What are mangroves?
Mangroves are tropical trees and shrubs that live in the tidal zone between the low and high tide marks. Mangroves also grow along brackish streams, where seawater mixes with freshwater. In this way, mangroves occur in both marine (ocean) and terrestrial (land-based) environments. While every species of mangrove is different, all mangroves have adaptations that allow them to survive in a harsh coastal environment frequently flooded by water.
Mangroves live in saline conditions, with higher salt concentrations than freshwater. Mangroves live in saline environments by filtering salt through their specialised roots. When mangroves absorb excess salt, they can expel it through salt glands in their leaves.
Mangroves also grow in soils and sediments deprived of oxygen, called anoxic soils. In some cases, highly anoxic soils become acidic (have a low pH); however, the acidity of mangrove soils varies widely. Some mangroves have special adaptations to survive in acidic soils.
Some mangrove species have pneumatophores, roots that stick upwards out of anoxic soils to allow the tree to “breathe.” Other species have prop roots that hold the tree above the water and have spaces in their bark called lenticels that help them to access fresh air.
Mangrove root systems are dense and tangled, allowing them to withstand the forces of the tides. The roots also slow tidal water flow, which causes sediments and organic matter floating in the water to sink and become trapped in the roots of the mangrove forest. The ability to trap sediments and organic matter makes mangrove forests some of the most carbon-rich ecosystems on Earth, often sequestering carbon at a rate up to ten times faster than terrestrial rainforests and storing three to five times more carbon overall.
Mangrove Ecosystem Hydrology
Hydrology is the circulation and distribution of water in an ecosystem. Mangroves are unique for their ability to withstand hydrology that can be deadly to other trees. The constantly changing tides and coastal storms mean mangroves must be able to remain rooted even when they are surrounded by moving water.
Mangroves with healthy, intact hydrology receive both input from the tides and enough freshwater to relieve the stress of living in saline and anoxic conditions. When the ecosystem’s hydrology is not intact—for example, when seawater or freshwater flow is blocked—the mangroves can become stressed and die. Natural causes such as severe weather or human impacts like coastal development can degrade a site’s hydrology.
Healthy soil conditions require adequate freshwater flow, as sediments and soils move with flowing water. If sediments or soils become trapped or inundated with water, this can increase soil acidity and decrease oxygen levels.
Soils also provide nutrients and physical support for mangrove growth. The supply of soil nutrients relies on a process called sedimentation, where sand, silts and clays become trapped by forest roots. When too much sedimentation occurs, the mangroves can become isolated from tidal flow, which can stress the mangroves. However, when erosion occurs, and too many sediments are lost, mangroves can become inundated by water and drown.
An imbalance in sedimentation or erosion can occur due to human activities such as coastal development, dredging, and deforestation. High rates of erosion or over-active sedimentation can cause a shift in soil nutrients, salinity, oxygen levels, and pH.
Healthy mangrove ecosystems rely on a freshwater supply and a steady and slow accumulation of sediments to survive.
Mangrove Reproduction
Many mangroves reproduce through a unique process called vivipary. Wind or insects carry pollen from one tree to another, fertilising mangrove seeds. After pollination, mangrove seeds grow on their parent tree before dropping to the ground as a ready-to-grow propagule. A mangrove propagule can float and survive in the water for some time, allowing them to be carried long distances by the tides.
Each mangrove species has a different “dispersal period,” or the period between the propagule leaving the parent tree and establishing itself in a favourable location. At the end of the dispersal period, if the propagule is in a location with the correct water depth and conditions, it will develop roots and leaves and grow into a mature mangrove tree. In this way, mangrove species can travel and become established in new locations.
Mangrove Diversity
There are around 54 species of mangrove trees and shrubs, all with different tolerances to salinity, acidity, and anoxic conditions. Some mangrove scientists say the “true” mangrove species (what we call mangrove trees) come from seven genera: Avicennia, Bruguiera, Ceriops, Kandelia, Lumnitzera, Rhizophora, and Sonneratia. In addition to the true mangroves, there are mangrove “associate” species that grow among mangrove forests but are not mangroves, such as buttonwood trees.
The diversity of mangrove forests varies around the world. For example, there are only three mangrove species in the Caribbean, in addition to the associate Buttonwood tree species. Some mangrove forests may consist of 30 or more species in South-East Asia.
The Fair Carbon Mangrove Tree Distribution map shows where different mangrove species are found. Fair Carbon’s mangrove ID guide provides users with field-ready pages with pictures and information on identifying the true mangrove species.
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Adaptation |
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Roots |
Prop Roots
These specialised roots support the tree, holding it above water and providing stability on unstable soils. |
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Pneumatophores
Pneumatophores are roots that grow from the soil and above the water to access air and provide it to the rest of the root system. They have small air holes in their tissues. |
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Reproduction |
Viviparous Young
Many mangrove seeds grow on the parent tree into a ready-to-germinate seedling called a propagule before dropping to the ground. |
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Leaves |
Salt Secreting Glands
Some mangroves cope with salt stress by excluding salt at their roots, and excreting excess through their leaves. |
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Mangroves Provide Many Ecosystem Services
Mangroves provide critical habitats for hundreds of species both within the water and on land. Mangrove forests are key resting, foraging and nesting habitats for migratory and local bird species. They also provide nursery habitats for fish and marine invertebrates living among their roots, including endangered sharks and rays. In addition, nurseries in and around mangrove forests make many commercial fishing operations possible, giving fish a place to grow to maturity before they are caught.
Mangroves’ sturdy root systems also provide ecosystem services for coastal human communities. During storms and floods, mangroves act as natural barriers against storm surges, decreasing wave energy and protecting communities from severe damage. The stabilisation of soil by the roots of the trees prevents sediment loss, minimising coastal erosion and protecting against sea-level rise. Mangroves are also a source of food, building materials, firewood, medicines, and dyes, or may have significant cultural value to local communities.
Despite many benefits to coastal communities and biodiversity, mangrove forests are threatened worldwide. Approximately 20% of the world’s mangroves have been destroyed since 1980, and mangroves are still declining in many parts of the world. Mangrove conservation and restoration are essential to protect biodiversity and support local communities. Blue carbon projects are one critical tool to finance the protection of the remaining mangrove ecosystems.
Fair Carbon has developed a Beta Mangrove Biodiversity tool to predict mangrove animal species in some locations. This information is essential both for conservation organisations aiming to protect mangroves, and for blue carbon project developers to understand how their activities may potentially impact local species.
To provide a common voice to advocate for mangroves, Fair Carbon partnered with the Global Mangrove Alliance to co-author the State of the World's Mangroves. With contributions from over 27 organisations, including The Nature Conservancy, IUCN, WWF, and many others, the report guides NGOs, scientists, and governments on best practices to recover mangrove forests.
Suggested Citation: Francis, E., Wilkman, A., Beeston, M. "Introduction to Mangrove Ecosystems". Geneva, Switzerland: Fair Carbon, 2023. https://faircarbon.org/content/fc/mangroves101
Photo Credits:
Mangrove Photo: Matt Curnock on Unsplash; Photo by: Shaueel Persadee on Unsplash; "File:Rhizophora mangle (prop roots).jpg" by Hans Hillewaert is licensed under CC BY-SA 3.0. ; "Pneumatophores of mangrove plant" by Jaisen Nedumpala is licensed under CC BY-SA 3.0.; "Mangrove seeds" by BXGD is licensed under CC BY-NC 2.0.; "Avicennia germinans-salt excretion" by Ulf Mehlig licensed under CC BY-SA 2.5.
