Age-related vertical soil water redistribution and root water uptake pattern of artificial forest in the Loess Plateau of China

Understanding how stand age regulates soil water balance components is essential for sustainable afforestation in semi-arid regions. However, the long-term dynamics of deep soil water depletion and root water uptake patterns at different forest growth stages remain insufficiently understood. In this study, soil water content in 0–300 cm depth for young (14-year-old) and old (28-year-old) black locust forest stands in the Loess Plateau was observed during 2014–2021, which were combined with HYDRUS-1D model simulations to quantify variations in soil water storage (SWS), soil desiccation index (SDI), actual evapotranspiration (ET a ) and root water uptake (RWU) under wet, normal, and dry years. The SWS exhibited pronounced age-dependent vertical redistribution during the eight years: the old forest maintained higher soil water in the shallow layer (0–40 cm), whereas the young forest stored substantially more soil water in the middle (40–100 cm) and deep (including 100–200 cm and 200–300 cm) layers. The SDI values confirmed stronger and more persistent deep-soil desiccation in old forest, particularly within the 100–300 cm profile, with limited recovery even during wet years. The ET a was consistently 5–12% higher in the old forest, reflecting greater canopy development and transpiration demand. The RWU patterns further revealed age-dependent water use strategies: the old forest relied primarily on shallow soil water, while the young forest showed greater water uptake from middle and deep soil layers, particularly under dry conditions. This pattern suggests that long-term deep soil water depletion in the old forest may limit effective deep-water uptake despite deeper root development. These findings indicate that forest maturation is associated with vertical soil water redistribution, characterized by stronger deep soil desiccation and greater reliance on shallow soil water in the old stand, rather than only reducing total soil water availability. This suggests that deep soil water status should be considered when evaluating the sustainability of older forest plantations in semi-arid environments.