What was done
The author grew the noxious agricultural weed Canadian thistle (Cirsium arvense L. Scop.) in environmental chambers maintained at atmospheric CO2 concentrations of 280, 380 and 720 ppm for about two months to study the effects of elevated CO2 on photosynthesis and growth in this invasive species.

What was learned
Increasing the atmospheric CO2 concentration from 280 to 380 ppm enhanced rates of photosynthesis and total plant biomass production in Canadian thistle by 45 and 126%, respectively.  In addition, increasing the CO2 content of the air from 380 to 720 ppm stimulated photosynthetic rates by 49% and enhanced total plant biomass production by 69%.  Moreover, elevated CO2 exposure increased the numbers and lengths of leaf spines on both immature and mature leaves.

What it means
As the air's CO2 content increases, Canadian thistle plants likely will display enhanced rates of photosynthesis and biomass production.  In addition, leaves on this plant should include longer and more numerous spines, which could reduce herbivory in this weedy species.  Thus, this study indicates a positive response to atmospheric CO2 enrichment for this noxious thistle, which can be put into better perspective by reading our summary on Growth Response to CO2: Weeds.