What was done
Working at the Swiss Canopy Crane facility in a species-rich deciduous forest 15 km south of Basel, Switzerland, where the 100-year-old stand reaches canopy heights of 30 to 35 meters, the authors measured light-saturated rates of net photosynthesis between 8:30 am and 12:20 pm at ambient (380 ppm) and elevated (550 ppm) atmospheric CO2 concentrations, the latter of which were maintained throughout all daylight periods over the course of the eighth growing season of their long-term study -- just as they had been similarly maintained over the prior seven years -- in three Quercus petraea trees, three Carpinus betulus trees, one Tilia platyphyllos tree and one Acer campestre tree, after which they compared the results of their measurements with those that had been obtained in earlier years of the experiment.

What was learned
Bader et al. report that the mean net photosynthetic rate of the upper-canopy foliage was 48% greater in the CO2-enriched foliage than in the ambient-treatment foliage in July and 42% greater in September, yielding an average increase of 45% in response to the 45% increase in the air's CO2 content, which is a truly amazing result. And they go on to say that "in the same stand, Zotz et al. (2005) found 36 and 49% photosynthetic enhancement in the mid and late growing season, respectively," which represents a mean response of 42.5%, which is actually slightly less than what Bader et al. observed most recently.

What it means
In the words of the three Swiss scientists, "the lack of photosynthetic down-regulation is consistent with the findings for mature and understory sweetgum trees growing at the ORNL- and Duke-FACE sites, respectively, and three poplar species growing at short-rotation coppice at the POP-FACE site, as well as for aspen and birch at the ASPEN-FACE stands (Herrick and Thomas, 2001; Sholtis et al., 2004; Liberloo et al., 2007; Uddling et al., 2009)." And in their final comment about their own study, they say their findings suggest that "the enhancement of photosynthesis may persist in these mature deciduous trees under high future atmospheric CO2 concentrations," while adding in the abstract of their paper that they will likely do so "without reductions in photosynthetic capacity."

References
Herrick, J.D. and Thomas, R.B. 2001. No photosynthetic down-regulation in sweetgum trees (Liquidambar styraciflua L.) after three years of CO2 enrichment at the Duke forest FACE experiment. Plant, Cell, and Environment 24: 53-64.

Liberloo, M., Tulva, I., Raim, O, Kull, O. and Ceulemans, R. 2007. Photosynthetic stimulation under long-term CO2 enrichment and fertilization is sustained across a closed Populus canopy profile (EUROFACE). New Phytologist 173: 537-549.

Sholtis, J.D., Gunderson, C.A., Norby, R.J. and Tissue, D.T. 2004. Persistent stimulation of photosynthesis by elevated CO2 in a sweetgum (Liquidambar styraciflua) forest stand. New Phytologist 162: 343-354.

Uddling, J., Teclaw, R.M., Kubiske, M.E., Pregitzer, K.S. and Ellsworth, D.S. 2008. Sap flux in pure aspen and mixed aspen-birch forests exposed to elevated concentrations of carbon dioxide and ozone. Tree Physiology 28: 1231-1243.

Zotz, G., Pepin, S. and Korner, C. 2005. No down-regulation of leaf photosynthesis in mature forest trees after three years of exposure to elevated CO2. Plant Biology 7: 369-374.