Spatiotemporal variation in the origin of C₄ grasses: δ¹³C analysis of grass pollen from the southeastern United States

The C₄-plant functional type is a major evolutionary and ecological success. However, the relative importance of environmental factors, such as climatic conditions and atmospheric CO₂ concentrations, in driving the origin of C₄ grasses in different parts of the world remains poorly understood. We determined the carbon-isotope composition of 612 individual grains of grass pollen in seven sediment samples from the southeastern United States from the middle/late Eocene. By applying a Bayesian model to these data, we determined the probability of rejecting (at p<0.05) the null model that each sample contained pollen from only C₃ grasses. We could not reject the null hypothesis for any sample when using a range of potential values of δ¹³C of atmospheric CO₂. These results suggest that C₄ grasses were not present on the landscape during the middle/late Eocene, a conclusion consistent with a limited number of studies from more northern and western regions of North America. In contrast, prior studies indicate that C₄ grasses existed during this time in parts of Europe and South America. Together these results imply variations in the location, time, and environmental conditions associated with the origin of C₄ grasses.