A Yucatan forest “trapped in time” reveals why an ancient threat can get worse

If you venture deep into the heart of the Yucatan Peninsula, you just might find a relic of Earth’s ancient past along a swampy riverbed.

In a study published Monday in the journal Proceedings of the National Academy of Sciences, researchers reveal a remarkable river ecosystem of red mangrove trees that is, in essence, a time capsule. It has been “trapped in time for more than 100,000 years,” explained co-author Octavio Aburto-Oropeza, a marine ecologist at Scripps Institution of Oceanography at UC San Diego, in a statement.

And it’s unlike anything else. Mangroves exist on coastlines — this mangrove forest is 105 miles from the nearest ocean and genetically distinct from other red mangrove populations. It has existed in isolation, weathering changes to the environment.

Its analysis reveals how living organisms can shed light on our past, revealing how nature adapted to ancient climate change and survived.

“We hope our results convince the government of Tabasco and Mexico’s environmental administration of the need to protect this ecosystem,” the study team writes.

“The story of Pleistocene glacial cycles is written in the DNA of its plants waiting for scientists to decipher it but, 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.”

What we can learn from ancient climate — Discoveries made about the past don’t just transport us back in time — they also reveal how Earth has historically been shaped by climate shifts.

This isn’t to say the climate change we’re currently experiencing is simply par for the course. While Earth has undergone disruptive climate changes throughout its history, what is happening now is unprecedented because it’s fueled by human influence.

Scientists study past climate in an effort to:

• Understand how climate systems work generally
• What happens when the climate changes
• Establish what we can do to do prepare for human-caused climate change

For example, paleoclimatologists drill for ice cores in Greenland and take sediment samples from the ocean so they can analyze changing carbon levels and assess the impact of climate change. Similarly, scientists compare leaves from modern ginkgo trees to 100-million-year-old leaves, searching for clues about climate during the last age of the dinosaurs.

This study on ancient mangroves goes a step further, showing how climate change influenced Earth through the survival of one overlooked, yet vital ecosystem: red mangrove trees, whose roots thrive in high-saline water, even when they’re far from the Earth’s salty oceans.

How they made the discovery — Mangroves are forests or shrubs which typically live in the region where the ocean meets the land between high and low tides. Red mangroves, or Rhizophora mangle, are one of the 80 different species of mangrove trees.

They exist in subtropical and tropical areas around the world: But this is the only red mangrove forest known to exist far from the ocean.

In 2018, the study team began investigating red mangrove trees along the San Pedro Mártir River. This river starts in the El Petén rainforest in Guatemala and ends in the Reforma Waterfalls of Tabasco, Mexico. (Their exploration is detailed in this 2021 short film.)

The study site was chosen after co-author Carlos Burelo, a botanist at the Universidad Juárez Autónoma de Tabasco, repeated a question he has asked himself ever since he played among this forest as a boy: How precisely did it end up so far from the ocean?

So the team collected samples from this forest of red mangrove (Rhizophora mangle) trees. Through a unique combination of this fieldwork, along with genetic analysis and simulation models, scientists were able to analyze the mangroves and their history in greater detail.

The process yielded another perplexing question: How did this mangrove still host flora and fauna typically of coastal ecosystems?

The answer, they suspected, lay in the mangrove’s ancient past.

The discovery — During the Last Interglacial Period (a time roughly 115,000 to 130,000 years ago), when global temperatures were warmer than they are today, ice sheets melted and sea levels rose.

The study team hypothesized red mangroves grew further inland along the river’s lakes and dams as sea levels rose during this period, and have remained there to this day, even after sea levels receded. The researchers were banking on finding evidence of the mangrove’s journey inland in the plant’s genomes.

Through their experiments, scientists confirmed their hypothesis, writing:

“San Pedro River mangroves are a relict of a coastal ecosystem that colonized the river’s tufa lakes, possibly during the Last Interglacial, and stayed behind along the riverbanks after the oceans receded during the Wisconsin glaciation.”

The scientists found the elevation of their mangrove samples matched the six to nine feet in ancient sea-level rise in this area reported by other studies.

The findings, in turn, confirm prior research suggesting the red mangroves in this region are a relict — an organism that has survived from the ancient past.

Why this matters — These findings help us understand how plants like the red mangrove adapted to ancient climate change, turning into thriving ecosystems that are still home to diverse wildlife.

Red mangroves generally support essential ecosystems ranging from Guatemala to the southeastern United States, sheltering a wide variety of birds, mammals, and even endangered species like the Florida Panther.

The red mangroves along the San Pedro Mártir River have withstood an onslaught of modern agricultural developments threatening their survival.

“In spite of the farming, in spite of the cattle ranching... despite the clearing of forests, there are the mangroves,” the narrator of Memories of the Future explains, a video documentary on these red mangroves.

But the mangrove’s survival is far from guaranteed. The researchers note that government-backed deforestation for cattle farming wiped out the plains surrounding the San Pedro River in the 1970s. The swampiness of the red mangroves proved too much of a headache for developers at the time, so they survived the initial development.

Due to the ongoing threat of deforestation, the future of the red mangroves remains in jeopardy. The study team writes that they hope their work will encourage governments to act to preserve this one-of-a-kind ecosystem with roots in the ancient past — literally.

Abstract: Climatic oscillations during the Pleistocene played a major role in shaping the spatial distribution and demographic dynamics of Earth’sbiota, including our own species. The Last Interglacial (LIG) or EemianPeriod (ca. 130 to 115 thousand years B.P.) was particularly influential because this period of peak warmth led to the retreat of all ice sheets with concomitant changes in global sea level. The impact of these strong environmental changes on the spatial distribution of marine and terrestrial ecosystems was severe as revealed by fossil data and paleogeographic modeling. Here, we report the occurrence of an extant, inland mangrove ecosystem and demonstrate that it is a relict of the LIG. This ecosystem is currently confined to the banks of the freshwater San Pedro Mártir River in the interior of the Mexico–Guatemala El Petén rainforests, 170 km away from the nearest ocean coast but showing the plant composition and physiognomy typical of a coastal lagoon ecosystem. Integrating genomic, geologic, and floristic data with sea level modeling, we present evidence that this inland ecosystem reached its current location during the LIG and has persisted there in isolation ever since the oceans receded during the Wisconsin glaciation. Our study provides a snapshot of the Pleistocene peak warmth and reveals biotic evidence that sea levels substantially influenced landscapes and species ranges in the tropics during this period.