RETURN TO A GREENER DAY: MIOCENE PLANT FOSSILS AND PRIMARY PRODUCTIVITY IN A HIGH CO2 WORLD (Invited Presentation)

Global greening refers to the effect increased atmospheric carbon dioxide (CO₂) has on vegetation and the resulting enhanced primary productivity. There are three pathways that can lead to a “greening” effect: direct carbon fertilization, increased plant water-use efficiency, and poleward expansion of forests. Future global greening is constrained by the speed that CO₂ and temperature change and the rate at which plants respond. The Miocene had higher-than-modern CO₂ and Miocene plant fossils document the resulting biosphere greening. Here we explore Miocene primary productivity from leaf fossils using (1) empirically reconstructed ecosystem net primary productivity (NPP) from ~90 leaf megafossil sites around the world, and (2) mechanistically reconstructed leaf-level gross productivity and intrinsic water-use efficiency (iWUE) from fossil leaf cuticle gas-exchange modeling. Reconstructed local NPP is higher than modern for 87% of the Miocene megafloras investigated, with particularly high NPP for the Middle Miocene Climatic Optimum. 88% of sites investigated indicate a wetter climate in the Miocene, and 62% of sites reveal a warmer climate in the Miocene. This suggests a larger role for water availability and iWUE, as compared to CO₂-temperature effects, in facilitating high Miocene NPP. Reconstructed Miocene gross leaf-level productivity is higher than modern, in particular for evergreen species, which suggests that a direct carbon fertilization effect took place. Miocene leaf-level gas-exchange modeling reveals especially high iWUE compared to modern trees, indicating the important role of stomatal diffusion regulation in Miocene enhanced NPP. Plant fossils indicate that the Miocene world was indeed greener and that the most important drivers of higher-than-modern Miocene NPP were water availability and iWUE.