Recent studies have pointed to the potential importance of evolutionary change in enhancing the success and spread of invasive species. Developing techniques to systematically explore the role of life-history evolution in the process of invasion will prove to be invaluable in gaining deeper insight into what makes an exotic species invasive. With this in mind, we conducted a coordinated common garden experiment in four gardens using Lythrum salicaria L. (purple loosestrife), an aggressive wetland invader in North America and native to Eurasia, as a model system. Plants in the experiment originated from twelve populations, three from the region of each garden. Two of the gardens were located in the United States and two in Germany. Four environmental treatments, varying hydrology and nutrient availability, were imposed in all gardens. The goal was to determine the relative importance of genetic change versus phenotypic plasticity in explaining differences in life-history traits of plants in native versus invasive habitats. Plants in the experiment were censused for a number of morphological traits throughout the growing season. At the end of the growing season additional information was obtained for flowering characteristics and production of above ground biomass partitioned between vegetative and reproductive portions of the plant. These data were used to determine if there were significant shifts in life-history traits in the invasive plants as compared to plants originating from the native provenance.

Results/Conclusions

The response of plants to the treatment combinations were complex, depending upon garden, environmental treatment combination, and source population, with high level interactions exhibiting a significant impact on the outcome of the experiment. Even so, we found a systematic shift in the production of flowering biomass differentiating plants from the invasive provenance from those originating in the native range. Invasive plants produced proportionately less flowering biomass when adjusted for total above ground biomass, even though they tended to have greater total biomass at the end of the growing season. This coincided with a shift towards later flowering in plants from the invasive provenance. A separate study has also shown that invasive L. salicaria plants are longer lived, at least in mesic habitats. These results are consistent with theoretical predictions that plants released from native herbivores will be longer lived with delay reproduction and clearly indicate that significant evolutionary change in L. salicaria has taken place during the process of invasion.