What was done
In the spring (November) of 2000 and 2001 in a FACE study conducted in a temperate pasture on the west coast of the North Island of New Zealand, Allard et al. measured changes in the nitrogen [N] concentrations of the leaves of the several different species that comprised the pasture, as well as changes in species composition, that arose in response to the imposition of an approximate 105-ppm increase in atmospheric CO2 concentration, after which they determined the implications of these changes for the digestibility of the total mix of pasture forage.

What was learned
The researchers report that "under elevated CO2, leaves of the individual species exhibited lower N concentrations and higher water-soluble carbohydrate (WSC) concentrations."  They also discovered "there was a significantly greater proportion of legume in the diet at elevated CO2."  In addition, they found that this "shift in the botanical composition towards a higher proportion of legumes counterbalanced the N decrease observed at the single species scale, resulting in an N concentration of the overall diet that was unaffected by elevated CO2."

In contrast to this ultimate balancing of CO2-induced nitrogen increases and decreases, Allard et al. report that "changes at the species level and at the sward level appeared to combine additively in relation to WSC."  Hence, they note that "as there was a significant correlation between WSC and digestibility (as previously observed by Dent and Aldrich, 1963 and Humphreys, 1989), there was also an increase in digestibility of the high CO2 forage," which result, in their words, "matches that found in a Mini-FACE experiment under cutting (Teyssonneyre, 2002; Picon-Cochard et al., 2003)," where "digestibility also increased in response to CO2 despite reduced crude protein concentration."

What it means
In the words of the authors, "these data, and the strong relationship between soluble sugars (rather than N) and digestibility suggest that the widespread response to CO2 of increased soluble sugars might lead to an increase in forage digestibility [generally]."

References
Dent, J.W. and Aldrich, D.T.A.  1963.  The inter-relationships between heading date, yield, chemical composition and digestibility in varieties of perennial ryegrass, timothy, cooksfoot and meadow fescue.  Journal of the National Institute of Agricultural Botany 9: 261-281.

Humphreys, M.O.  1989.  Water-soluble carbohydrates in perennial ryegrass breeding. III. Relationships with herbage production, digestibility and crude protein content.  Grass and Forage Science 44: 423-430.

Picon-Cochard, C., Teyssonneyre, F., Besle, J.M. et al.  2003.  Effects of elevated CO2 and cutting frequency on the productivity and herbage quality of a semi-natural grassland.  European Journal of Agronomy, in press.

Teyssonneyre, F.  2002.  Effet d'une augmentation de la concentration atmospherique en CO2 sur la prairie permanete et sur la competition entre especes prairiales associees.  PhD thesis, Orsay, Paris XI, France.