Influence of rising atmospheric CO 2 since 1900 on early growth and photosynthetic response of a noxious invasive weed, Canada thistle ( Cirsium arvense )

Using climate-controlled growth chambers, the growth, morphology and photosynthetic response of Canada thistle ( Cirsium arvense L. Scop.), a recognized invasive weed, was determined at CO2 concentrations ([CO2]) of 285, 382 and 721 µmol mol–1 during the vegetative state. These concentrations correspond roughly to ambient [CO2] from 1900, 2001 and those projected for 2100, respectively. At the end of the vegetative stage, leaves grown at 721 µmol mol–1 still had significantly higher photosynthetic rates under the growth conditions, relative to the 285 or 382 µmol mol–1 CO2 treatment. Significant differences in leaf morphology, particularly the number and length of leaf spines, were also observed, with spine number and length increasing as a function of [CO2]. At the end of the vegetative stage, at 54 d after sowing (DAS), whole plant biomass had increased by 69% for the elevated (720 µmol mol–1) CO2 treatment relative to current ambient CO2. However, biomass had increased by 126% for current ambient (380 µmol mol–1) relative to the concentration that existed in 1900 (285 µmol mol–1). Data from this study indicate that rising [CO2] may have already had a substantial influence on the vegetative development of Canada thistle, by significantly stimulating growth and photosynthesis, as well as altering leaf defenses to potential herbivory. Overall, these changes could have important implications for growth and control of this noxious weed as atmospheric CO2 continues to increase.