Big bluestem (Andropogon gerardii) is the ecologically dominant grass in tall grass prairies of the Midwest. With wide distribution across a west/east precipitation gradient (40-119cm/yr), and a south/north temperature gradient (15-5oC/yr), we expect ecotypic variation in drought and thermal tolerance. Understanding ecotypic variation will help predict how a dominant prairie grass may respond to current and predicted climate change. Current practice uses species distribution modeling to predict an organism’s response to climate change but fails to incorporate intraspecific ecotypic variation within a species. Our study utilizes phenotypic data from 37 geographically distributed populations across the Midwest to explore species-climate and phenotypic-climate relationships. For each of the 37 geographically distributed populations, we grew plants from seed under greenhouse conditions and measured phenotypes (blade width, height, biomass, and chlorophyll absorbance). Generalized mixed linear modeling was used to identify statistically significant differences among population phenotypes and principle component analysis was used to explore variance within data as related to climate variables. We used phenotype data to create a phenotypic distribution model using IPCC climate projections to predict current and future phenotypes across the Midwest. These predictions were compared to results from a species-level ecological niche model (Maxent).
There was a significant main effect among population phenotypes (height, width, biomass, and chlorophyll absorbance all p<0.001). PCA analyses show a phenotypic cline across populations that can be partially explained by longitude, mean annual precipitation, and vegetation type. These results support evidence for ecotypic variation in drought tolerance of big bluestem across the climate gradient of the Midwest. Phenotypic distribution models, for the year 2070, show phenotypes from dry areas (short stature, low biomass, narrow leaves, increased chlorophyll absorbance) are predicted to expand through the Midwest, eclipsing phenotypes from wet areas (robust, wide leaves, decreased chlorophyll absorbance), provided adequate migration. This novel phenotypic distribution model greatly refines current species distribution models that assume no intraspecific ecotypic variation and may accurately predict species’ response to climate change.