The mangroves the ice left behind

In the waters of a river in Tabasco grows a population of red mangrove: a lost world whose origins can be traced back to the last glaciation.

I heard trees of splendid age say: “What are you doing here? We are secretly illiterate. 
Learn to read so you can write about us.” 
CARLOS PELLICER's Sketch for a Tropical Ode. 1933

Carlos Burelo anticipated his question, even when simple, wouldn’t have an easy answer:  “I knew I needed a multidisciplinary approach,” he says in an interview with Botany One. “I began looking around and found someone who I believed would be perfect to work with for this project,” Burelo tells me, “so I sent him an email and it took him three months to reply that he didn’t believe me.”

Burelo, a botanist and conservation ecologist from the Herbarium of the Universidad Juárez Autónoma de Tabasco (UJAT), had contacted ecologist Exequiel Ezcurra from the University of California Riverside. Ezcurra’s skepticism was justified: “[Burelo] told me in his email that he’d found red mangrove in Tabasco, 170 km away from the coast,” Ezcurra says during our interview, “and that clearly couldn’t be true.” It was unheard of, to say the least. Red mangrove, Rhizophora mangle, usually grows along the coastline in tropical regions and in wetlands; either in seawater or brackish waters, but always near the coast. At times, it may be found a few kilometres inland, following the river. The distance Burelo reported was frankly unbelievable. 

Red mangrove, Rhizophora mangle, in the calm, calcium-rich, fresh waters of the San Pedro Mártir River, Tabasco, Mexico. Credit: Octavio Aburto-Oropeza.

“But I was certain,” Burelo says, “I know these mangroves really well.” And this is because he grew up in this area of the Yucatán Peninsula, in the city of Balancán, where the most powerful river in Mexico — the Usumacinta — flows. “My father took me and my siblings upstream, to the San Pedro River,” a tributary of the Usumacinta, “to swim and fish,” Burelo remembers.

Even when red mangroves lined the river and could easily be seen back then, Burelo didn’t know how to properly identify them. It was only after he became a biologist and later got his Ph.D. in Botany that he was able to recognise the twisted roots popping from the fresh water as Rhizophora mangle. And like good botanists usually do, Burelo recorded them with his camera. 

At the Herbarium of the Universidad Juárez Autónoma de Tabasco, botanist and conservation ecologist Carlos Burelo and his students display a selection of specimens from the rich relict flora of the San Pedro Mártir River. Credit: Octavio Aburto-Oropeza.

He then sent some of these photographs in a second email to Ezcurra. “I was very surprised,” Ezcurra confesses, “I immediately asked Carlos for his phone number, and then went to Google Earth to see exactly where these mangroves were.” At that moment, Ezcurra realised the importance of his colleague’s simple question: How could those mangroves get there? 

This brief, but fruitful exchange of emails happened in 2017. Since then, Burelo, Ezcurra and their colleagues in Mexico and the U.S. have been studying the area around the San Pedro River looking for clues to answer that question. One of the hypotheses that sparked a deep discussion among the group of researchers revolved around the possibility that the plants might have been brought by the Mayans. The ruins of Aguada Fénix, the oldest Mayan city found to date, is only a short distance from the river. But the answer lay within the mangroves themselves — more precisely, in their genes.

The team took leaves not only of the mangrove trees from the San Pedro River, but also of those growing on the coast of Tabasco and in other sites of the Yucatán Península. Part of their multidisciplinary approach included sequencing and comparing the DNA of each of the populations of Rhizophora mangle

Botanist Carlos Burelo, in hat and glasses, project scientist Paula Ezcurra, in hat and dark glasses, and members of the local community navigating the San Pedro River on their way to find and collect specimens of an inland population of Rhizophora mangle in Tabasco, Mexico. Front to back: Neil Morales Rodríguez, with camera, “Chito” Montiel in orange shirt, Rodrigo Morales in black, Manuel Campos, unidentified boat driver. Credit: Octavio Aburto-Oropeza.

“The mangroves of Términos Lagoon are closest to those of the San Pedro River,” Ezcurra explains. He’s referring to a genetic distance though, not a physical one. This means that the mangrove trees of the Términos Lagoon, separated by 87 km from those of the San Pedro River, are the closest relatives of the interior population. Ezcurra, Burelo and his colleagues report today October 4 in an article published in the Proceedings of the National Academy of Sciences. 

DNA can be more than a family photo: It may act as a history book and a molecular clock. The mutations between two populations occur more or less at the same rate, similar to the hands on a clock, but a clock that marks the time in tens of thousands of years. By comparing and measuring the amount of mutations unique to each mangrove population, researchers can estimate how long ago they diverged. “We worked on a simulation,” says Ezcurra to explain how they could estimate the time using a molecular clock, and concluded that “these populations separated from each other around 100,000 years ago, […] long before any human population arrived in the area,” Burelo adds. 

So, if the Mayans didn’t transport the mangroves to the San Pedro River, how did they get there? “We began to review what was happening on Earth at that time,” Ezcurra recalls, “and it turns out that 120,000 years ago, the last interglacial period occurred.” It was during the Pleistocene, in the Sangamonian Stage or Eemian, to be specific: a very hot period in which almost all glaciers had melted “and sea level rose up between 6 and 9 metres,” Ezcurra says. 

