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Heat negatively affects photosynthesis, thus tree growth and eventually survivability. The light-dependent photosynthesis reactions are considered highly heat sensitive; therefore, we studied the impact of supra-optimal temperatures on photosystem II (PSII) in five important tree species (Abies alba, Fagus sylvatica, Picea abies, Pinus sylvestris and Quercus petraea agg.). We simulated short-term heat stress at different levels and assessed PSII thermotolerance via fluorescence parameters derived from the OKJIP transient. To capture the seasonal changes in PSII thermostability and the stress tolerance index, the measurements were performed twice: at the beginning of the vegetation season before the natural heat events occurred and during the peak of the vegetation season. Moreover, the content of photosynthesis-related pigments was analysed. We confirmed the relationship between the appearance of the K …