What was done
The authors studied the response of whole-plant transpiration to atmospheric CO2 enrichment in the woody legume Acacia farnesiana (L.) Willd.  Plants were grown for one year in greenhouse bays continuously maintained at atmospheric CO2 concentrations of 385 and 980 ppm, after which whole-plant transpiration was assessed via sap flow measurements.  The CO2 concentration of the air surrounding the plants was then switched, so that the transpiration rate of the plants grown for a year at 385 ppm CO2 could be assessed under short-term exposure to air of 980 ppm CO2 and vice versa.

What was learned
When the plants that had been grown for a year at an atmospheric CO2 concentration of 385 ppm were briefly exposed to air of 980 ppm, their transpiration rate dropped to just a little over half of what it had been at 385 ppm.  Likewise, when the plants that had been grown for a year at an atmospheric CO2 concentration of 980 ppm were briefly exposed to air of 385 ppm, their long-term transpiration rate at 980 ppm CO2 also was found to be just a little over half of what it was during the plants' brief exposure to air of 385 ppm CO2.  When the transpiration rates of the plants were compared at their long-term growth concentrations, however, it was found that the transpiration rate of the plants that had been grown for a year at an atmospheric CO2 concentration of 980 ppm was only about a fourth of the transpiration rate of the plants that had been grown for a year at an atmospheric CO2 concentration of 385 ppm.  The long-term transpiration response of the plants to atmospheric CO2 enrichment was thus approximately twice as dramatic as their short-term response.

What it means
When assessing the impact of atmospheric CO2 enrichment upon a long-lived woody plant, it is important that the experiment be carried out over a long period of time, as it may take such plants a long time to acclimate themselves to a new CO2 concentration.  In the case of Acacia farnesiana, for example, which occurs throughout the south-central United States and is one of the most aggressive woody-plant invaders of grasslands worldwide, the long-term reduction in whole-plant transpiration caused by an increase in the air's CO2 concentration was twice as dramatic as what was observed over the first few days of a similar increase in atmospheric CO2 concentration.

So, if you bet that long-term acclimation to a large increase in atmospheric CO2 concentration would tend to reduce the immediate short-term effect of such a CO2 increase, you were wrong.  The effect got bigger with time and acclimation.  And this huge long-term -- and, hence, real-world-like -- CO2-induced reduction in the whole-plant transpiration rate of this species may well account for its observed increasing ascendancy over grasses (see Trees (Range Expansion) in our Subject Index) as the air's CO2 content continues to rise.