What if mangroves lined every tropical coast?

Most trees grow on dry land. Mangroves grow right at the edge of the sea!

What if mangroves grew along every warm coast in the world?

Most trees would die if salty seawater washed over their roots twice a day. Mangroves don't: they grow along warm coasts in the zone between high and low tide, standing on tangled roots that look like stilts.

So here is a thought experiment: what if mangroves lined every tropical coast? Could they calm storm waves, give more young fish a home and lock away more carbon? And could they really grow everywhere? You can follow each path and find out.

Mangroves are a group of trees and shrubs adapted to the coastal intertidal zone: waterlogged, low-oxygen soils and brackish or salt water. They occur only at tropical and subtropical latitudes.

This thought experiment asks what would follow if mangrove forests fringed every tropical shoreline. It weighs three well-studied services, wave reduction, nursery habitat and carbon storage, against the environmental conditions mangroves need, which determine where such forests could actually exist.

Mangroves grow on warm coasts. Their roots stand in the water like stilts.

The sea comes up over their roots, then goes out again, every day.

NOAA, the U.S. ocean agency, says there are about 80 kinds of mangrove trees. They only grow in tropical and subtropical places near the equator, because they can't survive freezing temperatures. Most get flooded by the tide at least twice a day.

In 2020, about 147,000 square kilometers of mangroves were mapped worldwide, about the size of Bangladesh, according to the Global Mangrove Alliance. But mangroves are being lost: farms for shrimp and fish, oil palm and rice caused up to 43 percent of mangrove losses from 2000 to 2020.

NOAA: about 80 mangrove species grow in low-oxygen soils where slow water lets fine sediment settle; they are limited to tropical and subtropical latitudes because they cannot withstand freezing, and NOAA's estuaries tutorial places them where sea surface temperature never falls below 16 °C.

The Global Mangrove Alliance's State of the World's Mangroves 2024 reports 147,256 km² mapped as of 2020. A Food and Agriculture Organization study it cites attributes up to 43 percent of losses between 2000 and 2020 to conversion for aquaculture, oil palm and rice, and half of the world's mangrove provinces are classed as threatened.

Let's pretend mangroves grow along every warm coast.

Their roots might calm the waves. Baby fish might hide in them. And they might keep lots of carbon in the mud!

Imagine a green wall of mangroves along every warm coast, from beaches to river mouths. Their roots might slow waves before they reach the shore, more young fish and shrimp might find shelter, and more carbon might be stored in the muddy ground.

But here's the catch: mangroves need calm, muddy shores where slow water lets fine mud settle. Rocky coasts and beaches pounded by big waves aren't places they can grow. So a mangrove on every tropical coast is impossible, but more mangroves where they can grow is an idea scientists take seriously.

Extending mangroves along every tropical coast would, in principle, add wave attenuation, nursery habitat and soil carbon storage wherever they grew. Each benefit is documented in existing forests.

The constraint is habitat. Mangroves need low-energy shores where slow-moving water lets fine sediment accumulate (NOAA); the TNC and Wetlands International review notes they are usually found on shores with little incoming wave energy. High-energy sandy and rocky coasts could not host them, so the realistic version of this thought experiment is protecting and restoring mangroves where conditions suit them.

Mangrove roots slow down the water. Mud sinks and piles up around them.

Some mangroves can even take salt out of seawater!

A mangrove's tangle of roots slows the water as the tide moves in and out. When water slows, the mud it carries sinks and piles up, building up the muddy ground. NOAA says this helps hold the coast together against storm surges, waves and tides.

Mangroves have clever ways of living with salt and soggy mud. Some filter salt out of the water through their roots; others push salt out through their leaves, leaving salt crystals on top. Many have roots that poke up into the air to breathe.

Dense prop roots increase drag on flowing water, so suspended sediment settles and the muddy bottom builds up; NOAA notes mangrove forests stabilize coastlines against storm surge, currents, waves and tides.

Adaptations include salt exclusion by ultrafiltration in the roots or salt secretion through leaf glands, and aerial roots called pneumatophores that take up oxygen for root tissue in waterlogged, anoxic soil (NOAA). The McIvor et al. review found wave height falls fastest at the forest edge and that tree structure, especially root and branch density, controls how quickly waves are reduced.

People cut down mangroves to make room for fish farms and fields.

In many places, people are now working to protect and plant mangroves again.

Mangroves are being lost as coasts are cleared for fish and shrimp farms, oil palm, rice and building. NOAA warns that when mangroves are destroyed, the carbon stored in their mud can be released into the air.

Scientists also stress what mangroves can and can't do. A review by The Nature Conservancy and Wetlands International found that 100 meters of mangrove forest reduced wave height by 13 to 66 percent, but most studies looked at fairly small waves. Mangroves help protect coasts, but they aren't a wall against every storm.

NOAA: coastal habitats, including mangroves, are being lost rapidly, largely to coastal development, and destroying them can release stored carbon. The Global Mangrove Alliance report adds conversion for aquaculture, oil palm and rice as major drivers.

The McIvor et al. review (2012) concluded mangroves can reduce wind and swell wave height by 13 to 66 percent over 100 m, with the effect depending on forest structure, water depth and wave conditions. Mangroves may also face larger waves during storms; the review frames them as one tool in coastal defense, not a guarantee.