Atmospheric CO₂ concentration had increased from a pre-industrial level of 270 μmol/mol to 350 μmol/mol in 2005 and continues to increase at 1.9 μmol/mol per year on average. It is well known that atmospheric CO₂ enrichment may influence the invasiveness of introduced plant species. Identifying the effects of elevated CO₂ on invasiveness of exotic plants is very important for improving our ability to predict and control potentially invasive species. Four closed-top chambers were used to control CO₂ concentration, ambient atmospheric CO₂ concentration (control) and doubled atmospheric CO₂ concentration (700 μmol/mol). To determine the effects of atmospheric CO₂ enrichment on invasiveness of Chromolaena odorata, a noxious invasive perennial herb or subshrub in many countries of Asia, Oceania and Africa, we compared C. odorata and its phylogenetically related indigenous plant Eupatorium heterophyllum in terms of morphology, growth, biomass allocation, and photosynthesis at two CO₂ concentrations. Fourteen traits related to morphology, growth, biomass allocation, and photosynthesis were measured when C. odorata and E. heterophyllum were treated for nearly three months.
At ambient CO₂ concentration, total biomass, height, stem diameter, and total leaf area were significantly higher and branch number were lower for invasive C. odorata than for native E. heterophyllum, which contribute to form dense monoculture for the invader, outshading native plant species. CO₂ enrichment significantly increased total biomass, height, stem diameter, branch number, and total leaf area in both species. For C. odorata, total biomass, height, stem diameter, branch number and total leaf area were increased by 92%, 41%, 60%, 325%, and 148%, respectively, much higher than 32%, 14%, 30%, 64%, and 79% for E. heterophyllum. Consistently, growth advantage of the invader over the native became more evident at doubled atmospheric CO₂ concentration.
CO₂ enrichment decreased root mass fraction (RMF) and increased stem mass fraction (SMF) and leaf mass fraction (LMF) for both the invasive and native species. However, the responses of these traits to CO₂ enrichment were not significantly different between C. odorata and E. heterophyllum. C. odorata allocated more biomass to stems and leaves and less to roots than E. heterophyllum at either ambient CO₂ concentration or doubled atmospheric CO₂ concentration. Higher LMF and SMF may help C. odorata to increase carbon assimilation, and lower RMF may help C. odorata to reduce respiratory carbon loss, facilitating biomass accumulation. The more efficient strategy of biomass accumulation adopted by the invader might still provide stronger competitive ability against the native. In addition, atmospheric CO₂ enrichment significantly increased net photosynthetic rate (P_max) for both species, and the increases were not significantly different between C. odorata and E. heterophyllum. At both low and high CO₂ concentration, P_max was not significantly different between two species. This indicates that higher biomass accumulation of C. odorata may not be associated with photosynthesis, and benefit from other aspects at doubled atmospheric CO₂ concentration.
In conclusion, our results indicate that CO₂ enrichment stimulates growth more greatly for invasive C. odorata than for native E. heterophyllum, and that in the future with high atmospheric CO₂ concentration invasions by C. odorata may become more serious.