Plant response to a global greenhouse event 56 million years ago.

PREMISE OF THE STUDY: The fossil record provides information about the long-term response of plants to CO2-induced climate change. The Paleocene-Eocene Thermal Maximum (PETM), a 200000-yr-long period of rapid carbon release and warming that occurred ∼56 million years ago, is analogous to future anthropogenic global warming.

METHODS: We collected plant macrofossils in the Bighorn Basin, Wyoming, United States, from a period spanning the PETM and studied changes in floristic composition. We also compiled and summarized published records of floristic change during the PETM.

KEY RESULTS: There was radical floristic change in the Bighorn Basin during the PETM reflecting local or regional extirpation of mesophytic plants, notably conifers, and colonization of the area by thermophilic and dry-tolerant species, especially Fabaceae. This floristic change largely reversed itself as the PETM ended, though some immigrant species persisted and some Paleocene species never returned. Less detailed records from other parts of the world show regional variation in floristic response, but are mostly consistent with the Bighorn Basin trends.

CONCLUSIONS: Despite geologically rapid extirpation, colonization, and recolonization, we detected little extinction during the PETM, suggesting the rate of climate change did not exceed the dispersal capacity of terrestrial plants. Extrapolating the response of plants from the PETM to future anthropogenic climate change likely underestimates risk because rates of climate change during the PETM may have been an order of magnitude slower than current rates of change and because the abundant, widespread species common as fossils are likely resistant to extinction.