Increasing evidence suggests that fine roots are particularly sensitive to environmental changes, making them essential in responding and adapting forest ecosystems to climate change. However, we still lack a fundamental understanding of the underlying mechanisms that control fine root plasticity. The objective of this study was to determine the influence of soil moisture changes on fine root dynamics and morphology of European beech (Fagus sylvatica L.). We conducted a 30-month study of fine root traits, i.e., fine root biomass (FRB), productivity, mortality, turnover, specific root length (SRL), specific root area (SRA), and root tip frequency (RTF), along a soil moisture gradient from dry, intermediate, and wet conditions in a near-natural mature beech forest. Sequential root coring with accompanying soil measurements was carried out at three study sites reflecting the gradient in soil water availability. For most fine root traits, we found significant differences between the upper 10 ​cm and lower soil depths. FRB showed significant differences between study sites, with the lowest FRB at the dry site. However, productivity, turnover, SRL, SRA, and RTF showed no significant differences between sites, but a high variability between seasons, suggesting an adaptation to short-term fluctuations but not to long-term gradients in soil water content (SWC). Linear mixed models revealed that decreasing SWC led to a significant increase in SRL, SRA, and RTF (standardized coefficients: −1.0 ​± ​0.46, −1.1 ​± ​0.46, and −1.1 ​± ​0.43, respectively). Our observations indicate an adaptation strategy of beech to low availability of soil water and drought by forming thin absorptive roots and by maintaining a high seasonal plasticity to tolerate fluctuations in soil moisture. By highlighting the belowground morphological adaptations of mature forests to low soil water availability, our results provide novel insights into the structure and dynamics of forest ecosystem adaptations to climate change.