In brief
Mangrove forests grow in sheltered tidal shores in warm regions. Their roots solve two problems at once: staying upright in soft mud and getting air in waterlogged soil. The forests shelter young fish, protect shores and store large amounts of carbon, but a large share has been cleared.
What it is
“Mangrove” names a way of life, not a single family of plants. Dozens of tree and shrub species from several plant families have independently adapted to live where the tide floods their roots. Mangroves grow mostly between about 30 degrees north and 30 degrees south of the equator, because most species cannot survive hard frost. A well-known global map-based estimate counted about 138,000 square kilometres of mangrove forest in 118 countries and territories around 2000.
How mangroves cope
Mangrove mud is salty, soft and has almost no oxygen. Each of these is a problem a mangrove must solve.
| Problem | Adaptation | Example |
|---|---|---|
| Soft mud; trees could topple | Wide, branching prop roots spread weight | Red mangroves (Rhizophora) |
| No air in waterlogged mud | Roots that grow up into the air and breathe through small pores: pneumatophores | Black mangroves (Avicennia) and Sonneratia |
| Too much salt | Roots that exclude most salt; glands on leaves that excrete salt, which dries to crystals on the leaf | Roots in Rhizophora; leaf glands in Avicennia |
| Seedlings could be swept away | Seeds start to grow while still on the tree (viviparity); a long propagule falls and can root or float | Rhizophora and relatives |
Mangrove roots slow water, so mud collects between them. Leaf litter and fallen wood feed crabs and microbes, and the roots give surfaces for oysters, sponges, barnacles and algae to grow on.
Why mangroves matter
- Nurseries. The tangle of roots is a shelter for the young of many fish and shrimp that later live on reefs or in open water.
- Coast protection. Mangrove forests reduce wave heights and slow water. Scientists agree the effect is real for everyday waves and storm surges; the evidence is more mixed for the biggest waves such as tsunamis, and researchers continue to debate it.
- Carbon. Mangrove soils bury large amounts of carbon, so they are a major part of blue carbon.
- Livelihoods. People harvest fish, crabs, honey, timber and firewood from mangroves in many countries.
Loss and recovery
Mangroves have been cleared for shrimp ponds, farmland, ports and development, and cut for timber and charcoal. A widely cited review (Valiela and others, 2001) estimated that at least about a third of the world’s mangrove area was lost in the last decades of the twentieth century. A 2007 commentary asked what a “world without mangroves” would look like and warned of large losses of services if the trend continued.
Restoration is possible but is often harder than it looks. Planting seedlings in the wrong place, where the mud is too low or too high, can fail. Restoring natural water flow so that seedlings can arrive on their own often works better than planting.
Because warming reduces frost, some mangrove species are expanding to slightly higher latitudes, where they replace salt marsh plants in places.
Field exercise
If you can visit a mangrove shore from a boardwalk or boat, sketch the root types you see and note how they differ from the water’s edge to the land. If you cannot, compare a photograph in a library book with the table above and label the prop roots and the pneumatophores. Do not walk on roots or take propagules.
Check yourself
Read the question, think of your answer, then open it with a tap or the Enter key.
Why are mangrove roots a special adaptation?
They hold the tree up in soft mud and bring air to roots in waterlogged soil.
What is a pneumatophore?
A root that grows up into the air and has pores through which the root system can take in air.
What is viviparity in mangroves?
The seed starts to grow into a seedling while still attached to the parent tree.
Why do most mangroves live only in warm regions?
Most species cannot survive hard frost.
How do some mangroves cope with salt?
Roots exclude most of the salt, and some species excrete salt through glands in their leaves.
Is the evidence for coastal protection equally strong for every kind of wave?
No. It is strong for ordinary waves and storm surges and more mixed for the largest waves such as tsunamis.
Basis of the facts
Where these facts come from. Mangrove adaptations and zonation follow Tomlinson (1986). The area figure (about 137,760 square kilometres in 118 countries and territories, circa 2000) is from Giri and others (2011), and is quoted rounded. The loss estimate is from Valiela and others (2001); Duke and others (2007) is the source of the “world without mangroves” warning. The tsunami protection debate is described without figures because published studies disagree. See the sources list.