What was done
The authors constructed open-top chambers in a native shortgrass steppe ecosystem in Colorado, USA, and exposed the enclosed ecosystems to atmospheric CO2 concentrations of 360 and 720 ppm for two six-month growing seasons to determine the effects of elevated CO2 on gas exchange and growth in this specialized grassland community.

What was learned
In spite of an average air temperature increase of 2.6°C, which was caused by the presence of the open-top chambers, elevated CO2 significantly enhanced photosynthetic rates and biomass production in this grassland community.  In both years of treatment, for example, elevated CO2 increased aboveground biomass by an average of 38%; and when 50% of the standing green plant biomass was defoliated to simulate grazing halfway through the growing season, atmospheric CO2 enrichment still increased aboveground biomass by 36%.

It was also determined that the communities enriched with CO2 tended to have greater amounts of moisture in their soils than communities exposed to ambient air.  This phenomenon likely contributed to the greater (less negative and, hence, less stressful) plant water potentials that were measured in CO2-enriched as compared to ambiently-grown plants.

Interestingly, elevated CO2 did not preferentially stimulate the growth of C3 species over that of C4 species in these communities.  Hence, elevated CO2 did not significantly affect the percentage composition of C3 and C4 grasses in these grasslands; and they maintained their current level of vegetative biodiversity.

What it means
As the CO2 content of the air rises, it is likely that shortgrass steppe communities will respond by exhibiting increases in photosynthesis and biomass production, regardless of whether or not they are being defoliated by natural or mechanical means.  In addition, because elevated CO2 increased soil moisture contents and plant water potentials, it is also likely such grasslands will be better able to cope with intermittent periods of drought.  Moreover, the rising CO2 concentration should not alter species composition in these grasslands, but will likely act to maintain the biodiversity that presently exists among its C3 and C4 component species.  Finally, to dispel any erroneous notions of global warming-induced reductions in these positive responses, the authors concluded that "in future CO2-enriched and warmer climates, production on the shortgrass steppe should be enhanced significantly from that occurring now."