Mangroves, expanding with the warming climate, are re-shaping the Texas coast

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This story is part of the Pulitzer Center’s nationwide Connected Coastlines reporting initiative. For more information, go to pulitzercenter.org/connected-coastlines.

PORT ARANSAS — Dead mangroves cover Harbor Island near this coastal city, creating a bleak landscape that contrasts with the calm, blue water that laps at the shore. The intense 2021 winter freeze killed these plants, which can tolerate some cold but not for that long. A few leafy, green saplings now sprout among them.

Black mangroves like these were expanding along the Texas coast for years before the freeze. The shrubs are native to the state, but, as climate change pushed temperatures generally higher, scientists saw them growing in greater numbers and spreading farther north than their typical range.

Biologists who study mangroves say little can be done about the plant’s expansion. Instead, they are analyzing what changes the mangroves bring as they spread to new areas — good and bad.

In some cases, mangroves have shaded out salt marsh plants that some fish, shrimp, whooping cranes and other species rely on. And even though the freeze killed off many mangroves along the Texas coast, researchers expect them to return and keep growing in fits and starts as periodic freezes punctuate the generally warmer weather.

The way mangroves are re-making the Texas coastline is one more example of how human-caused climate change is already altering our environment. Like other animals and plants, mangroves can now live farther north because temperatures are warming.

“The expansion and also the contraction [of mangroves] is a really striking and powerful example of the role of climate,” said Michael Osland, a research ecologist with the U.S. Geological Survey.

Anna Armitage, a professor at Texas A&M University at Galveston and a leading Texas mangrove researcher, said scientists have been evaluating how mangroves affect different species and the environment along the coast.

“Mangroves do some things better, and marshes do some things better,” she said. “But they’re not the same.”

Why mangroves can be good: They could help protect against sea level rise because their stick-like roots help build up the soil height and their falling leaves decompose into soil. One study determined that they were better than salt marsh succulent plants at protecting against erosion. They offer habitat for migratory songbirds.

Why mangroves can be bad: They displace the marsh habitat that acts as a nursery for some fish and shrimp and where endangered whooping cranes spend winters. And when mangroves die during a freeze, they form a skeleton forest, as one scientist described it, leaving the shoreline vulnerable to the erosion they once helped prevent.

Mangroves expanded in Texas before the 2021 freeze

The white, shallow-water boat hummed where Victoria Congdon had navigated it to Harbor Island on a short, speedy trip from the Mission-Aransas National Estuarine Research Reserve office. Congdon, the reserve’s research coordinator, was showing a reporter places where mangroves were plentiful and places where they had more slowly creeped in.

Congdon wore Chacos and toe rings on her feet. She grew up Lockhart and wanted to be a marine biologist as a kid. Today, she’s passionate about plants because — although they lack the charisma and likability of, say, a turtle — they play important roles for entire ecosystems.

She gestured toward the thick tangle of mangroves that she thought could eventually rebound. “This is the future, essentially,” she said.

The mangroves here used to provide habitat and protect the shoreline, which made losing them a negative thing. But she said scientists could learn from that loss — and from their potential recovery.

“It is what it is; we can’t do anything about it. All we can do is learn from it.”

Black mangroves have been in Texas forever, said Alejandro Fierro-Cabo, an associate professor at the University of Texas Rio Grande Valley, who specializes in restoration ecology. They don’t stretch as continuously along the Texas coastline as they do farther south in Tamaulipas, Mexico, but grow in patches.

Fierro-Cabo said it’s possible that the Texas coast may begin to look more like Mexico’s mangrove-covered shore as temperatures continue to warm.

Last year, the state’s average temperature was the hottest ever recorded: 68.1 degrees. And last winter was the mildest ever recorded based on the coldest minimum temperature on average, according to research by Texas State Climatologist John Nielsen-Gammon.

By 2036, Nielsen-Gammon’s latest study predicts, the average annual surface temperature in Texas will be 3 degrees warmer than the 1950-1999 average.

“We need to assume and accept and embrace that this change is happening in the coastline, with change of vegetation from salt marsh to mangrove,” Fierro-Cabo said. “Some species will be displaced. … But the ecosystems, the coastal ecosystems, will keep functioning, will keep providing ecosystem services for us and things like that, like carbon sequestration, like storm protection."

The plant transition can clearly be seen in the middle portion of the Texas coast. Katie Swanson, who is the Mission-Aransas National Estuarine Research Reserve’s stewardship coordinator and acting manager, has been monitoring the transition of a study site on Mud Island from a mix of marsh and mangrove to mangrove-dominated. She said they were seeing a lot of that pattern on the mid-Texas coast.

The Texas Parks and Wildlife Department is using aerial images to determine how coastal habitat has changed. Comparing images from 2015 to 2018 around Aransas Bay with images from 2004, the agency found a net increase of 632 acres of mangrove, said Jacob Harris, a coastal ecologist on the agency’s habitat assessment team.

Walking down a slope on the back side of the Texas A&M University-Corpus Christi campus, marine ecology professor Ed Proffitt pointed out the healthy, bushy mangroves near the edge of Oso Bay — one of the locations where he and students study mangroves. Marsh plants stretched out around and behind the mangroves.

Before the 2021 freeze, the researchers documented how plentiful and big the shrubs grew; some rose higher than graduate student Max Portmann, who is 6’7”. After the freeze, they thought many of the mangroves they studied were dead. But some sprouted up again from their old roots, reaching skyward. The scientists estimated about 40% of those they studied on campus survived.

Here the mangroves’ numerous, small leaves extended from long skinny branches. The leaves are a darker shade of green than the lime-colored marsh plants. Three years after the winter storm, some of the mangroves reached waist-high or higher.

