We present the results of a 5-year examination of variation in the stable carbon isotope composition (δ) of three C3 graminoid species from a Sandhills prairie: Agropyron smithii, Carex heliophila and Stipa comata. Although consistent species-specific patterns for mean δ were seen, there was also significant and substantial among-year and within-season variation in δ. A smaller contribution to variation in δ came from topographic variation among sampling sites. Effects of species, year, season and topography contribute to variation in δ in an additive manner. An association between climate and δ exists that is consistent with previous work suggesting that δ reflects the interplay between photosynthetic gas exchange and plant water relations. Within the growing season, the time over which δ integrates plant response to the environment ranges from days to months.