Background
Because atmospheric CO2 enrichment tends to reduce leaf stomatal apertures and can thereby curtail the rates of transpirational water loss from essentially all types of plants, it has long been speculated that the ongoing rise in the air's CO2 content will lead to higher soil moisture levels and greater amounts of water runoff to streams and rivers (Idso and Brazel, 1984).

What was done
To further assess the viability of this concept, the authors used three different sets of measurements (sap flow, soil moisture and canopy temperature) to quantify water savings under elevated CO2 in a 100-year-old mixed deciduous forest near Basel, Switzerland -- comprised primarily of Quercus petraea, Fagus sylvatica and Carpinus betulus -- which stood about 35 meters tall and was exposed to 540 ppm CO2 during the daylight hours of two 22-day periods in 2004 and 2005 through the use of free air CO2 enrichment (FACE) technology and the help of the Swiss Canopy Crane (SCC).

What was learned
Leuzinger and Korner write that their CO2 treatment resulted in "a ca. 14% reduction of tree transpiration during the growing season," which for comparison with the bulk of the world's CO2 enrichment studies that employ a 300-ppm increase in CO2 roughly equates to a growing-season transpiration reduction of 26%. In addition, they say that in line with this finding, "soil moisture at 10 cm depth decreased at a slower rate under high-CO2 trees than under control trees during rainless periods, with a reversal of this trend during prolonged drought when CO2-treated trees take advantage [of] initial water savings."

What it means
The two Swiss researchers conclude their paper by stating that whether the reduced rates of forest evapotranspiration they observed "will scale into greater run-off will depend on rainfall regimes and soil moisture storage capacity." In this regard, we merely note that the findings of Gedney et al. (2006) and Betts et al. (2007) would seem to suggest that, on a global basis, CO2-induced reductions in forest transpiration rates do indeed appear to lead to greater run-off and subsequent streamflow throughout the world.

References
Betts, R.A., Boucher, O., Collins, M., Cox, P.M., Falloon, P.D., Gedney, N., Hemming, D.L., Huntingford, C., Jones, C.D., Sexton, D.M.H. and Webb, M.J. 2007. Projected increase in continental runoff due to plant responses to increasing carbon dioxide. Nature 448: 1037-1041.

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.