Droughts are expected to become more intense and frequent in the American West. However, many ecosystems contain dominant species which may be well adapted to variable climatic conditions and help buffer the ecological effects of changing climate. Yet little is known about intraspecific diversity in the dominant species of xeric grasslands. It is imperative that ecologically and economically important dominant species be investigated for their genetic and phenotypic diversity, plasticity, and adaptations to aridity across different populations. The shortgrass steppes of the American West are strongly dominated by blue grama (Bouteloua gracilis), a warm-season grass which comprises up to 95% of production. We selected fifteen populations spanning from New Mexico to South Dakota where blue grama was a dominant species across an aridity gradient. 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. Two additional populations were sequenced from more mesic areas to provide comparison of extreme range edges.
Results/Conclusions
Genetic diversity was extremely high among populations, especially when comparing mesic and xeric populations, suggesting blue grama has genetic diversity on which selection can take place under future climate change. Overall, many traits among populations varied significantly from one another and were often related to population aridity. In a common environment, we observed that drier populations exhibited higher growth rates, greater biomass, greater overall fitness, and fewer osmolytes compared to northern, more mesic populations. When water limitation was removed, we observed greater plasticity in drier populations for biomass, height, and fitness which could be related to intra-annual precipitation variability response. We also considered differences in physiological drought response strategy, where we found populations differed in terms of anisohydric (passive) versus isohydric (active) water budgeting strategies. Our results confirm presence of genetic diversity across populations of blue grama and suggest local adaptation for drier and possibly colder populations. Elucidation of plasticity and diversity across different populations of blue grama will help inform models predicting how this species will respond to climate change and increasingly unpredictable droughts. Incorporating genetic and phenotypic variation will also better inform management and restoration practices in the shortgrass steppes of the American West.