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 cover 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. One of the most surprising discoveries was that forests did not decline at the Paleocene-Eocene thermal maximum. Just before rapid warming began, forest canopies reached their highest density in hundreds of thousands of years, likely reflecting favorable growing conditions as atmospheric carbon dioxide began to rise. One leading theory about the source of that carbon dioxide involves volcanic eruptions. However, that flourishing forest did not last. As temperatures rose, heat and drought negated the benefits of higher carbon dioxide levels. The crown thinned rapidly as the trees died and remained much thinner for more than 100,000 years. Tree canopy is often measured using the leaf area index. This graph of the Paleocene-Eocene Thermal Maximum, 56 million years ago, shows how canopy cover shrank as temperatures increased, and the timeline starts with the oldest period at the bottom. The bars on the right show the percentage of different plant types in Wyoming forests as the mix changed with canopy cover, based on fossilized pollen and other palynomorphs. R. Dunn, et al., 2026 Forests functioned very differently in this diminished state, and that affected the surrounding environment. Ancient soils gave way to thicker fluvial deposits, suggesting that the loss of cover altered the way water and sediment moved through the basin. The changing climate changed the forest and the forest changed the landscape. Lessons for today This sequence contains an important lesson for today. Higher levels of carbon dioxide like those the world is now experiencing can stimulate plant growth, but only as long as temperatures and water remain within the limits that trees can tolerate. Beyond those limits, heat, drought, insects, pathogens and wildfires can negate any growth-boosting fertilization effects. Around the world, many forests are already showing signs of decline as temperatures rise, with deforestation for timber, crops and grasslands further reducing their resilience. The forests recovered, but it took more than 100,000 years. The story of ancient forests from 56 million years ago does not end with collapse. Over time, the increasing breakdown of rocks in the warmer climate, known as erosion, gradually pulled carbon from the air and stored it in marine sediments. That allowed the climate to cool and water to become more available. Forest canopies recovered and eventually became even denser than before warming began. As forests expanded, they likely restored their ability to stabilize soils, regulate the water cycle, and extract carbon from the atmosphere, helping to reduce the greenhouse effect and drive the planet’s long-term recovery. The study’s authors, Regan Dunn and Ellen Currano, are working on a sediment core extracted from Wyoming’s Hanna Basin by colleagues at the U.S. Geological Survey. Cores like this capture layers of fossil pollen and leaf material dating back to the past, revealing how environments changed. Regan Dunn Our study shows that carbon dioxide emissions have pushed forests beyond their physiological limits in the past, triggering changes that extend from vegetation to rivers and entire landscapes. It also shows that forests are remarkably resilient when given time to recover, but what counts as time is much longer than the human lifespan: it requires thousands of generations. Today, human-caused carbon emissions and warming are developing much faster than during the PETM. The fossil record reminds us that forests can recover, but only if humanity avoids pushing them beyond thresholds that take tens of thousands of years to recover. Regan E. Dunn, associate curator of the La Brea Tar Pits and Museum; Associate Professor of Earth Sciences, USC Dornsife College of Letters, Arts and Sciences. This article is republished from The Conversation under a Creative Commons license. Read the original article.
