HÚDOKOVÁ H. 2019. Comparison of photosystem II thermostability of economically important tree species: master thesis. Zvolen: Technical University in Zvolen. Faculty of Forestry. 2019. 62 pp.Climate change caused by anthropogenic activity leads to declines in development, growth and production of biomass. The increase of the heat stress in plants living under natural conditions is one of the most important problems in forest ecosystems. Photosynthesis is physiological process in cells of plants sensitive to high temperature. Therefore we observe the impact of supraoptimal temperatures on primary physiological processes on five tree species in two different terms during vegetation season for observation of seasonal dynamic before and after influence of high temperatures. Fast kinetic of chlorophyll fluorescence (OJIP-curves) is simple and effective method of assessment of photochemical efficiency parameters, threshold it is possible to measure bigger amount of plant samples. We measured fluorescence of chlorophyll in two terms at the beginning of the vegetation season and in the middle of vegetation season (before and after high temperature period). We observed reaction and change of PSII thermostability during vegetation season. The reaction of PSII thermostability was assessed on economically important tree species (European beech, Pine, Oak, Silver fir, Norway spruce). Simulation of the heat stress was done by using water bath. It was made also statistical analysis of measured parameters. We calculated parameters such as T15 represents temperature at which Fv/Fm declines 15% from the maximum value, T50 – the temperature at which assessed parameters (PI and RC/ABS) decline by 50% from the reference value, Tc – critical temperature for PSII stability expressed as temperature, when the basal fluorescence F0 abruptly increases and parameter TWk – critical temperature at which the variable fluorescence at the K-step abruptly increases. Detection of the K-step in fluorescence OJIP curve is typical for heat stress influence and represents irreversible damage of the PSII. Differences during season and between species were observed by fluorescence induce curves (FIC) with simulation of the heat stress. That is the main reason we tried to assess influence of the heat stress on forest ecosystems. Then we compare our oucomes with photochemical processes in different conditions and different plants or woody species.