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Ozone (O3) is the primary air pollutant responsible for visible foliar injury and reduced growth in trees the world over. Most studies of the subject suggest it gains entrance to leaves through their stomata, whereupon it interferes with the process of photosynthesis and thereby reduces plant productivity. The global significance of the phenomenon was described in some detail by Fowler et al. (1999), who estimated O3 to have been negatively impacting a full quarter of earth’s forests at the close of the 20th century, and who calculated it to have the potential to negatively impact fully one-half of the planet’s forests by 2100.