What was done
Noting that "few studies have been completed where trees are allowed to develop to canopy closure and where a 'stable' response to CO2 is likely," the authors actually reach that elusive experimental state in a study of various properties of the leaves of three Populus genotypes - P. alba, P. nigra and P. deltoides x P. nigra = P. euramericana - at the POPFACE facility of Miglietta et al. (2001) in central Italy, where trees that grew to canopy closure were exposed to a target CO2 concentration of 550 ppm over a period of three years.

What was learned
Tricker et al. report that leaf area index was enhanced by the elevated CO2 up until the point of canopy closure, and that "increases in leaf size almost certainly contributed to this effect."  After canopy closure, however, they found that leaf area index was no longer significantly stimulated; but specific leaf area (SLA, leaf area per mass) declined, as leaves thickened and "bulked up," packing more mass between the upper and lower surfaces of each unit area of leaf surface.  Specifically, and averaged across all species studied, SLA declined by 29% and 5%, respectively, in sun and shade leaves in the CO2-enriched air compared to ambient air.  Also, they report that "autumnal senescence was delayed in elevated CO2 with a 10% increase in the number of days at which 50% leaf loss occurred in elevated as compared with ambient CO2."  Together with increased net photosynthesis rates (Bernacchi et al., 2003), these several phenomena led to significant increases in above-ground harvested biomass in all three species at the conclusion of the experiment (Calfapietra et al., 2003).

What it means
In their concluding sentence, the group of ten British and Italian scientists state that "a continued enhancement of biomass productivity will be likely when intensively managed forest plantations are exposed to a future rising concentration of atmospheric CO2."  We agree.

References
Bernacchi, C., Calfapietra, C., Davey, P., Wittig, V., Raines, C. and Long, S.P.  2003.  Photosynthesis and stomatal conductance responses of poplars to free-air CO2 enrichment (POPFACE) during the first growth cycle and immediately following coppice.  New Phytologist 159: 609-621.

Calfapietra, C., Gielen, B., Galema, A.N.J., Lukac, M., Moscatelli, M.C., Ceulemans, R. and Scarascia-Mugnozza, G.  2003.  Free-air CO2 enrichment (FACE) enhances biomass production in a short-rotation poplar plantation (POPFACE).  Tree Physiology 23: 805-814.

Miglietta, F., Peresotti, A., Vaccari, F.P., Zaldei, A., de Angelis, P. and Scarascia-Mugnozza, G.  2001.  Free-air CO2 enrichment (FACE) of a poplar plantation: the POPFACE fumigation system.  New Phytologist 150: 465-476.