Mangroves Are Vital Carbon Reservoirs

Mangroves in Florida Everglades. Photo: © Riandi (CC BY 2.0)

All you need to know about the mangrove forest biome. I explain why mangroves are important, how they survive in salty water, how they cope with oxygen shortages, and how they anchor themselves in water. I also look at biodiversity in the saline mangal swamps and woodlands. As well as providing critical protection from storms, mangroves are regarded as a critical carbon sink due to their storage of ‘blue carbon’. They sequester more carbon per hectare than tropical rainforests.

In this article I examine the importance of mangroves and mangrove ecosystems, which constitute one of the planet’s most valuable reservoirs of so-called ‘blue carbon.’ They also absorb pollution and help to protect coastlines from the effects of the ocean.

Mangroves — one of the most underrated features of the biosphere — are species of trees and shrubs, that live along the shorelines of coasts, rivers, and estuaries, in the tropics and subtropics. Here, they form unique intertidal habitats — part swamp, part forest — which are easily recognizable by their dense tangle of stilt-like prop roots. These roots help them cope with the daily rise and fall of tides, which submerge them daily. The roots also slow down the tidal movements, causing sediments carried in the water to sink down and accumulate on the muddy bottom.

Although they prefer sheltered coastlines where there is little high-energy wave action, mangroves are an amazingly hardy species. Apart from being the only trees that are capable of thriving in salt water, they endure twice-daily flooding by ocean tides, and they live in a habitat with a low-oxygen soil. On top of all this, they grow on the very edge of the coast, thus bearing the brunt of coastal pollution as well as ocean-borne hurricanes, typhoons and tsunamis. The word ‘mangrove’ comes from the Portuguese word ‘mangue’, meaning tree, combined with the English word ‘grove’ which means a medium-sized clump of trees. (1)

In This Article:

• Mangrove Biome
• Why Are Mangroves So Important? Blue Carbon!
• Location
• Mangrove Species and Diversity
• How Do Mangroves Survive in Salty Water?
• How Do Mangroves Cope With Oxygen Shortages?
• How Do Mangroves Anchor Themselves in the Water?
• How Do Mangroves Reproduce?
• Biodiversity Within the Mangrove Biome
• Threats to Mangrove Biome From Climate Change and Human Activity
• Human Threats
• Climate Threats
• References

Mangrove Biome

The mangrove biome, or ecosystem, consists of saline forested swamps (mangals) located on tropical shorelines and river estuaries. It’s a critical coastal habitat that forms the join between land and sea — between the terrestrial and marine environments.

Mangals consist of dense entanglements of trees and woody shrubs, all sprouting thick and highly functional roots, to secure their footing and help them absorb oxygen. Most mangrove forests have muddy soil, but they can also be found on peat, sand and coral. Most mangals occupy no more than 5 miles of shoreline, although in some ideal conditions they can grow into massive aquatic forests.

One of the world’s largest mangrove biomes is the sprawling Sundarbans Forest, a UNESCO World Heritage site situated at the confluence of the Ganges, Brahmaputra and Meghna Rivers in the Bay of Bengal. The Sundarbans is a network of mudflats and waterways covering about 3,900 square miles (10,000 square km) of India and Bangladesh.

Why Are Mangroves So Important? Blue Carbon!

The mangrove biome is one of the most productive and biologically complex ecosystems on the planet. Most importantly, it helps to combat global warming by reducing the greenhouse effect in the lower atmosphere. It achieves this through the capture and storage of “blue carbon“. (2) This term describes carbon dioxide (CO2) captured from the atmosphere by coastal ecosystems around the world, through photosynthesis and the accumulation and burial of organic matter in the soil. It’s called blue carbon because it’s stored underwater. (3)

Mangrove forests are extremely proficient at capturing and storing carbon from the atmosphere. As mangrove trees grow, they absorb CO2 from the atmosphere and use it to build their trunks, branches, leaves and roots. When leaves are shed, and old branches and trees die, they fall to the seafloor, where this carbon-rich plant litter then becomes buried in the surrounding soil. Every year, mangroves shed around 9 metric tons of leaves and branches per hectare (4 tons/acre). (4)

