PRELIMINARY ESTIMATES OF CANOPY STRUCTURE FROM THE LATE PALEOCENE TO EARLY EOCENE IN THE BIGHORN BASIN

Canopy structure—the openness of a canopy—is a critical component of ecosystems. Changes in canopy structure play a role in ecological interactions (e.g., productivity, landscape stability, the composition of faunal communities, and mammal evolution) and can influence the Earth’s climate (e.g., albedo, hydrological and carbon cycling). Despite being a vital component of terrestrial ecosystems, deficiencies in existing proxy methods have necessitated higher-resolution analyses of canopy structure throughout deep time. Here, we study the relationship between canopy structure and climate change during the late Paleocene-early Eocene (~59-53 Ma) in the Bighorn Basin, Wyoming. This interval may provide good analogs for future climate change as Earth transitioned from a ‘warmhouse’ to a ‘hothouse’ punctuated by several hyperthermals with elevated atmospheric CO₂ levels. We used a new canopy structure proxy based on leaf cell morphology to reconstruct leaf area index, a measure of canopy density (LAI; foliage area (m²)/area of ground(m²)) for eight localities. Our preliminary results find an ~51% decrease in LAI from the late Paleocene to the Paleocene-Eocene Thermal Maximum (PETM) hyperthermal (Paleocene, 2.47 m²/m²; PETM, 1.20 m²/m²) and an increase in LAI to 3.92 m²/m² within the PETM recovery interval. LAI subsequently decreases during the Early Eocene Climatic Optimum (EECO, 1.70 m²/m²). A decrease in LAI during the PETM and EECO is coincident with evidence for increased mean annual temperature and increased water stress within the BHB. Importantly, our results provide a new lens to evaluate the changes in flora and fauna and may provide insight into current and future ecosystem change. Future work will focus on reconstructing LAI in the Bighorn Basin during ETM2 and the Eocene cool period to provide a more complete analysis of the impact of climate change on forest canopy structure.