What was done
The authors conducted a massive review of the scientific literature published between 1980 and 1997 to compare the responses of wild C4 and C3 grasses to atmospheric CO2 enrichment.

What was learned
After analyzing approximately 40 and 80 individual responses of C4 and C3 grasses to elevated CO2, respectively, it was determined that both types of grasses respond favorably to atmospheric CO2 enrichment.  Photosynthetic rates, for example, increased by an average of 25 and 33% for C4 and C3 grasses, respectively, in response to a doubling of the atmospheric CO2 concentration.  In addition, atmospheric CO2 enrichment increased total biomass of C4 and C3 grasses by 33 and 44%, respectively.  Thus, it is abundantly clear that C4 plants can (and do!) respond robustly to increases in the CO2 content of the air.

What it means
As the atmospheric CO2 concentration continues to rise, C4 plants will likely exhibit significant increases in photosynthesis and biomass production that will closely parallel those of C3 plants, which often have been implicated to respond much more favorably to elevated CO2 than do C4 plants.  Consequently, this literature review suggests, and its authors state, that "it may be premature to predict that C4 grass species will lose their competitive advantage over C3 grass species in elevated CO2."  Thus, as the atmospheric CO2 content of the air continues to rise, it is highly unlikely that C3 plants will displace C4 species.  Indeed, rising atmospheric CO2 concentrations should help to maintain biodiversity in ecosystems where C4 and C3 plants coexist.