Drivers of Intraspecific Trait Variation and Drought Response of a Dominant US Great Plains Grass: Disentangling the Role of Climate and Genetic Background

ABSTRACT Intraspecific trait variation (ITV) is critical for plant adaptation, particularly under increasing drought. ITV arises from phenotypic plasticity and genetic differentiation, so identifying its sources improves understanding of population responses to drought. This study investigates how climate and genetics shape ITV and drought responses in Andropogon gerardii , a foundational Great Plains grass. To quantify climatic determinants of ITV, we conducted a common‐garden greenhouse experiment with 25 A. gerardii populations spanning wide temperature (4°C–21°C; Minnesota–Texas) and precipitation (350–1400 mm year −1 ; Colorado–North Carolina) gradients, measuring 17 functional traits. Populations were also genotyped to assess the genetic basis of ITV, and greenhouse–field trait comparisons were used to separate genetic and environmental effects. To test drought as a selective pressure, eight populations from a precipitation gradient (470–1350 mm year −1 ) were exposed to experimental drought (15% moisture vs. 30% control). We hypothesized that climate of origin, particularly precipitation, predicts ITV: dry‐origin populations would express conservative traits (higher water‐use efficiency, shorter stature), whereas wet‐origin populations would exhibit competitive traits (greater height, biomass). We further predicted that drought would reduce growth and delay flowering, with stronger effects in wet‐origin populations, and that trait variation would correspond with genetic differentiation. Precipitation was the strongest predictor of ITV. Principal component analyses revealed that wet‐origin populations displayed competitive traits, while arid populations showed conservative traits. Congruent genetic and trait PCAs confirmed a genetic basis for ITV, and consistent greenhouse–field patterns reinforced genetic control. Under drought, wet‐origin populations exhibited greater declines in height and photosynthesis, indicating drought as a key selective force. Overall, ITV in A. gerardii is primarily shaped by precipitation through genetically based trait responses, offering a predictive framework for grassland adaptation under future climate change.