Mangals make up less than 2 percent of marine environments but account for 10–15 percent of carbon storage. One acre of mangrove forest stores about 1,450 pounds of carbon per year. One study estimates global carbon storage by mangrove ecosystems at 75 billion pounds (34 million metric tons) of carbon per year. (5)

As well as mangroves, other ‘blue’ ecosystems include salt marshes, seagrasses, seaweeds and micro-algae. According to a recent NASA-led satellite study, mangrove forests move significantly more CO2 from the atmosphere into long term storage, than other forest ecosystems, making them “among the planet’s best carbon scrubbers.” 6 7 The role of coastal marine ecosystems in this natural form of carbon capture and storage is another example of how oceans influence climate on a regular basis.

Mangrove ecosystems provide several other valuable benefits, all of which help to combat the effects of global warming on humans and other species. For example, they bear the brunt of storm surges and other extreme weather events that affect coastlines, minimizing damage to homes and crops. They restrain and even reverse coastal erosion, stabilizing the coastline in the process. Like coral reefs, mangals offer food and shelter to a variety of marine animals, thus helping to stem the loss of biodiversity from both climate change and human action. (8)

Location

Mangrove forests and swamps (mangals) are found on all continents with tropical and subtropical coasts — that is, between latitudes of 25 degrees north and 25 degrees south, although this varies. Some mangrove species thrive on Atlantic coastlines as far north as 32 degrees, and on Australian Pacific coastlines as far south as 38 degrees. Temperature is the critical factor. Freezing temperatures can kill some species after just a few hours of exposure.

However, rising temperatures due to climate change have prompted mangroves to expand their ranges towards the poles, encroaching on temperate coastal wetlands in the process. These encroachments are not welcome on some tropical islands, like Tahiti and Hawaii, where mangroves are viewed as invasive species.

Marine heatwaves, another of the harmful effects of global warming on the oceans, is also believed to be responsible for several mass die-offs in Northern Australia.

Globally, mangroves can be found on the southern coastlines of the United States (e.g. Florida); most Caribbean islands; Central America; South America, notably the Atlantic coast; West Africa and the east coast of Africa from Oman to South Africa, including Madagascar. But the most diverse mangrove habitats are found on the coasts of the Indian subcontinent and throughout Southeast Asia, notably Myanmar, Thailand, Malaysia, Indo-China, the Philippines, Indonesia, Papua New Guinea, numerous Pacific islands, and northern coasts of Australia. Mangals are common sights on the leeward side of tropical islands, atolls and estuaries.

Mangrove Species and Diversity

Originating in Southeast Asia, mangroves once populated three-quarters of the world’s tropical coastlines. Even today, mangrove swamps still cover between 55,000 and 75,000 square miles (142,000–194,000 square km).

There are about 80 recorded species of mangroves in total, of which 60 are found only on coasts between the high/low-tide lines. Most diversity of mangrove species is in Southeast Asia — with only around 12 species surviving in the Americas. Mangroves range in size from small shrub-like bushes to the huge 60-meter (200 ft) specimens found in the province of Manabi, Ecuador. In the mangrove forest ecosystem, different species have different roles. Those able to cope with tidal submergence grow in the open water, and on fringe islands. Trees best equipped for drier, less salty soils are usually found further away from the shoreline. Other mangrove trees thrive best on the banks of tidal estuaries, sometimes quite far inland. (9)

How Do Mangroves Survive in Salty Water?

How do mangroves cope with salt? Only by adapting. Mangroves aren’t natural seawater plants — they need freshwater to live. So, to survive, they must create freshwater from seawater. They do this in one of three ways: by filtering out most of the salt, as they draw seawater into their roots; by excreting salt through glands in their leaves; or by extracting and storing salt in older leaves or bark, which they duly shed. (10)

Mangrove leaf showing excreted salt crystals. Photo: © Ulf Mehlig, Wikimedia Commons.

