Intraspecific variation in tallgrass big bluestem Andropogon gerardii across a precipitation gradient of the Central US grasslands: Response to experimental drought

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Paige Wiebe, Biology, Kansas State University, Manhattan, KS, Loretta Johnson, Matthew Galliart and Jacob Alsdurf, Division of Biology, Kansas State University, Manhattan, KS, Megan Keenan, Honors Program, Manhattan High School, Manhattan, KS

Background/Question/Methods
Evolutionary ecology seeks to understand factors that contribute to population genetic divergence, formation of ecotypes, and ultimately new species. Habitats are often spatially variable, causing different selection pressures and ultimately, genetic divergence. Andropogon gerardii, dominant grass of the Great Plains, is used on 5 million acres of restoration and accounts for $8 billion in cattle production within Kansas. Despite its dominance, A. gerardii has not been adequately studied across its range in the Great Plains, leaving underexplored what natural variation can reveal about adaptation to climate. We characterized intraspecific variation between populations of A. gerardii from different rainfall regions and assessed population response to drought. Because populations putatively experience strong selection from rainfall, we hypothesized 1) trait differences should be primarily due to rainfall of population origin, 2) dry populations should be more drought-tolerant, and 3) population differences should be supported by underlying genetics. Nine populations spanning rainfall 437-1020 mm annually (CO to IL) were planted into 1.2L pots in greenhouse for 6 months. Droughted plants received 200mL water every 3 days, while controls received water daily. We report on vegetative morphology, above- and belowground biomass, flowering date, physiology (water potential, SPAD) and underlying genetics.
Results/Conclusions
We see clear evidence for strong population differences depending on rainfall regime of origin and lesser effects of experimental drought. Populations from wetter areas had significantly longer and thicker leaves, greater canopy area, and produced more above- and belowground biomass compared to populations from drier areas. Furthermore, when plants from wetter populations were exposed to drought, leaves were disproportionately narrower, canopy area and above-and belowground biomass reduced and had less favorable water balance--all indicating greater sensitivity to drought in populations from wetter areas. Populations from drier areas flowered earlier suggesting escape from late season drought. Intraspecific variation was corroborated by genetic differences with greatest genetic distances between CO and Il populations. Combined, results suggest that populations of A. gerardii have functionally different growth strategies, with populations from the west limited by water, and in contrast, populations from the east limited by light and competition from neighbors. More broadly, intraspecific variation in form and function across the rainfall gradient contributes to our understanding of broad patterns of increased primary productivity and greater root biomass leading to more soil organic matter with increasing rainfall across the Great Plains. Finally, intraspecific differences should be considered in climate-matching populations for restoration across the Great Plains.