Climatic controls on C₄ grassland distributions during the Neogene: A model-data comparison

Grasslands dominated by taxa using the C₄ photosynthetic pathway first developed 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 isotope measurements (of fossil vertebrate herbivores and paleosols) and the plant microfossil record (pollen and/or phytolith assemblages). 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, quantitative proxies for key abiotic drivers of these hypotheses (e.g., temperature, precipitation, pCO₂) are still in development, not yet widely applied at the continental or global scale or throughout the late Cenozoic, and/or remain contentious. Testing these hypotheses globally therefore remains difficult. To understand better the potential links between changes in the Earth system and the origin of C₄ grasslands, we undertook a global-scale comparison between observational records of C₄ plant abundances in Miocene and Pliocene localities compiled from the literature and three increasingly complex models of C₄ physiology, dominance, and abundance. The literature compilation comprises > 2,600 δ¹³C-values each of fossil terrestrial vertebrates and of paleosol carbonates, which we interpret as primarily proxies for the abundance of C₄ grasses, based on the modern contribution of C₄ grasses to terrestrial net primary productivity. We forced the vegetation models with simulated monthly climates from the HadCM3 family of coupled ocean-atmosphere general circulation models (OAGCMs) over a range of pCO₂-values for each epoch to model C₄ dominance or abundance in grid cells as: (1) months per year exceeding the temperature at which net carbon 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 mm/month; Collatz model); and (3) the Sheffield Dynamic Global Vegetation Model (SDGVM), which models multiple plant functional types (PFTs) (C₃ and C₄ grasses, evergreen, and deciduous trees). Model-data agreement is generally weak, although statistically significant for many comparisons, suggesting that regional to local ecological interactions, continent-specific plant evolutionary histories, and/or regional to local climatic conditions not represented in global scale OAGCMs may have been equally strong or stronger in driving the evolution of C₄ grasslands as global changes in the Earth system such as decreases in atmospheric pCO₂ and late Cenozoic global cooling and/or aridification.