What was done
The authors grew initially-fertilized Avena barbata Link (slender oat) and Hemizonia congesta DC. ssp. luzulifolia (DC) Babe. & H.M. Hall (an annual C3 forb) from seed for 143 days in 95-cm-deep by 15-cm-diameter microcosms filled with a 1:1 mix of sand and field soil placed within open-top chambers maintained at ambient and approximately double-ambient atmospheric CO2 concentrations at the Jasper Ridge Biological Preserve near Woodside, California, USA, while measuring a number of plant physiological parameters, soil water parameters and, at the end of the study, final biomass production.

What was learned
Defining intrinsic water use efficiency as the ratio of assimilation rate (A) to stomatal conductance (g), Moore and Field report that A/g rose by 40% in A. barbata and by 114% in H. congesta in response to their approximate doubling of the air's CO2 content, while at the end of the experiment yield water use efficiency (the ratio of total biomass produced to water lost during the season) was determined to have been increased by 15% in A. barbata and by 41% in H. congesta in the doubled-CO2 treatment. As an integral part of this favorable phenomenon, aboveground and belowground biomass production were increased by 1% and 35%, respectively, in A. barbata and by 37% and 69% in H. congesta. In addition, Moore and Field found that for A. barbata, "increases in water use efficiency under elevated CO2 translated into water savings and increased soil moisture," a phenomenon that the two researchers say "has been widely reported in this and other grassland systems (Field et al., 1997; Niklaus et al., 1998; Owensby et al., 1999; Lund, 2002)." What is more, noting that "the A/g values from this experiment fall in line with the [linear] relationship between A/g and a broad range of CO2 concentrations measured in a water-limited grassland with completely different species composition (Anderson et al., 2001; Polley et al., 2002)," they repeat the suggestion of the researchers involved in that work that "water-limited ecosystems will continue responding to increasing CO2."

What it means
As the air's CO2 content continues to rise, we can expect to see significant continued increases in the productivities and water use efficiencies of the world's grasslands.

References
Anderson, L.J., Maherali, H., Johnson, H.B., Polley H.W. and Jackson R.B. 2001. Gas exchange and photosynthetic acclimation over subambient to elevated CO2 in a C-3-C-4 grassland. Global Change Biology 7: 693-707.

Field, C.B., Lund, C.P., Chiariello, N.R. and Mortimer, B.E. 1997. CO2 effects on the water budget of grassland microcosm communities. Global Change Biology 3: 197-206.

Lund, C.P. 2002. Ecosystem Carbon and Water Budgets under Elevated Atmospheric Carbon Dioxide Concentration in Two California Grasslands. Ph.D. Thesis, Stanford University.

Niklaus, P.A., Spinnler, D. and Korner, C. 1998. Soil moisture dynamics of calcareous grassland under elevated CO2. Oecologia 117: 201-208.

Owensby, C.E., Ham, J.M., Knapp, A.K. and Auen, L.M. 1999. Biomass production and species composition change in a tallgrass prairie ecosystem after long-term exposure to elevated atmospheric CO2. Global Change Biology 5: 497-506.

Polley, H.W., Johnson, H.B. and Derner, J.D. 2002. Soil and plant-water dynamics in a C3/C4 grassland exposed to a subambient to superambient CO2 gradient. Global Change Biology 8: 1118-1129.