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The effects of one to two seasons of ozone (O 3 ) exposure ranging from subambient to 1.7 times ambient on the gas exchange of sugar maple (Acer saccharum Marsh.) and yellow-poplar (Liriodendron tulipifera L.) were studied in standard 3 m diameter open-top chambers. Cumulative O 3 exposures based on SUM00 index ranged from 16 to 107 ppm⋅h in 1990 and 31 to 197 ppm⋅h in 1991. During the 120-day exposure in 1990, rates of net photosynthesis and stomatal conductance of sugar maple foliage were not affected by O 3 . Based on estimates from light-response curves, net photosynthesis at saturating light, dark respiration, compensation irradiance, and quantum efficiency were not affected in sugar maple leaves exposed to elevated O 3 . Gas exchange rates of leaves of yellow-poplar grown in elevated O 3 were significantly reduced in 1990 and 1991, when compared with foliage of seedlings grown in charcoal-filtered air (CF air). In 1990, seasonal monthly net photosynthesis of yellow-poplar leaves was reduced 11.7% in 1.5 times ambient O 3 air compared with leaves of seedlings grown in CF air. The light response of yellow-poplar foliage generally was not affected by exposure to O 3 during either growing season; with the exception in 1991 of net photosynthesis at saturating light being reduced 13 to 42% in leaves of seedlings exposed to 1.7 times ambient O 3 air compared with leaves of seedlings grown in CF air. Stomatal conductance rates were generally not affected by exposure to O 3 during either exposure season. In 1991, the response of newly mature (node 3 to 5 from apex) and older leaves (node 8 to 12 from apex) of ozone-exposed yellow-poplar were compared and responded similarly to elevated O 3 . Ozone effects on seasonal diurnal photosynthetic response of newly mature and older yellow-poplar foliage were observed in 1991. Reductions (21-42%) in net photosynthetic rates of leaves of yellow-poplar exposed to elevated O 3 were first observed after 42 days of treatment (113 ppm⋅h exposure). Predawn and post-sunset respiration and stomatal conductance were not affected by exposure to elevated O 3 . Despite significant reductions of net photosynthesis of O 3 -exposed yellow-poplar during both seasons, significant negative growth effects were not observed until the final harvest in June 1992. Root/shoot ratios of yellow-poplar seedlings exposed to 1.7 times ambient O 3 were reduced 30% compared with those seedlings exposed to CF air.

The chronic effects of ozone (O 3 ) alone or combined with elevated carbon dioxide (CO 2 ) on the foliar physiology of unfertilized field-grown yellowpoplar (Liriodendron tulipifera L.) seedlings were studied from 1992 to 1996. Within open-top chambers, juvenile trees were exposed to the following: charcoal-filtered air (CF); 1 ambient ozone (1XO 3 ); 1.5 ambient ozone (1.5XO 3 ); 1.5 ambient ozone plus 700 ppm carbon dioxide (1.5XO 3 +CO 2 ); or chamberless open-air (OA). Seasonal 24-h mean ambient O 3 concentrations ranged from 32 to 46 ppm over the five seasons. Averaged over 5 years, midseason net photosynthesis at saturating light (A sat ) was reduced by 14% (P=0.029) and stomatal conductance (g s ) was reduced, albeit non-significant, by 13% (P =0.096) in upper canopy foliage exposed to 1.5XO 3 -air relative to CF controls. There were no significant differences over the 5 years in A sat and g s between trees grown in 1XO 3 -and 1.5XO 3 -air. Our results support the hypothesis that the magnitude of O 3 effects on A sat and g s decreases as saplings age. When averaged over the five seasons of exposure, total chlorophyll concentration (chl) was not significantly affected by exposure to elevated O 3 ; however, in 1.5XO 3 +CO 2 -air, foliar chl was reduced by 33% relative to all others (P<0.001). In 1.5XO 3 +CO 2 -air, A sat was 1.4-1.9 times higher (P<0.001) and g s was 0.7 times lower (P=0.022) than all others. O 3 uptake in juvenile trees exposed to elevated O 3 plus elevated CO 2 (1.5XO 3 +CO 2 -air) was most comparable to trees exposed to ambient air (1XO 3 ) throughout the study. These findings suggest that elevated CO 2 may minimize the negative effects of O 3 by reducing O 3 uptake through decreased stomatal conductance.

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