How Mangrove Forests Are Great for the Planet

"Mangroves are different in the way that they're able to adapt to deal with really adverse conditions," says Kathy Worley, biologist and director of environmental science for the Conservancy of Southwest Florida. "They deal with salinity changes, low oxygen and they grow in areas that most plants can't. They are also really resilient because they take the brunt of storms. They get knocked back severely and yet they keep coming back. They're amazing in that respect."

Mangroves are so resilient due to a few factors. First of all, they can deal with saltwater, in some cases by filtering out as much as 90 percent of the salt in the seawater where they live. Other species of mangrove excrete the salt through glands in their leaves, and others concentrate salt in older leaves that they then shed. Mangroves are also able to hoard freshwater so they can go for long times without it.

Their root systems are unique in other ways too. These systems, called prop roots, arch out over the water, providing both structural support and oxygen for the plant. Some of them also have pencil-sized roots that they use as breathing tubes which allow them to get oxygen in spite of being flooded daily.

Even though mangroves are so resilient, they are also facing a number of unprecedented challenges that threaten their survival. One of these is developments built next to mangroves. When this happens, it alters their hydrology, meaning the distribution of water in their environment.

"You mess with their hydrology, and it can literally kill them," says Worley. "That's their Achilles heel."

These developments built near mangrove forests isolates them and water gets impounded without tides in and out. Then, these developments also tend to shunt their stormwater into mangrove systems. Because a mangrove's upper trunk and leaves live above the waterline, when the water gets too high, they essentially drown. And because the gaseous exchange in their root system gets blocked, the soil gets bad and it results in a sulphury, eggy smell as well.

Climate change is also a big threat to mangroves. As storms become more common, mangroves must deal with more and more stressors. And while they are generally very hardy plants, the more impacts they have to deal with, the harder it will be to bounce back from each one.

"Mangroves have adapted to take hurricanes," Worley says. "In the case of Florida trees, when you have a hurricane come in, the larger, more mature trees get hit harder, so it opens up the canopy and new growth comes in. Before climate change and sea level rise, they could get back to a state where they could take another hurricane. But if the frequency of those storms and the intensity increases, are they going to have enough time to recover between those events?"

Sea level rise is also a problem. Mangroves naturally build up soils in their root systems which help them maintain their stability and not drown in water. This process is called accretion. If the accretion rate of soil keeps up with sea level rise, then the mangroves can stay where they are. If not however, the mangroves will want to move inland. If there is no development impeding them, they can move inland, but they will out-compete the freshwater plants and other native species.

Mangroves suffer under the effects of climate change, but they also protect us from these very same impacts. Because they are so resilient, they offer vital protection during hurricanes. During Hurricane Irma in 2017, the Nature Conservancy found that mangroves prevented $1.5 billion in direct damages.

"If we hadn't had that mangrove system in place, the damage would have been substantially larger," Worley says.

In addition, they sequester carbon even better than the rainforest, with the world's mangrove forests taking in more than 6 billion tons (5.44 billion metric tons) of carbon each year. Mangrove forests also act as nurseries for fish species, which are important both for food security and economically in many places. And they maintain water quality by filtering pollutants.

"They've already proven their worth both ecologically and economically," Worley says. ""They really do a lot for us, and we just don't think about it."

As sea levels rise due to climate change, mangroves' ability to stabilize shorelines, prevent erosion, and protect the coast from large waves and floods has already become increasingly important.

"They are really cool trees. People haven't always appreciated them that much, but they're getting better at it," Worley says. "So I'm very encouraged."