Print PageBig bluestem is a widely-distributed dominant C4 grass, whose productivity is dependent upon precipitation. With wide distribution across a sharp precipitation gradient (400-1200mm yr-1 in Kansas to Illinois), we expect ecotypic variation in drought tolerance and that selection will promote local drought adaptation. A better understanding of ecotypic variation to drought will help predict how a dominant prairie grass may respond to climate change and will inform prairie restoration. We investigate the contribution of genotype and environment to phenotypic variation using reciprocal common gardens across the precipitation gradient. Sites were planted in Carbondale, Illinois, Manhattan and Hays KS and a site in Colby, KS (to test limits of tolerance into drier areas). At these four locations, plants of three ecotypes (each comprised of seeds collected from four pristine populations in Hays, Manhattan, and Illinois) were reciprocally planted in replicate blocks with each plant growing singly and in replicated artificially assembled seeded communities (16m2 plots). We measured growth (height, blade width, canopy area, and biomass) in single plants to investigate variation in drought adaptations across ecotypes and sites. We estimated establishment and cover (using frequency quadrats and counts) in seeded plots to assess if ecotypes respond differentially across the precipitation gradient.
Results/Conclusions
Our data demonstrate a strong phenotypic cline in drought tolerance of the three ecotypes in the singly grown plants and seeded communities. The westernmost ecotype (Hays) exhibits drought-adapted features of a dwarfed stature in height and canopy area. For single plants, canopy area and height were significantly different between ecotype (p<0.0001) and site (p=0.0003). Interaction effects were also significant. All ecotypes in Colby are reduced in height and canopy, with Illinois and Manhattan ecotypes disproportionately smaller there. However, in the Illinois site, all ecotypes were four-fold increased with the Illinois ecotype disproportionately larger. The reduced canopy and height are thus adaptations for reduced transpiration in drier regions. Furthermore, establishment and cover in the seeded plots showed a significant ecotype (p<0.0001), site (p<0.0001) and interaction effect (p<0.0001). The Hays ecotype had disproportionate cover in western regions relative to the Illinois and Manhattan ecotypes (GXE), indicating local adaptation to drought. Thus, the Hays ecotype had 2x the cover compared to the other ecotypes in the Hays site, and 3x the cover in Colby (driest). Contrary to expectations, plants in seeded plots in Illinois did poorly regardless of ecotype. These results (based on 2009 establishment and 2010 cover) were consistent over time.
