Abstract: The evolution of increased competitive ability hypothesis predicts that invasive plants may reallocate the resources formerly used for defenses to grow and reproduce in responses to enemy release in introduced ranges, contributing to increased competitive abilities and therefore to invasiveness. To explore the roles of evolution in invasion success of noxious Chromolaena odorata, eight invasive and eight native populations were compared at two nutrient levels in a common garden in terms of growth and quantitative defense compounds such as tannin, total phenolics, hemicellulose and cellulose. The results indicated that plants from invasive populations showed lower hemicelllulose concentrations in leaf and stem than plants from native populations at both nutrient levels. Plants from invasive populations exhibited lower leaf cellulose concentration at high nutrient level but higher leaf and stem phonolics concentrations at lower nutrient level. Plants from invasive and native populations were not significantly different in total biomass at both nutrient levels. Thus, we concluded that C.odorata may not evolve to increase growth in introduced ranges, while showing genetically based changes in quantitative defense compounds. The decreased hemicellulose and cellulose concentrations in plants from invasive populations of C.odorata are likely to be associated with the evolutionary responses to the lack of specialist enemies in introduced ranges, while the increased total phenolics concentration may help C.odorata defend generalist enemies in introduced ranges.

Key words: Chromolaena odorata, biological invasions, EICA hypothesis, growth, quantitative defense compounds