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Genetic differentiation in reproduction in the wide-ranging Schizachyrium scoparium (Poaceae) has been demonstrated in uniform gardens. However, the fine-tuning of flowering phenology and biomass allocation in relation to spatial and temporal fluctuations in the local environment is best accomplished by plastic responses to local variability. An earlier central New Jersey study suggested that S. scoparium populations in old fields of 2 to 40 years differed in plasticity. To test this apparent effect of ecological history on the development of different levels of plasticity, genotypes were collected from high- and low-fertility sites in New Jersey (forest biome) and in Oklahoma (grassland biome). Three greenhouse experiments manipulating light and nutrients were used to partition variation into genetic and environmental components. High light or high nutrients resulted in plasticity for increased biomass, greater reproductive allocation, and more tillers. Earlier flowering was induced by high light, but nutrient treatments had no effect. Populations were more likely to differ in plasticity across regions than within regions, and Oklahoma populations were consistently more plastic than New Jersey populations. In response to nutrients, populations from high-nutrient sites were often more plastic than those from low-nutrient sites. There were fewer differences in plasticity in response to light between high- and low-nutrient populations. The greater plasticity in Oklahoma populations is suggested to be the result of historically greater environmental unpredictability and K-selection factors such as density-dependent selection and greater competition for resources. A native grass population is more than just a Latin binomial. Evolutionary forces create an ecological unit unique and irreplaceable at the local level.