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The extraordinary drought during the summer of 2003 in Central Europe allowed to examine responses of adult beech trees (Fagus sylvatica) to co-occurring stress by soil moisture deficit and elevated O₃ levels under forest conditions in southern Germany. The study comprised tree exposure to the ambient O₃ regime at the site and to a twice-ambient O₃ regime as released into the canopy through a free-air O₃ fumigation system. Annual courses of photosynthesis (A _max), stomatal conductance (g _s), electron transport rate (ETR) and chlorophyll levels were compared between 2003 and 2004, the latter year representing the humid long-term climate at the site. ETR, A _max and g _s were lowered during 2003 by drought rather than ozone, whereas chlorophyll levels did not differ between the years. Radial stem increment was reduced in 2003 by drought but …