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In coastal areas in the tropics and subtropics, few trees play a more important role than mangroves. Mangroves grow along shorelines, and they protect coastal areas from erosion and storm surge. They also store large amounts of carbon, which helps slow climate change. But as NPR's Greg Allen reports, new research suggests the effects of our changing climate may wipe out large mangrove forests.
GREG ALLEN, BYLINE: In 2017, Hurricane Irma's eye passed over this area in the Florida Keys. Big Pine Key saw 130-mile-per-hour winds and an 8-foot storm surge which wrecked homes and battered the island's lush vegetation.
JAN SVEJKOVSKY: These are some of the oldest - or were some of the oldest mangroves in this entire area.
ALLEN: Three years after the hurricane, most of the shrubs and trees have come back on Big Pine Key. A notable exception are the mangroves. Jan Svejkovsky, an oceanographer who lives on the island, takes me to an area where large mangrove trees, some over 50 years old, are dead.
SVEJKOVSKY: So it weathered many hurricanes before. And Irma, unfortunately, it wasn't able to stand against, so...
ALLEN: Along the water's edge, a dense forest of mangroves, once lush and green, is now brown and bare. Using high-definition satellite imagery, Svejkovsky surveyed the widespread die-off of mangroves here and on nearby islands. In some areas, he says, more than 40% of the mangroves died and show little sign of coming back. As for why, Svejkovsky says researchers are still figuring it out. He says it appears to be related to a fine sediment left behind by the hurricane storm surge that covered the mangrove roots.
SVEJKOVSKY: When it dries, it actually dries very hard, almost like cement. And so all the aerial roots that, of course, the mangroves depend on to breathe were covered with this stuff.
ALLEN: In other parts of the Florida Keys where Irma had less of an impact, the mangroves are doing fine. But here and on nearby islands, Svejkovsky says the dead mangrove forests will decay and eventually become mudflats, leaving the land vulnerable to flooding that will only get worse as sea levels rise.
Lynn Wingard is a research geologist with the U.S. Geological Survey. Mangroves, she says, help build up a berm that protects the islands.
LYNN WINGARD: I kind of describe it like the rim of a bowl. In many of these islands, the center parts of the islands are below sea level. If you erode away the rim of the bowl, then the water starts getting in.
ALLEN: Wingard says with the die-off of mangroves, an area on one island she studied that used to be dry some of the time is now underwater, a lagoon. It's a trend she and others worry could get worse. There's evidence mangroves are beginning to move into areas that used to be too cold for them, and some experts think they could migrate inland. But new research suggests sea levels may rise faster than mangroves can adapt.
Miriam Jones, also a research geologist at USGS, says since the end of the Ice Age, huge mangrove forests have often been wiped out by hurricanes and drought. Climate change, she says, is likely to bring more of this extreme weather.
MIRIAM JONES: In a place like Florida, where you do experience a lot of these storms, it's potentially going to cause the demise of many of the mangroves along the Florida coastline.
ALLEN: A decline in mangroves around the world would have implications for efforts to combat climate change. Along with salt marshes and sea grasses, mangroves capture and hold huge amounts of carbon at a much faster rate than forests on land.
Greg Allen, NPR News, Miami.
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