What was done
The biomass and herbage quality responses of 72 monoliths (0.4 x 0.4 x 0.4 m) of a fertile but nitrogen-limited C3 grassland community from southeastern France that was exposed from August 1998 to June 2001 to ambient (350 ppm) and CO2-enriched (600 ppm) air via the Mini-FACE system described by Miglietta et al. (2001) were studied under two cutting regimes (three cuts per year and six cuts per year).

What was learned
The effect of the extra CO2 on dry matter yield, in the words of the authors, "increased markedly through time."  As they describe it, "the relative response to CO2 was negative in October 1998, on average nil in 1999, reached +30% in 2000 and rose up to +38% in spring 2001."  They also determined that the yield responses to elevated CO2 were typically twice as great under the high-frequency cutting regime as under the low-frequency cutting regime.  With respect to the first of these findings, they report that several other studies have reported "an increased responsiveness to elevated CO2 through time, both with relatively low fertility calcareous (Leadley et al., 1999) or semi-arid grasslands (Morgan et al., 2001) and with sown grasses grown with different N fertilizer supplies (Daepp et al., 2000)."

As for herbage quality, Picon-Cochard et al. report that an increase in the water soluble carbohydrate (WSC) content of the bulk forage under elevated CO2 and a corresponding decline in cell wall neutral detergent fiber (NDF) contents were observed, and that "the decline in NDF was correlated with an increased in-vitro dry matter digestibility."  Also, they report that a CO2-induced increase in forb content favored herbage quality "because of a higher digestibility of the forb shoots and, indirectly, through the reduction in the mass of the grass stems."

What it means
With respect to the conduct of CO2 enrichment studies, the authors say their results suggest "that short-term experiments that impose an instantaneous change in CO2 concentration may not be sufficient to predict the long-term productivity response to elevated CO2 of semi-natural grasslands."  With respect to the world of nature, their results suggest that as the air's CO2 content continues to rise, we may expect to see an increase in the production of more-readily digestible forage on earth's temperate C3 grasslands.

References
Daepp, M., Suter, D., Almeida, J.P.F., Isopp, H., Hartwig, U.A., Frehner, M., Blum, H., Nosberger, J. and Luscher, A.  2000.  Yield response of Lolium perenne swards to free air CO2 enrichment increased over 6 years in a high N input system on fertile soil.  Global Change Biology 6: 805-816.

Leadley, P.W., Niklaus, P.A., Stocker, R. and Korner, C.  1999.  A field study of the effects of elevated CO2 on plant biomass and community structure in a calcareous grassland.  Oecologia 118: 39-49.

Miglietta, F., Hoosbeek, M.R., Foot, J., Gigon, F., Hassinen, A., Heijmans, M., Peressotti, A., Saarinen, T., van Breemen, N. and Wallen, B.  2001.  Spatial and temporal performance of the Mini-FACE (Free Air CO2 Enrichment) system on bog ecosystems in northern and central Europe.  Environmental Monitoring and Assessment 66: 107-127.

Morgan, J.A., Lecain, D.R., Moiser, A.R. and Milchunas, D.G.  2001.  Elevated CO2 enhances water relations and productivity and affects gas exchange in C3 and C4 grasses of the Colorado shortgrass steppe.  Global Change Biology 7: 451-466.