Fine roots play a crucial role in many ecological and biogeochemical processes in temperate forests. Generally, fine root biomass is expected to increase during the growing season, when water and nutrient demands are high, but information on seasonality is still scarce. Here, seasonal differences in root length of European beech (Fagus sylvatica L.) were analysed at eight sites within its north-eastern distribution range. Fine roots of mature trees were monitored using minirhizotrons. Scans were taken at the beginning of winter, the end of winter and over the summer for two consecutive years, and analysed automatically by an AI-algorithm (RootDetector). An additional experiment was carried out to show that the RootDetector was unaffected by changes in soil moisture. Root length density was 40 % higher at the beginning of winter and 51% higher at the end of winter than in summer, indicating a net root loss during early summer, which was compensated afterwards during more favourable conditions in autumn. Our study indicates that fine root length in temperate forests is seasonally more variable and, so far, less predictable than previously assumed. Profound information on seasonal variability is important for modelling terrestrial biogeochemical processes and global carbon fluxes.