Forests across the world could be heading towards a future of thinner canopies, widespread tree loss and browning landscapes, according to a new study that has reconstructed ancient forests from 56 million years ago.
The research, published in Science, examines the Paleocene-Eocene Thermal Maximum (PETM), a period when a massive release of carbon dioxide caused rapid global warming. Scientists say the event offers one of the closest natural comparisons to today’s climate change, although humans are now releasing carbon into the atmosphere far faster than the natural processes that drove the PETM.
Lead author Dr Regan Dunn, a paleobotanist at the Natural History Museum of Los Angeles County said: ‘We are putting CO₂ into the atmosphere faster than any known natural process. The Earth has never experienced a carbon release at the pace we’re creating today.’
To understand how forests responded to the ancient warming, researchers developed a new method for estimating the density of prehistoric forest canopies using tiny fragments of fossilised leaves.
The technique focuses on the shape of microscopic cells on the surface of leaves. Leaves growing beneath a dense canopy receive less sunlight, causing their cells to become longer and narrower. By comparing these fossilised cell shapes with measurements from modern forests, researchers were able to estimate the ancient forests’ leaf area index (LAI)—a measure of how densely leaves cover the landscape.
The team applied the method to fossil material collected over more than a decade from Wyoming’s Hanna Basin. The area contains unusually well-preserved organic-rich rocks, including coal and lignite, which hold fragments of leaves from the PETM.
The results suggest that forests became considerably more open as temperatures rose and rainfall declined. Large trees became less common, plants migrated northwards and canopy cover decreased, contributing to changes in erosion, nutrient cycling and the terrestrial water cycle.
Dr Ellen Currano, a paleoecologist at the University of Wyoming and co-author of the study, said the findings show that excessive carbon dioxide can ultimately harm forests despite its initial fertilising effect on plants.
‘Understanding how forest structure changed during the PETM is really important because it tells us about how plant growth, biomass and productivity are affected by adding a lot of carbon dioxide to the atmosphere. It turns out that too much carbon dioxide is a bad thing for forests because the accompanying warming and drying stresses the trees and kills many of them.
‘Our work shows that during the PETM, forest canopies became more open, with fewer big trees, and this change affected climate, nutrient cycling, weathering, and, of course, the animals that inhabited the forests. We are starting to see similar changes to forests occurring today, particularly in the Amazon, and the Wyoming plant fossil record gives us insight into where Earth might be headed.’
The researchers say there are already signs of a similar shift today. Satellite observations show that the Earth initially became greener as rising carbon dioxide stimulated plant growth, but that trend is now reversing in some regions as heat and drought intensify.
Dunn added: ‘Earth has been on a greening trajectory because of anthropogenic carbon dioxide emissions that have fertilized plants. But as Earth has heated up because of those emissions, this greening trend is reversing, and many parts of Earth are now browning. This suggests that we’ve passed a critical threshold where heightened temperatures are now adversely affecting forests.’
The scientists stress that modern forests face additional pressures that did not exist during the PETM, including deforestation, habitat fragmentation and land-use change.
They say the ancient record provides a warning of what could happen if warming continues—and highlights the importance of protecting forests, which are themselves a crucial natural defence against climate change.
Dunn said: ‘Trees are awesome. All those forests are taking up hundreds of millions of tons of carbon for us every day. But when you start losing the forests, then you’re losing those critical carbon sinks. The PETM reminds us that when forests decline, the consequences ripple through the climate system. We would be wise to protect and restore them while they can still help buffer the unprecedented pace of human-caused climate change.’
Photo: Dave Robinson