Fragments of fossilized leaves like these helped the authors recreate the forest canopies of 56 million years ago. Credit: Dr. Regan Dunn

A study finds that during the PETM, Earth’s landscapes became widely browner amid elevated atmospheric CO₂, global warming, and reduced rainfall caused by volcanically driven carbon release, with trees playing a central role in these dramatic changes.

Fifty-six million years ago, a massive pulse of carbon entered the atmosphere during the Paleocene Eocene Thermal Maximum (PETM), producing one of the best ancient comparisons for understanding how forests respond to rapid warming.

In research published in Science, scientists reconstructed the forest canopy from that period using fossilized leaf cells for the first time, providing a detailed record of how vegetation changed during Earth’s most recent episode of comparable global warming.

“We are putting CO₂ into the atmosphere faster than any known natural process,” says lead author and paleobotanist Dr. Regan Dunn, Assistant Deputy Director and Associate Curator of the Samuel Oschin Global Center for Ice Age Research at La Brea Tar Pits and the Natural History Museum of Los Angeles County. “The Earth has never experienced a carbon release at the pace we’re creating today. The PETM gives us our best window into how Earth’s climate and ecosystems respond to a massive carbon injection before humans began reshaping the planet.”

(Left) Fisheye view of a forest canopy with a Leaf Area Index of 2.8 above a corresponding estimate of forest structure. (Top Right) Leaf cuticle collected from modern forest soil. (Bottom Right), Fossilized leaf cuticle from the Hanna Basin. Credit: Dr. Regan Dunn

Ancient warming turned landscapes browner

The reconstruction indicates that large areas of Earth became browner during the PETM. Volcanically driven carbon emissions increased atmospheric CO₂, raised global temperatures, and reduced rainfall.

As conditions changed, plant species shifted northward, forest canopy cover decreased, erosion intensified, and the terrestrial water cycle was disrupted. Today, human activity is adding CO₂ to the atmosphere at roughly an order of magnitude faster than during the PETM, and changes to trees appear central to many of these effects.

The field crew excavating a trench through PETM strata in the Hanna Basin, WY. Credit: Marieke Dechesne, USGS

“There are significant tree mortality events everywhere on Earth right now,” says Dunn. “Forests are in decline because of warming temperatures, drought stress, pathogen and insect infestations, and wildfires—when you start losing the trees, our canaries in the coal mine, you’re in trouble. It’s almost universally true in all of Earth’s five major extinction events.”

Fossil cells reveal vanished forest canopies

Reconstructing forests that disappeared more than 50 million years ago required the researchers to examine microscopic pieces of fossilized leaves known as leaf cuticle fragments.

The researchers developed a new way to use these fragments to calculate leaf area index (LAI), a measurement of how much of the sky is covered by leaves and therefore how dense a forest canopy is. Dunn and her colleagues first measured LAI in several modern forests across South and Central America. They pointed a camera fitted with a fisheye lens upward from a tripod while simultaneously collecting leaf cuticles from soil directly below the canopy.

“What excites me about LAI is that it lets us move beyond somewhat subjective descriptions of vegetation, like ‘open forest’ or ‘woodland.’ Instead, the leaves themselves provide a quantitative estimate of canopy structure, allowing us to compare ecosystems using an objective, reproducible metric,” says Dunn. Assigning a numerical measurement makes it possible to compare changes in canopy structure more precisely through time.

Example of fossilized leaf cuticle used in the study exhibiting cell shapes of a denser, shadier forest. Scale bar is 50 microns (0.05 millimeters) in length. Leaf cuticle fossil stained red for greater visibility under a microscope. Credit: Dr. Regan Dunn

Dunn previously estimated LAI using phytoliths, microscopic plant cells filled with silica. Because leaf cuticle fragments are preserved differently, however, the researchers needed a separate calculation to account for those differences. To reconstruct how forests responded to the enormous rise in CO₂ during the PETM, Dunn examined cuticle fragments preserved within organic-rich sedimentary rocks from Wyoming.

A leaf’s epidermal cell shape records how much sunlight it received during growth. Leaves growing lower in a tree under heavy shade develop differently from leaves near the top of the canopy that receive direct sunlight. Dunn found that these cell shapes are closely related to vegetation density (LAI). “Leaves that grow in denser forests—in shadier habitats–have cells with higher aspect ratios, or more elongate and skinny cells than leaves receiving more sunlight,” says Dunn.

