Usually mangroves only grow on the coast of the sea – but there is one astonishing exception: over 170 kilometers from the sea, a mangrove forest thrives on a river in the heart of the Yucatan Peninsula. Now a study shows that it is a relic from the interglacial period, when the sea level was higher than it is today. The mangroves were still on the coast over 100,000 years ago. As the sea retreated, the coastal ecosystem moved inland – but it was apparently able to adapt to the new conditions, the scientists report.
The stilt-like roots are their trademark: Mangrove trees grow along many of the world’s tropical coasts. Thanks to their special adaptations, they can withstand the salty water and the fluctuating water levels. The red mangrove (Rhizophora mangle) is typical for Central America. Although the trees also grow in the area of ​​river mouths, areas and regions in the inland that are permanently characterized by fresh water do not normally colonize them. But that is exactly the case in northern Guatemala: Far from any sea coast, a mangrove forest thrives on the banks of the San Pedro River.
On the trail of forest history
An interdisciplinary team of researchers has now investigated what this mysterious ecosystem is all about. For the study, the scientists carried out botanical surveys, genetic studies of plants, geological analyzes and, finally, geographical modeling. “The fascinating history of this ecosystem was ultimately only revealed when the individual results were combined: Each researcher involved helped us with their specialist knowledge to unravel the secret,” says co-author Exequiel Ezcurra from the University of California at Riverside.
The results showed that the ecosystem originally emerged as a typical coastal mangrove forest – but over 120,000 years ago. As the researchers explain, the last so-called interglacial period prevailed at that time. During this warm phase, the earth had comparatively small ice masses and so the sea level was six to nine meters above today’s level. Apparently this led to the area of ​​the current San Pedro river lying on the coast. This was confirmed by the geological investigation results on site, in which the researchers found marine fossils, among other things.
Fascinating eco time capsule
The area of ​​the mangrove forest of San Pedro attained its current inland location when the earth’s climate cooled down again and water was bound in masses of ice, the researchers explain. As a result, the sea level sank and the coast slowly retreated in the area of ​​the mangrove forest. At the end of this process, the area then reached a distance of around 170 kilometers to the nearest ocean. Apparently, the mangroves on the banks of the San Pedro River could cope with this relocation and thus formed a relic of the former coastal ecosystem. This was confirmed by comparisons of the genome of the trees with that of mangroves on the nearest coasts: Genetic peculiarities made it clear that the mangroves of San Pedro have been isolated inland for thousands of years.
While these trees are the most striking representatives of this amazing ecosystem, the scientists’ studies of the vegetation also found around 100 other plant species that are also typical of the coast. “Not only the mangroves, but the entire coastal lagoon ecosystem of the last interglacial has apparently found refuge here,” says senior author Felipe Zapata from the University of California in Los Angeles. His colleague Octavio Aburto-Oropeza from the University of California in San Diego adds: “There is certainly more to be found out about how the many species in this ecosystem have adapted to the environmental conditions over the past 100,000 years”.
As the researchers finally emphasize, the significance of the study goes beyond biology: It illustrates the far-reaching effects of a climate change process in the past on the world’s coasts. Specifically, it becomes clear that a large part of the coastal lowlands on the Gulf of Mexico was under water during the last interglacial period. The results also open up opportunities to better assess the effects of sea level rise if man-made climate change continues, according to the scientists.
Source: University of California – San Diego, Article: PNAS, doi: 10.1073 / pnas.2024518118