Ozone (O 3) is well known to affect photosynthesis on tree seedlings under experimental conditions. In natural conditions, young trees grow under a competitive environment that may alter light availability. Such conditions were simulated in an open-top chamber (OTC) experiment carried out in Southern Switzerland (Lattecaldo). The experimental set-up consisted of Fagus sylvatica (beech) seedling plots (planted within the same chamber) either in a monospecific population (Mono, leaves grown in full sunlight conditions—FL) or under competition with taller plants of Viburnum lantana (Mixed, shaded leaves—SH). These cultures were treated with ambient air (NF, not filtered, 92% of ozone in ambient air) and with charcoal filtered air (CF, 50% of ozone in ambient air). Measures of net photosynthesis ( P N) and chlorophyll a fluorescence (with the analysis of the OKJIP transient) were performed over a growing season, from June to September. Net photosynthesis ( P N) and the maximum quantum yield of electrons reaching the acceptor side ( φ Ro = RE 0/ABS) were higher in Mono conditions (full light leaves). Vice versa, quantum yield of primary photochemistry ( φ Po = TR 0/ABS = F V/ F M) and Performance Index on absorption basis (PI ABS) were higher in Mixed plots (shaded leaves). Changes due to ozone occurred only at the end of the growing season (September). Ozone affected mainly P N, that was lower in the NF-OTCs, both Mono (−19%) and Mixed (−28%) plots, compared to the CF ones. Fluorescence parameters were affected much more by light conditions (full light vs. shaded leaves) than ozone exposure, but ozone enhanced the action of high light by lowering the values of TR 0/ABS and PI ABS. The most specific effect of ozone concerned the decrease in RE 0/ABS, that indicates the inactivation of the end acceptors of electrons, so producing an excess of oxidative pressure. Results are discussed in relation to the impact of ozone on the processes of the regeneration of forests.