Background
The authors write that "dynamic global vegetation models simulate feedbacks of vegetation change on ecosystem processes," but they say that "experimental evidence for feedbacks that result from atmospheric CO2 enrichment is rare." Therefore, based on their hypothesis that "feedbacks from species change would amplify the initial CO2 stimulation of aboveground net primary productivity (ANPP) of tallgrass prairie communities," they set out to explore the subject experimentally.

What was done
Polley et al., as they describe it, "grew communities of perennial forb and C4 grass species for 5 years along a field CO2 gradient (250-500 ppm) in central Texas (USA) on each of three soil types, including upland and lowland clay soils and a sandy soil," while measuring a number of plant physiological properties, processes and, most importantly, ecosystem ANPP.

What was learned
The three researchers with the U.S. Department of Agriculture's Agricultural Research Service discovered that CO2 enrichment from 280 to 480 ppm "increased community ANPP by 0-117% among years and soils and increased the contribution of the tallgrass species Sorghastrum nutans (Indian grass) to community ANPP on each of the three soil types," while noting, in this regard, that the "CO2-induced changes in ANPP and Sorghastrum abundance were linked." More specifically, they report that "the slope of ANPP-CO2 regressions increased between initial and final years on the two clay soils because of a positive feedback from the increase in Sorghastrum fraction," which feedback "accounted for 30-60% of the CO2-mediated increase in ANPP on the upland and lowland clay soils during the final 3 years and 1 year of the experiment, respectively."

What it means
In the words of Polley et al., "by favoring a mesic C4 tall grass, CO2 enrichment approximately doubled the initial enhancement of community ANPP on two clay soils." And as a result, they conclude that "CO2-stimulation of grassland productivity may be significantly underestimated if feedbacks from plant community change are not considered."