Background
The authors write that "levels of atmospheric CO2 and O3 have increased rapidly in the last five decades," and they say that "it is predicted that at the end of this century, the average levels of CO2 and O3 in the Earth's atmosphere are going to reach 700 ppm and 80 ppb, respectively (IPCC, 2007)."

What was done
In an experiment designed to evaluate the opposing effects of these two atmospheric trace gases on Chinese pine (Pinus tabulaeformis) trees at the year AD 2100, Xu et al. grew four-year-old trees in loamy soil with no extra fertilizer within twelve open-top chambers in May 2006 within the populated central area of Shenyang city in northeastern China, where the trees were exposed to either current ambient air of about 400 ppm CO2 and 40 ppb O3 or 700 ppm CO2 and 80 ppm O3, plus all combinations thereof.

What was learned
The four Chinese researchers report that elevated CO2 by itself "did not significantly affect net photosynthetic rate, stomatal conductance, chlorophyll content, the maximum quantum yield of photosystem II, or the effective quantum yield of photosystem II electron transport after 90 days of gas exposure." But they did find that it increased growth. Elevated O3 by itself, on the other hand, "decreased growth, net photosynthetic rate and stomatal conductance after 90 days of exposure," but they say that "its negative effects were alleviated by elevated CO2 [italics added]."

What it means
In taking stock of what was found, elevated CO2 had no negative effects and only one positive effect; but the latter was an important one on growth. Ozone, on the other hand, had multiple negative effects, but they were all alleviated by elevated CO2; and these findings suggest a huge win for CO2 in its battle with O3 to impact the growth and development of Chinese pine trees.

Reference
IPCC. 2007. Climate Change 2007. Working Group I Report: The Physical Basis of Climate Change. IPCC, Geneva, Switzerland.