Climatic Controls on C-4 Grassland Distributions During the Neogene: A Model-Data Comparison

Abstract

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.