Ancient forests in what is now Wyoming took more than 100,000 years to recover after a period of intense global warming around 56 million years ago, according to a new study published in Science.
Researchers examined fossil leaves and microscopic plant cuticles to reconstruct the density of forests during the Paleocene Eocene Thermal Maximum (PETM), when global temperatures rose by as much as 6°C.
The study found that Wyoming forest canopy declined by about 60% as extreme heat and drought placed increasing stress on trees. As forests became more open, sunlight reached the ground more easily, while changes in vegetation also affected water and sediment movement across the landscape.
Ferns and heat tolerant plants, including palms expanded as the climate warmed, replacing several tree species that had previously dominated the region.
Researchers said increased carbon dioxide initially supported forest growth but the benefits were eventually outweighed by heat and water stress. The forests gradually recovered as natural processes removed carbon dioxide from the atmosphere and temperatures cooled.
Scientists say the findings offer an important warning for the present. Human activities are increasing atmospheric carbon dioxide far faster than during the PETM. While forests can recover from climate stress the fossil record shows that complete recovery can take tens of thousands of generations.