What was done
Liu et al. grew 3- and 4-year-old European beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) Karst.) seedlings for five months in well-watered and well-fertilized soil in containers located within walk-in phytotrons maintained at either ambient or ambient + 300 ppm CO2 (each subdivided into ambient and double-ambient ozone concentration treatments, with maximum ozone levels restricted to <150 ppb), in both monoculture and in competition with each other.  At the end of the study, the authors examined the effect of each treatment on leaf non-structural carbohydrate levels (soluble sugars and starch).

What was learned
The effects of elevated O3 alone on non-structural carbohydrate levels were small for both beech and spruce when grown in monoculture, as well as for spruce when grown in mixed culture.  When beech was grown in mixed culture, however, elevated O3 slightly enhanced its leaf sugar levels, but reduced its starch levels by a full 50%.  With respect to elevated CO2 alone, for beech grown in monoculture and for beech and spruce grown in mixed culture, levels of sugar and starch were significantly enhanced.  When spruce was grown in monoculture, however, its starch levels were significantly enhanced, but its sugar levels remained unchanged.  Consequently, when they significantly affected non-structural carbohydrate levels, elevated CO2 tended to enhance them, whereas elevated O3 tended to degrade them.

The combined effects of elevated CO2 and O3 acting together were shown to produce a significant increase in leaf non-structural carbohydrates of both tree species under both mixed and monoculture conditions, which was similar to what was observed under CO2 enrichment alone.  As a result, the authors concluded that "since the responses to the combined exposure were more similar to elevated pCO2 than to elevated pO3, apparently elevated pCO2 overruled the effects of elevated pO3 on non-structural carbohydrates."

What it means
In the words of the authors, "elevated pO3 is considered to be one of the most detrimental environmental factors for plant growth and development."  Yet, at the concentrations used in this study, elevated O3's only negative impact was to decrease the starch levels of beech trees growing in mixed culture.  As for why a negative effect was not more widespread, the authors postulate it may be due to the fact that although elevated O3 reduces photosynthetic efficiency, it also limits phloem-loading of sugars, which "limitation of carbohydrate export from the leaves is likely to be more pronounced than O3-reduction of photosynthesis, thereby, enhancing leaf non-structural carbohydrate levels."  In any event, it would appear that when the growth-enhancing effects of elevated CO2 are factored into the equation, O3's detrimental effects may sometimes be totally thwarted.  These results are certainly good news for the biosphere (and especially for beech and spruce trees).