Abstract: Invasive plant species pose signifcant ecological and economic threats due to their establishment and dominance in non-native ranges. Previous studies have yielded mixed results regarding the plants’ adaptive mechanisms for thriving in new environments, and particularly, little is known about how the phenotypic plasticity of growth and defense-related traits may facilitate plant invasion. This study addressed these uncertainties by employing the aggressive weed Reynoutria japonica as a study model. We examined the differences in growth, defenserelated traits and biomass allocation between R. japonica populations from native and introduced ranges grown in two common gardens with distinct climate conditions. Our results demonstrated that while the introduced populations did not exhibit increases in height and total dry mass, nor reductions in leaf defense levels, their investment in leaf production was signifcantly higher compared to the native populations. Additionally, introduced populations displayed greater phenotypic plasticity in clonal ramet but less phenotypic plasticity in biomass production than native populations across varying environments. These fndings highlight the roles of phenotypic plasticity and specifc trait adaptations, such as clonality, in the successful invasion of R. japonica. This study has important implications for managing invasive plant species under changing environmental conditions.

Key words: plant invasion, common garden, plant defense, biomass allocation, phenotypic plasticity, Reynoutria japonica