What was done
The authors grew the C3 grass Pascopyrum smithii, the C4 grass Bouteloua gracilis, and the C3 legume Medicago sativa for 20 days post-defoliation in growth chambers receiving atmospheric CO2 concentrations of 355 and 700 ppm and low or high levels of soil nitrogen to determine how these factors affect the regrowth of these forage species.

What was learned
After 20 days post-defoliation, Medicago sativa plants grown in elevated CO2 had attained total dry weights that were about 62% greater than those reached by ambiently-grown plants, regardless of soil nitrogen.  Pascopyrum smithii plants, however, were more sensitive to soil nitrogen; and whole plant regrowth was stimulated by 150 and 68% under conditions of high and low soil nitrogen, respectively.  Last of all, elevated CO2 had no positive effect on the regrowth of the C4 grass, which fared better under high vs. low soil nitrogen conditions.

What it means
As the CO2 concentration of the air increases, C3 forage species subjected to natural or mechanical defoliation will likely respond by significantly increasing their regrowth biomass in a plant-dependent and soil-nitrogen-dependent manner.  Leguminous nitrogen-fixing C3 species, for example, will likely exhibit enhanced regrowth irrespective of soil nitrogen content, while non-nitrogen-fixing C3 species will likely exhibit greater regrowth on soils containing more, rather than less, nitrogen.  Moreover, the regrowth of C4 species may be less responsive to atmospheric CO2 enrichment than that of C3 species.  Hence, future increases in the air's CO2 content should promote or, at the very least, not impair prairie regrowth following defoliation by either animals or machinery.