Differential responses to fertilization and competition among invasive, noninvasive alien, and native Bidens species.
Comparative studies of invasive, noninvasive alien, and native congenic plant species can identify plant traits that drive invasiveness. In particular, functional traits associated with rapid growth rate and high fecundity likely facilitate invasive success. As such traits often exhibit high phenotypic plasticity, characterizing plastic responses to anthropogenic environmental changes such as eutrophication and disturbance is important for predicting the invasive success of alien plant species in the future. Here, we compared trait expression and phenotypic plasticity at the species level among invasive, noninvasive alien, and native Bidens species. Plants were grown under nutrient addition and competition treatments, and their functional, morphological, and seed traits were examined. Invasive B. frondosa exhibited higher phenotypic plasticity in most measured traits than did the alien noninvasive B. pilosa or native B. bipinnata. However, differential plastic responses to environmental treatments rarely altered the rank of trait values among the three Bidens species, except for the number of inflorescences. The achene size of B. frondosa was larger, but its pappus length was shorter than that of B. pilosa. Two species demonstrated opposite plastic responses of pappus length to fertilization. These results suggest that the plasticity of functional traits does not significantly contribute to the invasive success of B. frondosa. The dispersal efficiency of B. frondosa is expected to be lower than that of B. pilosa, suggesting that long-distance dispersal is likely not a critical factor in determining invasive success.