Mangrove forest

Mangrove forests, also called mangrove swamps, mangrove thickets or mangals, are productive wetlands located in tropical and subtropical intertidal zones. Approximately 80 mangrove species exist, all adapted to areas where slow-moving water allows for the deposition of fine sediment and low-oxygen soil conditions. These trees cannot endure freezing temperatures, which restricts their distribution to warmer climates.
Their distinct, stilt-like roots allow the trees to slow the movement of tidal waters, causing the settlement of suspended sediments and acting as a protective barrier to reduce coastal erosion. These roots also provide an important habitat for many species.
Mangrove forests live at the interface between the land, the ocean, and the atmosphere, and are centres for the flow of energy and matter between these systems. They have attracted much research interest because of the various ecological functions of the mangrove ecosystems, including runoff and flood prevention, storage and recycling of nutrients and wastes, cultivation and energy conversion. The forests are major blue carbon systems, storing considerable amounts of carbon in marine sediments, thus becoming important regulators of climate change. Marine microorganisms are key parts of these mangrove ecosystems. However, much remains to be discovered about how mangrove microbiomes contribute to high ecosystem productivity and efficient cycling of elements.

Overview

Mangrove forests, are productive wetlands located in tropical and subtropical intertidal zones. Approximately 80 mangrove species exist, all adapted to areas where slow-moving water allows for the deposition of fine sediment and low-oxygen soil conditions. These trees cannot endure freezing temperatures, which restricts their distribution to warmer climates.
Their distinct, stilt-like roots allow the trees to slow the movement of tidal waters, causing the settlement of suspended sediments and acting as a protective barrier to reduce coastal erosion from storm surge and waves. These roots also provide a habitat for many species and act as nurseries for juvenile fish, including species that are recreationally and commercially important.
The main contribution of mangroves to the larger ecosystem comes from litter fall from the trees, which is then decomposed by primary consumers. Bacteria and protozoans colonise the plant litter and break it down chemically into organic compounds, minerals, carbon dioxide, and nitrogenous wastes. The intertidal existence to which these trees are adapted represents the major limitation to the number of species able to thrive in their habitat. High tide brings in salt water, and when the tide recedes, solar evaporation of the seawater in the soil leads to further increases in salinity. The return of tide can flush out these soils, bringing them back to salinity levels comparable to that of seawater. At low tide, organisms are exposed to increases in temperature and reduced moisture before being then cooled and flooded by the tide. Thus, for a plant to survive in this environment, it must tolerate broad ranges of salinity, temperature, and moisture, as well as several other key environmental factors—thus only a select few species make up the mangrove tree community.
A mangrove swamp typically features only a small number of tree species. It is not uncommon for a mangrove forest in the Caribbean to feature only three or four tree species. For comparison, a tropical rainforest biome may contain thousands of tree species, but this is not to say mangrove forests lack diversity. Though the trees are few in species, the ecosystem that these trees create provides a habitat for a great variety of other species, including as many as 174 species of marine megafauna.
Mangrove plants require a number of physiological adaptations to overcome the problems of low environmental oxygen levels, high salinity, and frequent tidal flooding. Each species has its own solutions to these problems; this may be the primary reason why, on some shorelines, mangrove tree species show distinct zonation. Small environmental variations within a mangal may lead to greatly differing methods for coping with the environment. Therefore, the mix of species is partly determined by the tolerances of individual species to physical conditions, such as tidal flooding and salinity, but may also be influenced by other factors, such as crabs preying on plant seedlings.
Once established, mangrove roots provide an oyster habitat and slow water flow, thereby enhancing sediment deposition in areas where it is already occurring. The fine, anoxic sediments under mangroves act as sinks for a variety of heavy metals which colloidal particles in the sediments have concentrated from the water. Mangrove removal disturbs these underlying sediments, often creating problems of trace metal contamination of seawater and organisms of the area.
Mangrove swamps protect coastal areas from erosion, storm surge, and tsunamis. They limit high-energy wave erosion mainly during events such as storm surges and tsunamis. The mangroves' massive root systems are efficient at dissipating wave energy. Likewise, they slow down tidal water enough so that its sediment is deposited as the tide comes in, leaving all except fine particles when the tide ebbs. In this way, mangroves build their environments. Because of the uniqueness of mangrove ecosystems and the protection against erosion they provide, they are often the object of conservation programs, including national biodiversity action plans.

