Ancient forests took well over 100,000 years to recover from a major global warming episode 56 million years ago, according to a study published in the journal Science. Researchers analyzing fossil leaves and soils from Wyoming’s Hanna Basin found that forest canopies thinned by roughly 60% during the Paleocene-Eocene Thermal Maximum, or PETM, as rising temperatures and severe droughts overwhelmed the growth-boosting benefits of high atmospheric carbon dioxide.
The PETM represents one of the most intense periods of greenhouse warming in Earth’s history and serves as a natural analog for modern climate change. However, scientists note that human-driven carbon emissions are currently warming the planet roughly 10 times faster than natural processes did millions of years ago. Understanding how ancient ecosystems responded to extreme heat provides critical context for the thresholds facing modern forests today.
“The hotter things are, the more stressed trees are,” said paleobotanist Regan Dunn of the Natural History Museum of Los Angeles County, who led the research alongside colleagues investigating how ancient landscapes adapted to environmental shocks.
Reconstructing Ancient Forest Canopies Through Fossil Leaves
While plant fossils routinely help researchers identify which species inhabited a region millions of years ago, answering deeper questions about ecosystem structure requires evaluating forest density. Ecologists typically measure modern forest density using the leaf area index, a metric that tracks how much foliage occupies a canopy and controls the amount of light reaching the forest floor.
To measure canopy density in ecosystems that disappeared 56 million years ago, the research team examined microscopic plant cuticles—the thin, waxy outer skins of leaves preserved in organic-rich sedimentary layers. Under magnification, these fossil fragments display the distinct shapes of epidermal cells, which function like puzzle pieces reflecting the amount of sunlight a leaf received during development.
Leaves growing under dense shade stretch out to capture light, resulting in elongated cells. In contrast, leaves exposed to direct sun develop shorter, rounder cells. By calibrating this cellular relationship against modern forests across Central and South America with known leaf area indices, the scientists created a reliable tool to estimate ancient canopy cover from fossil leaf litter.
Volcanic Warming and the Collapse of Wyoming Forests
The study revealed that the ancient forests of southern Wyoming did not enter the PETM in decline. Just before rapid warming commenced, driven largely by volcanic activity releasing massive quantities of greenhouse gases, local forest canopies reached their greatest density in hundreds of thousands of years. Lush communities featuring relatives of elms, walnuts, dawn redwoods, and avocados thrived as atmospheric carbon dioxide levels ticked upward.
That flourishing state vanished as global temperatures climbed by as much as 11 degrees Fahrenheit (6 degrees Celsius). Heat and drought placed extreme stress on trees, causing widespread die-offs that opened up the canopy and allowed warmth-loving plants like palms to spread northward while ferns briefly flourished in the understory.
Based on their cellular analysis, researchers found that the Hanna Basin forest canopy thinned by an average of 35% overall during the warming event, with the sharpest decline seeing the leaf area index drop by approximately 61% at the onset of the PETM. This loss of canopy fundamentally altered the surrounding environment, shifting ancient soils toward coarser river deposits as the movement of water and sediment across the landscape transformed.
Implications for Modern Forest Resilience and Climate Feedback
The findings offer a sobering glimpse into the future of contemporary ecosystems. Although rising carbon dioxide emissions have historically stimulated global plant growth, scientists warn that this fertilization effect is rapidly being outpaced by heat and drought stress.
In the ancient past, it required more than 100,000 years of enhanced rock weathering to draw down carbon, cool the climate, and allow forest canopies to recover their density.
Researchers stress that while forests possess remarkable resilience, the pace of modern human-induced emissions far exceeds the natural recovery timelines recorded in the fossil record. Recognizing these ecological thresholds remains essential for protecting modern forest ecosystems before they are pushed beyond their physiological limits.