“That would have caused the sea to reach the Reforma waterfalls, which is where the mangrove population of the San Pedro River begins,” says Burelo excitedly. Knowing what kind of evidence to look for, the team quickly found data that corroborated their hypothesis: The sea had risen so much that the coastline extended as far as the area around the San Pedro River — an ideal place for red mangrove.

And it wasn’t only the sea level rise that showed that the coastline had changed. In the area the group of researchers also found 131 species of plants usually found in coastal habitats, mainly orchids and legumes. Among the species they found are Acrostichum aureum, Myrmecophila tibicinis, Coccoloba barbadensis and Acoelorrhaphe wrightii. The full floristic account, compiled by students and researchers from UJAT, accompanies the main text as supplementary material. 

When digging the soil, they also found fossils of mussels and mollusks that only live in the sea. “I told a friend who has a ranch in the area,” says Burelo, “he told me: ‘There’s something I want to show you.'” Burelo and his colleagues went to his friend’s corn field, and dug about 30 cm into the ground. “We found white, velvety sand,” Burelo recalls. After some analyses, it turned out to be from the ocean floor.

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. Credit: Octavio Aburto-Oropeza

“The story had come full circle,” a smiling Ezcurra says. “It’s like reading an H. G. Wells novel about a lost world!” It may not have been a whole world, but it was a full ecosystem. “It was not just the mangroves,” Burelo adds, “the entire ecosystem of the coast rose so much as to reach the San Pedro River.”

The surprising results are only the beginning of future research. For example, it’s unclear how the San Pedro River mangroves manage to survive in freshwater, but they don’t seem to cope particularly well. Normally, Rhizophora mangle grows in seawater, and even when they filter a lot of this salt, the minerals play an important role in the proper functioning of leaf tissue and, therefore, in the photosynthetic process. This might explain why the mangrove population of the San Pedro River shows clear signs of stress like low stature and reduced populations. “They don’t make up patches,” Burelo describes, “you see groups of three or four trees, and then, after some kilometres you spot another one.”

Although the questions that Ezcurra and Burelo are asking themselves are part of a research agenda that seeks to unravel the mysteries of these mangroves, Burelo is clear on something: “The main purpose of this story is to propose the creation of a protected area.”

This is because, throughout its history, Tabasco had the highest deforestation index in Mexico. (That place is now taken by the neighbouring states of Campeche and Yucatán). The loss of vegetation was a consequence of the Balancán-Tenosique Plan, which the Mexican government implemented in 1972 hoping to transform the rainforest into an area for agriculture and livestock. Although the Plan never achieved the expected success, by 1990 Tabasco had already lost 41% of its forests.

An aerial view of the San Pedro Mártir River in Tabasco, Mexico, highlights the river’s unique and naturally occurring tufa dam structures. Credit: Ben Meissner.

Now, the Mexican government is working on another mega project: the Maya Train. The train will make a stop in Burelo’s city: Balancán. The project has been criticised for various reasons, but mainly because of environmental concerns, since the studies for its development range from non-existent to misleading. However, Burelo believes that the mega project could benefit this small region of Tabasco now, years after losing its tropical forests. “There is no rainforest here, there are pastures. Neither the forests nor the jaguars will be destroyed, perhaps poverty and marginalization will.”

More than half of the population living near the San Pedro River experiences some level of poverty. Sixty seven percent of homes lack indoor plumbing and there are no solid waste treatment programs, nor sewage treatment plants.

Therefore, the river and its unique mangroves could become a tourist attraction, something that may help improve the quality of life of the local community. Burelo, smiling, already pictures the slogans: “Navigate a Pre-Hispanic ecosystem,” “Visit a one-of-its-kind mangrove“. For this to happen, precautions should be taken and, on this, Ezcurra and Burelo agree: “The authorities are not preparing.” Even when it’s true that the Maya Train won’t affect the rainforest  — as there’s none left — the researchers want to make sure the project won’t imperil the relictual mangrove populations.

“We’ve talked with local authorities, with the dean of the University, as well as with the National Commission of Natural Protected Areas,” says Ezcurra, “we are doing everything we can so that the San Pedro River is declared a natural protected area.”

For the plan to work — and the researchers know this well —  government, universities and local people must work together. Burelo adds anxiously: “I’m just waiting for the pandemic to end, so that I can go visit the rest of the communities and tell them about these mangroves, about this ecosystem.”


O. Aburto-Oropeza, C.M. Burelo-Ramos, E. Ezcurra, P. Ezcurra, C.L. Henríquez, S. Vanderplank, F. Zapata. “Relict inland mangrove ecosystem reveals last interglacial sea levels”. PNAS. 04 October, 2021.


Tudela, F. Recursos naturales y sociedad en el trópico húmedo tabasqueño. In: Leff, E. (Coord.) 1990. Medio ambiente y desarrollo en México. Vol. I. Centro de Investigaciones Interdisciplinarias en Humanidades, UNAM; Ed. Porrúa. México. Pp.:149-227.

Agustín Ávila-Casanueva is director of communications at the Centre for Genomic Sciences of the National Autonomous University of Mexico, UNAM. Along with the Science Beat collective, he won the National Journalism Award in 2018 for their science communication work. He was awarded the Robert L. Breen Scholarship for Mexican Journalists to attend the Under the Volcano program in 2020. He’s also a member of the Mexican Network of Science Journalists. His work has been published in Tec Review, La Revista de la Universidad, Nexos, Chilango and Este País. Follow him on Twitter.

English translation by Lorena Villanueva Almanza

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