Background
The authors note that "increased LAI [leaf area index] in elevated CO2 results in earlier canopy closure, and thus in enhanced competition for light among shoots," which might be expected to lead to reduced growth stimulation by elevated CO2 following canopy closure.

What was done
Liberloo et al. studied the regrowth of three species of poplar tree - white poplar (Populus alba L., Clone 2AS-11), black poplar (Populus nigra L., Clone Jean Pourtet), and robusta poplar (Populus x euramericana (Dode) Guinier, Clone I-214) - under unfertilized and fertilized conditions for two years after coppicing in a FACE experiment conducted in central Italy, where the air's CO2 concentration was increased by approximately 160 ppm.  One of their stated objectives was to answer the question: "will a possible growth stimulation be sustained after canopy closure?"

What was learned
"During the first and second growing seasons after coppicing," in the words of the seven European scientists, "FACE significantly increased LAI," with relative differences between the CO2-enriched and control trees ranging from +1.7 to +38.7%, +4.7 to +38.5% and +3.9 to +45% for P. alba, P. nigra and P. x euramericana, respectively, under unfertilized and fertilized conditions, respectively.  In addition, they report that the increased LAI "supported increased aboveground biomass production," but only in the fertilized treatment.

What it means
According to the authors, their positive finding about sustained CO2-induced growth enhancement after canopy closure, if it continues to hold true in subsequent years, "will have important implications for the carbon balance of terrestrial ecosystems, because forests could behave as a larger carbon sink under future atmospheric conditions."