What was done
The authors utilized the same trees as those employed by Norby et al. (2001) in their FACE study of the growth of a closed canopy of sweetgum (Liquidambar styraciflua L.) trees that were planted in the spring of 1988 and began receiving fumigations of air of different CO2 concentrations -- 394 ppm (ambient) and 533 ppm (enriched) -- in April of 1998.  In this study, which extended from April through September of 1999, they used a compensated heat-pulse technique to measure the sap velocity of four trees in each of two ambient and two elevated CO2 FACE rings.  Using trees just outside the study plots, they also determined the mean sapwood areas of trees of the same sizes as those inside the rings; and with the same heat-pulse technique they determined the fractions of the sapwood that were active in transporting water within those trees.  From this information, as well as knowledge of the stand densities of the trees within the FACE rings, they calculated rates of whole-canopy transpiration for the half-year period of the study.

What was learned
Averaged across the six-month growing season, sap velocities in the CO2-enriched trees were 13% less than those in the ambient-treatment trees, which led to an approximate 10% reduction in season-long stand transpiration in the CO2-enriched plots.  Over this same time period, aboveground tree biomass increased by 15% more in the enriched plots than in the ambient plots; and combining this CO2-induced increase in growth with the CO2-induced decrease in transpiration indicates that stand water use efficiency was increased by fully 28% by the 35% increase in atmospheric CO2 concentration employed in the study.

What it means
This FACE study of a real-world monoculture stand of 12-year-old sweetgum trees clearly demonstrates that a modest 35% increase in atmospheric CO2 concentration can enhance the water use efficiency of a closed-canopy forest by an almost equal amount (28% in this specific instance).  If other trees respond similarly -- and there are many reasons for believing that they do (see the various Journal Reviews dealing with trees that are archived under the heading Water Use Efficiency in our Subject Index) -- this effect should have tremendous positive implications for the ability of earth's trees to cope with stresses induced by inadequate levels of soil moisture availability as the air's CO2 content continues to rise.  If the climate of the planet warms and dries, for example, earth's forests should be better able to hold their own; while if things climatic remain pretty much the same, many areas not presently able to sustain forests should be able to do so in the future.