Phenotypes of big bluestem (Andropogon gerardii Vitman) vary throughout the central grasslands of North America, a consequence of genetically-distinct ecotypes within the species. This study sought to distinguish between genetic and environmental variation of photosynthesis and water potential in big bluestem ecotypes during an extreme drought. Four ecotypes of big bluestem were measured in common gardens in western, central, and eastern Kansas. Experimental plots contained seeded assemblages to represent prairie communities. Photosynthesis rates and water potential were measured three times during the 2012 growing season. The role of precipitation was assessed with rainout shelters that reduced ambient rainfall by 50 percent. There were prominent differences between sites that correlated with available soil moisture, with only minor genetic differences among ecotypes. The mesic site in Manhattan, KS had higher photosynthesis and water potentials compared to drier sites in Colby and Hays, KS. Photosynthetic rates decreased in all sites as the growing season progressed. Extreme drought in Colby and Hays reduced photosynthesis rates to near zero by late summer, whereas photosynthesis in Manhattan remained above 6 µmol CO₂ m^-2 s^-1 in late summer. The big bluestem ecotype from the driest environment had higher photosynthetic rates compared to mesic ecotypes, evident at the mesic site in Manhattan. Plant water potentials followed soil moisture availability across sites. Mean water potentials were as low as -7 MPa in Hays and Colby, but were never lower than -1.3 MPa in Manhattan. Although significant differences were detected among ecotypes of A. gerardii, these differences were minimal compared to environmental effects during a severe drought.