Understanding how subsurface water storage regulates ecosystem responses to hydroclimatic variability is central to ecohydrology, but the extent to which lithology mediates seasonal sensitivity of forests to climatic water deficit (CWD) through soil–regolith water storage remains poorly understood. Using four satellite vegetation metrics and meteorological reanalysis (2000–2023), we quantify seasonal sensitivity to CWD across the hydro-lithological regions of the Qinling Mountains. Results show that forest functional responses to CWD were more pronounced than structural greenness, implying that greenness-based metrics may overlook substantial drought impacts on forest ecosystems. Temperature and precipitation affected drought sensitivity in different ways across seasons and regions. Higher temperatures generally reduced drought sensitivity in spring and summer, especially where soils and bedrock can store more water, but increased sensitivity in autumn. More precipitation increased sensitivity to CWD in summer and autumn, although this effect differed across regions. Bedrock-stored water exerts a dual effect. In summer, it helped reduce drought stress by providing extra water during the growing season. In spring, however, it could increase vegetation sensitivity because it encouraged canopy development, which raised water demand. In regions where access to bedrock water was limited, even a small amount of bedrock-water access was associated with higher sensitivity to CWD. Our findings demonstrate that drought assessment and forest management should account for hydro-lithological properties and bedrock water dynamics.