Background
The authors write that "some researchers report that down-regulation of photosynthesis under elevated CO2 is strongly linked to an increased carbon:nitrogen ratio of the photosynthesizing leaves, when the increased uptake of CO2 cannot be matched by a sufficient nutrient supply," which is the crux of the progressive nitrogen limitation hypothesis. But is this concept correct?

What was done
Working with photosynthesis data they and others collected over a period of eleven years at the Aspen FACE site near Rhinelander, Wisconsin (USA), Darbah et al. evaluated this hypothesis for two different quaking aspen (Populus tremuloides Michx.) clones (42E and 271), which were exposed to all combinations of ambient and elevated (560 ppm) CO2 and ambient and elevated (1.5 times ambient) ozone (O3). And as an added bonus, they investigated the same idea to see if it might apply to leaf stomatal conductance.

What was learned
In a crisp and clear report of what they learned, the eight researchers say their results "suggest no long-term photosynthetic and stomatal acclimation to elevated CO2, O3 or CO2 + O3 in aspen trees exposed to elevated CO2 and/or O3 gases for 11 years." And they add that the aspen trees "have sustained their maximum instantaneous photosynthesis stimulation for over a decade."

In commenting on their findings, Darbah et al. say they support the observations of (1) Liberloo et al. (2007), who measured a 49% increase in net photosynthetic rate in poplar trees after six years of exposure to elevated CO2, (2) the findings of Sholtis et al. (2004), who reported a 44% stimulation of net photosynthesis in sweetgum trees after three years of exposure to elevated CO2, (3) Crous and Ellsworth (2004), who found a photosynthetic enhancement of 51-69% in Pinus taeda trees after six years of exposure to elevated CO2, as well as (4) Davey et al. (2006) and (5) Paoletti et al. (2007), of whose work Darbah et al. state "there was no photosynthetic acclimation (down-regulation) occurring in Quercus ilex under long-term CO2 enrichment." In addition, they remark that (6) even in white clover (Trifolium repens), Ainsworth et al. (2003) found that photosynthetic stimulation "remained after nine years of exposure to elevated CO2."

What it means
As ever more long-term experiments are conducted on long-lived plants growing out-of-doors and rooted in the earth, where their roots are not artificially confined to a limited volume of soil, it is becoming abundantly clear that they generally do not experience any significant decline in the initial photosynthetic stimulation provided them by the extra CO2 to which they are exposed in CO2 enrichment studies.

References
Ainsworth, A.E., Rogers, A., Blum, H., Nosberger, J. and Long, S.P. 2003. Variation in acclimation of photosynthesis in Trifolium repens after eight years of exposure to free air CO2 enrichment (FACE). Journal of Experimental Botany 54: 2769-2774.

Crous, K.Y. and Ellsworth, D.S. 2004. Canopy position affects photosynthetic adjustments to long-term elevated CO2 concentration (FACE) in aging needles in a mature Pinus taeda forest. Tree Physiology 24: 961-970.

Davey, P.A., Olcer, H., Zakhleniuk, O., Bernacchi, C.J., Calfapietra, C., Long, S.P. and Raines, C.A. 2006. Can fast growing plantation trees escape biochemical down-regulation of photosynthesis when growing throughout their complete production cycle in the open air under elevated carbon dioxide? Plant, Cell and Environment 29: 1235-1244.

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.

Paoletti, E., Seufert, G., Della Rocca, G. and Thomsen, H. 2007. Photosynthetic response to elevated CO2 and O3 in Quercus ilex leaves at a natural CO2 spring. Environmental Pollution 147: 516-524.

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: 243-254.