Population differentiation in phenotypic and genetic traits of an invader: The recent high elevation expansion of Bromus tectorum

Local adaptation can be an important mechanism of spread into new environments. We evaluated the adaptive and neutral evolutionary changes associated with the recent expansion of Bromus tectorum (cheatgrass) into high elevation habitat in its introduced range.  It is not known if this range expansion is due to environmental change, genetic change, morphological or physiological change (plasticity), or if B. tectorum is a generalist that can tolerate a broad range of environmental conditions and is opportunistically expanding its range. Using microsatellites, we genotyped individuals from low and high elevation populations in the Rocky Mountains.  We found no indication of genetic differentiation at these loci between high and low elevation sites, but we did find a significant amount of variation within and among sites.  A common garden revealed differences in size and growth between plants from low and high elevation sites.  The difference in performance traits but not neutral genetic traits suggests B. tectorum is responding to different selective regimes at high and low elevations. This is the first step in a series of experiments addressing the roles of local adaptation, phenotypic plasticity, and environmental changes (e.g., in nitrogen, temperature, and precipitation) in this recent range expansion.