Plants depend upon both genetic differences and phenotypic plasticity to cope with environmental variation over different timescales. The spatial variation in foliar δ13C levels along a moisture gradient represents an overlay of genetic and plastic responses. We hypothesized that such a spatial variation would be more obvious than the variation arising purely from a plastic response to moisture change. Leymus chinensis and Stipa spp. were sampled from Inner Mongolia along a dry-wet transect, and some of these species were transplanted to an area with a moisture gradient. For Stipa spp., the slope of foliar δ13C and mean annual precipitation along the transect was significantly steeper than that of foliar δ13C and mean annual precipitation after the watering treatment. For L. chinensis, there was a general decreasing trend in foliar δ13C under the different (increasing) watering levels; however, its populations showed an irregular relationship between foliar δ13C and moisture origin. Therefore, support for our hypothesis was obtained from Stipa spp., but not from L. chinensis.