Trait divergence, not plasticity, determines the success of a newly invasive plant

Phenotypic plasticity and genetic differentiation both play important roles in the establishment and spread of species after extra-range dispersal; however, the adaptive potential of plasticity and genetic divergence in successful invasions remains unclear.

We measured six anatomical traits associated with drought tolerance in contrasting water environments for individuals from the invasive and native range of the bunchgrass Brachypodium sylvaticum. To represent sources contributing to admixed genotypes in the invasive range accurately, we used unique alleles to determine probabilities of genetic contribution, and utilized these as weights in our analyses. The adaptive values of plasticity and genetic differentiation were assessed using regression.

No plasticity was found in response to water availability for any of the measured traits. Bulliform cell area and three traits related to xylem morphology displayed genetic differentiation between invasive and native ranges, indicating a shift in the invasive range towards drought-tolerant phenotypes. Genetic divergence was not consistently in the direction indicated by selection, suggesting that responses are limited by trade-offs with other traits or physical constraints.

Our results indicate that invasive adaptation is the consequence of post-introduction selection leading to genetic differentiation. Selection, rather than plasticity, is driving B. sylvaticum success in its invaded range.