Where the forest meets the tide

Most trees would die if salt water washed over their roots. Mangroves live with it every day. Along tropical and subtropical coasts, these trees and shrubs grow in the zone between high and low tide, where land and sea take turns.

A forest on stilts

There are about 80 species of mangrove trees, according to NOAA. Many mangrove forests are easy to recognise by their dense tangle of prop roots, which make the trees look as if they are standing on stilts above the water.

Those roots let the trees cope with the rise and fall of the tide. Most mangroves are flooded at least twice a day. They grow where water moves slowly and fine sediments settle, in soil that holds little oxygen.

Mangroves only grow at tropical and subtropical latitudes near the equator, because they cannot survive freezing temperatures.

Living with salt

Mangroves are salt-tolerant trees, and different species deal with salt in different ways. The University of Florida describes several strategies: some limit the amount of salt that enters through their roots, while others get rid of salt after it has been taken up. Black mangroves, for example, release salt through their leaves, which can look coated with tiny crystals.

Species also sort themselves along the shore. In Florida, red mangroves tolerate flooding best and grow closest to the water, while white mangroves grow further inland and are the least flood-tolerant of the three common species there.

Builders and defenders

Mangrove roots do more than hold the tree up. By slowing tidal water, they make sediment settle out and build up the muddy bottom. In that way, mangrove forests help stabilise the coastline and reduce erosion from storm surges, currents, waves and tides.

The same tangle of roots creates a maze of shelter. NOAA notes that fish and other organisms are drawn to mangrove forests for food and for protection from predators.

Part of a bigger system

Mangroves do not stand alone. In the Florida Keys National Marine Sanctuary, where mangroves line more than 1,800 miles of shoreline, NOAA describes coral reefs, seagrass meadows, hard-bottom habitats and mangrove forests as ecologically connected. Nurse sharks, for instance, can be seen hunting for small crustaceans in the shallow water near the mangroves.

The sanctuary also names the main local threat clearly: in the Florida Keys, human development is the greatest danger to mangrove communities.

Why it matters

A mangrove forest is a coastal defence, a nursery and a meeting point between land and sea. Where mangroves are left to grow, the coast is steadier and life in the shallows has somewhere to shelter.