Forest fossils from Wyoming suggest that rapid and prolonged greenhouse warming during the Paleocene–Eocene Thermal Maximum (PETM) caused forests to thin, drastically reshaped plant communities, and reduced overall vegetation carbon storage. The findings could foreshadow challenges facing modern forests in our rapidly warming world. Although rising atmospheric carbon dioxide initially stimulated plant growth across much of the globe, that trend has reversed in recent decades as increasing heat and drought place growing stress on vegetation. This shift raises concerns that forests may lose their capacity to store carbon, thereby accelerating climate change. However, the long-term effects of sustained greenhouse warming on forest structure and carbon storage remain poorly understood, leaving uncertainty about the future of today's forests in our warming world.

To understand how forests respond to prolonged atmospheric warming, Regan Dunn and colleagues examined fossil evidence from Wyoming’s Hanna Basin. They wanted to know how forests responded to extreme greenhouse warming during the PETM – often considered one of the closest natural analogs to modern human-driven climate change – about 56 million years ago. By combining fossil pollen, sediment records, geochemical data, and a new proxy for reconstructing Leaf Area Index (LAI) from fossil leaf cuticles, Dunn et al. reconstructed changes in plant communities and vegetation structure during this period of rapid carbon release and global warming. Dunn et al. found that PETM warming caused rapid forest canopy opening, increased landscape erosion, and major shifts in forest composition, including the replacement of temperate broad-leaved angiosperm plants with more heat-tolerant species, such as palms and ferns. The findings show that prolonged warming can reduce forest density and carbon storage, even when elevated carbon dioxide initially promotes plant growth, offering insight into how modern forests may respond as rising temperatures and drought increasingly threaten their ability to absorb and store carbon.