Using vegetation model-to-data comparisons to test the role of abiotic factors in the Neogene and Quaternary origins of modern C4 grasslands

Thursday, 18 December 2014
David L. Fox1, Caroline Strömberg2, Stephanie Pau3, Lyla Taylor4, Caroline Lehmann5, Colin Osborne4, David John Beerling6 and Christopher J Still7, (1)University of Minnesota Twin Cities, Minneapolis, MN, United States, (2)University of Washington, Biology & Burke Museum of Natural History & Culture, Seattle, United States, (3)Florida State University, Tallahassee, FL, United States, (4)University of Sheffield, Sheffield, United Kingdom, (5)University of Edinburgh, School of GeoScienes, Edinburgh, United Kingdom, (6)University of Sheffield, Sheffield, S10, United Kingdom, (7)Oregon State University, Corvallis, OR, United States
Grasslands dominated by taxa using the C4 photosynthetic pathway evolved on several continents during the Neogene and Quaternary, long after C4 photosynthesis first evolved among grasses. The histories of these ecosystems are relatively well documented in the geological record from stable carbon isotopes (fossil vertebrate herbivores, paleosols) and the plant microfossil record (pollen, phytoliths). The distinct biogeography and ecophysiology of modern C3 and C4 grasses have led to hypotheses explaining the origins of C4 grasslands in terms of long term changes in the Earth system such as increased aridity and decreasing atmospheric pCO2. However, proxies for key parameters of these hypotheses (e.g., temperature, precipitation, pCO2) are still in development, not yet widely applied, or remain contentious, so testing the hypotheses globally remains difficult. To understand better possible links between changes in the Earth system and the origin of C4 grasslands on different continents, we are undertaking a global scale comparison between observational records of C4 grass abundances in Miocene and Pliocene localities compiled from the literature, and three increasingly complex models of C4 dominance and abundance. The literature compilation comprises >2,600 δ13C values of both fossil vertebrates and of paleosol carbonates and >6,700 paleobotanical records. We are using paleoclimate output from the HadCM3L GCM over a range of pCO2 values for each epoch to model C4 dominance or abundance in grid cells as (Model 1) months per year exceeding the temperature at which net assimilation is greater for C4 than C3 photosynthesis (crossover temperature); (Model 2) the number of months per year exceeding the crossover temperature and having sufficient precipitation for growth (≥25 cm/yr; Collatz model); and (Model 3) the Sheffield Dynamic Global Vegetation Model (SDGVM), output from which includes biomass (g C/m2/yr) for distinct structural components (roots, stems, leaves) of multiple plant functional types (C3 and C4 grasses, evergreen and deciduous trees). Statistical comparisons of the isotopic and paleobotanical databases with the paleoclimate and vegetation model outputs allows us to assess the possible role of abiotic factors in the evolution of modern C4 grasslands during the late Neogene.