Source: The Conversation – USA (2)
Fifty-six million years ago, Earth’s forests reached a tipping point. They had grown dense, lush canopies at the start of one of Earth’s most intense episodes of greenhouse warming. But those canopies began to thin.
As global temperatures rose by as much as 11 degrees Fahrenheit (6 degrees Celsius), heat and drought put stress on the forests, killing large numbers of trees. Forest canopies opened, exposing the ground to more sunlight and altering the movement of water through the landscape.
In southern Wyoming, ferns briefly flourished where relatives of elms, walnuts, dawn redwood and avocado trees once thrived. Then palms and other warmth-loving plants spread northward.
In a new study in the journal Science, my colleagues and I show how those Wyoming forests lost 60% of their canopy during this period, known as the Paleocene-Eocene Thermal Maximum, or PETM, and how it took them well over 100,000 years to recover.
Regan Dunn
The PETM was Earth’s closest natural analog to the warming the world is experiencing today, although humans are releasing carbon dioxide roughly 10 times faster than the planet’s natural processes did then.
Understanding what happened to the forests may help humanity recognize similar thresholds before the planet crosses them again.
Reading the forest from fossil leaves
As paleobotanists, my colleagues and I use plant fossils to identify which species once lived in a place. We wanted to answer a harder question: What did the forest itself look like and how did it change?
The structure of a forest – and importantly its canopy – controls the amount of light that reaches the forest floor, the temperature, water habitat and amount of carbon the forest can store, making it one of the clearest indicators of ecosystem function.
But how do you measure the density of a forest that disappeared 56 million years ago?
Ecologists measure canopy density using what’s known as leaf area index. Dense forests with multiple layers of leaves intercepting sunlight have a high leaf area index score, while open forests that allow more light to reach the forest floor have a lower score. Because the forest canopy influences shade, temperature, water loss and photosynthesis, the index provides a powerful measure of forest function.
Our clues to the density of forest canopies millions of years ago came from microscopic plant cuticles – the thin, waxy outer skin of leaves that can survive for millions of years in organic-rich sediments.
You can still see the shapes of epidermal cells in these fossil leaf fragments, and that’s important.
The cell shape reflects the amount of sunlight the leaf received while growing. Leaves that grow in shade develop longer, more elongated cells as they stretch out seeking sunlight. Those exposed to more sun develop shorter, rounder ones.
R. Dunn, et al., 2026
We turned that relationship into a tool for reconstructing ancient forests. To calibrate it, we collected soils from forests across Central and South America spanning a wide range of canopy densities. Each handful of soil contains cuticles shed by many different plants across the canopy, reflecting the structure of the forest as a whole.
R. Dunn, et al., 2026
The fossil record preserves this same fragmented leaf litter. Comparing the shapes of thousands of epidermal cells with the leaf area index we measured revealed a remarkably strong relationship: The more elongated the cells, the denser the forest canopy above them.
That relationship allowed us to reconstruct the structure of Wyoming’s forests millions of years ago and show how they changed over time.
When forests reach their limits
One of the most surprising discoveries was that the forests did not enter the Paleocene-Eocene Thermal Maximum in decline.
Just before rapid warming began, the forest canopies reached their greatest density in hundreds of thousands of years, likely reflecting favorable growing conditions as atmospheric carbon dioxide began to increase. A leading theory for the source of that carbon dioxide involves volcanic eruptions.
That flourishing forest did not last, however. As temperatures climbed, heat and drought overwhelmed the benefits of higher carbon dioxide levels. The canopy rapidly thinned as trees died, and it remained much thinner for over 100,000 years.
R. Dunn, et al., 2026
The forests functioned very differently in this diminished state, and that affected the surrounding environment. Ancient soils gave way to coarser river deposits, suggesting that the loss of canopy altered how water and sediment moved through the basin.
The changing climate changed the forest, and the forest changed the landscape.
Lessons for today
This sequence carries an important lesson for today.
Higher carbon dioxide levels like the world is experiencing now can stimulate plant growth, but only while temperatures and water remain within the limits that trees can tolerate.
Beyond those limits, heat, drought, insects, pathogens and wildfire can overwhelm any fertilization effect that would boost growth.
Around the world, many forests are already showing signs of diminishing as temperatures rise, in addition to deforestation for timber, crops and rangeland that further reduce their resilience.
Forests recovered, but it took over 100,000 years
The story of the ancient forests of 56 million years ago does not end with collapse.
Over time, the increased breaking down of rocks in the warmer climate, known as weathering, gradually pulled carbon from the air, storing it in marine sediments. That allowed the climate to cool and water to become more available.
Forest canopies recovered, eventually becoming even denser than before the warming began. As the forests expanded, they likely restored their ability to stabilize soils, regulate the water cycle and draw carbon from the atmosphere, helping reduce the greenhouse effect and boost the planet’s long-term recovery.
Regan Dunn
Our study shows that carbon dioxide emissions have pushed forests beyond their physiological limits before, triggering changes that ripple from vegetation to rivers and across entire landscapes. It also shows that forests are remarkably resilient when given time to recover, but what counts as time is far longer than a human lifespan – it requires thousands of generations.
Today, human-caused carbon emissions and warming are unfolding vastly faster than during the PETM. The fossil record reminds us that forests can recover, but only if humanity avoids pushing them beyond thresholds from which recovery takes tens of thousands of years.
Regan E. Dunn does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.