Successful invasive species can exhibit phenotypic variation in their native and invasive ranges as the result of plastic or genetic differences. Determining the source of phenotypic variation in terms of growth and reproduction may help explain species invasiveness and their potential for further spread. Using reciprocal transplant common gardens, we examined phenotypic variation of two common New England invasive plants, Berberis thunbergii and Celastrus orbiculatus, to explore whether populations have undergone adaptive evolution in their introduced ranges. The experimental design allows for estimation of plastic and genetically determined differences in plant performance in various parts of their current or potential range and investigation of whether particular genotypes might be better able to survive at range extremes. Clonally replicated cuttings from wild populations in both native and invasive ranges were used to establish gardens in the species’ native range (Japan) and in northern (Vermont) and southern (Connecticut) regions of the introduced range in New England. Plant size and fecundity were assessed at the beginning and end of each growing season for two years. Individually-based hierarchical Bayesian models were used to estimate the response of each clone, population, and region to differences in garden and source population location.
Results of final above-ground biomass indicate that C. orbiculatus from both the US and Japan grew robustly regardless of where they were grown. However, neither source region within a country, nor source population, nor garden location significantly influenced growth of C. orbiculatus, indicating low overall phenotypic plasticity. In contrast, B. thunbergii results show significant phenotypic plasticity. Other attributes of plant success including fecundity, branch number and basal stem diameter are also investigated for similar patterns of plasticity. Since these species are still expanding their invasive ranges northward, understanding the phenotypic responses of these plants will give us insight into how they might respond to novel site conditions and whether propagule source regions influence their success. In addition to range expansion, there are also implications for how these species may respond to future climate change.