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March of the Mangroves

Mangroves, once axed for development, are expanding across Florida due to climate change. Will Floridians learn to live with the iconic coastal tree?

David Rahahę-tih Webb’s aquarium was more than just a collection of small gobies, brackish water and the buoyant “prop roots” of mangroves, the coastal trees that dominated south Florida’s shorelines before development.

His aquarium was a tangible connection to his many adventures with his grandfather, paddling around the mangroves of southwest Florida. His grandfather would stand in his canoe, pushing with a tall pole alongside the barrier of leggy trees.

The salt encrusting the edges of the glass etched Webb’s memories. He credited experiences like these, all narrated by his family’s oral history, with teaching him how to live in harmony with the world around him.

“We grew up to respect the mangroves,” said Webb, a native of Sanibel Island and author of the book The Spanish Seminole.

Not everyone respected the mangroves, though. The iconic trees once covered much of southern Florida; Miami Beach was not so much a beach, but a mangrove forest before Carl Fisher turned it into hotel-lined white sand for winter holidays. More than a century later, the Florida Marine Research Institute reports up to 86% of mangroves have vanished from parts of Florida since the 1940s.

Today, Floridians have another chance to define the state’s relationship with its mangrove coasts. Mangroves are on the move. Climate change is making the state more tropical. Warming average temperatures with fewer freezes in north Florida and rising sea levels throughout the peninsula are creating conditions for the salt-loving trees to thrive.

Will Floridians learn to respect the mangroves as Webb’s grandfather taught him? As the trees prove part of the solution to the impacts of climate change in Florida and around the globe, increasing numbers of residents and communities are heralding a new appreciation for the once-demolished tree.

The Walking Tree

Mangroves have been expanding both north and south since 1984, not just because of a warming climate, but less-frequent freeze events. A harsh freeze below 24.8 degrees Fahrenheit can push mangroves back. Consistent cold weather like that simply hasn’t occurred as much in Florida – allowing mangroves to flourish.

Historically, mangroves in eastern North America grew no further than about 30 degrees N, just north of St. Augustine. Salt marshes traditionally dominated the region–coastal grasses that provide food, refuge and nursery habitat for more than 75 percent of fisheries species. Now, the habitat has to compete with mangroves, which are starting to take over.

“They effectively get above the canopy of the grass and then take all the light themselves,” said Todd Osborne, an associate professor of biogeochemistry at the University of Florida’s Whitney Laboratory for Marine Bioscience in St. Augustine.

Along the Gulf Coast, mangroves sprawl across Texas and Louisiana. They are even moving more inland. In Florida alone, more than 600,000 acres of mangroves now stretch across the coasts.

As they begin to replace salt marsh, the need to better understand how mangroves will change local ecology and landscape becomes ever more important.

While there are around80 species of mangroves around the world, only four dominant types live in the United States. Red mangroves, with elaborate root systems, are mostly in the water. White mangroves tend to grow the most inland. Black mangroves lie in between. The buttonwood is restricted to mainly south Florida, being the most sensitive to cold. Red, white and buttonwood mangroves are considered the most tropical.

Mangroves thrive in brackish estuaries, a mixture of salt and freshwater that are some of nature’s most productive ecosystems.

“The estuary is kind of the nursery of the ocean,” said Veronica Frehm, the chief executive officer of Friends of MacArthur Beach State Park. The mangrove’s root systems provide homes for smaller fish and animals that would otherwise be preyed upon in open water. Bright green canopies provide food and habitat for creatures above the water too, like the Key deer, which eat the leaves from red mangroves for a source of freshwater.

Angela Witmer, an associate professor of biology and oceanography at Palm Beach Atlantic University, said other species will begin to move with the mangroves.

The mangrove box jellyfish is a newcomer to Florida waters. It’s translucent and floats along mangrove ecosystems in the Caribbean but doesn’t share the same lethal sting as its related box jellyfish species. Instead, the small predator hunts for microorganisms like copepods near mangroves.

Witmer attributes their presence in warm Florida waters to climate change and mangrove productivity. The little jellies are a grape-sized sign that as mangroves grow into new regions, other creatures will follow.

Julie Walker, a visiting assistant professor of biology and environmental studies at St. Mary’s College of Maryland, who is studying the march of the mangroves, calls the coastal trees a “climate barometer.”

“There’s been a long history of mangroves moving in and out of the area,” Walker said.

That longer arc of history can help show how past people lived with mangroves.

People and Mangroves

Mangroves can also be barometers of how people live with and impact their environment. Archaeologist William Marquardt, curator emeritus with the Florida Museum of Natural History and author of The Calusa and Their Legacy, said that mangroves give scientists a way to look back into Florida’s climate history.

“Charred wood and even shells from leftover meals can flesh out a picture of climate change and human interactions with climate,” said Marquardt.

From around 250 BC to AD 400, the world experienced the Roman Warm Period, a time of unusually warm weather. It gave mangroves an opportunity to expand, Marquardt said, like they are now.

Archaeologists can see what material people used for fuel by examining burned remnants of branches. Mangroves indicate a warmer, more tropical-like environment. The Calusa almost always used mangroves for fires, Marquardt said. But when the environment got colder, it was more common for them to use pine wood.

