According to research published in the journal Science, ancient forest canopies in Wyoming lost 60% of their density during the Paleocene-Eocene Thermal Maximum (PETM) 56 million years ago, taking more than 100,000 years to recover as global temperatures climbed by up to 11 degrees Fahrenheit (6 degrees Celsius). The findings, led by paleobotanists including Regan Dunn of the La Brea Tar Pits and Museum and Ellen Currano, provide a stark historical precedent for how modern climate change pressures can push contemporary forest ecosystems beyond their physiological limits.
Reconstructing Ancient Canopies From Fossil Leaf Cuticles
To understand what happened to forests millions of years ago, researchers had to look beyond mere species lists and examine the actual structure of the prehistoric ecosystem. According to Dunn and her colleagues, forest canopies control light penetration, temperature, and carbon storage, making them critical indicators of overall ecosystem function. Because complete ancient trees rarely fossilize, the team turned to microscopic plant cuticles—the thin, waxy outer skin of leaves preserved in organic-rich sediments.
By comparing fossil leaf fragments with modern samples from Central and South America, the researchers discovered a reliable link between epidermal cell shapes and forest density. According to the study, leaves grown in heavy shade develop elongated cells while stretching for sunlight, whereas sun-exposed leaves develop shorter, rounder cells. Measuring thousands of these cells allowed the research team to track how Wyoming’s canopy cover shifted over time.
Did you know? Fossilized pollen and palynomorphs recovered alongside leaf cuticles from Wyoming’s Hanna Basin reveal that as the canopy shrank, regional flora shifted dramatically—fern populations briefly flourished where relatives of elms, walnuts, and avocado trees once grew.
The Tipping Point of Global Warming During the PETM
The PETM stands as Earth’s closest natural analog to current anthropogenic warming, although human-driven carbon emissions are occurring roughly 10 times faster than natural processes did 56 million years ago. According to the research team, one of the most surprising discoveries was that the forests did not enter this thermal event in decline. Just before rapid warming began, driven by a suspected surge in carbon dioxide likely sourced from volcanic eruptions, forest canopies reached their greatest density in hundreds of thousands of years under favorable growing conditions.
However, that flourishing state collapsed as extreme heat and drought overwhelmed the fertilization benefits of elevated carbon dioxide. According to the findings, the canopy rapidly thinned as large numbers of trees died, exposing the ground to intense sunlight and altering how water and sediment moved through the basin, transforming ancient soils into coarser river deposits. In southern Wyoming, warmth-loving plants like palms eventually spread northward as ecological stress fractured the established ecosystem.
Lessons for Modern Forests and Climate Resilience
The historical sequence recorded in Wyoming’s sedimentary layers carries clear implications for modern forestry and climate policy. According to the study authors, higher carbon dioxide levels can stimulate plant growth only up to the specific thermal and hydrological thresholds that trees can tolerate. Beyond those boundaries, compounding pressures such as extreme heat, severe drought, insects, pathogens, and wildfires can completely neutralize any growth-boosting fertilization effect.
Global forests are already exhibiting signs of stress from rising temperatures, a vulnerability compounded by widespread deforestation driven by agriculture, timber, and commercial rangeland development. While the fossil record proves that ecosystems possess remarkable resilience, the timeline for recovery poses a stark warning. According to the research, natural weathering processes eventually pulled excess carbon from the air and stored it in marine sediments, allowing the planet to cool and forest canopies to regenerate.
Frequently Asked Questions
What caused the Paleocene-Eocene Thermal Maximum?
According to researchers, the PETM was driven by massive releases of carbon dioxide into the atmosphere, with volcanic eruptions serving as a leading theory for the source of the carbon.
How long did it take ancient forests to recover after the PETM?
Fossil evidence shows it took well over 100,000 years for forest canopies to fully recover their density after global temperatures and environmental conditions stabilized.
How do scientists measure the canopy density of prehistoric forests?
Paleobotanists measure ancient canopy density by analyzing the shape of epidermal cells preserved in microscopic leaf cuticles found within sedimentary rock layers and soil cores.
How does the PETM compare to modern climate change?
While the PETM is Earth’s closest natural analog to current warming, human-caused carbon emissions are currently unfolding roughly 10 times faster than the natural processes that drove the ancient thermal event.
Worth a look