Characterizing the soil moisture environment in the plantation ecosystems may provide substantial information for judging the stability of forests in relation to climate change. In this study, we explored the level and the variations of water availability, as well as their influence on tree water use in the long-term. This was achieved by monitoring in particular the volumetric soil water content (θ), the groundwater level, the atmospheric vapor pressure deficit (VPD), and the sap flux-based forest transpiration (T_r) in a sand-fixation plantation of Mongolian Scots pine in northern China. The results showed a significant inter- and intra-annual variability of annual precipitation (P_r), for an average (±standard deviation) of ~ 476 ± 139 mm over the past 16-year study period. The inter-annual variations of the groundwater level showed a general downward trend, decreasing from −4.82 to −6.13 m. The relative extractable soil water (REW) between 0−1 m and between 1−2 m depth along soil profile was only 0.29 ± 0.14 and 0.16 ± 0.08, respectively, indicating a decreasing of soil moisture capacity and of the moisture depletion patterns with depth. The statistics relative to the past seven years (between 2013−2019) indicate an average T_r of 193 ± 59 mm for Mongolian Scots pines. Notably, their transpiration capability (T_r/VPD) declined with the decrease of REW_0−1 m: the boundary curve showed a clear exponential decline. The threshold value of REW_0–1 m (deduced from an abrupt decrease of T_r/VPD) was ~ 0.41 and 0.35 at the daily and monthly timescales, respectively. The average T_r/P_r value was 0.42 (maximum = 0.60) and T_r accounted on average for 55% of the total evapotranspiration (maximum = 77%). Soil drought prevailed at the study site; however, we did not notice a heavy use of soil water resources by Mongolian Scots pine. The results, overall, underlined the causes rather than the consequences of the observed soil moisture conditions, which are leading to the degradation of Mongolian Scots pine. Forest management measures causing moderate disturbance are hence recommended for mitigating the water competition between Mongolian Scots pine and sub-canopy vegetation.