FOREST CANOPY RESPONSE TO GREENHOUSE WARMING AT THE PALEOCENE-EOCENE THERMAL MAXIMUM

At the current rate of greenhouse gas emissions, atmospheric carbon dioxide (pCO₂) levels will increase to values not known on Earth in the last 50 million years. How will vegetation, more specifically, primary productivity respond to this exponential increase in pCO₂ and the associated climate changes? Much of our present knowledge about the effects of pCO₂ on plant growth comes from modeling studies and small-scale experimental work. How whole ecosystems will be affected on longer temporal scales remains largely unknown.

This study tests how increased pCO₂ during the Paleocene-Eocene Thermal Maximum (PETM ~ 56 million years ago) affected forest ecosystems in the Hanna Basin of south-central Wyoming, USA. We demonstrate that canopy density, quantified as Leaf Area Index (LAI – foliar area/area of ground) can be reconstructed using light-dependent cellular morphology of leaf epidermis in modern environments. We then present a reconstructed LAI record from fossilized dispersed leaf cuticles from middle Paleocene–early Eocene strata using the newly developed proxy method. Our preliminary results show consistently dense forest habitats throughout most of the Paleocene with an abrupt increase in LAI early in the PETM, and a high amount of variability during and after the event. These results suggest that increased pCO₂ had a fertilizing effect on forest ecosystems in the Hanna Basin where wet, swampy conditions prevailed for much of the Paleocene and Eocene and where thick coal beds accumulated. Like other basins (e.g. the Piceance Creek Basin), the PETM in the Hanna Basin corresponds with sedimentological evidence for increased fluvial discharge and sediment flux. Though the overall LAI pattern during the PETM and early Eocene shows increased values, periods of decreased LAI precede the deposition of large fluvial channel bodies suggesting decreased plant cover may have altered hydrological and sedimentological signals during the climate event.