Stands of mangroves frequently dot coastlines, their roots sipping from salty waters. But now researchers have spotted these salt-tolerant trees in an unexpected place: 170 kilometers away from the nearest ocean in southern Mexico’s rain forest. By analyzing the DNA of the trees, the team concluded that these mangroves are relics from the Last Interglacial Period, roughly 115,000 years ago when sea levels were 6–9 meters higher than they are today. This finding is a “memory of the future,” the team proposed, given predicted increases in sea level.
Carlos Manuel Burelo-Ramos, a biologist at Juárez Autonomous University of Tabasco in Mexico and a coauthor of the recent study, remembered fishing on Central America’s San Pedro Mártir River with his father in the 1980s and seeing odd-looking trees. “They didn’t resemble any type of plant from that region that I had ever seen,” Burelo-Ramos said in a video produced about the team’s research. Those plants were mangroves, Burelo-Ramos realized as an adult, but he had a difficult time convincing colleagues of his discovery.
That’s because mangroves characteristically thrive in salty or brackish waters found near coastlines, said Isamar Cortés, a mangrove researcher at Montclair State University in New Jersey not involved in the research. “Normally mangroves are found within coastal environments.” Finding mangroves along the inner reaches of the San Pedro Mártir River, which traverses parts of Mexico and Guatemala, was therefore unexpected.
In 2018 and 2019, Octavio Aburto-Oropeza, a marine ecologist at the Scripps Institution of Oceanography in La Jolla, Calif., and lead author of the new study, and his colleagues made four trips to the stands of red mangroves (Rhizophora mangle) located along inland sections of the San Pedro Mártir River. They snipped samples of the mangroves’ leaves and drilled core samples from the trees’ trunks. Back in the laboratory, the researchers compared the mangroves’ DNA with that of seven other populations of mangroves found along the coasts of Mexico’s Yucatán Peninsula. On the basis of the degree to which the inland mangroves’ DNA differed from that of their nearest neighbors, the team estimated how long it’d been since the two populations shared a common ancestor (their divergence time).
The researchers found that the San Pedro Mártir River’s mangroves diverged from their closest geographic relatives roughly 100,000 years ago. That’s consistent with the timing of the Last Interglacial Period, an interval in Earth’s history when there was relatively more carbon dioxide in the atmosphere. “Not as high as now, but higher compared with other periods,” Aburto-Oropeza said. The planet was between 1°C and 2°C warmer then, and sea levels are believed to have been several meters higher.
Trees from the Past
These mangroves are likely relics from that time period, the team concluded. They would have thrived in the salty waters that inundated the region at the time. (Oyster shells found nearby provided additional evidence of the area’s oceanic past.) The researchers suggested that slowly, over generations, the trees would have adapted to the calcium carbonate–rich freshwater that characterizes the inland reaches of the San Pedro Mártir River today. That’s plausible, because modern mangroves are known to survive in freshwater high in calcium carbonate, said Aburto-Oropeza. “Many mangroves are distributed in areas with coral reefs.”
The results were published in October in Proceedings of the National Academy of Sciences of the United States of America.
Finding these relic mangroves provided a snapshot of the past, which will in turn become our future unless we reverse climate change, said Aburto-Oropeza. “What we are seeing is a memory of what will happen in the future.”
—Katherine Kornei (@KatherineKornei), Science Writer