Canada thistle was grown under field conditions in 2000 and 2003 at ambient and elevated (∼ 350 μmol mol⁻¹ above ambient) carbon dioxide [CO₂] to assess how rising [CO₂] alters growth, biomass allocation, and efficacy of the postemergent herbicide glyphosate. By the time of glyphosate application, approximately 2 mo after emergence, elevated CO₂ had resulted in significant increases in both root and shoot biomass. However, the relative positive effect of [CO₂] was much larger for root, relative to shoot growth, during this period (2.5- to 3.3-fold vs. 1.2- to 1.4-fold, respectively) with a subsequent increase in root to shoot ratio. Glyphosate was applied at 2.24 kg ae ha⁻¹ in 2000 and 2003. Subjective classification of leaf damage in shoots after spraying indicated no significant difference in the extent of necrosis in aboveground tissue as a function of CO₂ concentration. After a 6-wk regrowth period, significant reductions in shoot and root biomass relative to unsprayed plots were observed under ambient [CO₂]. However, the decrease in the ratio of sprayed to unsprayed biomass was significantly less at elevated relative to ambient [CO₂] conditions for roots in both years, and no difference in shoot biomass was observed between sprayed and unsprayed plots for Canada thistle grown at elevated [CO₂] in either year. The observed reduction in glyphosate efficacy at the enriched [CO₂] treatment did not appear to be associated with differential herbicide uptake, suggesting that tolerance was simply a dilution effect, related to the large stimulation of root relative to shoot biomass at elevated [CO₂]. Overall, the study indicates that carbon dioxide–induced increases in root biomass could make Canada thistle and other perennial weeds that reproduce asexually from belowground organs harder to control in a higher [CO₂] world.

Nomenclature: Canada thistle, Cirsium arvense L. Scop. CIRAR.