An early successional plant community was exposed to various ozone concentrations for two growing seasons (1994)(1995) in open-top chambers in Auburn, Alabama, USA. The ozone treatments were : AA, ambient air (open plots) ; CF, carbon-filtered air (c. 0n5iambient air), 1i, non-filtered air, and 2i, twice ambient air. Vegetative canopy cover exhibited a pattern of accumulation in the spring, with maximum canopy cover attained in summer, then senescence of foliage in the autumn 1994. This pattern was not observed in 1995 as a result of a drought during the spring and summer. Varying ozone exposures caused shifts in the competitive interactions between plants, thereby altering community structure. Higher canopy cover, vertical canopy density (layers of foliage), species richness, diversity, and evenness existed in the CF treatments than in the other treatments. In addition, winged sumac (Rhus copallina L.) became a major component of the CF treatments only during 1995. Surprisingly, blackberry (Rubus cuneifolius Pursh.), a species considered ozone-sensitive, based on visible injury, dominated canopy cover within the 2i treatments, 41 and 33 % of total canopy cover in 1994 and 1995, respectively. From these results it is concluded that plant communities existing in areas where lower ozone concentrations are prevalent might be more complex and diverse than those existing in areas with higher ozone concentrations.

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