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.
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.
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.
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, Associate Curator at La Brea Tar Pits and Museum; Adjunct 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.
