Droughts are expected to intensify in coming years with changing climate. Climate stress may be buffered in biodiverse communities through increased richness or presence of different species. However, many ecosystems contain dominant species which are disproportionately responsible for ecological function. Genetic diversity in dominant species has been shown to significantly impact function in these systems. The shortgrass steppes of the American Midwest are strongly dominated by blue grama (Bouteloua gracilis), a C4 grass which comprises up to 90% of ground cover. It is imperative that ecologically and economically important native rangeland grasses like blue grama be investigated for their genetic and phenotypic diversity across populations. We selected ten populations where blue grama was a dominant species across a precipitation and elevation gradient in Boulder County, Colorado, plus five sites along a larger north-south gradient from central New Mexico to South Dakota. From each population, fifteen clones were transplanted to controlled greenhouse conditions (common garden) and subjected to a water-limited and water-abundant treatment. Height, fitness, biomass allocation, and physiological traits were measured on each clone within population. Each clone within population was also genotyped using reduced-representation sequencing.
When water limitation was removed, we observed plasticity compared to field phenotype in all populations. However we observed that southern populations exhibited higher growth rates compared to northern and Boulder populations. We also considered differences in physiological drought response strategy (hydroscape), where we found populations to differ in terms of anisohydric (passive) versus isohydric (active) water budgeting strategies. Finally, we observed differences in fitness (flower mass and length) across populations and clones, suggesting the potential tradeoff between clonal and sexual reproduction is specific to population or individual clones. Height was sometimes responsive to water availability in the controlled environment, indicating different capacity for plasticity across populations. Fitness plasticity was consistent across populations, however. Our results confirm genetic diversity across populations of blue grama and suggest potential local adaptation of growth rate for drier populations. We also confirm populations are distinct according to plant genotype. Elucidation of phenotypic diversity along with research highlighting genetic diversity across different populations of blue grama may help inform models predicting how this species will respond to climate change and increasingly unpredictable droughts. Incorporating genetic and phenotypic distinctions will also be critical for management and restoration practices in the shortgrass steppe ecoregion.