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Mangroves are prevalent in tropical south Florida, but the plants have been moving farther north as climate change makes freezing weather less common. Hari Sreenivasan reports on the plant's encroachment and what it means for the future of coastal ecosystems. This story is produced in partnership with Climate Central, and is part of our series "Peril and Promise: The Challenge of Climate Change."
The effects of climate change can be hard to see – but in Florida there is a clear visual marker: the mangrove. As freezing cold weather in some parts of the state become less common, this tropical plant has been taking root –and further north of its usual range.
It's a transition that scientists are closely following and a climate change story where there may actually be some positive side effects.
This story is produced in partnership with climate central, a nonprofit science and news organization and is part of our ongoing series, "Peril and Promise: The challenge of climate change."
This is the site of an outdoor laboratory on the banks of the intracoastal waterway in northeast Florida. It's part of a research project on the interaction of climate change and coastal ecosystems. And getting here is not easy.
That is not warm!
That's a little chilly.
It's not so coincidentally called the WETFEET project.
If you sink at all you wont go far…You won't go… but you can sink up to your waist sometimes.
Samantha Chapman is an ecology professor at Villanova University and the primary investigator on this project. I'm following her and her team into this marsh where they're studying the impact of climate change on a tropical plant that's been migrating further and further north: the mangrove.
We're in this sort of encroachment and spread phase where they're moving northward and taking over rather rapidly.
There are three species of mangroves that grow in florida: white, black, and red. What they have in common is an ability to grow with their roots submerged in salty water and soil. An environment that kills many other plants. Mangroves grow around the world, thriving in tropical conditions generally within a set distance of the equator. But this spot in Florida, near St. Augustine, is north of that range.
Here in the Guana Tolomato Matanzas Research Reserve, or GTM, mangroves are growing, in a region that was previously thought to be too cold for them to permanently take root. Matt Hayes is post-doctoral researcher at villanova.
Historically they have been up here before. So we know from looking at historical records from early botanists and explorers they have spoken about seeing mangroves around here even just north of here. But then once these cold snaps came that was enough to kill them off and then they'd be pushed right back again further south of here.
As climate change makes these cold snaps less frequent, researchers are studying how permanently warmer temperatures will affect this ecosystem.
Each of these chambers is a glimpse into the future. By shielding it from the elements, the temperature inside is a couple of degrees centigrade warmer. By doing this, scientists can study what happens to the salt marsh and the mangrove in here at these higher temperatures versus what's outside.
What we see at least in our first six months of data is that the mangroves seem to be growing faster in the warming chambers.
The WETFEET Project is a multi-year initiative, funded by the National Science Foundation. Researchers pull water samples to measure data, including salinity.
It's kind of like getting blood from a stone, right?
They use bluetooth-connected sensors to track environmental conditions like temperature and humidity; both inside and outside of the chambers.
Is this mangrove going to replace that marsh?
Likely, yes. Unless there's a deep freeze, it's going to overtop the marsh and as it does so it's going to shade out the marshes under it and it will just take over.
But a "take over" might not necessarily be a bad thing. Mangroves are the foundation of an extremely productive coastal ecosystem in their own right. To see the benefits that these plants provide, we traveled more than 180 miles south to a research site near the coastal town of Ft. Pierce. Here, it's a more tropical environment, and a place where mangroves traditionally thrive.
Ilka “Candy” Feller:
This is solid mangrove. We have pretty much 100 percent cover of mangroves.
Our guide is Candy Feller, an emeritus scientist at the Smithsonian Environmental Research Center. She's been studying mangroves for more than three decades.
Mangroves are known as a nursery of the sea so in Florida it's been shown that most of the commercial fishes spend part or all of their their lives in the mangroves so if you get rid of the mangroves and you've lost a whole lot of your catch.
In addition to anchoring a whole ecosystem, mangroves play an important role in defending in shorelines from the sea. A 2017 study found that about 5.5 feet of mangroves can reduce wave height by about 90 percent. By comparison, it takes 57 feet of salt marsh to do the same.
In the 1980s, Florida adopted its first rules to protect mangroves, but globally they are threatened by agriculture and aquaculture, including shrimp farming, as well as coastal development.
The Achilles heel of mangroves is that they're occupying that part of the landscape that people really covet.
Researchers estimate that as much as a third of the world's mangrove habitat has been lost since 1980. But about 80 miles up the Florida coast, Feller took us to an area that has seen a mangrove explosion in the last 15 years.
Merritt Island National Wildlife Refuge – near NASA's Kennedy Space Center – is home to hundreds of bird species from Glossy Ibises to Great Blue Herons. It's also part of what's known as the ecotone: part salt marsh, part mangrove environment.
I remember the day when I was looking out there and it was just about as much salt marshes as it was mangroves but it looks like it's pretty much all mangroves now doesn't it?
The transformation here has been so fast that an informational sign is now completely out-of-date.
