Phenotypic plasticity of invasive knotweed across Europe: a distributed common garden experiment

Biological invasions expose populations of introduced species to strong selection forces in abiotic and biotic factors, which is expected to lead to adaptive differentiation of populations, particularly for large-scale invaders. To better understand population differentiation and phenotypic plasticity of invasive Japanese knotweed (Reynoutria japonica) in Europe, we compared the performance of 45 European knotweed populations, collected across a 2000 km latitudinal transect, in three common gardens with contrasting climatic conditions, one at the southern edge, one in the center, and one at the northern edge of the species European distribution. The plants exhibited strong phenotypic plasticity across the three gardens, with a more acquisitive growth strategy in the southern garden, and a more conservative strategy and change of architecture in the climatically unfavorable north. Although we observed phenotypic selection on several leaf traits, and on the plasticity of leaf thickness, with some selection differences between the gardens, there was little evidence of population differentiation, and none for local adaptation. Only for plant architecture (shrubbiness), we detected heritable population variation in trait means and plasticity. However, variation in plasticity was unrelated to climatic variability of origin, and to plant fitness. Our results suggest that evolutionary processes - local adaptation and evolution of plasticity - did not play a key role in the success of Japanese knotweed in Europe. Instead, its high baseline plasticity appears to be the key factor that makes it such a strong invader across a broad range of environments.