This Bachelor thesis deals with dynamics of stem circumference changes of the beech (Fagus sylvatica L.) in changing environmental conditions situated in the third oak-beech forest vegetation zone, in Bienska dolina. In the research area established in 2012, 12 representative beech samples were selected, on which changes in the girth of the trunks were recorded using automatic dendrometers. A digital meteorological station was established near the research area, and based on its measurements, we obtained values of environmental conditions. In the selected period from 01.04. - 30.09.2018 we analyzed changes in beech trunk girths depending on the water potential of the soil and current meteorological parameters with emphasis on the occurrence of available water deficits in the soil. It was found that meteorological parameters significantly affect stem circumference changes. They were positively affected by favorable values of available soil water and rainfall, which was reflected in an increase on the girth of the trunk from mid-April to the end of June with an average value of 8.5 mm. From the end of June to the end of August, the trunk shrinked due to the reduced water potential of the soil. On average, the trunks shrunk by more than 1.5 mm. The change occurred with an increased rainfall and a decrease in air temperature, when we recorded an increase in proportion size by tissue rehydration. We compared the detailed course of reversible changes of circumference during the day in two periods. One period was with favorable soil moisture and the other with soil water deficit. Under favorable soil moisture conditions, when the water potential values reached numbers close to zero, we detected three phases on the stem circumference change curve: the trunk shrinkage phase, the trunk expansion phase and the increment phase. In the second period, with a deficit of available water, there was only a shrinkage of the trunks and subsequent expansion of the trunk, which, however, did not reach the proportion from the previous day - the beech trunks gradually shrunk.