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

• δ13C of grass pollen indicates no evidence of C4 grasses in middle/late Eocene.
• Lack of C4 is consistent with prior studies in North America.
• Results contrast with those from Europe and South America.
• There was variation in the location and time of the origin of C4 grasses.

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.