Background
Plants grown for long periods of time in CO2-enriched air sometimes undergo a process of acclimation, whereby they exhibit a lower degree of photosynthetic stimulation than they did when first exposed to the higher-than-normal atmospheric CO2 concentration.  However, as the authors of this paper note, "down-regulating responses are often due to restrictive experimental conditions such as growth in pots which do not allow a free development of belowground biomass or growth in chambers which affects the normal development of the aerial portion and/or indirectly modifies the microclimate."

What was done
Working with a number of Robusta poplar (Populus x euramericana) trees that had been growing in a FACE experiment (which restricts neither above- nor below-ground growth) in central Italy for a period of five years, Calfapietra et al. measured their photosynthetic responses to an approximate 200-ppm increase in the air's CO2 concentration in mid-July of the study's fifth year, comparing their results with what was observed at the beginning of the experiment, both with and without supplemental nitrogen fertilization.

What was learned
In the words of the authors, "even after such a long period of exposure, leaves of Populus x euramericana have not shown clear signs of photosynthetic acclimation."  In addition, they report that "FACE significantly decreased stomatal conductance both on upper and lower canopy leaves," which together with the continued stimulation of photosynthesis implies a significant sustained increase in leaf water use efficiency throughout the trees' canopies.

What it means
In the concluding sentence of their paper, Calfapietra et al. say their results "suggest that the photosynthetic acclimation of poplar plantations is unlikely to occur in an atmosphere enriched in CO2 and thereby will not influence the response of poplar plantations to increasing atmospheric CO2 concentrations either over the long term or under conditions of nitrogen deposition."