Adaptive Trait Variaiton and Genetic Divergence of a Widespread Native Grass Andropogon gerardii Across a Strong Climate Gradient in the US Great Plains

Midwest grasslands, dominated by the forage grass big bluestem, Andropogon gerardii, grow across a strong precipitation gradient (500-1200 mm/yr from western KS to IL). Three ecotypes (from central KS, eastern KS, and Illinois) were reciprocally planted in Colby, Hays, and Manhattan, KS, and Carbondale, IL The reciprocal gardens were used to investigate ecotype differences in vegetative and reproductive traits and then characterize genetic divergence among ecotypes.. We evaluated ecotypic differences in vegetative and reproductive features across planting sites, predicting ecotypes would perform best at in their location or origin. Canopy area and height increased from west to east, with no evidence for ecotype differences in Colby and Hays. In Carbondale, the IL ecotype showed an increase indicating local adaptation. In Carbondale and Manhattan, the CKS ecotype flowered 20 days earlier than other ecotypes with greater probability of seed in western sites relative to other ecotypes. IL ecotype produced greatest seed in its home site. The CKS ecotype shows traits consistent with drought tolerance—reduced canopy, short stature, and early flowering. PCA of traits shows variation sorting along a moisture gradient. To characterize genetic divergence, we used GBS to identify 4,023 SNPs. PCA of genetic distance showed the IL ecotype sorted strongly from admixed CKS and EKS ecotypes. STRUCTURE showed evidence for K=3. The IL ecotype existed as a distinct genetic group while the two Kansas ecotypes showed admixture. Results provide insight into the plasticity of adaptive trait variation and ecotypic genetic divergence and may help inform seed sourcing for restoration in future drier climates.