Wearing sandals, a wide-brimmed hat and a peach-colored Columbia fishing shirt, Proffitt, who has researched mangroves for 30 years, explained how he expects the mangroves to continue to fill in the shoreline by Oso Bay so long as there isn’t another big freeze.

“They’re coming back in some places faster than others, but they’ll probably come back in all these places,” Proffitt said. “It’s just going to take a while. The question becomes: Are they going to come back before there’s erosion and subsidence that will convert it all to something else like open water?”

Portmann squelched in scuba boots across the marsh muck to get closer to the plants, squatted down and sorted through the tangle of green to show how the mangroves’ roots spread more than three feet from the plant’s trunk. This would help hold soil in place better than a dead mangrove — which could be blown away in a strong storm.

Portmann sees the mangrove’s expansion as just one piece of a changing coastline: “With climate change, the marsh isn’t going to stay the same way it is regardless,” he said, as the duo walked back to the parking lot.

And some animals here like the mangroves. When Texas Parks and Wildlife researchers used nets to collect species in the bays along the central coast, they found that spotted seatrout and brown and white shrimp preferred marsh. Red drum and blue crabs preferred the mangroves.

“It’s not something that we can necessarily control — or should we be even controlling?” said Harris, the TPWD ecologist, adding, “There’s a lot of mixed emotions with it because mangroves do provide very useful habitat. … But is it a good thing that some salt marsh is being converted into mangroves?”

That’s something Harris and other scientists will be watching closely as mangroves march north.

“It’s going to be good for some species and it won’t be good for others.”

Mangroves could be a problem for whooping cranes

One species in particular that advocates worry about when it comes to mangroves is whooping cranes, which have been a success story after being pushed to the brink of extinction.

Carter Crouch, director of Gulf Coast Programs for the International Crane Foundation and Katie Fernald, an ecologist with the foundation, told the story as Crouch drove on a recent afternoon to get gas for the foundation’s boat, “Crane Seeker.”

During the winter of 1941 into 1942, the number of whooping cranes in Texas dwindled to 15 or 16. Six other whooping cranes lived in Louisiana.

How did their numbers get so small? There probably weren’t too many of the birds to begin with, maybe 10,000 before their decline. And people hunted the large birds, which stand around five feet tall, for their meat and feathers. They also snatched up their eggs.

Intense conservation efforts focused on teaching people why they needed to stop shooting the creatures — it was illegal. As it happened, land at the Aransas National Wildlife Refuge where the cranes spent the winter was also federally protected in 1937.

The cranes in Louisiana died, but the number of endangered whoopers in Texas rose from near extinction to around 550 in Texas. They make up the last wild population in the world.

Crouch wore a wide-brimmed hat and aqua fishing shirt as he put the “Crane Seeker” into the water and motored toward the refuge. He loved to hunt and fish growing up in Wichita Falls and has long been a “pretty big bird nerd,” he said. He spent time at the same reservoirs and rivers where whooping cranes sometimes stop when they migrate.

Crouch and Fernald are bird people: They paused to admire an American oystercatcher on its nest. They debated whether it was a sandwich tern (with a yellow-tipped bill that looks like it was dipped in mustard) or a gull-billed tern flying overhead against the overcast sky. Fernald heard a marsh wren that she said sounded like a sewing machine.

At the refuge, Crouch slowed the boat to point out the lush green marsh habitat where whooping cranes spend the winter. It was mid-April now and most, if not all, of the cranes had left for Canada. One could see clear across the marsh.

This is why whooping crane people are nervous about mangroves: The birds are known to prefer open landscapes, like a duck prefers water or a songbird might prefer a forest. Scientists aren’t sure if the birds will avoid areas with a lot of mangroves.

“I don’t think we really know what would happen,” Crouch said.

In some spots, dead mangroves dotted the shoreline. Crouch and Fernald got off the boat at another island covered more densely by dead mangroves. They expected mangroves would continue to expand again.

“It’s tough because they are a native species; they’re not intrinsically bad,” Fernald said earlier in an interview. “Mangrove habitat is also habitat for other wildlife species. It’s just not what was typically and historically here.”

The mangroves are also just one of a number of problems they have to consider, Crouch said. Cranes are losing habitat to development and sea level rise. Drought can make the bay waters that the birds drink saltier, which can also mean fewer blue crabs for them to eat. The wetlands where they stop when they migrate are drying up, and the permafrost where they breed is melting. People still shoot them.

“It’s a weird species to work with because there’s a lot of reasons to be very optimistic, given (nearly) 70 years of data,” Crouch said. “But … we’re not out of the woods, and the threats are increasing fairly quickly still.”

Scientists at the refuge have been aware of the mangrove expansion for years, said Andy Stetter, the refuge’s supervisory wildlife biologist. But with 116,000 acres of land in their care, they don’t really have good options to keep them out if they wanted to. And right now they’re not sure that they do.

Stetter said that although mangroves theoretically may harm whooping cranes by taking away their habitat, how the plants could impact the marsh in the future remains an area of research for scientists. The mangroves may actually be helpful in combating sea level rise and benefit the blue crabs that whooping cranes eat.

“We’re managing the habitat for the benefit of not only whooping cranes but other wildlife, of course,” Stetter said, adding, “We just don’t understand the relationship between mangroves and how it’s going to impact whooping cranes, and that’s something we’re trying to understand.”

Disclosure: Texas A&M University and Texas Parks And Wildlife Department have been financial supporters of The Texas Tribune, a nonprofit, nonpartisan news organization that is funded in part by donations from members, foundations and corporate sponsors. Financial supporters play no role in the Tribune's journalism. Find a complete list of them here.

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