What was done
Canada thistle (Cirsium arvense L. Scop.), which the author describes as "the most frequently listed noxious weed species in surveys of the continental United States and southern Canada," was grown in pots that were watered to the drip point daily with one of three complete nutrient solutions that differed only in nitrogen (N) concentration (3.0, 6.0 or 14.5 mM) in controlled environment chambers maintained at 287 and 373 ppm CO2 from seeding until flowering, which occurred at 77 days after seeding (DAS).

What was learned
The author reports that "N supply did not affect the relative response to CO2 for any measured vegetative parameter up to 77 DAS."  Hence, averaged across the three nitrogen treatments, we find that the 86 ppm increase in atmospheric CO2 concentration employed in this study increased total plant biomass by 65.5%, which for the full 100 ppm CO2 increase experienced over the course of the Industrial Revolution (initial value ~275 ppm, current value ~375 ppm) translates into an approximate 76% biomass increase.

What it means
To assess the significance of this finding, it is necessary to compare it with what would be expected for crops with which Canada thistle competes.  Fortunately, Mayeux et al. (1997) have obtained this information for two varieties of wheat (Triticum aestivum L. cv. Seri M82 and Yaqui 54).  From their data it can be calculated that the 100 ppm increase in atmospheric CO2 concentration experienced over the course of the Industrial Revolution should have produced yield increases of 70% and 74%, respectively, for the Seri and Yaqui cultivars growing under well-watered conditions comparable to those employed in Ziska's study.  In addition, based on the voluminous data summarized by Idso and Idso (2000) for the world's major food crops, the calculations we have made for well-watered wheat can be scaled to derive comparable well-watered mean CO2-induced growth enhancement values of 84% for other C3 cereals, 74% for legumes, and 80% for root and tuber crops.  Hence, it would appear that the CO2-induced growth enhancement likely experienced by Canada thistle over the course of the Industrial Revolution was not much different from the growth enhancements that were likely experienced by many of the crops with which it competes.

References
Idso, C.D. and Idso, K.E.  2000. Forecasting world food supplies: The impact of the rising atmospheric CO2 concentration.  Technology 7S: 33-55.

Mayeux, H.S., Johnson, H.B., Polley, H.W. and Malone, S.R.  1997.  Yield of wheat across a subambient carbon dioxide gradient.  Global Change Biology 3: 269-278.