Distribution

Worldwide there are about 80 described species of mangroves that live along marine coasts. About 60 of these species are true mangroves which live only in the intertidal zone between high and low tides. "Mangroves once covered three-quarters of the world's tropical coastlines, with Southeast Asia hosting the greatest diversity. Only 12 species live in the Americas. Mangroves range in size from small bushes to the 60-meter giants found in Ecuador. Within a given mangrove forest, different species occupy distinct niches. Those that can handle tidal soakings grow in the open sea, in sheltered bays, and on fringe islands. Trees adapted to drier, less salty soil can be found farther from the shoreline. Some mangroves flourish along riverbanks far inland, as long as the freshwater current is met by ocean tides."
Mangroves can be found in 118 countries and territories in the tropical and subtropical regions of the world.
The largest percentage of mangroves is found between the 5° N and 5° S latitudes. Approximately 75% of world's mangroves are found in just 15 countries. Estimates of mangrove area based on remote sensing and global data tend to be lower than estimates based on literature and surveys for comparable periods.
In 2018, the Global Mangrove Watch Initiative released a global baseline based on remote sensing and global data for 2010. They estimated the total mangrove forest area of the world as of 2010 at, spanning 118 countries and territories. Following the conventions for identifying geographic regions from the Ramsar Convention on Wetlands, researchers reported that Asia has the largest share of the world's mangroves, followed by Latin America and the Caribbean, Africa, Oceania, and Northern America.

Sundarbans

The largest mangrove forest in the world is in the Sundarbans. The Sundarban forest lies in the vast delta on the Bay of Bengal formed by the super confluence of the Brahmaputra and Meghna rivers with distributaries of the Ganges. The seasonally flooded Sundarbans freshwater swamp forests lie inland from the mangrove forests on the coastal fringe. The forest covers of which about are in Bangladesh.
The Sundarbans is intersected by a complex network of tidal waterways, mudflats and small islands of salt-tolerant mangrove forests. The interconnected network of waterways makes almost every portion of the forest accessible by boat. The area is known as an important habitat for the endangered Bengal tiger, as well as numerous fauna including species of birds, spotted deer, crocodiles and snakes. The fertile soils of the delta have been subject to intensive human use for centuries, and the ecoregion has been mostly converted to intensive agriculture, with few enclaves of forest remaining. Additionally, the Sundarbans serves a crucial function as a protective barrier for millions of inhabitants against floods that result from cyclones.
Four protected areas in the Sundarbans are listed as UNESCO World Heritage Sites. Despite these protections, the Indian Sundarbans were assessed as endangered in 2020 under the IUCN Red List of Ecosystems framework. There is a consistent pattern of depleted biodiversity or loss of species and the ecological quality of the forest is declining.

Ecosystem

The unique ecosystem found in the intricate mesh of mangrove roots offers a quiet marine habitat for young organisms. In areas where roots are permanently submerged, the organisms they host include algae, barnacles, oysters, sponges, and bryozoa, which all require a hard surface for anchoring while they filter-feed. Shrimp and mud lobsters use the muddy bottoms as their home. Mangrove crabs eat the mangrove leaves, adding nutrients to the mangal mud for other bottom feeders. In at least some cases, the export of carbon fixed in mangroves is important in coastal food webs. Mangrove plantations host several commercially important species of fish and crustaceans.
In Puerto Rico, the red, white, and black mangroves occupy different ecological niches and have slightly different chemical compositions, so the carbon content varies between the species, as well between the different tissues of the plant. There is a clear succession of these three trees from the lower elevations, which are dominated by red mangroves, to farther inland with a higher concentration of white mangroves.
Mangrove forests are an important part of the cycling and storage of carbon in tropical coastal ecosystems. Knowing this, scientists seek to reconstruct the environment and investigate changes to the coastal ecosystem over thousands of years using sediment cores. However, an additional complication is the imported marine organic matter that also gets deposited in the sediment through the tidal flushing of mangrove forests.
Mangrove forests can decay into peat deposits because of fungal and bacterial processes as well as by the action of termites. It becomes peat in good geochemical, sedimentary, and tectonic conditions. The nature of these deposits depends on the environment and the types of mangroves involved. Termites process fallen leaf litter, root systems and wood from mangroves into peat to build their nests. Termites stabilise the chemistry of this peat and represent approximately 2% of above ground carbon storage in mangroves. As the nests are buried over time this carbon is stored in the sediment, and the carbon cycle continues.
Mangroves are an important source of blue carbon. Globally, mangroves stored of carbon in 2012. Two percent of global mangrove carbon was lost between 2000 and 2012, equivalent to a maximum potential of of CO₂ emissions. Globally, mangroves have been shown to provide measurable economic protections to coastal communities affected by tropical storms.