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Inland mangroves reveal a tumultuous climatic past — and hint at our future

The aquatic life of the San Pedro Mártir River in Tabasco, Mexico, finds refuge in the submerged roots of the red mangrove forests. Image by Octavio Aburto.

The aquatic life of the San Pedro Mártir River in Tabasco, Mexico, finds refuge in the submerged roots of the red mangrove forests. Image by Octavio Aburto.

  • A new study concludes that the presence of inland mangroves along a river in southern Mexico was the result of climate change-driven sea level rise during the Pleistocene Epoch, some 115,000 to 130,000 years ago.
  • The researchers’ analysis of the genetic history of the mangrove trees suggests that they are closely related to trees found on the coastline, and sediments nearby are similar to those found in ocean environments.
  • Publishing their work Oct. 12 in Proceedings of the National Academy of Sciences, the team notes that their research highlights the impacts of global climate change.

Exequiel Ezcurra was dubious when he first heard about the possibility of mangroves on the San Pedro Mártir River in southern Mexico from Carlos Burelo-Ramos, a botanist at Mexico’s University of Tabasco. The red mangrove tree (Rhizophora mangle) does inhabit freshwater environs — Cuba’s Bay of Pigs and the Florida Everglades are two notable examples.

But mangroves sitting at least 170 kilometers (106 miles) inland? That seemed unlikely.

“You probably got your botanical identification wrong because it doesn’t seem plausible that mangroves are growing in a river with freshwater at such a distance from the coast,” Ezcurra, a professor of ecology at the University of California, Riverside, told Burelo-Ramos.

But a confident Burelo-Ramos pushed back, respectfully telling Ezcurra, “I’m a good taxonomist. I know my plants, and this is red mangrove.”

A tall red mangrove provides habitat to various epiphytic cacti and orchid species, creating an ecosystem unique in the world. Image by Octavio Aburto.

“That sort of stirred my curiosity,” Ezcurra told Mongabay. Sure enough, on a trip to visit the San Pedro Mártir with his daughters, he found red mangrove, which, unlike other mangrove species that grow in the Gulf of Mexico, can take up residence in habitats free of salt, at least the sodium chloride variety pervasive in the world’s oceans. The trees exist atop calcium-rich limestone known as karst, which previous research has shown supports salt-loving mangrove trees.

Red mangrove wasn’t the only shoreline-dwelling species he found in the middle of the state of Tabasco at the base of the Yucatán Peninsula, however. He found coastal palms and rosewoods, golden leather fern (Acrostichum aureum), Barbados sea grape (Coccoloba barbadensis), and the pleat-petaled mangrove orchid (Myrmecophila tibicinis). In short, it was as if “a whole coastal ecosystem” had been transplanted up from the Gulf of Mexico to this spot on the San Pedro Mártir River.

Ezcurra’s first move was to get back in touch with Burelo-Ramos. “You’re absolutely right,” he told him. “We should study what is going on.”

An aerial view of the San Pedro Mártir River in Tabasco, Mexico. Image by Ben Meissner.

The interdisciplinary team they brought together would uncover not only how the ecosystem arrived in this place, but new insights into the power that a changing climate can exert on all life, including us. Their findings, published Oct. 12 in the Proceedings of the National Academy of Sciences, suggest that higher sea levels resulting from global ice melt from higher temperatures about 115,000 to 130,000 years ago were responsible for the mangroves’ occurrence in this part of Mexico. Essentially, they thought that this ecosystem sat on an ancient coastline in a warmer world.

Past research has put forth several “crazy hypotheses” to explain how mangroves end up inland, Ezcurra said. Perhaps hurricanes or birds carried them inland. Mangrove’s seeds are large and bitter, though, making it less likely that they arrived on the wind or in the belly of a bird. Others speculated that the Maya may have planted them here. But hauling them a couple of hundred kilometers inland would have required a lot of effort for little apparent benefit, Ezcurra said.

The mangroves’ own natural life cycle offered the team a more plausible route. Seedlings can bob like corks on the water for a year or more before sinking to the bottom and throwing down a spidery tangle of roots in muddy coastal soils. Ezcurra and his colleagues wondered if the waters of the Gulf of Mexico might have risen enough to carry the mangrove seeds to where they exist now on the San Pedro Mártir River.

“Was there any event in which the ocean could have risen 10 meters [33 feet] higher than the level we have now?” Ezcurra said.

