What Are Mangroves? Trees That Solved Living in Salt Water
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Almost no tree can live with its roots in seawater. Salt pulls water out of plant tissue rather than into it, and waterlogged mud contains almost no oxygen, so a coastal mudflat should be lethal to anything woody. Mangroves are the group of unrelated trees that each solved both problems independently, and the forests they build turn out to be among the most valuable ecosystems per hectare anywhere.
How they survive the salt
Mangrove is an ecological description rather than a family, covering around seventy species from several unrelated plant lineages that converged on the same set of solutions:
- •Exclusion at the root, where an ultrafiltration system in the root membranes blocks most salt from entering at all, so the water drawn up is far fresher than what surrounds it
- •Excretion through glands on the leaves, which push salt out onto the surface where it crystallises and is washed away, visible as white crusts
- •Sacrifice, concentrating salt in older leaves that are then shed
- •Succulence, storing water in thick leaves to dilute what does get through
- •Aerial roots for oxygen, including pneumatophores that stick up out of the mud like pencils, stilt roots arching down from the trunk and knee roots looping above the surface, all carrying pores that admit air to the buried root system
- •Viviparity, in which the seed germinates while still attached to the parent tree and drops as a developed seedling that can root immediately or float for months until it finds somewhere suitable
What the forest does
The tangle of roots that makes a mangrove forest almost impassable is also what makes it valuable. It slows water, so suspended sediment drops out and accumulates, building land rather than losing it, and the same drag absorbs wave energy, with measurements showing substantial wave height reduction across a few hundred metres of forest and documented reductions in storm surge damage behind intact mangrove belts. The roots are a nursery: a large proportion of commercially fished species in the tropics spend part of their juvenile life among them, sheltered from larger predators, which links mangrove area directly to fishery yields. The waterlogged oxygen-poor soil means organic matter does not fully decompose, so carbon accumulates in the sediment for millennia, and mangroves store several times more carbon per hectare than most tropical forests, the great majority of it below ground rather than in the trees. That carbon, along with the equivalent in saltmarsh and seagrass, is called blue carbon and is now traded in offset markets.
What lives there
The species assemblage reflects the fact that the habitat is both marine and terrestrial and is neither for long. Mudskippers are fish that spend much of their time out of water, breathing through skin and mouth lining and defending territories on the mud. Fiddler crabs process enormous quantities of sediment and aerate it with their burrows. Archerfish shoot down insects with jets of water. Proboscis monkeys in Borneo are mangrove specialists, and Bengal tigers in the Sundarbans occupy the world's largest contiguous mangrove forest, where they swim between islands. Bird life is heavy, with breeding colonies of herons, egrets and storks. Saltwater crocodiles, snakes and a great range of molluscs and crustaceans complete a food web supported by leaf litter falling into the water, which is broken down by crabs and microbes and feeds everything upward from there. Because the tide advances and retreats through it, the same space is used by different animals at different hours.
The losses and the responses
Mangroves have been cleared faster than most forest types, with global extent reduced substantially since 1980, though the rate of loss has slowed markedly in recent years. The main driver has been conversion to aquaculture, particularly shrimp ponds, alongside rice paddies, salt pans, coastal development and charcoal production. The economic argument behind clearance was always partial, since a shrimp pond generates income for a few years before acidification and disease commonly force abandonment, while the forest it replaced supplied fisheries, timber, storm protection and carbon storage indefinitely. Studies valuing those services have consistently found intact mangrove worth more than the converted use once protection and fisheries are counted, which is why restoration has become policy in many countries. Restoration frequently fails when it is done as plantation, planting seedlings in unsuitable places at the wrong elevation, and succeeds far more often when hydrology is restored first so that natural recolonisation can occur, a lesson learned repeatedly and expensively.
The takeaway
Mangroves are around seventy unrelated tree species that independently solved salt and waterlogging, excluding salt at the root, excreting it through leaf glands and breathing through aerial roots, with seeds that germinate on the parent tree. Their root tangle traps sediment, absorbs wave energy and shelters juvenile fish, and their oxygen-poor soil stores several times more carbon per hectare than most tropical forest. Clearance for shrimp ponds drove heavy losses, and restoration works best by repairing water flow rather than planting.