SPATIAL AND TEMPORAL VARIATION IN CARBON-ISOTOPE DISCRIMINATION IN PRAIRIE GRAMINOIDS

We present the results of a 5-year examination of variation in the sta ble carbon isotope composition (delta) of three C3 graminoid species f rom a Sandhills prairie: Agropyron smithii, Carex heliophila and Stipa comata. Although consistent species-specific patterns for mean delta were seen, there was also significant and substantial among-year and w ithin-season variation in delta. A smaller contribution to variation i n delta came from topographic variation among sampling sites. Effects of species, year, season and topography contribute to variation in del ta in an additive manner. An association between climate and delta exi sts that is consistent with previous work suggesting that delta reflec ts the interplay between photosynthetic gas exchange and plant water r elations. Within the growing season, the time over which delta integra tes plant response to the environment ranges from days to months.