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

Abstract:

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.