Recent changes in the groundwater regime are often regarded as the most influential factor of forest decline and decreased productivity of pedunculate oak forests in Croatia. A groundwater monitoring system was established in all main pedunculate oak forest complexes to collect information and trace changes in the groundwater regime. Research was undertaken to determine the influence of decrease in the groundwater level on forest growth and dieback on a short-term seasonal scale and a long-term scale of 13 years of permanent monitoring. Fluctuations with similar cyclical behaviour of groundwater recharges and seasonal radial growth were found with the aid of Fast Fourier Transform, which confirmed a mutual relationship. Patterns of radial growth were analysed in order to explore the possibilities of early seasonal prediction of decrease in forest productivity influenced by drought. According to these results an improvement of forest managerial practice by using groundwater monitoring data could be done in order to recognise and reduce seasonal drought impact.