I think this is extraordinary. This is what we're looking at here. It says the marsh to the east is a grassland community. That is no longer a grassland community. It's a mangrove forest.
The changes look just as dramatic from above. Researchers measure the expansion of mangroves using satellite images, and in this region alone mangrove cover increased by nearly 70 percent from 2003 to 2010.
They are really flexible like snorkels too.
Back at the WETFEET Project, about two hours up the coast, researchers bring us to another site where mangroves are encroaching on salt marsh.
Here's the little root that sticks out.
This is a mangrove propagule, the plant's seedling.
And then it essentially lifts up its head and then it pops out its first two real leaves.
It's not just the growth of mangroves popping up in this salt marsh that researchers are studying. They are also measuring a hidden benefit of mangroves that's trapped in the ground beneath them.
The mud that I'm standing in, this really dark stuff, that is thousands of years worth of carbon that's been captured and locked in by this salt marsh. Now as these mangroves work their way up the coast researchers are trying to figure out how that affects the rate at which that carbon is stored.
These wetlands are like little strips and so by area they're not the biggest carbon stores, but they can take up carbon really quickly and lock it away.
As mangroves grow they absorb carbon dioxide and other climate change-causing greenhouse gases from the atmosphere. These elements are converted by the mangroves to organic matter and trapped in the soil below. This stored carbon is known as blue carbon, and by growing quickly and decomposing slowly, mangroves store more of it, on a per area basis, than any other type of forest on earth.
I think this system is one in which we're seeing some hope. The vegetation, the plants are adapting to climate change. They are moving into a place where they couldn't be before because it's warmer and that may, and that's what we are trying to figure out, end up helping us with issues like carbon storage and sea level rise.
But the transition from grassy salt marsh to bushy mangrove is not seen as beneficial by everyone.
It's not that either one is better than the other, it's just how is it going to change
Nikki Dix is the research director at GTM, and a scientist working on the WETFEET Project.
Do the people that live around here understand the difference between an invasive species and what a mangrove is, what a mangrove does?
Yes, some do some don't. So we try to educate people the best we can. When people are not used to seeing mangroves out their backyard they're used to this nice flat marsh view. And then you start getting trees blocking your view. They immediately think oh it's an invasive we've got to cut it down. So we work a lot with different folks trying to get the word out but it's hit or miss.
You can't touch it. People don't like that. They don't like being told no especially people that can afford to live on the water.
To "touch" a mangrove in Florida, you usually need the help of someone like Danny Lippi. He's an arborist near where the WETFEET Project is underway and he's licensed by the state to trim mangroves. He had to travel all the way to south Florida to learn about the state regulations around mangroves.
When I was at the class I said, 'hey, I'm from St. John's County' and they said, 'what are you doing here? You know, you don't have mangroves where you are.' I said, 'yeah we do. And some of them are 20 feet tall.' They had no clue.
He recently trimmed these mangroves from around 12 feet down to 8 feet, after a months-long regulatory process. The penalty for removing or improperly pruning a mangrove is up to 5,000 dollars per plant.
Every once in a while you'll get a pretty good client that understands the benefits. But I'd say most of my clients don't care about any of that. They just want to have a view this plants in their way and the state says that they can't remove it when they want to.
Our final stop with Candy Feller was nearly 40 miles north of the WETFEET Project on Amelia Island, near the border with Georgia. She's brought us to the northernmost known mangrove on the eastern seaboard.
That tree got there, about 15 years ago. We think that it probably arrived during or after the 2004 hurricanes.
Researchers believe the plants' northern march is spurred by extreme weather events like hurricanes physically spreading mangrove propagules. That push from a hurricane, combined with no significant freezes, means this plant has been able to take root.
The predictions are that we're going to have less intense and fewer freezes. We're going to have more and more intense hurricanes, which means that mangroves who can be pushed further and further up the coast. So these become like a sentinel of, a biological sentinel of the effects of climate change.
Back at the GTM reserve, there are more than 72,000 acres of protected land, including almost 40 miles of coast. as climate change continues, researchers believe this ecosystem will see more and more mangroves. It's an encroachment that could permanently change this landscape in ways that researchers are racing to understand.
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Hari Sreenivasan joined the PBS NewsHour in 2009. He is the Anchor of PBS NewsHour Weekend and a Senior Correspondent for the nightly program.
Sam Weber has covered everything from living on minimum wage to consumer finance as a shooter/producer for PBS NewsHour Weekend. Prior joining NH Weekend, he previously worked for Need to Know on PBS and in public radio. He’s an avid cyclist and Chicago Bulls fan.
Connie Kargbo has been working in the media field since 2007 producing content for television, radio, and the web. As a field producer at PBS NewsHour Weekend, she is involved in all aspects of the news production process from pitching story ideas to organizing field shoots to scripting feature pieces. Before joining the weekend edition of PBS Newshour, Connie was a Peace Corps volunteer in Thailand where she trained Thai English teachers.
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