Failure to invest below-ground may limit the northern expansion of invasive knotweed: lessons from a two-phase transplant experiment.

The ecological and evolutionary processes determining species distributions and range limits remain poorly understood. Ultimately, range limits depend on the species' abilities to persist under heterogeneous conditions, by adaptive differentiation and phenotypic plasticity, including transgenerational effects. To investigate ecological differentiation and transgenerational effects in the clonal invasive knotweed, Reynoutria japonica, in its introduced range in Europe, we analyzed biomass production in a two-phase transplant experiment: plants sampled along a latitudinal gradient were planted in three sites located at the northern range margin, mid-range and near the southern range margin, and then re-transplanted among all three sites after two years. Biomass production and allocation in the second transplant phase were generally not associated with latitude of origin, and previous growth at the same site did not promote performance. We therefore found no evidence that adaptive differentiation or adaptive transgenerational effects contribute to the wide distribution of R. japonica in Europe. However, at the northern site, with a 25% shorter season, knotweed plants invested much less biomass below-ground, even though overwintering below-ground rhizomes are essential for the species' survival and spread. The pattern was further strengthened in plants that had grown in the northern site in the previous generation. We further explored limiting climate conditions in a species distribution model for the European range and found that mean annual temperature and temperature annual range are the main predictors of the European distribution of R. japonica. Taken together, our study suggests that low temperatures and associated short growing seasons may pose a limit to the broad environmental tolerance of R. japonica and restrict its northward spread by reducing below-ground biomass accumulation.