Background
Well over a quarter of a century ago -- in a paper entitled "Rising atmospheric carbon dioxide concentrations may increase streamflow" -- Idso and Brazel (1984) suggested that CO2-induced streamflow increases were already occurring, based on their study of twelve drainage basins in Arizona, USA, and as a consequence of CO2-induced reductions in plant transpirational water losses directly to the atmosphere. Twenty-two years later, Gedney et al. (2006) confirmed that such was also true for the globe as a whole; and one year after that development, Betts et al. (2007) demonstrated that this welcome phenomenon may be reproduced by an ensemble of simulations with a global climate model that included "a vegetation component to assess the contribution of physiological forcing to future changes in continental runoff," finding that "the physiological effect of doubled carbon dioxide concentrations on plant transpiration increases simulated global mean runoff by six percent relative to pre-industrial levels."

What was done
Building upon this historical foundation, Warren et al. used "a combination of measurements, synthesis of existing literature, and modeling to address the consequences of climate change on ecohydrologic processes in forests, especially [their] response to elevated CO2 (eCO2)," employing data that had been obtained from five free-air CO2 enrichment or FACE studies that had been conducted in temperate forest ecosystems.

What was learned
The six scientists report that at two of the FACE sites, where the studied forests "were in the early stages of stand development," there was "a strong eCO2-stimulation of canopy leaf area [that] led to enhanced stand water use." In the three closed-canopy forests, on the other hand, they say that "eCO2-reduced stomatal conductance led to declines in canopy transpiration and stand water use." In the sweetgum FACE experiment conducted in Oak Ridge, Tennessee (USA), where most of their work was conducted, for example, they found that eCO2 reduced seasonal transpiration by 10-16%.

What it means
Warren et al. conclude that "the direct effect of eCO2 on forest water balance through reductions in transpiration could be considerable, especially following canopy closure and development of maximal leaf area index," noting that in the case of temperate deciduous forests, "reductions in canopy transpiration and stand water use due to direct effects of CO2 on stomatal conductance will be reflected in increased soil water content and potential water yield," which should result in significant contributions to groundwater, streams, lakes and rivers.

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.