A mangrove forest with light shining through a gap

The Mexican mangroves used to be on the coastline, but are now more than 170 kilometres inland. Image © Shutterstock / Kai Grim

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Mexican forest offers glimpse into 110,000-year-old lost world

A lost world in Mexico has offered scientists a glimpse of the planet before the last Ice Age. 

The mangrove forest on the San Pedro Martir River was once on the shores of a vast ocean when Earth was much warmer. As the last remnant of this world, it provides clues about how the environment may change in response to climate change. 

A mangrove forest in Mexico is the last remnant of an ancient ecosystem dating back over 110,000 years. 

The trees, which are normally found by the ocean, are now more than 170 kilometres inland. They are believed to mark the ancient coastline of a warmer world where melting ice caused seas to rise over nine metres higher than they are today. 

The forest offers a snapshot of the ancient past, allowing scientists to understand how environments adapted to changing sea levels as the global climate changed. 

'The most amazing part of this study is that we were able to examine a mangrove ecosystem that has been trapped in time for more than 100,000 years,' said study co-author Dr Octavio Aburto-Oropeza. 'There is certainly more to discover about how the many species in this ecosystem adapted throughout different environmental conditions over that time.  

'Studying these past adaptations will be very important for us to better understand future conditions in a changing climate.' 

The paper, led by researchers at the University of California, was published in PNAS.

A mangrove shown above and below water

Mangroves provide vital habitat and ecosystem services such as lessening the impact of storms. Image © Shutterstock / Damsea

Trees on the move 

Mangroves are a group of trees which are found around the equator, typically along coastlines and tidal rivers. They have a variety of specialist adaptations to take in extra oxygen as well as removing salt which allows them to tolerate conditions that would kill most plants

They provide an important habitat for a wide range of marine species such as fish which shelter among the roots for protection, while other animals live on the leaves and among the branches. 

Being adapted for life in saltwater, the mangroves of the San Pedro Martir River are something of an oddity. The freshwater river runs from Guatemala to Mexico hundreds of kilometres from the nearest coastline. 

Dr Sandra Knapp, a researcher in plant sciences at the Museum who was not involved with the study, says, 'I've always been intrigued by these inland mangroves, and you see them in Belize as well.  

'If you saw a similar example in Britain, it would be like finding the same plants in the Thames estuary up the river in Oxford.' 

A team of American and Mexican scientists set out to investigate the trees. They took genetic and geological samples to help them work out exactly how the mangroves made it from the coast to the river. 

Analysing the genetics of the trees, they found that the San Pedro Martir mangroves are closely related to those at the coastal Términos Lagoon in southern Mexico. This suggests that these trees migrated from a coastal population in the distant past, with the age of divergence estimated at around 100,000 years ago. 

'The way they have dated them is really interesting, in addition to finding out how these populations are related,' says Sandra says.

'There's a lot of uncertainty around molecular dating, but they've used a variety of different methods to ensure that even the uncertainty in their generation times still date around the time of the last interglacial.' 

A single tree amid water

Deforestation poses a threat to mangroves. Image © Shutterstock / Charles Barrios

Water world 

This period of history, known as the Eemian or Last Interglacial, represents a time when the world was around three to five degrees centigrade warmer than it is now. These higher temperatures melted many icesheets around the world, causing sea levels to rise by between six and nine metres. 

As southern Mexico is relatively flat, such a change in sea level would see a dramatic changes as seawater would flood over the land. Evidence of this change is found in the geological samples, with aquatic shells and rounded pebbles indicating that the land was underwater during the Eemian. 

At the time, far from being isolated from the sea, the mangroves of the San Pedro Martir River were once on the coastline of a large ocean. Following cooling temperatures as the last Ice Age began the sea levels declined, leaving the mangroves isolated in the river. 

'This discovery is extraordinary,' says co-author Professor Felipe Zapata. 'Not only so the mangroves here have their origins printed in their DNA, but the whole coastal lagoon ecosystem of the last interglacial has found refuge here.' 

The mangroves are the last vestige of these ancient ecosystems, having lived through a period of intense climate change. By studying them, researchers may be able to find out more about how rising sea levels caused by greenhouse emissions will affect coastlines and ecosystems in the future. 

However, even this survival is uncertain. The area lost most of its tree cover in the 1970s as the government of the time sought to encourage pasture for raising cattle. The mangroves only survived because the area they thrive in is too boggy for bulldozers to work, but are still at risk even now. 

'The story of Pleistocene glacial cycles is written in the DNA of its plants waiting for scientists to decipher it,' the authors said in a statement.

'More importantly, the San Pedro mangroves are warning us about the dramatic impact that climate change could have on the coastal plains of the Gulf of Mexico if we do not take urgent action to stop the emission of greenhouse gases.'