<p>This study aimed to analyze changes in the production capacity of mountain forests that have faced decreased rainfall and drought occurrence in recent decades, with field sampling, a long-term time series analysis of satellite imagery and climate data. To achieve this goal, first during field sampling and when measuring the crown diameter of all the trees in the sample plots, the allometric equations developed for <i>Quercus brantii</i> Lindle in mountain forest habitats were used, and the aboveground biomass (AGB) value of forests was calculated for 2020. To investigate changes in the AGB amount, a regression model was established between the Ratio Vegetation Index (RVI; extracted from the 2020 Landsat satellite images) and the AGB amount in 2020. By running the developed regression model on the 35-year time series (1986–2020) of RVI maps, the 35-year time series of AGB was prepared. To prepare the 35-year time series (1986–2020) of the Standardized Precipitation Index (SPI), the 35-year time series (1986–2020) of monthly rainfall data was applied. The analysis of changes in drought occurrence revealed that 2007 was the most important change point in the studied time series, with a significant difference between the mean rainfall values before and after 2007. Hence, rainfall increased somewhat after 2007. An examination of the relations between AGB changes and drought occurrence variations during the study period demonstrated that there was a suitable correlation (<i>R</i>² = 0.64) between these two variables, and the amounts of both biomass and rainfall displayed similar increasing trends during the study period.</p>