by the tide and ideally eventually mature in another location, increasing the number of these amazing trees.

Although red mangroves (scientific name Rhizophora mangle) have fruit that can be eaten, they are reportedly quite bitter. On black mangrove trees, the sprouted propagules are edible if cooked – eaten raw they are toxic.

The mangrove apple tree (Sonneratia caseolaris) bears fruit that is edible, as are it’s leaves. In some regions, like the Maldives, people eat the fruit as food. The fruit’s pulp is combined with coconut milk extract to create a milkshake in Sri Lanka, where the name of the fruit is kirala gédi.

Different Types of Mangrove Trees

Manngroves are divided into true mangrove species and mangrove associates, and there is some disagreement where exactly the dividing line falls. As the British botanist Philip Tomlinson wrote in his 1986 book The Botany of Mangroves,

“The ecological literature seems incapable of being reduced to a simple set of rules to account for the diversity of vegetation types within the broad generic concept of mangroves.”

True mangroves include the families Avicenniaceae (grey mangrove, Indian mangrove and black mangrove), Arecaceae (Mangrove palm), Combretaceae (white mangrove, white-flowered black mangrove,), and Rhizophoraceae (oriental mangrove, yellow mangrove, rib-fruited mangrove, Asiatic mangrove, red mangrove, upriver orange mangrove).

Various species of mangroves considered non-true occur in the families Euphorbiaceae (milky mangrove, blind-your-eye mangrove, and river poison tree), Malvaceae (kapok magrove), Myrtaceae (mangrove myrtle), Primulaceae (black mangrove, river mangrove) and Pelliciera (tea mangrove).

Red and black mangroves, along with Buttonwood and white mangoves, are the most common species found in the United States in general, as well as along southern Florida shorelines.

References:

Daniel C. Donato, J. Boone Kauffman, Daniel Murdiyarso, Sofyan Kurnianto, Melanie Stidham & Markku Kanninen. Mangroves among the most carbon-rich forests in the tropics. Nature Geoscience, 2011; DOI: 10.1038/ngeo1123

Cragg, Simon & Hendy, Ian. (2010). Mangrove forests of the Wakatobi National Park

Duke, N.C. 1984. Mangrove Floristics and Biogeography. In A.I Robertson and D.M.Alongi (eds.). Tropical Mangrove Ecosystems. Coastal and Estuarine Geophysical Union, Washington DC

Hendy, I., Eme, J., Dabruzzi, T. F., Nembhard, R. V., Cragg, S., & Bennett, W. A. (2013). Dartfish use teredinid tunnels in fallen mangrove wood as a low-tide refuge. Marine Ecology Progress Series, 486, 237-245

K. Kathiresan, B.L. Bingham, Biology of mangroves and mangrove Ecosystems, Advances in Marine Biology, Volume 40, 2001, Pages 81-251

NOAA. What is a mangrove forest? Accessed Sept 21 2002

Robertson, A. I. (1991). Plant-animal interactions and the structure and function of mangrove forest ecosystems. Austral Ecology, 16(4), 433–443.doi:10.1111/j.1442-9993.1991.tb01073.x

Tomlinson, P. B. (2016). The botany of mangroves. Cambridge, United Kingdom: Cambridge University Press

V. Selvam. Trees and Shrubs of the Maldives. FAO Regional Office for Asia and the Pacific