Climate change promotes migration of mangroves from Florida to Georgia

Even seemingly small discoveries can illustrate the enormous impacts of global warming.

This was the case recently when three curious naturalists confirmed that an important tropical plant species had found its way to the coast of Georgia.

Ches Vervaeke, a coastal ecologist with the National Park Service, and his two companions had spent a day searching the coast of northern Florida for mangroves, the saltwater-loving trees whose barbed-wire-like roots have long lined tidal banks in the southernmost part of the state, as well as in the Caribbean and Central America.

Vervaeke and his companions – Candy Feller, an ecologist at the Smithsonian Institution who has studied mangroves for nearly half a century, and a reporter from Scientific American magazine – began their exploration near Amelia Island in Florida, then the northernmost occurrence of the now migratory plants.

“We went up the Intercostal Waterway by boat and started finding more mangroves,” Vervaeke recalled in a telephone interview on Monday. “So technically every mangrove we found was the new northernmost mangrove along the east coast.”

He estimated that the scene repeated itself about 15 times.

“At the end of the day, it was actually pretty weird,” Vervaeke added.

Meanwhile, the explorers had reached Fernandina Beach, directly on the Florida side of the state border.

“Georgia was right there and it was late in the day, so we said, 'Hey, you know what? Let's just quickly look up here and see if there are any,'” Vervaeke said.

They searched the Florida side of the St. Marys River before crossing into Georgia and heading into Cumberland Sound.

“All three of us found a mangrove forest at the same time and the reaction was simply: 'My goodness! I can't believe it. There are mangrove forests in Georgia,'” said Vervaeke. “In my research, I suspected for years that they were there. And we actually found them.”

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“Migration is expected to increase steadily”

Climate change provides attractive conditions for the migration of plant and animal species from tropical areas to the north.

“Increasing temperatures, including milder winters, are thought to be the reason for the expansion of mangrove habitat,” notes Jaynie L. Gaskin, wetland biologist with the Georgia Department of Natural Resources' Division of Coastal Resources. “Mangroves are (also) sensitive to cold temperatures and cannot survive prolonged periods of freezing temperatures. Their distribution is limited to areas where winter frosts are less frequent or last longer.”

In Jacksonville, south of the newly discovered mangroves in Georgia, average winter temperatures have risen by 3.4 degrees over the past half century, according to Climate Central.

And at nearby Fernandina Beach, temperatures have fallen below 30 degrees only once in the last decade, according to data from the National Weather Service.

These figures provide statistical evidence of climate change, which is caused primarily by the burning of fossil fuels for energy, heating and transport. But as convincing as they are, facts and figures are not enough to convince everyone.

“You know, you can talk about sea level rise until you're blue in the face,” Vervaeke said. “And if you visit (the beach) once a year, you might not understand it. But if you point to a mangrove forest that shouldn't be there, the educational value in terms of climate change is just enormous.”

And there's more to come, Gaskin added.

“Given projections of continued warming, mangrove migration along the East and Gulf Coasts is expected to increase steadily,” she said.

Dividing lines are shifting

Merritt Island National Wildlife Refuge, about 115 miles south of St. Augustine, has long been considered Florida's boundary between the more temperate north and the more tropical south, noted Dan Chapman, an Atlanta-based spokesman for the U.S. Fish and Wildlife Service.

Mangroves lined the coast in the south, while salt marshes dominated the north. However, as temperatures rose, this climatic boundary began to shift rapidly around the turn of the century.

This means that the northward spread of mangroves is still a relatively new phenomenon, so its final impacts are unclear.

“Due to mangrove migration northward along Florida's east coast and across the Gulf Coast, we expect mangroves and salt marsh grasses to coexist,” Gaskin said. “It may take a century or more for mangroves to become the dominant species in today's salt marsh system.”

Like salt marshes, mangroves create habitats for wildlife and fish, improve water quality, reduce storm damage and erosion, and even support economic growth through tourism.

“Scientists don't know exactly how the transition from salt marshes to mangroves will occur,” Gaskin added. “So we lack an understanding of how these ecosystem services might change.”

Georgia’s geography, with its 14 large barrier islands, is also likely to have an influence on migration.

“The Georgia coast is lined with over 368,000 acres of tidal salt marshes that act as a natural buffer system between the ocean and the mainland,” Gaskin explained. “Mangroves spread into new habitats when seedlings from parent trees fall into the water and are transported by tides and currents. If ocean currents carry mangrove seedlings north from Florida wetlands, the outer marginal marshes are likely the site of initial deposition and possible establishment.”

This is how researchers believe seeds from Florida, carried by the winds of Hurricane Irma in 2017, reached the Georgia coast.

“More research is needed”

A study in areas in Texas where mangroves were first observed found that mangroves and swamps are not inherently interchangeable and have different benefits to the ecosystem.

Researchers from Texas A&M University in Galveston, the University of Houston and Florida International University concluded that while mangroves may be better at preventing coastal erosion, the carbon in mangrove leaves is not a good source of food for organisms at the bottom of the food chain, such as snails and fiddler crabs.

The researchers found that foraging birds had a hard time in mangroves, while breeding birds fared better among the trees.

They also suggested not to plant mangroves in areas where they do not naturally occur.

But Vervaeke, like Gaskin, believes that much more needs to be learned about the potential impact on Georgia.

“More research needs to be done to see if these mangroves are actually establishing themselves, proliferating and actually competing with the marsh this far north,” he explained. “There are areas in Jacksonville where the mangroves are definitely competing with the marsh and starting to crowd it out, and from Jacksonville to St. Mary's, Georgia, it's not that far.”

John Deem covers climate change and the environment on the Georgia coast. Reach him at 912-652-0213.

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