How Do Mangroves Cope With Oxygen Shortages?

Most mangroves suffer inundation and low-oxygen soils, a combination that kills most plants. Mangroves cope with this low oxygen environment by ‘breathing’ in a variety of ways. Some grow pencil-like cone roots (pneumatophores) that stick up out of the muddy ground like snorkels. These cone roots function as breathing tubes that suck in oxygen from the air.

Mangroves also develop distinctive aerial roots, the ones you see arching high over the water, that also provide oxygen for respiration. Oxygen enters the tree through thousands of tiny breathing pores (lenticels), that cover the surface of the roots. During high tide, the pores are tightly closed to prevent the trees from drowning.

Mangroves, not unlike desert plants, are also good at conserving fresh water in thick succulent leaves, whose waxy coating minimizes evaporation. In addition, Small hairs on the leaves help to deflect wind and sunlight, both of which can stimulate the loss of water through the tiny openings (stomata) used during photosynthesis.

How Do Mangroves Anchor Themselves in the Water?

The aerial root systems we mentioned, that arch out of the water, include various forms — such as stilt roots and plank roots that extend away from the trunk and bury themselves in the soil. The effect is to solidify and broaden the base of the mangrove, not unlike the stabilizing effect of the flying buttresses you see sprouting from the sides of medieval Gothic cathedrals. The roots help to anchor the shallow root system in the soft, muddy soil, whilst also providing a dense thicket of woody surfaces and sheltered spaces for a wealth of marine creatures and organisms.

How Do Mangroves Reproduce?

The unique characteristics of the mangrove habitat have engendered a unique method of reproduction. Seed pods germinate while they are still on the tree, so they are primed to take root whenever they fall. If a seed falls into the water during high tide, it floats and takes root as soon as it finds solid ground, although some have been known to drift along for a whole year before finally taking root. If it falls from the tree during low tide, it is often able to establish itself before the next tide comes in. (11)

Healthy seeds can grow up to 60cm (two feet) in the first year and, in the process, rapidly sprout a variety of aerial roots to stabilize their position. Within a decade, as those roots spread and sprout further, a single sapling can give rise to an entire thicket. Furthermore, mangrove growth stimulates the expansion of shorelines. As mud gathers around the dense network of roots, it creates mudflats that gradually extend outwards into the water, like a self-perpetuating ecosystem.

Biodiversity Within the Mangrove Biome

Mangrove forests and swamps contain an amazingly rich biodiversity of wildlife, including birds, fish, mammals, reptiles and amphibians. It also has a very large number of spiders, scorpions, ants, moths, termites, mosquitoes and bugs. All these creepy-crawlies feed and nest throughout the swamp, often in hollowed out tree trunks and branches. Snakes, lizards, proboscis monkeys, and even Mangrove Tree Crabs, crawl along tree limbs and roots, while aggressive saltwater crocodiles — the apex predator in the local marine food web — laze in the brackish water. Other creatures who lurk within the underwater tangle of mangrove roots, include: Mudskippers, Fiddler Crabs, and Mud Lobsters.

The Mud Lobster plays an important role in many mangrove forests in Southeast Asia and the Pacific islands. These lobsters dig out burrows for themselves, depositing the excavated nutrient-rich soil in large mounds above the water. This action recycles nutrients from the underwater sediments and creates a fertile platform for more mangroves.

Another species that contributes to the health of the mangal is the bat. Along with birds (and butterflies, bees and moths), bats are vital pollinators for the spread of seeds, and some species (like the mangrove Sonneratia) willingly collaborate by opening their flowers at dusk — ideal for night-time feeders like bats. As the bats fly in to sample the nectar, pollen from the flower sticks to their bodies and is duly transferred to other flowers.

