What was done
Open-top chambers were constructed on a pristine tallgrass prairie in Kansas, USA, and fumigated with ambient and twice-ambient concentrations of atmospheric CO2 for eight consecutive growing seasons to study the effects of long-term elevated CO2 exposure on biomass production and community composition in this particular prairie ecosystem.  Vegetation on the site was dominated by C4 warm-season perennial grasses, followed by C3 cool-season perennial grasses, C3 cool-season monocots, and C3 warm-season forbs.  Annual rainfall varied during the study, with three years experiencing substantially less rain than the 30-year average, three years experiencing greater than average precipitation, and two years receiving average or slightly below average amounts of rain.

What was learned
In every year of the study, CO2-enriched plots contained greater amounts of soil moisture than plots exposed to ambient CO2 concentrations, suggesting that CO2-enriched prairie ecosystems would have greater amounts of water at their disposal to cope with the adverse consequences of water stress.  Indeed, long-term atmospheric CO2 enrichment significantly increased both above- and belowground biomass in years of below average rainfall, while having little or no impact on growth during relatively wet years.

Elevated CO2 did not affect the basal coverage or species composition of the ecosystem's major C4 grasses during the eight-year study, contrary to one popular view, which suggests the replacement of C4 species by typically more CO2-responsive C3 species.  However, C3 cool-season monocots and C3 forbs did increase in basal cover and species composition, but it was at the expense of a reduction in the amount of C3 cool-season grasses.

What it means
As the CO2 content of the air continues to rise, it is likely that tallgrass prairie ecosystems will maintain a more favorable water status when subjected to periodic moisture stress resulting from less-than-average amounts of annual precipitation.  In addition, it is likely that biodiversity in tallgrass prairies will be maintained as the atmospheric CO2 concentration increases; for in this particular ecosystem, the assemblages and abundances of C4 species did not change in response to elevated CO2.  Thus, it is not likely that C4 species will be displaced by more photosynthetically CO2-responsive C3 species.