What was done
Working at the EUROFACE facility in central Italy near Viterbo, the authors grew three different species of poplar tree - robusta poplar (Populus x euramericana), white poplar (P. alba) and black poplar (P. nigra) - for two three-year periods, between which the trees were coppiced and allowed to re-grow, in either ambient air or air enriched with an extra 180 ppm of CO2 (an approximate 49% enhancement). No fertilization was applied to any of the plots over the first growth cycle, while half of the trees were fertilized over the second growth cycle. Then, during the last year of the last cycle, a number of plant processes and parameters were measured and compared with similar observations made throughout earlier years of the experiment.

What was learned
Liberloo et al. report that after six years of growth under elevated atmospheric CO2, the poplar trees did not experience any down-regulation of leaf net photosynthesis. And the long-term stimulation was huge. In response to the 49% increase in the atmosphere's CO2 concentration, the CO2-induced stimulation of net photosynthesis, averaged over the three species, was also 49%. In addition, they report there was no difference in CO2-induced net photosynthetic stimulation between sun and shade leaves, nor was there any difference in CO2 effects between the fertilized and non-fertilized trees.

What it means
Finding that "photosynthetic stimulation of poplar leaves was sustained in elevated CO2 after six years of fumigation, even under non-fertilized conditions," the five researchers concluded that "a sustained stimulation of photosynthesis in a future high-CO2 world ... could increase the feasibility of short-rotation bioenergy poplar plantations," and that "these results give optimistic perspectives for the future, as the maintained enhancement of photosynthesis in poplar trees is likely to continue over several rotations, thereby providing more carbon for growth in a closed canopy forest."