Recent research suggests that introduced plants become invasive following post-introduction evolutionary changes. Evolution can occur via founder effects, hybridization, or adaptation to new environments. The evolution of increased competitive ability (EICA) hypothesis suggests that release from natural enemies facilitates evolutionary reallocation of resources away from defense and towards greater growth and reproduction. Studies evaluating post-introduction evolution in invasive plant populations, including tests of the EICA hypothesis, have most often been conducted in one or a few common garden environments and have produced conflicting results. When such studies are conducted at only one or a few sites, populations from the native or invasive range may or may not be adapted to the environment chosen for the common garden, potentially leading to spurious conclusions about evolutionary changes. We compared the growth and plasticity of 10 introduced and 10 native range populations of the invasive grass Microstegium vimineum in the greenhouse under two shade treatments, and among 22 common gardens located across biotic and abiotic environmental gradients. Using this novel framework, we sought to determine if the success of Microstegium is driven in part by genetic differences between invasive and native populations.  

Results/Conclusions

Microstegium plants from introduced populations exhibited greater growth under both shade treatments in the greenhouse and at all 22 common garden locations. In the greenhouse, plants from introduced populations were on average 57% larger than native populations in the shade, and 46% larger in the sun. Across all common garden sites, plants from introduced populations had 45% greater aboveground mass. Plants from introduced populations were larger at all common gardens, suggesting introduced populations have evolved greater productivity over a wide range of site conditions. There were strong gradients in nitrogen, soil moisture, light, and resident herbaceous competitors across the common garden environments. For example, light availability, a key determinant of Microstegium growth, varied from 1.5% to 100% ambient light across the common gardens. Given the wide range of environmental conditions across the common garden sites, it is unlikely that our results are influenced by specific common garden conditions, as with other studies. These results suggest that introduced populations of Microstegium have evolved greater vigor and exhibit greater plasticity.