Mangrove forest, South Lido County Park, Lido Key, Sarasota, Florida. Credit: Robert S. Donovan
The five-year, 2010 study based on satellite images broke ground as the world’s most comprehensive mangrove assessment to date. Lead scientist Giri and an international team analyzed more than 1,000 LanSat scenes using digital image classification techniques to create a more accurate worldwide map of mangroves.
“Our study was a systematic study using satellite data for the year 2000,” explains Giri, whereas earlier estimates were based on a wide variety of old and new data collected from individual countries by the Food and Agriculture Organization of the United Nations. Some countries had complete, accurate data while data from others was incomplete or vague. Some of the previous data revealed only the land area of mangroves and not where they grew spatially. The satellite images allowed the team to create a far more comprehensive picture.
In the decade since the images were taken, the total area of mangroves has diminished even further, a fact confirmed by studies, the vast amounts of new shrimp farms cropping up and even anecdotal accounts. While Giri, who is originally from Nepal, was completing his master’s and doctorate in Thailand, he saw mangroves disappear rapidly to aquaculture and farming; the new farms would often be abandoned after two or three years as they became less productive. “It’s very hard to grow anything on that land — it’s acidic,” Giri says. “I witnessed that when I was a grad student and said ‘I need to do something.’”
Industrial shrimp farming — the number one cause of mangrove loss — exploded in the late 1970s and ’80s. “Mostly in Southeast Asia, the coastal areas of these countries have been carved up into farms,” Feller says, though Latin American countries also have extensive shrimp farming.
Mangroves also occupy prime waterfront real estate — land that developers covet for resorts, vacation homes and coastal destinations. Since mangroves accumulate peat — networks of fine root systems — developers must fill in the cleared area with sediment, often pumped in from ocean reef areas, which further contributes to the breakdown of the original ecosystem.
In mid-20th-century Florida, many mangrove wetlands were impounded and flooded to control mosquitoes. “It killed the mangroves because they need a fluctuating water level,” Feller says. At one site, a late-1970s hurricane blew a hole in the berm, which was later opened to allow normal tidal exchange, and the red mangroves returned. But rising waters in the last 60 years — about two millimeters a year, totaling 12 centimeters over the last 60 years — means a smaller area for mangroves as they return to the site.
“In a mangrove swamp, 12 centimeters is a lot of sea level rise,” says Feller, whose team recently won a grant from NASA and The National Science Foundation to investigate how mangroves are responding to climate change.
Around the world, climate change is re-drawing the lines of where mangroves can survive. As sea levels rise, mangroves will naturally move upland, if they have the space, explains Alfredo Quarto of the Mangrove Action Project. Buffer zones have been taken over by roads, buildings and other developments, so mangrove forests have nowhere to retreat. “They’re really between a rock and a hard place, in a sense,” Quarto says.
In addition to retreating upland, mangroves are migrating as growing zones shift northward. Researchers at the Smithsonian Environmental Research Center are tracking more than 100 miles of Florida coastline, where mangrove trees are gaining a foothold in salt marshes to the north. The typical transition from tropical mangrove to temperate salt marsh is at about 30 degrees latitude in both the Northern and Southern Hemispheres; in the Northern Hemisphere, that’s around St. Augustine in Florida. At that 30-degree latitude, mangroves and salt marshes intermingle in about a two-degree latitudinal band.
“As the climate is warming, the conditions are such that these little plants can get there and get established,” says Feller, who has noticed young mangroves taking hold north of St. Augustine. Some of these northward-drifting mangroves are becoming established enough to survive killing frosts.
In her office, Feller points to a photo taken at Merritt Island National Wildlife Refuge (near Cape Canaveral) of a sign that illustrates only salt marsh species. “Now, about 50 percent of what you see is mangrove,” she says, a change that has happened since the sign was created. “It’s happening faster than I think people realize.”
Exactly how climate change will affect mangroves is a question that researchers are working to understand. “We know it’s happening,” Feller says, “but how will it affect the fish or nutrient cycling or the ability to protect shorelines from wave energy?”
HOPE ON THE HORIZON
With mangroves under fire, our best bet is to try to conserve what we have left, says Quarto. Countries could better enforce conservation in protected intertidal zones and preserve currently unprotected areas.
Among the best things that individuals can do to preserve mangrove forests around the world is to eat fewer imported shrimp. “With less consumption, there’s less expansion of the industry,” Quarto says. If you’re going to eat shrimp, he advises, buy homegrown shrimp from Louisiana and Alaska, rather than imported shrimp, which also tend to be loaded with antibiotics.
While protecting existing mangroves, we can also restore mangrove forests, which will yield the bonus benefit of restoring ocean health — including fisheries and coral reefs — by improving water quality from runoff. But it’s important to remember that mangrove ecosystems must be restored in the right way, taking into account the area’s freshwater hydrology, ecology and land forms and planting a diverse variety of appropriate, native species. Any successful mangrove restoration project must consider incoming freshwater sources and flow and their balance with the nearby sea.
Carrie Madren writes from Great Falls, Virginia, and can be reached at email@example.com.
Working with the Putian Green Sprout Coastal Wetlands Research Center in China, American Forests Global ReLeaf has planted more than 600,000 mangrove seedlings in a number of critical sites on the Chinese coast. To these coastal communities, mangroves represent environmental health, inland safety from storms, habitat for migratory birds and economic stability for the local fishermen that rely on the rich variety of aquatic life supported within the mangroves’ underwater root system. Unfortunately, over the past 50 years, 70 percent of China’s mangrove forests have been lost, making restoration projects like this one all the more vital to coastal ecosystems and communities.
In addition to planting seedlings, the project raised public awareness of the importance of mangrove ecosystems to the health of the local community. Working with local partners, we organized an educational workshop on mangrove ecosystems and reached out to 10 different schools in the region. A number of students took part in the program, even volunteering to take part in the planting itself.
Learn how you can support Global ReLeaf projects like this at www.AmericanForests.org/ways-to-give.