Extraordinary drought of 2003 overrules ozone impact on adult beech trees (Fagus sylvatica)

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 fully recovered during the subsequent, humid year. Comparison of AOT40, an O₃ exposure-based risk index of O₃ stress, and cumulative ozone uptake (COU) yielded a linear relationship throughout humid growth conditions, but a changing slope during 2003. Our findings support the hypothesis that drought protects plants from O₃ injury by stomatal closure, which restricts O₃ influx into leaves and decouples COU from high external ozone levels. High AOT40 erroneously suggested high O₃ risk under drought. Enhanced ozone levels did not aggravate drought effects in leaves and stem.