Background
The authors write that "with very few exceptions, deceased stomatal conductance (gs) is one of the most consistent and conserved responses of leaves to growth at elevated CO2," but they go on to note that "less well understood ... is the extent to which leaf-level responses translate to changes in ecosystem evapotranspiration (ET)," and, hence, that it "is not certain that a decrease in gs will decrease ET in rain-fed crops." To answer this very basic and important question, therefore, was the primary purpose of their study.

What was done
The five researchers grew soybean (Glycine max) under field conditions in Free-Air CO2 Enrichment (FACE) plots maintained at ambient (~375 ppm) and elevated (~550 ppm) atmospheric CO2 concentrations for four complete growing seasons at the SoyFACE facility in central Illinois (USA), making season-long measurements of both leaf gs and canopy ET once complete canopy closure had been achieved, after which they conducted a number of analyses of the data thereby obtained.

What was learned
Bernacchi et al. report that "elevated CO2 caused ET to decrease between 9% and 16% depending on year and despite large increases in photosynthesis and seed yield," and that "ecosystem ET was linked with gs of the upper canopy leaves when averaged across the growing seasons, such that a 10% decrease in gs results in a 8.6% decrease in ET."

What it means
In discussing the implications of their meticulous and voluminous field observations, the researchers confirm that their findings "are consistent with model and historical analyses that suggest that ... decreased gs of upper canopy leaves at elevated CO2 results in decreased transfer of water vapor to the atmosphere." They also note that their findings are "consistent with the recent mechanistic modeling and statistical fingerprinting analysis of global trends in increasing river discharge across the 20th century [our italics]," which suggests last of all, in their words, that "suppression of plant transpiration due to the effect of rising CO2 on stomatal closure is the most consistent factor in explaining observed [river discharge] changes (Gedney et al., 2006)." And this conclusion, in turn, finally confirms what was suggested to be the case well over two decades ago by Idso and Brazel (1984) in a paper entitled "Rising atmospheric carbon dioxide concentrations may increase streamflow."

References
Gedney, N., Cox, P.M., Betts, R.A., Boucher, O., Huntingford, C. and Stott, P.A. 2006. Detection of a direct carbon dioxide effect in continental river runoff records. Nature 439: 835-838.

Idso, S.B. and Brazel, A.J. 1984. Rising atmospheric carbon dioxide concentrations may increase streamflow. Nature 312: 51-53.