CLIMATE, CO2 AND THE HISTORY OF C3 AND C4 GRASSES FROM THE LAST GLACIAL MAXIMUM TO PRESENT ACROSS NORTH AMERICA

C₄ grasses make up approximately 20% of Earth’s primary productivity, yet their potential responses to increasing temperature and precipitation patterns, as well as to increasing atmospheric CO₂ concentrations, are not well constrained relative to vegetation using the more common C₃ pathway. Bison (Bison bison) and Mammoth (Mammuthus columbi, Mammuthus primigenius) tissue records the isotopic composition of their diet (δ¹³C), and can be used as a proxy for the average abundance of C₃ and C₄ vegetation in the areas in which they lived. Here, we use the δ¹³C of bison hair, and bison and mammoth tooth enamel and collagen coupled with advanced statistical methods to model changes in the abundance of C₃ and C₄ grasses through the last glacial/interglacial transition in North America. Using the Random Forest multiple regression tree algorithm and eight climatic and soil predictor variables, the model explains 91% of the spatial variability in bison tissue δ¹³C values and determines that summer average temperature is the most important predictor of all climate variables. We apply this model to high-resolution gridded climate data and CMIP5 ensemble paleoclimate model outputs to produce a predictive isotope landscape model (‘isoscape’) for the current, Last Glacial Maximum (LGM) and Mid-Holocene Climatic Optimum (MHCO) average δ¹³C of vegetation across western North America. From the LGM to the modern, we find a large increase in the abundance of C₄ grasses in the central Great Plains, along with a decrease in the abundance of C₄ grasses along the Gulf coast. This model also allows us to directly quantify the effects of atmospheric CO₂, and predict a threshold of ~270 ppm, below which CO₂ concentrations begin to exert an influence on the distribution of C₃ and C₄ grasses. However, at low atmospheric CO₂ values, the importance of summer average temperature on grass abundance is three to four times greater than it is for atmospheric CO₂.