“This is the shade response of plants; they lengthen the leaf by lengthening each cell,” Dunn says. Picture each cell stretching itself to get sunlight, compared to a fat and happy cell bathed in full sunlight. “Once the leaves fall from the tree, and begin to break down in the soil, the cell shapes preserved in the fragments can tell us how dense the canopy was where the leaf originally grew.”

The researchers calculated LAI from photographs of modern forests, then measured thousands of epidermal cells preserved in leaf fragments collected from the soil and compared those measurements with the corresponding LAI values. “This is the process of setting up a proxy to reconstruct an environmental variable from fossil deposits. You go into modern environments, establish the relationship, and create a model from which you can reconstruct that variable, in this case LAI, from rocks 56 million years old,” says Dunn.

Cell shapes shown here indicate greater exposure to sunlight in open vegetation. Scale bar is 50 microns (0.05 millimeters) in length. Credit: Dr. Regan Dunn

“I love telling my undergraduate classes that Wyoming has the best fossil record in the world,” says co-author Dr. Ellen D. Currano, paleoecologist, paleobotanist, and professor at the University of Wyoming. “This work is another demonstration of that.”

“What’s unique about the Hanna Basin is that it’s a coal-forming basin. So you have all these organic-rich rocks, like lignites and coals, that are lacking in other places where the PETM interval is well-known, such as the Bighorn Basin in Northwestern Wyoming,” adds Dunn. “Contained within these organic-rich rocks are the leaf fragments we need to reconstruct the ancient forest canopy.”

Wyoming fossils record thinning forests

Ancient Wyoming supported forests dramatically different from the sagebrush landscape found there today. Giant dawn redwoods (Metasequoia) rose above sycamores, alders, palms, and other subtropical and tropical plants. Over more than a decade, Currano, Dunn, and their colleagues collected hundreds of samples from the Hanna Basin, searching systematically for fossil evidence of forests that existed before and after the PETM. Those samples allowed the researchers to reconstruct how the forests changed as the climate shifted.

“Understanding how forest structure changed during the PETM is really important because it tells us about how plant growth, biomass, and productivity are affected by adding a lot of carbon dioxide to the atmosphere. It turns out that too much carbon dioxide is a bad thing for forests because the accompanying warming and drying stresses the trees and kills many of them,” says Currano. “Our work shows that during the PETM, forest canopies became more open, with fewer big trees, and this change affected climate, nutrient cycling, weathering, and, of course, the animals that inhabited the forests. We are starting to see similar changes to forests occurring today, particularly in the Amazon, and the Wyoming plant fossil record gives us insight into where Earth might be headed.”

The authors compared the cuticle structure of modern leaves (pictured above under epifluorescence microscopy) with fossil leaf cuticles to create a model for canopy cover. Scale bar is 50 microns (0.05 mm). Credit: Dr. Regan Dunn

Modern forests show a similar warning

Satellite measurements of modern global LAI, available since the 1980s, show a pattern that the researchers compare with their PETM reconstruction.

“Earth has been on a greening trajectory because of anthropogenic carbon dioxide emissions that have fertilized plants,” says Dunn. “But as Earth has heated up because of those emissions, this greening trend is reversing, and many parts of Earth are now browning. This suggests that we’ve passed a critical threshold where heightened temperatures are now adversely affecting forests.”

The findings suggest that continued warming could move modern forests toward the widespread browning recorded during the PETM. Today’s forests, however, face several human-driven pressures at the same time. Along with rapidly increasing atmospheric CO₂ and rising temperatures, deforestation, increasingly frequent and severe wildfires, habitat fragmentation, invasive species, and land use change are weakening forest resilience and reducing opportunities for recovery.

Together, these pressures could intensify carbon release and ecosystem degradation beyond what occurred during the PETM. Protecting and restoring forests is therefore important both for preserving biodiversity and for maintaining one of Earth’s major natural defenses against accelerating climate change.

“Trees are awesome. All those forests are taking up hundreds of millions of tons of carbon for us every day,” Dunn adds. “But when you start losing the forests, then you’re losing those critical carbon sinks. The PETM reminds us that when forests decline, the consequences ripple through the climate system. We would be wise to protect and restore them while they can still help buffer the unprecedented pace of human-caused climate change.”

Reference: “Forest canopy decline under elevated CO₂ during the Paleocene-Eocene Thermal Maximum” by Regan E. Dunn, Marieke Dechesne, Brady Z. Foreman, Keifer Nace, Jenna M. West and Ellen D. Currano, 13 August 2026, Science.
DOI: 10.1126/science.aec4776

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