Red mangroves thrive in the rainforests in the Yucatán Peninsula's San Pedro Mártir River, over 200 kilometers from the sea. But how did these knotted trees, which usually flourish in saline water along coastlines, become trapped so far inland and in freshwater?
Carlos Burelo has been pondering a variation of that topic since he first visited the river 35 years ago on a fishing expedition with his father. As a child, he saw how mangroves differed from other trees because of their twisted aboveground roots, a discovery he carried into adulthood as a biologist at the Universidad Juárez Autónoma de Tabasco in Villahermosa, Mexico.
The red mangroves (Rhizophora mangle) are now part of a "relict ecosystem" that has survived for more than 100,000 years, according to genetic analysis, vegetation and sediment surveys, and simulations of sea-level variations. The lowlands of the Yucatán Peninsula flooded when sea levels rose 9 meters above present-day levels during the last interglacial era, which peaked around 130,000 years ago. As a result, Burelo and colleagues reported in the Proceedings of the National Academy of Sciences on Oct. 12 that the mangrove forest was displaced and developed inland by today's standards. When sea levels dropped as the world cooled again, the trees were left far from the coast.
"We were able to deduce [that the San Pedro mangroves] have been isolated for 120,000 years" by comparing the number of DNA mutations in the inland population with the other mangroves and determining the ages of the trees using tree cores, says Felipe Zapata, an evolutionary scientist at UCLA. According to Zapata, San Pedro's calcium-rich water and riverbed have aided the red mangroves' long-term survival.
In addition to the mangroves, the scientists discovered several plant species with a maritime ancestry in the inland area. According to the researchers, more than 30% of the entire flora growing along the river, or 112 species, is generally found in coastal lagoons or along shorelines, including orchids and legumes.
With such information in hand, the researchers opted to investigate the soil as well. According to Exequiel Ezcurra, an ecologist at the University of California, Riverside, the team expected to find "coastal sediments, ocean sediments, and possibly ocean fossils." Coastal gravels, shells of marine gastropods, clay sediments rich in shell fragments, coastal dunes, and big oyster shells were discovered during a geologic survey of residues near the mangroves, he says.
"The tremendous durability of these trees, in particular, is striking — that they've lasted all this time inland despite being generally acclimated to seawater," says Holly Jones, a conservation scientist at Northern Illinois University in DeKalb. The latter was not involved in the new study.
The team examined the DNA of leaves from the trees and other mangrove forests along the Caribbean Sea and Gulf of Mexico coasts to estimate where the mangroves may have been displaced from. The origins of the inland mangroves were discovered roughly 170 kilometers away along the Gulf of Mexico due to this research.
Together with estimates of past sea levels, these findings suggest that the ocean must have mingled with the lower basin of the San Pedro River at some point during the last interglacial period, forcing red mangroves and other coastal species inland, the researchers conclude.
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