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Effects of Elevated O3 and CO2 on Yellow Poplar Seedlings Over Five Seasons
Rebbeck, J., Scherzer, A.J. and Loats, K.V.  2004.  Foliar physiology of yellow-poplar (Liriodendron tulipifera L.) exposed to O3 and elevated CO2 over five seasons.  Trees 18: 253-263.

The authors note that studies of the combined effects of elevated ozone (O3) and carbon dioxide (CO2) on long-lived woody plants have been conducted before, but that "few have reported results for more than two or three seasons," and that few such studies of trees have been done "during their development from seedlings to saplings or mature trees."  Consequently, their five-year study - which does both of these things - is rather unique.

What was done
Rebbeck et al. grew yellow poplar (Liriodendron tulipifera L.) seedlings for five years within open-top chambers in a field plantation at the Northeastern Research Station's Forestry Sciences Laboratory at Delaware, Ohio, USA, exposing them continuously from mid-May through mid-October of each year to either (1) charcoal-filtered (CF) air to remove ambient O3, (2) ambient O3, (3) 1.5 times ambient O3, and (4) 1.5 times ambient O3 plus 350 ppm CO2 above ambient CO2, (target concentration of 700 ppm CO2), while they periodically measured a number of plant parameters and processes.  Throughout the study, the trees were never fertilized, and they received no supplemental water beyond some given in the first season.

What was learned
Averaged over the study's five growing seasons, the midseason net photosynthetic rate of upper canopy foliage at saturating light intensities declined by 10% when the trees were grown in ambient O3-air and by 14% when they were grown in elevated O3-air, when compared to the trees that were grown in the charcoal-filtered air, while seasonal net photosynthesis of foliage grown in the combination of elevated O3 and elevated CO2 was 57-80% higher than it was in the trees exposed to elevated O3 alone.  There was also no evidence of any photosynthetic down regulation in the trees exposed to the combination of elevated O3 and CO2, with some of the highest rates being observed during the final growing season.

What it means
In the words of the authors, "the results of this five-season field study suggest that elevated CO2 may ameliorate the negative effects of increased tropospheric O3 on yellow-poplar."  Indeed, their results suggest that a nominally doubled atmospheric CO2 concentration more than compensates for the deleterious effects of a 50% increase in ambient O3 levels ? and by several times over.

Reviewed 1 September 2004