Background
The 28 authors, hailing from seven different countries, write that "ground-level ozone (O3), if occurring at concentrations above the pre-industrial range, is potentially the most detrimental of all air pollutants for vegetation (Reich and Amundson, 1985; Bytnerowicz et al., 2003), including trees and forests (Matyssek and Sandermann, 2003)."

What was done
In what Matyssek et al. describe as "a unique 8-year free-air O3-fumigation experiment" that was conducted on adult beech (Fagus sylvatica) trees at Kranzberg Forest (Germany), five 60-year-old trees some 28 meters tall had 150 teflon tubes vertically suspended, at 0.5-m distances across their closed foliated canopy, through which O3 was released through pressure-calibrated capillary outlets at 0.3-m intervals at rates that resulted in a doubling of the ambient air's O3 concentration (2 xO3), while an equal number of untreated trees served as ambient (1xO3) controls.

What was learned
It was determined, in the words of the international research team, that "at stand level, the decline in stem productivity attributable to O3 was 10 m³/ha/year, on average, during the 8-year study period, representing a 44% loss relative to the 1xO3 trees."

What it means
Clearly, ozone well deserves to be called one of "the most detrimental of all air pollutants for vegetation, including trees and forests," as Matyssek et al. have written, and as they have found for themselves to be the case for beech trees. So what happens when one adds to a doubling of the atmosphere's debilitating ozone content a less-than-doubling of the air's CO2 concentration, which the U.S. Environmental "Protection" Agency (EPA) has recently labeled a dangerous air pollutant?

In earlier work that was also conducted with beech trees, Liu et al. (2005) found that "elevated CO2 overruled the effects of elevated O3 on non-structural carbohydrates," while Grams et al. (1999) found that "long-term acclimation to elevated CO2 supply counteracts the O3-induced decline of photosynthetic light and dark reactions," and Liu et al. (2004) found that all "adverse effects of ozone on carbohydrate concentrations and contents were counteracted when trees were grown in elevated CO2 [italics added]." And all of this really good news -- together with a host of other such findings archived under Growth Response to CO2 with Other Variables (Ozone) in our Subject Index -- makes us wonder how the EPA could possibly classify CO2 as a dangerous air pollutant, without completely taking leave of its collective senses.

The answer, of course, is simple. The facts be damned: political correctness rules!

References
Bytnerowicz, A., Arbaugh, M.J. and Alonso, R. 2003. Ozone air pollution in the Sierrra Nevada: distribution and effects on forests. In: Drupa, S.V. (Ed.), Developments in Environmental Science, Volume 2. Elsevier, p. 402.

Grams, T.E.E, Anegg, S., Haberle, K.-H., Langebartels, C. and Matyssek, R. 1999. Interactions of chronic exposure to elevated CO2 and O3 levels in the photosynthetic light and dark reactions of European beech (Fagus sylvatica). New Phytologist 144: 95-107.

Liu, X.-P., Grams, T.E.E., Matyssek, R. and Rennenberg, H. 2005. Effects of elevated pCO2 and/or pO3 on C-, N-, and S-metabolites in the leaves of juvenile beech and spruce differ between trees grown in monoculture and mixed culture. Plant Physiology and Biochemistry 43: 147-154.

Liu, X., Kozovits, A.R., Grams, T.E.E., Blaschke, H., Rennenberg, H. and Matyssek, R. 2004. Competition modifies effects of ozone/carbon dioxide concentrations on carbohydrate and biomass accumulation in juvenile Norway spruce and European beech. Tree Physiology 24: 1045-1055.

Matyssek, R. and Sandermann Jr., H. 2003. Impact of ozone on trees: an ecophysiological perspective. In: Progress in Botany, Volume 64. Springer-Verlag, Heidelberg, Germany, pp. 349-404.

Reich, P.B. and Amundson, R.G. 1985. Ambient levels of ozone reduce net photosynthesis in tree and crop species. Science 230: 566-570.