Hidden mangrove forest in the Yucatan peninsula reveals ancient sea levels

Deep in the heart of the Yucatan Peninsula, an ancient mangrove ecosystem flourishes more than 200 kilometers (124 miles) from the nearest ocean. This is unusual because mangroves -- salt-tolerant trees, shrubs, and palms -- are typically found along tropical and subtropical coastlines.

A new study led by researchers across the University of California system in the United States and researchers in Mexico focuses on this luxuriant red mangrove forest. This "lost world" is located far from the coast along the banks of the San Pedro Martir River, which runs from the El Petén rainforests in Guatemala to the Balancán region in Tabasco, Mexico.

Because the red mangrove (Rhizophora mangle) and other species present in this unique ecosystem are only known to grow in salt water or somewhat salty water, the binational team set out to discover how the coastal mangroves were established so deep inland in fresh water completely isolated from the ocean. Their findings were published Oct. 4 in the Proceedings of the National Academy of Sciences.

Integrating genetic, geologic, and vegetation data with sea-level modeling, the study provides a first glimpse of an ancient coastal ecosystem. The researchers found that the San Pedro mangrove forests reached their current location during the last interglacial period, some 125,000 years ago, and have persisted there in isolation as the oceans receded during the last glaciation.

The study provides a snapshot of the global environment during the last interglacial period, when the Earth became very warm and polar ice caps melted entirely, making global sea levels much higher than they are today.

"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 Octavio Aburto-Oropeza, a marine ecologist at Scripps Institution of Oceanography at UC San Diego and a PEW Marine Fellow. "There is certainly more to discover about how the many species in this ecosystem adapted throughout different environmental conditions over the past 100,000 years. Studying these past adaptations will be very important for us to better understand future conditions in a changing climate."

Combining multiple lines of evidence, the study demonstrates that the rare and unique mangrove ecosystem of the San Pedro River is a relict -- that is, organisms that have survived from an earlier period -- from a past warmer world when relative sea levels were six to nine meters (20 to 30 feet) higher than at present, high enough to flood the Tabasco lowlands of Mexico and reach what today are tropical rainforests on the banks of the San Pedro River.

The study highlights the extensive landscape impacts of past climate change on the world's coastlines and shows that during the last interglacial, much of the Gulf of Mexico coastal lowlands were under water. Aside from providing an important glimpse of the past and revealing the changes suffered by the Mexican tropics during the ice ages, these findings also open opportunities to better understand future scenarios of relative sea-level rise as climate change progresses in a human-dominated world.

Carlos Burelo, a botanist at the Universidad Juárez Autónoma de Tabasco and a native of the region, drew the attention of the rest of the team towards the existence of this relict ecosystem in 2016. "I used to fish here and play on these mangroves as a kid, but we never knew precisely how they got there," said Burelo. "That was the driving question that brought the team together."

Burelo's field work and biodiversity surveys in the region established the solid foundation of the study. His remarkable discovery of the ancient ecosystem is documented in "Memories of the Future: the modern discovery of a relict ecosystem," anaward-winning short film produced by Scripps alumnus Ben Fiscella Meissner (MAS MBC '17).

Felipe Zapata and Claudia Henriquez of UCLA led the genetic work to estimate the origin and age of the relict forest. Sequencing segments of the genomes of the red mangrove trees, they were able to establish that this ecosystem migrated from the coasts of the Gulf of Mexico into the San Pedro River over 100,000 years ago and stayed there in isolation after the ocean receded when temperatures dropped. While mangroves are the most notable species in the forest, they found nearly 100 other smaller species that also have a lineage from the ocean.

"This discovery is extraordinary," said Zapata. "Not only are the red mangroves here with their origins printed in their DNA, but the whole coastal lagoon ecosystem of the last interglacial has found refuge here."

Paula Ezcurra, science program manager at the Climate Science Alliance, carried out the sea-level modeling, noting that the coastal plains of the southern Gulf of Mexico lie so low that a relatively small change in sea level can produce dramatic effects inland. She said a fascinating piece of this study is how it highlights the benefits of working collaboratively among scientists from different disciplines.

"Each piece of the story alone is not sufficient, but when taken together, the genetics, geology, botany, and field observations tell an incredible story. Each researcher involved lent their expertise that allowed us to uncover the mystery of a 100,000+ year-old forest," said Ezcurra, an alumna of Scripps Oceanography (MAS CSP '17).

The field work was led by the ecologists on the team -- Octavio Aburto-Oropeza, Paula Ezcurra, Exequiel Ezcurra of UC Riverside, and Sula Vanderplank of Pronatura Noroeste. Visiting the study sites several times starting in 2016, they collected rocks, sediments and fossils to analyze in the lab, helping them pinpoint evidence from the past that is consistent with a marine environment.

