Low winter temperatures can limit a species’ distribution and may influence the current range of the invasive grass Bromus tectorum (cheatgrass) in North America. While B. tectorum is abundant across the Intermountain West, its historical distribution has been limited within the northern Great Plains. The range of B. tectorumhas recently expanded however in the Great Plains, potentially due to selection for genotypes more adapted to sub-freezing temperatures. To determine the importance of adaptive evolution to range expansion, we assessed freezing tolerance among populations and tested for its correlation with winter temperatures in the populations’ collection locales. We predicted that 1) Great Plains populations have greater freezing tolerance than Intermountain West populations and that 2) freezing tolerance is negatively correlated with the mean January minimum temperature of a population’s source locale. Plants were harvested from three widely separated populations collected in both the Intermountain West and Great Plains, and field-collected samples were grown in a common garden for one generation. Seedlings from each grow-out population were acclimated to cold temperatures (3°C) for five weeks before freezing. Seedlings were then exposed to freezing treatments ranging from -16 to -24°C; freezing tolerance was evaluated after three weeks.
Results/Conclusions
Freezing tolerance varies significantly among B. tectorum populations (p < 0.0001). The population from the northern Great Plains (North Dakota) displayed the greatest freezing tolerance (LT50 = -25.41°C), while the southern Intermountain West population (Southern Nevada) had the lowest freezing tolerance (LT50 = -17.93°C). Great Plains populations had generally greater freezing tolerance than the populations from the Intermountain West. Mean January minimum temperature at a population’s source locale and population freezing tolerance across the range are strongly correlated (p = 0.05; R2 = 0.55), suggesting low temperatures are an important force in selection. Populations displaying a greater freezing tolerance (e.g. North Dakota) may be more likely to expand their range in cold-limited environments. These cold tolerant populations should be a management priority, so that their long distance dispersal and continued regional spread in the Great Plains and the Intermountain West can be prevented. Variation in freezing tolerance and selection for freezing tolerant populations may provide an example of non-native species adaptive evolution that can be applied to the invasion process on a larger scale. The consequences of the phenotypic variation documented here for future invasions could be explored further through controlled-environment or reciprocal transplant experiments.