What Is Driving the Global Resurgence of Mangrove Forests?

From the bayous of Louisiana to the backwaters of the Irrawaddy in Myanmar, from the lagoons of West Africa to the billabongs of northern Australia, mangroves are making a dramatic and unexpected comeback. 

What a difference a couple of decades can make. 

In 2007, ecologists from six countries asked in a letter published in the journal Science whether we would soon be living in “a world without mangroves.” One of the world’s most distinctive, biodiverse, and carbon-rich ecosystems was in rapid retreat in the face of encroaching agriculture, construction of sea walls, ports, and marinas, and rising sea levels. The mangroves seemed doomed. 

But in recent months two separate studies have painted a very different picture. A high-resolution analysis of satellite imagery published in Science in June found that the stunted, salt-tolerant trees are now undergoing an “unexpected expansion,” healing after decades of destruction.

At the same time, a report on the State of the World’s Mangroves, published in September by a group of NGOs under the umbrella of the Global Mangrove Alliance, found that “mangroves are now increasing in extent, year on year,” said coauthor Mark Spalding of The Nature Conservancy. 

For the first time, mangrove forests are expanding faster than they are declining, says a coastal ecologist.

The new findings are the surprising outcome of the most detailed analysis yet attempted of satellite imagery of mangroves over the past four decades, says Pete Bunting, a remote sensing analyst at Aberystwyth University in Wales, who worked on both studies. It shows that previous assessments had provided “a substantial underestimation of mangrove gains” over the past 15 years, says the Science study’s lead author Zhen Zhang of Tulane University in Louisiana. 

“For the first time, these unique ecosystems are expanding at a faster rate than they are declining,” says Catherine Lovelock, a coastal ecologist at the University of Queensland, who was not involved in either study.

However, not all ecologists are celebrating. 

For one thing, mangroves are recovering in some places thanks to environmental degradation elsewhere. For another, says Norman Duke, an ecologist at James Cook University in Queensland, Australia, and lead author of the 2007 letter, mangrove quantity should not be confused with quality. Lost old-growth mangrove forests, which harbor the most wildlife, are being replaced by less biodiverse and resilient new stands, he says. Contrary to the new studies, “the world’s mangroves are not bouncing back.”

Mangroves are unique trees, evolved with specialized roots that anchor in shallow, intertidal waters with slow currents, no frost, and plenty of sediment. Mangrove forests are critical biodiversity hotspots, supporting everything from barnacles to tigers, and are vital fish nurseries for the oceans. For coastal communities, they yield rich harvests of crabs and shellfish and form a protective barrier, absorbing winds and tidal surges during storms, and even moderating the energy of tsunamis. They also store large amounts of carbon — typically four times more than nearby tropical rainforests.

Zhang has recorded the biggest revivals in the river estuaries of northern Australia and river deltas in India. In each case, the new growth extends across more than 450 square miles. But the most surprising turnaround has been in Southeast Asian countries, which were once the scene of rapid mangrove destruction. Since 2010, they have accounted for more than 40 percent of global gains. 

Myanmar is “still the most severely deforested country among major mangrove-holding nations,” says Zhang. But since 2010, its mangroves have grown in area by 10 percent. Indonesia tells a similar story. After major losses in past decades, due primarily to a boom in shrimp and fish aquaculture, mangroves have been stable since 2005, when the boom began to abate. The country remains home to around a quarter of the world’s mangroves. 

“Some 65 percent of mangrove gains are in new places where mangroves have not grown before,” says an analyst.

So, what is going on? Why the dramatic upturn for a global ecosystem once considered all but doomed by human invasion and rising seas? The new remote-sensing analysis, despite its greater resolution, allows only tentative conclusions. “We don’t fully understand what is driving these gains,” says Bunting. His mapping cannot distinguish planted trees from natural recovery, for instance. But there is growing evidence that natural regeneration is powering the resurgence. 

Sometimes, mangroves are recolonizing areas previously taken by humans but since abandoned. During the late 20th century, coastal communities across the tropics ripped out millions of acres of mangroves in East and Southeast Asia alone to make way for ponds where they raised fish and shrimp for lucrative international markets. But many of these ponds soon became unproductive through disease, pollution, or poor management, and were abandoned. 

But such recolonization is only part of the story. “Some 65 percent of mangrove gains are in new places where mangroves have not grown before,” the State of the World’s Mangroves report finds. Sometimes climate change is helping extend the range of mangroves polewards. On the Mississippi Delta, they are overtaking salt marshes as warmer winters eliminate mangrove-killing frosts, restricting habitat for birds and foraging mammals that need open ground.

The majority of the gains, however, “seem to occur in river mouths and deltas and are being driven by sediment coming off the land,” says Bunting. On reaching the ocean, the sediment settles, creating mudbanks that mangroves swiftly colonize. The unsettling paradox is that, in many cases, mangroves along coastlines are returning thanks to the destruction of their forest counterparts inland. The loss of trees and their roots destabilizes soils, causing erosion and the flow of sediment into rivers.

