TERRESTRIAL FLORAL CHANGE DURING THE PALEOCENE-EOCENE THERMAL MAXIMUM

During the Paleocene-Eocene Thermal Maximum (PETM), global temperatures rose 5-8 °C in a few thousand years in response to increased atmospheric CO₂. Precipitation regimes changed in many regions.

Megafossils from the northern Rocky Mountains show high rates of extirpation of local plant populations during the PETM. 88% (46 of 52) of the taxa present in the last 100 kyr of the Paleocene do not occur in the PETM. Of the 6 taxa that range through the PETM, 4 only occur in the last part of the hyperthermal when temperature and CO₂ declined and precipitation increased. Almost 90% of late Paleocene plants do have post-PETM occurrences in the region, demonstrating these taxa survived the event elsewhere. The northern Rockies also experienced high levels of immigration during the PETM, with 77% (20 of 26) of PETM taxa having no Paleocene occurrences in the region. Some PETM immigrants occur in the Paleocene of Europe and southern North America, suggesting large range changes. Only a few PETM immigrants persisted in the northern Rockies following the event.

Outside the northern Rockies PETM floral changes were more subtle, but have only been documented by pollen and/or biomarkers which have lower taxonomic resolution and a greater chance of being reworked than do megafossils. Palynofloras do not show high rates of local extirpation during the PETM, but high-latitude sites do record range extensions of plants from lower latitudes. Cupressaceous conifers decreased in high northern latitudes and Podocarpaceae probably declined in high southern latitudes. Decreased conifer abundance at high latitudes, where precipitation likely increased, may show they were less able than angiosperms to take advantage of higher temperatures and/or CO₂. During the PETM tropical floras began a major increase in diversity, possibly reflecting evolutionary diversification in many separate angiosperm lineages.

Plants survived the PETM without major extinction, perhaps surprising given the rate and magnitude of environmental change, and massive changes in distribution and abundance. Given the parallels between the PETM and the Anthropocene, the absence of a plant mass extinction is good news, but it is critical to note that current global change is possibly ten times faster, and that climate is far from the only problem facing plant populations.