Invasive plant transformers substantially change the abiotic environment of invaded ecosystems and thus habitat suitability to resident species. Post-invasion environmental alteration can also modify the selective pressure acting on native plants. Here, I explored whether the decrease in light availability due to an invasion of Heracleum mantegazzianum drives evolution of reproductive strategies of Veronica chamaedrys, a perennial plant combining sexual and clonal reproduction.

Using a common garden experiment with plant material of V. chamaedrys from 23 sites with distinct invasion history and light conditions, I searched for changes in reproductive allocation. I also asked whether evolution of the two modes can be constrained by the genetic trade-off between them. Furthermore, the phenotypic trade-off between the two modes was explored in a field experiment.

I found that invaded populations increased investments in clonal reproduction, a shift that was driven by decreased light availability, particularly in the early invasion phases. However, as light availability rebounded in the more advanced phases of invasion, so decreased allocation to clonal structures. In terms of relative allocation, increased investment in ramets was paralleled by reduced seed production, and the changes were underpinned by genetic trade-off. Finally, the phenotypic trade-off was demonstrated in the field experiment by showing that plants producing more ramets were also less likely to flower.

These results suggest that an exotic plant invasion can drive evolution of reproductive allocation, here observed on a timescale of tens of years. This knowledge is important not only to predict long-term invasion impacts but also, more generally, to provide novel insights into the process of adaptation of plants to changing abiotic conditions.