That is what has happened along the Yangon estuary in southern Myanmar. Part of the giant Irrawaddy Delta, the estuary stretches some 25 miles from the ocean to the country’s biggest city, Yangon. The estuary lost more than half its mangroves between 1988 and 2011 as rice farmers moved onto its shores and replaced mangroves with rice fields, erecting dikes to protect them from the salty water. But the mangroves have since made a dramatic comeback. By 2023, their coverage had increased almost tenfold to more than 13,000 acres, according to Phyu Phway Thant of the Myanmar Maritime University in Thanlyin, who has mapped the growth from satellite imagery. 

The rice fields have not gone. Instead, almost all the growth was mangroves colonizing new mudbanks created from sediment dislodged by upstream sand mining and deforestation, which was driven by illegal logging and the expansion of agriculture along tributaries of the Yangon River. 

Millions of dollars have been spent planting mangroves across the tropics, but “the majority of these projects have failed,” a report found.

“Rising sea levels should in theory restrict mangrove growth, as seas invade the intertidal areas where they grow,” says Lovelock, but at the mouths of many rivers, sediment deposition is creating land faster than the seas are engulfing it, creating new habitats that mangroves swiftly colonize. And once established, the dense networks of mangrove roots trap more sediment — enough, the State of the World’s Mangroves report concludes, to keep pace with current rising tides in many places. 

Of course, this positive feedback loop can be confounded by upstream dams, which drastically reduce the amount of sediment being carried toward the ocean, stopping the growth of intertidal mud flats, and causing deltas to be drowned.

For this reason, there are serious concerns among ecologists about the fate of East Africa’s largest expanse of mangroves, on the delta of the River Rufiji in Tanzania. In recent years, new mangrove stands have grown on mudflats in the outer delta, says Bunting. But that growth could now abruptly halt. In August, the Tanzanian government completed the Julius Nyerere hydroelectric dam some 100 miles upstream. The environmental group WWF has estimated that the dam will trap most of the 16.6 million tons of sediment flowing down the river each year — initiating a cycle of delta retreat and mangrove loss. 

The growth of new mangrove stands has been accompanied by a growing awareness among some governments and coastal communities of their value. This is especially true in Indonesia and other countries bordering the Indian Ocean that were hit by a tsunami in 2004 that killed more than 200,000 people. Villages that had kept their mangroves were much better protected than those that had converted them to fishponds.

A resulting fashion for planting new mangroves has had mixed results, however. Tens of millions of dollars have been spent on planting mangroves across the tropics in recent years, according to the NGO Wetlands International. But “the majority of these restoration projects have failed,” it said in a recent report. More than 80 percent of seedlings have died or been washed away by the tides. 

The new mangrove stands colonizing the tropics may not be as resilient as their predecessors to climate change, an ecologist warns.

In response, some conservation groups have begun helping communities manage coastal landscapes to encourage natural regeneration. On the north shore of Java, villagers have erected some 10 miles of near-shore brushwood barriers to create calm water where seedlings cast off by mangroves have become established. 

And in the West African state of Guinea-Bissau, Wetlands International is recruiting locals to encourage the natural return of mangroves to abandoned rice fields. “We open up dikes and clean the old natural canals” to allow water carrying mangrove seedlings from nearby stands to return, says Abdoulaye Ndiaye, who runs the program. Nature does the rest. More than 6,000 acres have been restored since 2015. 

Meanwhile, the greener landscapes seen in satellite images do not necessarily indicate that biodiversity has recovered, says Uday Pimple of King Mongkut’s University of Technology in Bangkok, who has been doing extensive field research in the mangrove swamps of Thailand.

In Phang Nga province in western Thailand — an area where tin mining, charcoal burning, and shrimp farming previously decimated coastal mangroves — he found that 90 percent of mangrove recovery since the 1980s has been through the natural regeneration of a range of mangrove species, creating biodiverse forest. But restored mangrove forests in Trat province in eastern Thailand, which appear identical on satellite images, were largely planted with a single species of mangrove, resulting in much less biodiversity.

Clearly, the prognosis for mangroves remains contested. Bunting says, “It is quite possible that the future trend will be similar to the last 15 years,” with gains continuing to exceed losses. And in a study of the likely fate of mangroves in a world 2 degrees Celsius warmer, Daniel Friess, a colleague of Zhang at Tulane University, found reasons to believe the trees could thrive. Because, at least in some places, they seem well able to keep pace with rising sea levels. Because increased rainfall will often increase soil erosion inland and sediment deposition along shorelines, giving mangroves even more room to expand. And because warmer temperatures should push mangroves further outside the tropics, replacing many more salt marshes in Australia and the southern U.S., for instance, for better or for worse.

But Duke argues that the new mangrove stands colonizing the tropics may not be as resilient as their predecessors to the more severe weather triggered by climate change. Or as useful in capturing and holding onto carbon. “They are immature stands, not fully developed in either biomass or biodiversity,” he says. That will leave them ever more vulnerable “as damaging [weather] events become more severe and more frequent.” Recent research appears to support his pessimistic presumption.