Mangrove forests are climate treasure chests, storing more carbon acre-per-acre than virtually any other natural landscape. Yet they are gradually disappearing. Roughly 35% of the world’s mangrove forests have been lost over the last half century, thanks to logging, encroaching farms and extreme storms, among other things.

But at least some of that treasure can be regained. Mangrove forests planted by people can hold as much as three-quarters of the carbon found in natural mangrove forests, according to research out today.

The results suggest that campaigns to restore mangrove forests could make good on at least some of their promises. But it comes with a cautionary note. Even after 40 years, replanted mangrove forests still can’t match the carbon capacity of the natural ones.

Mangrove planting has become something of a cause célèbre among environmental campaigners. The trees are renowned for their ability to thrive in brackish water along coastlines. They act as storm barriers, sheltering land from storm-driven waves. Their distinctive root systems, which extend like fingers from a trunk suspended above tidewaters, trap huge amounts of sediment and detritus, a reason why these trees create such carbon-rich ecosystems.

But while people are jumping on the mangrove bandwagon, less is known about how much carbon these restoration projects help capture. Some research has estimated newly planted forests can match the carbon levels in pristine forests in as little as 20 years, while others have put it at a century or more.

A more definitive answer would help inform a host of important calculations. It might clarify how many carbon credits could be sold from a mangrove reforestation project, or how much it would count towards a country’s commitments under the 2015 Paris climate agreement.

To come up with better estimates, 24 scientists from the universities and government agencies in the U.S., Africa, Europe and Asia combed through data collected from studies of more than 1,200 mangrove stands, some natural and some restored or planted for the first time. The performance of these forest plantings was tracked for as much as 40 years.

A common pattern emerged from the far-flung forests. In newly planted spots the captured carbon shot up over the first two decades until the trees’ roots held 70% of the carbon compared to a natural, mature forest, and 63% of natural levels in the trunk and leaves. Over the next 20 years, the growth slowed significantly, inching up to 73% for carbon below the ground and 71% above the ground, the scientists reported today in Science Advances.

The carbon trapped in the soil beneath the trees followed a similar pattern, although it peaked even sooner. Soil carbon was at half the natural levels before new trees were planted, then jumped to around 75% within five years. It then remained largely unchanged for the next 35 years. It’s possible the collection of more carbon in the soil happens so slowly that it wasn’t captured over a 40-year time frame, the scientists wrote.

More than 6,600 square kilometers of mangrove habitat is ripe for restoration. By these new estimates, that much forest could capture 46 million metric tons of carbon over two decades – not counting the carbon trapped in the soil. That’s roughly equal to the greenhouse gas emissions in a year from cars in the United Kingdom, according to the researchers.

But that performance also depends on mangrove plantings actually taking root. In many cases, past efforts to regrow mangrove forests have fallen flat. In just one example, when scientists visited planting sites in Indonesia after a devastating 2004 tsunami wiped out forests, they found nearly all of the trees had died.   

So perhaps the main lesson to take from this new research is that while planted mangroves can capture a lot of carbon, the best route is to keep the forests that nature already put there.

Bourgeois, et. al. “Four decades of data indicate that planted mangroves stored up to 75% of the carbon stocks found in intact mature stands.” Science Advances. July 5, 2024.