Background
In spite of the fact that "ozone (O3) is the most important phytotoxic gaseous air pollutant with a regional distribution," and that "it is the cause of widespread occurrence of visible symptoms of injury on plants across Europe," Volk et al. note that "the use of unenclosed O3 fumigation systems has so far been restricted to the investigation of effects on trees and agricultural crops," and that "in spite of a growing amount of data characterizing the specific sensitivity to O3 of species typical of seminatural grasslands, it remains highly uncertain how effects observed with individual plants under controlled or semicontrolled conditions relate to effects at the community level in a natural environment."

What was done
To fill the important research void noted above, the five Swiss scientists conducted the first-ever study to use, in their words, "a newly designed, chamberless free-air fumigation system...to determine the long-term response, in terms of productivity and functional group composition, of a 30-year-old temperate grassland community under low intensity management to moderately increased O3 exposure [1.5 x ambient] based on a 5-year study (1999-2003)."

What was learned
After adjusting for slight differences in initial yields of the ambient-air and O3-enriched plots, Volk et al. report that "the trend in normalized yields revealed growing differences between treatments," such that although there was little difference between the plant dry matter production of the two treatments the following year, by the end of the final year of the study the annual dry matter production of the O3-fumigated plots was fully 31% less than that of the ambient-air plots. In addition, they found that the percentage of legumes in the grassland community dropped from an initial value of 12% to a final value of 4%.

What it means
The researchers say their results "show that a moderately elevated O3 level reduces the productivity of intact grasslands during a 5-year exposure under real field conditions," and that "this loss may become even larger if O3 stress continues." With respect to the O3-induced loss of legumes, they conclude that this phenomenon "could have negative implications for the maintenance of biological diversity in rural landscapes across large areas of Europe." In addition, they note that "through the effect on legumes, which contribute nitrogen via symbiotic N2 fixation, O3 stress may affect the plant community by lowering soil fertility," which could exacerbate the decline in productivity driven by the more direct effects of O3 pollution. Hence, we can be thankful indeed that the air's CO2 concentration continues to rise, for the many materials archived under Growth Response to CO2 with Other Variables (Ozone) in our Subject Index demonstrate that the negative effects of O3 pollution are generally totally thwarted by the positive effects of atmospheric CO2 enrichment.