A stand of red mangroves sits in the calm, calcium-rich, fresh waters of the San Pedro Mártir River, Tabasco, Mexico. Image by Ben Meissner.

They knew that a little over 100,000 years ago, glaciers, ice caps and ice sheets had retreated during a spate of warm temperatures — what scientists call the Last Interglacial period during the Pleistocene Epoch. That melting led to a rise of 6-9 m (20-30 ft) in sea levels, potentially enough to account for the mangroves’ current position. Later on, computer modeling suggested this was a likely scenario.

Still, the team wanted more evidence to bolster this “working hypothesis,” Ezcurra said. For example, could they show that the red mangroves on the San Pedro Mártir were related to the ones that existed downriver?

To answer that question, the team probed the trees’ DNA and compared it with red mangrove populations from around the Yucatán Peninsula. Indeed, the results showed that the mangroves on the banks of the San Pedro Mártir were more similar to those found along the fringes of the Gulf of Mexico, rather than red mangroves from the Caribbean side of the peninsula.

But their analyses didn’t stop there. By drilling down to the level of DNA bases, those chemical compounds that are the basis for subtle (and sometimes not so subtle) differences within and between species, the team was able to determine just how long ago the coastal and riverine variants of the mangrove had split from each other. The team was using the plant’s genetic code as a “molecular clock,” Ezcurra said.

“There is a whole history of the Pleistocene written in the DNA,” he said. Then analysis pegged the time of the split at around 100,000 years ago, give or take a few thousand years. In other words, it was within the window of time during which new glaciation began, touched off by cooler temperatures. Receding sea levels soon effectively cut the coastal and river mangroves off from each other and allowed them to evolve separately from that point forward.

Project scientist Paula Ezcurra collecting a specimen from a red mangrove tree along the San Pedro Mártir River, Tabasco, Mexico. Image by Ben Messiner.

The researchers also examined the sediments in the area of the mangroves on the San Pedro Mártir. The rounded gravel they found in local quarries looks more like what you would find in the ocean than on the bed of a river. They also noticed the presence of oyster shells that are indistinguishable from the ubiquitous eastern oyster (Crassostrea virginica) that haunts estuaries across the Gulf to this day. These findings led the scientists to conclude “unequivocally” that the mangroves exist on the edges of what was once an ancient coastal lagoon, conspiring to leave behind this “relict” of a different time on our planet.

More recently, however, the mangrove ecosystem’s continued existence on the San Pedro Mártir was much less certain. In the 1970s, the push to turn over the forests of Central and South America to agriculture and livestock pasture found its way to Tabasco. Much of the state’s standing forests fell, but the crews’ bulldozers were stymied by the swampy soils of the mangroves, and they left them standing.

The farms that replaced them soon sputtered, however, once the thin layer of fertile topsoil found in the former rainforest withered away.

“It leaves behind just eroded fields where magnificent rainforest grew,” Ezcurra said. “It’s a tragic story of misguided development.”

The decline of the landscape has left the people of Tabasco impoverished, he said.

“It’s a really sad story,” Ezcurra said, “but we would like to think that perhaps the findings on the San Pedro River bring a hopeful note.”

Researchers explore the exposed quarries in the lands surrounding the San Pedro Mártir River in Tabasco, Mexico, to better understand the geologic history of this region. Image by Octavio Aburto.

The mangroves are still there, their twisted root systems anchoring the river banks and helping to provide clean water for the people who remain. And as they do along coastlines around the globe, they incubate populations of young fish that serve as an important source of food.

They serve as a reminder of the important services mangroves provide wherever they’re found, Ezcurra said. But the discovery of how they ended up along the San Pedro Mártir River is also a portent, presaging how the current round of climate change could alter the face of the world we inhabit.

“We should take these things more seriously, without being alarmist,” Ezcurra said. “Sea level rise is happening.”

Banner image: The aquatic life of the San Pedro Mártir River in Tabasco, Mexico, finds refuge in the submerged roots of the red mangrove forests. Image by Octavio Aburto.

John Cannon is a staff features writer with Mongabay. Find him on Twitter: @johnccannon

Citation:

Aburto-Oropeza, O., Burelo-Ramos, C. M., Ezcurra, E., Ezcurra, P., Henriquez, C. L., Vanderplank, S. E., & Zapata, F. (2021). Relict inland mangrove ecosystem reveals last interglacial sea levels. Proceedings of the National Academy of Sciences, 118(41), e2024518118. doi:10.1073/pnas.2024518118

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