The authors note that the region surrounding the study sites was systematically deforested in the 1970s by a misguided development plan; the banks of the San Pedro River were only spared because the bulldozers could not reach it. The area is still threatened by human activities, so the researchers stressed the need to protect this biologically important area in the future.

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Climate change threat to tropical plants

Tropical plants closer to the equator are most at risk from climate change because it is expected to become too hot for many species to germinate in the next 50 years, UNSW researchers have found.

Their study analysed almost 10,000 records for more than 1300 species from the Kew Gardens' global seed germination database.

The research, published in the journal Global Ecology and Biogeography recently, was the first to look at the big picture impact of climate change on such a large number of plant species worldwide.

Lead author Alex Sentinella, UNSW PhD researcher, said past research had found that animal species closer to the equator would be more at risk from climate change.

"The thought was that because tropical species come from a stable climate where it's always warm, they can only cope with a narrow range of temperatures -- whereas species from higher latitudes can cope with a larger range of temperatures because they come from places where the weather varies widely," Mr Sentinella said.

"However, this idea had never been tested for plants.

"Because climate change is a huge issue globally, we wanted to understand these patterns on a global scale and build upon the many studies on plants at an individual level in their environment."

Seeds a key indicator of survival

The researchers examined seed germination data from the Millennium Seed Bank Partnership Data Warehouse, hosted by Kew Royal Botanic Gardens in London, to quantify global patterns in germination temperature.

They analysed 9737 records for 1312 plant species from every continent except Antarctica and excluded agricultural crops.

Mr Sentinella said they chose seed data because it was widely available and relevant to the ability of a species to cope with different temperatures.

"With seeds, you can experiment on them quickly, there are a lot of studies about them and importantly, germination directly relates to how a species will survive, because if the seed doesn't germinate the plant won't live," he said.

"So, we collated the data from the Kew Gardens database, examined all experiments on the same species from the same locations, and then determined the range of temperatures each species could tolerate in order to survive."

The researchers also examined climate data for the same locations as the plant species used in the study.

They looked at current temperature -- the average temperature of the warmest three months from 1970 to 2000 -- and predicted temperature for 2070.

The researchers then compared the temperatures the plants were experiencing now with the forecasted 2070 temperatures.

Tropical plants to hit or exceed temperature limits

The study discovered tropical plants do not have narrower temperature tolerances but were more at risk from global warming, because it would bring them close to their maximum seed germination temperatures.

Mr Sentinella said, on average, the closer a plant was to the equator, the more at risk it would be of exceeding its temperature ceiling by 2070.

"These plants could be more at risk because they are near their upper limits. So, even a small increase in temperature from climate change could push them over the edge," he said.

"The figures are quite shocking because by 2070, more than 20 per cent of tropical plant species, we predict, will face temperatures above their upper limit, which means they won't germinate, and so can't survive."

Mr Sentinella said the researchers also found that more than half of tropical species are expected to experience temperatures exceeding their optimum germination temperatures.

"That's even worse because if those plants can survive it would be at a reduced rate of germination and therefore, they might not be as successful," he said.

"If a seed's germination rate is 100 per cent at its optimum temperature, then it might only manage 50 or 60 per cent, for example, if the temperature is higher than what's ideal."

Mr Sentinella said he was surprised to find that climate change would threaten so many tropical species.

"But our most unexpected discovery was that the hypothesis often used for animals -- that those near the equator would struggle to survive the impact of climate change because they have narrower temperature tolerances -- was not true for plants," he said.

"We found that regardless of latitude, plant species can germinate at roughly the same breadth of temperatures, which does not align with the animal studies."

The researchers also found 95 per cent of plant species at latitudes above 45 degrees are predicted to benefit from warming, because environmental temperatures are expected to shift closer to the species' optimal germination temperatures.

Findings to help target conservation efforts

Mr Sentinella said it was possible for some plants to slowly evolve to increasing temperatures, but it was difficult to predict which ones would survive.

"The problem with the quick change in temperatures forecasted, is that some species won't be able to adapt fast enough," he said.

"Sometimes plants can migrate by starting to grow further away from the equator or, up a mountain slope where it's cooler. But if a species can't do that it will become extinct.

"There are almost 400,000 plant species worldwide -- so, we would expect a number of them to fail to germinate between now and 2070."

Mr Sentinella hopes the researchers' findings will help to conserve plant species under threat from climate change.

"Ideally, we would be able to conserve all ecosystems, but the funding is simply not there. So, our findings could help conservation efforts target resources towards areas which are more vulnerable," he said.

"We also hope our findings further strengthen the global body of research about the risks of climate change.

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