"Significance. The Paleocene–Eocene Thermal Maximum (PETM; ~56 Ma) was marked by rapid global warming, making it a valuable test bed for the effects of extreme climate change on the environment. Using pollen and spores preserved in a laminated sedimentary sequence, we reconstruct vegetation change at decadal time-scales. Our results, integrated with existing vegetation reconstructions, reveal a widespread geologically synchronous shift to highly disturbed terrestrial ecosystems and biomass loss, that occurred within decades to centuries after massive carbon release during the PETM-onset and lasted millennia. Modeling suggests that carbon release from such perturbed terrestrial reservoirs, including biomass, soils, and buried kerogen, acted as significant positive feedback, underscoring the need to include land carbon reservoirs in future (PETM) carbon cycle assessments."