Map of Mangrove Height Reveals Carbon-rich Coastal Forests
(Inside Science) -- Lola Fatoyinbo had studied mangroves -- the nearly impenetrable tangled-trunk forests that fringe warm coastlines worldwide -- for almost a decade, and was pretty sure that the trees topped out around 45 meters high -- roughly the height of the Statue of Liberty without her pedestal. So, when Fatoyinbo, an ecologist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, heard in 2011 about mangrove trees in the Central African nation of Gabon that were more than 10 meters taller, she knew she had to visit.
Mangroves pack away massive amounts of carbon, both in their woody trunks and in the soil where their roots decay. The trees and soil contain more carbon per acre than tropical rainforests. And because the stored carbon can stay out of the atmosphere for many decades, environmentalists have long hoped that protecting mangrove forests could help slow climate change while the world gets a handle on fossil fuel emissions.
Fatoyinbo got a grant to travel to Gabon to gather on-the-ground data about the ability of the mangroves there to store carbon. She and her colleagues hired a motorboat to take them up an estuary into Pongara National Park. Dense, crisscrossing roots emerged from the shallow brown water, which teemed with crabs and fish. When the tide rolled out, the scientists scrambled under roots that stuck 3 meters or more out of the exposed mud and aimed laser devices at the tops of the tallest trees they could find. Working in mangrove forests “is very challenging. It’s like a real-life jungle gym,” Fatoyinbo said. “Sometimes it’s scary. You don’t know what’s in the water.”
But the adventure was worth it. The trees she and her colleagues measured were up to 65 meters high -- taller than any tree in the U.S. east of the Mississippi River, and topping any mangrove tree ever reported in the scientific literature. “We couldn’t believe it,” Fatoyinbo said. Combining field measurements from the Gabon trip and others with satellite data, the researchers created the first global map of mangrove height and above-ground carbon.
The research will help identify conservation hotspots and provide long-needed data for climate change-fighting programs, said Jennifer Howard, a biologist at Conservation International in Washington, D.C. and cofounder of the Global Mangrove Alliance. “I’ve been very excited and waiting for it ever since Lola told me she was doing this research,” Howard said. “It’s a huge deal.”
Globally, mangroves occupy an area roughly equal to that of Bangladesh. In the past few decades, however, half the world’s original mangrove forests have been logged or destroyed to make way for fish farms and coastal development. During the 1990s, as much as one percent of the world’s mangroves were lost annually; that rate is down by half or more today, though still much higher in parts of Southeast Asia. While conservation groups have hoped to get the mangroves’ stored carbon included in climate-change fighting plans, which could provide funding for conserving and restoring the forests, they have been hindered by sparse data on how tall trees in different places are -- a crucial measure for determining a forest’s total mass and stored carbon.
To provide that data, Fatoyinbo and her colleague Marc Simard at NASA’s Goddard Space Flight Center led a team that used measurements from two satellites: the Shuttle Radar Topography Mission, which flew in 2000 and gathered elevation data globally using radar beams, and the Ice, Cloud and land Elevation Satellite (ICESat), which from 2003 to 2010 took more precise height measurements at certain points using lasers. To calibrate the satellites’ measurements, the researchers also measured tree heights from the ground at 14 sites in seven countries and used field data gathered by others.
They found that the tallest trees grow in Gabon and other Central African nations, and in Latin American countries such as Colombia and Venezuela. Mangroves’ carbon density varied dramatically, with an average of 244 metric tons of aboveground carbon per hectare in Gabon, versus less than 100 tons per hectare in Brazilian and Nigerian forests. The tallest mangroves grow where there is abundant freshwater and minimal human development, the researchers found.
“The numbers are really, truly exceptional,” said Robert Twilley, an ecologist at Louisiana State University in Baton Rouge who in 40 years in the field had never seen mangrove trees topping 60 meters.
The study “seems like a real significant improvement over anything that had been out there before” for aboveground mangrove carbon, said Jonathan Sanderman, a biogeochemist at Woods Hole Research Center in Falmouth, Massachusetts. Sanderman studies mangroves’ soil carbon, which outweighs aboveground carbon but cannot be directly measured via satellite. He welcomes the better number for aboveground carbon, but would have liked to see more field data from subtropical locales such as central and northern Florida.
The satellite data the team used are less accurate for the shorter mangroves that grow in such areas, added Daniel Friess, a geographer at the National University of Singapore. Two recently launched NASA laser instruments -- the Global Ecosystem Dynamics Investigation and ICESat-2 -- should aid in refining the map, both Friess and Fatoyinbo said; a German satellite, TanDEM-X, could also help her team update their dataset with more recent radar measurements, Fatoyinbo added.
Howard, at Conservation International, said even the current map will further her alliance’s goal of increasing global mangrove cover by 20 percent by 2030. Conservation groups might now prioritize protecting the mega-tall Central African forests, which have mostly escaped damage so far. And many countries that had argued that they lacked the data needed to get credit for the carbon stored in mangrove forests should now be able to meet part of their greenhouse gas reduction goals under the 2015 Paris climate agreement by conserving and restoring mangroves, Howard said.
“I can look at your country and talk to Lola and say, more accurately than we’ve been able to in the past, exactly how much carbon is there. This is something you should be including in your greenhouse gas inventory,” she said.
The paper was published in late December in Nature Geoscience.