Threats to Mangrove Biome From Climate Change and Human Activity

Despite their environmental and ecological value, mangrove swamps and forests face several major threats. Aquaculture, commercial development, and industrial activity are rapidly eroding the mangrove biome and the ecosystems it supports. According to a 2005 assessment published by the World Resources Institute, around 35 percent of documented mangroves have been destroyed. (12) Since then, the rate of loss has declined, but Southeast Asia still lost between 3.5 percent and 8 percent of its mangals during the period 2000–2012. (13)

Human Threats

• Shrimp Farming
The biggest single human threat to mangroves is the development and expansion of shrimp (prawn) farms, largely in response to booming demand for shrimp in the United States, Japan, China and the EU. Every year, seemingly “worthless” mangrove forests were cleared to make way for shrimp ponds. (4)

• Clear Cutting
Another threat is clear cutting, due to the increased demand for the hard wood of the mangrove tree, which is termite resistant. It can also be utilized as charcoal and fuelwood.

• Pollution & Water Waste Treatment
The increasing upstream use of pesticides and fertilizers leads to increased run-off which ends up in coastal waters around the mangroves. The excess nitrogen and phosphorus cause oxygen depletion (deoxygenation) and excessive algae growth (eutrophication). A similar effect occurs after pollution from waste-water treatment.

Climate Threats

• Marine Heatwaves
Biodiversity can be drastically affected by both ocean warming and by more severe marine heatwaves, one of the worst effects of global warming across the tropics. In 2015–2016, heatwaves in the ocean led to a major mangrove die-back in the Gulf of Carpentaria. In 2019, scientists came across an even larger die-back along the same coast. (14) This last event coincides with a report which says that, while Australia’s mangroves, salt marshes and seagrass meadows are absorbing about 20m tonnes of carbon dioxide every year, deforestation of these “vegetated coastal ecosystems” was already responsible for 3 million tonnes of CO2 per year being released back into the atmosphere. (15)

• Sea Level Rise
As temperatures rise so do sea levels. In the past, mangroves responded to sea level rise by migrating further inland. However, in many places building development now prevents this, leaving mangroves at the mercy of the sea.

Ironically, the recent drop in drop in sea levels off the northeast coast of Australia — one of the effects of an El Niño-Southern Oscillation (ENSO) event in the Pacific Basin — led to equally damaging impacts. The mangrove diebacks in the Gulf of Carpentaria in the Northern Territory and at Exmouth in Western Australia, are believed to have been caused by a combination of a 35cm (14 inch) drop in sea level, a prolonged drought, and marine heatwave, all of which left mangroves exposed long enough to cause extensive fatalities. (4)

References

1. “Introduction to marine biology.” Karleskint G. 1998. Harcourt Brace College Publishers. p.378
2. “A blueprint for blue carbon: toward an improved understanding of the role of vegetated coastal habitats in sequestering CO2.” Elizabeth Mcleod, et al; Frontiers in Ecology and the Environment 2011.
3. “Blue Carbon Initiative.”
4. “Smithsonian Ocean.”
5. “Clarifying the role of coastal and marine systems in climate mitigation.” Howard, Jennifer, et al; (2017). Frontiers in Ecology and the Environment. 15. 42–50.
6. “New satellite-based maps of mangrove heights.” NASA
7. See also: “Mangrove canopy height globally related to precipitation, temperature and cyclone frequency.” Simard, M. et al; (2018) Nature Geoscience. 12 (1): 40–45.
8. “Mangrove Swamps” EPA.
9. “WWF.”
10. What’s a Mangrove? AMNH.
11. Mangrove Alliance.
12. “Ecosystems and Human Well-being: Synthesis.” Millennium Ecosystem Assessment (2005) (p.2) Island Press, Washington, DC. World Resources Institute ISBN 1–59726–040–1
13. “Creation of a high spatio-temporal resolution global database of continuous mangrove forest cover for the 21st century (CGMFC-21)“. Hamilton, Stuart E; Casey, Daniel (2016). Global Ecology and Biogeography. 25 (6): 729–38.
14. “Shocked scientists find 400km of dead and damaged mangroves in Gulf of Carpentaria.” Graham Readfearn. BBC News. Oct 3, 2019.
15. “Australian vegetated coastal ecosystems as global hotspots for climate change mitigation.” Serrano, O., et al. Nat Commun 10, 4313 (2019).