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

Successful large-scale biological invasions require introduced species to either rapidly adapt to a broad range of—partly novel—environmental conditions, or to possess a high level of phenotypic plasticity. Replicated common garden experiments across the introduced range provide a powerful framework to investigate these complementary mechanisms. To better understand population differentiation, adaptation and plasticity of invasive Japanese knotweed (Reynoutria japonica) in Europe, we compared the performance of knotweed plants from 46 European populations, collected across a 2000 km latitudinal transect, in three common gardens with contrasting climatic conditions, one at the southern edge, one in the centre 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 unfavourable north. Although we observed phenotypic selection on several leaf traits, with some differences in selection between the gardens, we found little evidence for population differentiation or local adaptation, that is variation in plant performance was not negatively related to climate or geographic distance. Greater plasticity in leaf thickness across gardens was linked to the production of fewer but larger shoots, indicating a trade-off between growth and clonal expansion. In addition, populations from higher latitudes and regions with greater inter-annual temperature variability displayed increased plasticity in shoot volume and shrubbiness, but reduced plasticity in shoot number. Synthesis. Our results suggest that local adaptation has not played a key role in the success of Japanese knotweed in Europe. Instead, its high overall phenotypic plasticity (‘general purpose genotype’), as well as evolutionary fine-tuning of plasticity, may have contributed to the species' invasion success across a broad range of environments.