Background
The authors write that "in 2008, atmospheric CO2 concentrations from the Mauna Loa, Hawaii, Observatory records exceeded 385 ppm, representing a 22% increase since 1959," and they say that "as CO2 has increased, most tree species have been able to use water more efficiently," as their "leaf stomatal apertures narrow during photosynthesis," resulting in "less transpirational water loss per biomass gained." The parameter representing this phenomenon is referred to as intrinsic water-use efficiency or iWUE (defined as the ratio of net CO2 assimilation to stomatal conductance," and it has been documented, as they describe it, "for various tree species in many parts of the world," citing in support of this statement the findings of Bert et al. (1997), Feng (1999), Tang et al. (1999), Arneth et al. (2002), Saurer et al. (2004), Waterhouse et al. (2004) and Liu et al. (2007).

What was done
The authors examined changes in -- and relationships between -- radial growth rates and the iWUE of ponderosa pine (Pinus ponderosa) trees, climate and atmospheric CO2 concentration in the western United States since the mid-nineteenth century, developing tree-ring chronologies for eight sites in three climate regions, while using carbon isotope data to calculate pentadal values of iWUE, after which they examined relationships among radial growth, climate, iWUE and CO2 via correlation and regression analyses.

What was learned
Soule and Knapp report finding significant upward trends in iWUE at all sites; and they say that "despite an absence of climate changes that would favor growth," upward radial growth trends occurred at five sites. In addition, they say that the highest iWUE values "were recorded in the last pentad at six of eight sites and follow a positive quadratic progression at all sites, suggesting that future increases in iWUE are likely for ponderosa pine within our study regions as CO2 levels increase." And they remark that they found "significant improvements in radial growth rates during drought years after 1950," when the air's CO2 content rose at an accelerating rate.

What it means
The two U.S. researchers say their findings suggest that "increased iWUE associated with rising CO2 can positively impact tree growth rates in the western United States and are thus an evolving component of forest ecosystem processes." And they conclude that "if potential climate changes lead to increasing aridity in the western United States, additional increases in iWUE associated with future increases in CO2 might ameliorate growth declines associated with drought conditions."

References
Arneth, A., Lloyd, J., Santruckova, H., Bird, M., Girgoryev, S., Kalaschnikov, Y.N., Gleixner, G. and Schulze, E. 2002. Response of central Siberian Scots pine to soil water deficit and long-term trends in atmospheric CO2 concentration. Global Biogeochemical Cycles 16: 10.1029/2000GB001374.

Bert, D., Leavitt, S. and Dupouey, J.-L. 1997. Variations of wood δ¹³C and water-use efficiency of Abies alba during the last century. Ecology 78: 1588-1596.

Feng, X. 1999. Trends in intrinsic water-use efficiency of natural trees for the past 100-200 years: A response to atmospheric CO2 concentration. Geochimica et Cosmochimica Acta 63: 1891-1903.

Liu, X., Shao, X., Liang, E., Zhao, L., Chen, T., Qin, D. and Ren, J. 2007. Species dependent responses of juniper and spruce to increasing CO2 concentration and to climate in semi-arid and arid areas of northwestern China. Plant Ecology 193: 195-209.

Saurer, M., Siegwolf, R. and Schweingruber, F. 2004. Carbon isotope discrimination indicates improving water-use efficiency of trees in northern Eurasia over the last 100 years. Global Change Biology 10: 2109-2120.

Tang, K., Feng, X. and Funkhouser, G. 1999. The δ¹³C of trees in full-bark and strip-bark bristlecone pine trees in the White Mountains of California. Global Change Biology 5: 33-40.

Waterhouse, J., Switsur, V., Barker, A., Carter, A., Hemming, D., Loader, N. and Robertson, I. 2004. Northern European trees show a progressively diminishing response to increasing atmospheric carbon dioxide concentrations Quaternary Science Reviews 23: 803-810.