Forests across what is now southern Wyoming were dramatically altered 56 million years ago when the planet experienced the Paleocene-Eocene Thermal Maximum (PETM). During that interval temperatures rose by up to six degrees Celsius, heat and drought killed large numbers of trees and the canopy thinned by roughly 60 per cent. The new research published in Science shows that it took well over 100,000 years for those forests to regain their former density.
Why the ancient collapse matters today
The PETM is the closest natural analogue to the warming the world faces now, but carbon emissions are occurring about ten times faster than the natural processes that drove the ancient event. Understanding how forests responded then helps scientists identify thresholds that, if crossed, could push modern ecosystems into long-term decline.
Reconstructing a vanished canopy
To gauge the structure of a forest that vanished millions of years ago, the team used the leaf area index, a measure of how much leaf surface blocks sunlight. They compared microscopic cuticles, the waxy skins of fossil leaves, with modern samples collected from forests in Central and South America. The shape of epidermal cells changes with the amount of light a leaf receives: shade-grown leaves have longer, more elongated cells, while sun-exposed leaves develop shorter, rounder cells. By matching cell shapes to known leaf-area-index values, the researchers could infer canopy density in the ancient Wyoming woods.
From dense to open and back again
Just before the PETM's rapid warming, the forest canopy reached its greatest density in hundreds of thousands of years, likely boosted by rising atmospheric carbon dioxide from volcanic activity. As temperatures climbed, the benefits of higher CO₂ were overwhelmed by heat stress and drought, leading to a rapid thinning of the canopy. The reduced forest altered the landscape, with coarser river deposits indicating changes in water and sediment flow.
What the findings mean for the future
The study demonstrates that while higher CO₂ can initially stimulate plant growth, once temperature and water limits are exceeded the fertilisation effect disappears and forests can enter a prolonged state of reduced function. Modern forests are already showing signs of stress from rising heat, drought, pests and fire, compounded by deforestation for agriculture and timber.
Recovery in the ancient record was possible because the climate eventually cooled as weathering removed CO₂ from the atmosphere, allowing soils to stabilise and new trees to re-establish. However, that process unfolded over tens of thousands of years, far longer than a human lifespan or a political cycle.
For policymakers and conservationists, the lesson is clear: avoiding the crossing of ecological thresholds is essential if we want forests to remain resilient. Protecting existing canopy, reducing emissions and allowing time for natural regeneration are the most effective ways to keep forests from entering a state that would take millennia to reverse.
Regan E. Dunn, associate curator at the La Brea Tar Pits and Museum and adjunct professor of Earth Sciences at USC, says 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."