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Chamber experiments on juvenile trees have resulted in severe injury and accelerated loss of leaves along with reduced biomass production under chronically enhanced O₃ levels. In contrast, the few studies conducted on adult forest trees in the field have reported low O₃ sensitivity. In the present study, young beech in phytotrons was more sensitive to O₃ than adult beech in the field, although employed O₃ regimes were similar. The hypotheses tested were that: (1) differences in O₃ uptake were caused by the ontogenetically higher stomatal conductance of young compared to adult trees, (2) the experimental settings in the phytotrons enhanced O₃ uptake compared to field conditions, and (3) a low detoxification capacity contributes to the higher O₃ sensitivity of the young trees. The higher O₃ sensitivity of juvenile beech in the phytotrons is demonstrated to relate to both the experimental conditions and the …