What was done
The authors enriched the air's CO2 concentration within the canopy of a 30-meter-tall species-rich forest just south of Basel, Switzerland, via the web-FACE technique of Pepin and Korner (2002) to a mean value of 520 ppm for an entire growing season to test, in their words, "whether elevated CO2 reduces water use in mature forest trees," making sap flow measurements of 14 broadleaved trees (3 Fagus, 4 Quercus, 4 Carpinus, 1 Tilia, 1 Acer and 1 Prunus) and their ambient-treatment counterparts via the constant heat-flow technique of Granier (1985, 1987) from 5 June to 1 October 2001.

What was learned
Over the entire growing season, the extra 150 ppm of CO2 reduced mean daily sap flow across all species by an average of 10.7%.  The reductions were high (22%) when the evaporative demand of the air was low (mean daily vapor pressure deficits less than 5 hPa) and small (2%) when the evaporative demand of the air was high (mean daily vapor pressure deficits greater than 10 hPa).

What it means
The authors say their results "suggest that daily water savings by CO2-enriched trees may have accumulated to [produce] a significantly improved water status by the time when control trees were short of soil moisture," such that "CO2-enriched trees would enter drier periods with a higher soil moisture capital, permitting prolonged gas exchange (for a few days)."

References
Granier, A.  1985.  A new method of sap flow measurement in tree stems.  Ann Sci For 42: 193-200.

Granier, A.  1987.  Evaluation of transpiration in a Douglas fir stand by means of sap flow measurements.  Tree Physiology 3: 309-320.

Pepin, S. and Korner, C.  2002.  web-FACE: a new canopy free-air CO2 enrichment system for tall trees in mature forests.  Oecologia 133: 1-9.