Abrupt climate changes in their more than 1,500 years on the coast forced the Calusa to adapt. Generally, when the mangrove, seagrass and other coastal ecosystems thrived, people tended to build more, especially closer to the coast. When the coasts faced pressures including freezes, sea rise or severe storms, Indigenous people responded by moving inland and changing their eating patterns during times of resource scarcity. More records of spearheads indicate consuming more deer and duck than fish, confirming the movement away from the volatile coastline that would’ve been disrupted by sea rise.

“Climate has been and will continue to change,” said Marquardt, who believes people in the 21st century can learn from the Calusa..

Carl Fisher, the man who cleared lush mangrove forests and swamps to build Miami Beach, was among the first Floridians to develop luxury tourism in the state. Hotels like the Nautilus, Flamingo and King Cole were self-contained luxury resorts that lured the rich and famous south. Rookeries were replaced with regattas. Estuaries became a place for elephant rides. Green mangrove forests were turned into golf courses.

It was the start of the modern-day transformation that mangroves exist in now.

People at the turn of the last century didn't see the benefit of mangroves. Public policy favored development and draining, particularly to build canals and homes.. In the late 20th century though, people began to realize the benefits of wetlands and coastal protection.

Mangrove roots hold onto sediment, keeping land and islands in place. The habitat they provide allows fisheries to flourish. They break winds and water action from strong storms and hurricanes, protecting coastal homes during major storm events.

“If you live 50 miles inland, these mangroves are actually protecting you. They’re helping you in some way, and you may not even know it,” said Frehm with Friends of MacArthur Beach State Park.

The Florida Legislature passed the state’s first mangrove laws in 1984. Over time, state statutes evolved to protect mangroves but allow homeowners to trim them. But some coastal owners still cut.

“We find homeowners move into Miami-Dade County and purchase waterfront property because they want to enjoy the beautiful views we have down here, and mangroves can often be seen as impeding those views,” said McKee Gray, chief of the county’s Natural Resources Division.

She argues, though, that they’re not a hindrance and can instead help save property and prolong the durability of surrounding seawalls. They help filter out some of the runoff that flows off the property before it goes into the bay.

Miami-Dade County is among the local governments trying to preserve the protective tree. Cutting or trimming mangroves without a permit can result in civil court action with penalties of up to $25,000 per violation.

Luckily, Gray said concerned citizens in Miami-Dade will often report mangrove trimming or alteration they believe to be unauthorized.

But increasing numbers of Florida residents—including John Schleede, a clam farmer who lives in North Florida’s Cedar Key—have started to realize the benefits of mangroves on their properties.

Schleede has been working on the deck of a boat since he was 8 years old – the clam business brought him down to Florida from Long Island 28 years ago.

“I want mangroves in front of my house,” said Schleede, boasting about how his tightly trimmed mangroves have acted as a “natural seawall.” Because of them, he said his property has not had erosion in 18 years.

Climate, Landscape and the Future 

While mangroves are protecting Schleede’s backyard, they are also changing North Florida’s coastlines. Whether or not that change is a good thing is something fisheries scientist Mike Allen, director of the University of Florida’s Nature Coast Biological Station, is trying to determine.

Climate change has given mangroves the opportunity to expand in Cedar Key, and that will continue without a major freeze, Allen said. While black mangroves have been in the fishing village since the 1950s, within the past decade, red and white mangroves have started moving in.

It’s why Walker called mangroves a “climate barometer” - explaining that, as most species have been adapting to warming temperatures, for the most part, mangroves haven’t. They’ll only move to where it’s warm enough - avoiding regions that have experienced harsh freezes.

The Florida march of the mangroves may portend a global shift in mangrove gains. After decades of worldwide decline in mangrove forests due to human impacts, the Global Mangrove Alliance that tracks these ecosystems reports that the rate of decline has slowed to just .04% a year. Around the world, mangrove forests have now shifted “from being one of the fastest-diminishing habitats on Earth to being one of the best protected,” the alliance reported.

That may be a good thing for keeping vulnerable coastal areas including Florida safe from the impacts of climate change. Studies suggest that mangroves and coastal wetlands store carbon, a greenhouse gas that greatly contributes to climate change, at a rate 10 times greater than mature tropical forests. Every mangrove preserved keeps that much more carbon out of the atmosphere. That makes mangroves a powerful “nature-based solution” for climate change, according to the United Nations and many other scientific and policy organizations.

Still, Allen said that while mangroves can be part of solutions, they are not cure-alls for the enormity of climate change.

“We’re losing tiny islands all the time – it’ll be on the navigational chart, but it’s just not there anymore,” he said.

The disappearing islands of Cedar Key remind us of the stories and rich history of the Calusa, who moved their homes based on the stability of their coastline. As the population in Florida continues to grow, so do the mangroves. Living with them instead of chopping them down may be even more important now, given the rising impacts of climate change.

Floridians have another chance with mangroves. Will we learn from Rahahę-tih Webb’s words to respect them?

Augustus is a reporter for WUFT News who can be reached by calling 352-392-6397 or emailing
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