Evolution meets ecology: Can native grasses evolve in response to cheatgrass invasion?

Background/Question/Methods

Widespread invasion by Bromus tectorum (cheatgrass) in the Intermountain West has drastically altered native plant communities. While cheatgrass invasion is considered a significant threat to native plant diversity and abundance, it does not result in the extinction of all native plants impacted; many are able to persist alongside invaders. Are certain native plants able to successfully coexist with invaders because they are evolving in response to invasion? Elymus multisetus (big squirreltail) has demonstrated a tolerance of cheatgrass invasion and this ability is greater in plants collected from cheatgrass invaded areas. To investigate whether big squirreltail is evolving in response to invasion and what traits may contribute to competitive ability, we conducted a common garden greenhouse experiment with seeds collected from invaded and nearby uninvaded areas with known differences in adult competitive ability. We measured big squirreltail seedling performance, cheatgrass biomass production, and family-level variation in competition experiments.  Then we used two methods to assess potentially adaptive growth traits: a) directly testing which traits correlate with competitive ability and b) comparing traits of successful plants—those that have persisted in invaded areas—with those of plants from uninvaded areas.

Results/Conclusions

Our findings are consistent with the hypothesis that cheatgrass is exerting a strong selection pressure on native grasses populations in the Intermountain West: we found that big squirreltail seed from invaded areas produced seedlings that were more tolerant of competition and better able to suppress cheatgrass than plants from adjacent uninvaded areas. Competitive ability—quantified by tolerance to cheatgrass competition—was correlated with early seedling growth traits, namely root:shoot ratio, root fork number, and fine root length; root forks differed among families, but none of these traits differed significantly across invasion status. Big squirreltail seedlings from invaded areas were smaller, allocated more biomass to roots, and produced a higher percentage of fine roots than seedlings from uninvaded areas, suggesting that resource capture may affect competitive ability. Several growth traits exhibited family-level variation, indicating that these traits are likely inherited.

Our study provides examples of two experimental methods with the potential to identify and assess potentially adaptive growth traits of native plants for persistence in invaded or otherwise disturbed environments. The ability to identify which traits are beneficial and the degree of genetic variation in these traits can help inform the selection of populations to use for restoration of native species in disturbed environments.