What was done
Working in central Texas (USA) with elongated field chambers designed to expose tallgrass prairie vegetation comprised of a mix of seven different species -- four C4 grasses (Bouteloua curtipendula, Schizachyrium scoparium, Sorghastrum nutans and Tridens albescens) and three forbes (Salvia azurea, Solidago canadensis and Desmanthus illinoensis) -- to a continuous atmospheric CO2 gradient spanning the pre-industrial to elevated CO2 concentration range of 250 to 500 ppm, the authors measured the concentrations of ten different elements found in the aboveground tissues of three of the C4 grasses (B. curtipendula, S. scoparium and S. nutans) that are common competitive dominants in assemblages of tallgrass prairie vegetation, which they grew on three different soil types over three different growing seasons.

What was learned
Polley et al. discovered, as they describe it, that "the CO2 effect on relative abundances of Bouteloua and Sorghastrum had far greater impact on element concentrations in grass stands than did change in element levels of individual species," adding that "elements that were most limiting relative to the nutritional requirements of cattle generally occurred at lowest concentrations in Bouteloua, the species most strongly disadvantaged at elevated CO2," while "CO2 enrichment favored a grass, Sorghastrum, with relatively high concentrations of these elements, thereby increasing mean concentrations of several elements in grass assemblages."

What it means
The four researchers say their results "highlight the importance of accounting for change in species abundances and composition when predicting CO2 effects on ecosystem functioning and services," as is also indicated by the work of Polley et al. (2010), while concluding that "by favoring one grass species over another, CO2 enrichment from pre-industrial to elevated levels increased concentrations of several nutritionally important elements in prairie grasses," while further noting that this "improvement in the nutritional quality of plants for herbivores" represents an "underappreciated impact that CO2 enrichment may have on ecosystem functioning by changing plant composition."

Reference
Polley, H.W., Morgan, J.A. and Fay, P.A. 2010. Application of a conceptual framework to interpret variability in rangeland responses to atmospheric CO2 enrichment. Journal of Agricultural Science 149: 1-14.