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The effects of ambient and elevated ozone levels on growth and photosynthesis of beech (Fagus sylvatica) were studied by exposing seedlings in open-top chambers for one growing season to three treatments : charcoal-filtered (CF), non-filtered (NF) and non-filtered air with addition of ozone (30 ppb ozone) on clear days for 8-10 h d −" (NFj). Ambient levels were relatively low and accumulated to an AOT40 (accumulated exposure over a threshold of 40 ppb) of 4055 ppb h (for the period 23 Apr-30 Sept). The NFj chambers received an AOT40 of 8880 ppb h. Throughout the growing season we measured growth and photosynthetic properties. The treatments did not cause strong effects : measurements of gas exchange (light-saturated assimilation rate, CO # and light-response curves) and chlorophyll fluorescence showed slight and mostly non-significant reductions of several parameters. No significant differences were found for growth, though in the NFj treatment (AOT40 8880 ppb h) the relative growth rate for diameter increment was at times reduced by 12 % compared with the control treatment.

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Beech seedlings (Fagus sylvatica L.) were exposed to episodes of O 3 in environmentally controlled growth chambers during one growing season. Three treatments were applied: charcoal-filtered air, charcoalfiltered air with the addition of 40 ppb O 3 for seven episodes of 5 days' duration (9000-1700 hours), and charcoal-filtered air with the addition of 100 ppb O 3 for seven episodes of 5 days' duration (9000-1700 hours). The accumulated exposure over a threshold of 40 ppb in the last treatment reached 13,911 ppb h. Throughout the growing season we measured growth as well as photosynthetic properties and related effects to external and calculated internal doses of O 3 , using stomatal conductance (g s ) data. Growth, measured as diameter increment and biomass, was not significantly affected by the O 3 treatments. In the 100-ppb treatment, light-saturated CO 2 assimilation rates and chlorophyll content were significantly reduced, and the chlorophyll fluorescence parameter F v /F m was significantly reduced at times of high uptake rates and coincided with strong reductions of assimilation rates. O 3 uptake was lowered in the 100-ppb treatment due to reduced g s . There was serious visible damage by the end of the exposure period in the 100-ppb treatment, while the treatment with 40 ppb O 3 did not seem to cause any significant changes.

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