What was done
Bracken (Pteridium aquilinum), which presents a serious weed problem and potential threat to human health in the United Kingdom and other regions, was grown for 19 months in controlled environment chambers receiving atmospheric CO2 concentrations of 370 and 570 ppm and normal or high levels of fertilization to determine the effects of elevated CO2 and soil fertility on growth in this species.

What was learned
Elevated CO2 consistently increased rates of net photosynthesis in bracken by approximately 30 to 70%, depending upon soil fertility and time of year.  However, elevated CO2 did not increase total plant dry mass nor the dry mass of any plant organs, including rhizomes, roots, and fronds.  The only significant interaction of elevated CO2 on plant growth was observed in the normal nutrient regime, wherein elevated CO2 actually reduced the average area of fronds.

What it means
The response of bracken to atmospheric CO2 enrichment reinforces an increasing trend in the literature, which reports that relatively fast-growing weedy species will not necessarily respond more favorably to elevated CO2 than slower-growing crop species.  Specifically, it suggests, if anything, that bracken may exhibit decreased growth as the CO2 content of the air continues to rise, as most natural and disturbed ecosystems where this species exists are not subjected to enhanced levels of soil fertility.

It is interesting that the observed CO2-induced photosynthetic enhancement did not lead to increased growth in bracken.  However, the excess carbohydrates generated by this phenomenon may have been secreted belowground into the rhizosphere, where it often stimulates soil microbial organisms in many beneficial ways.