Understanding how different tree species consume soil water is critical for land management, especially in areas with limited water resources. This study examined the soil water consumption characteristics of two exotic tree species—economic forest apple tree (Malus pumila) and ecological forest black locust (Robinia pseudoacacia)—and the effect of soil desiccation on plant transpiration in the 2018 growing season in a semi-arid region of the Loess Plateau. We investigated root distribution and deep soil water content in the vertical soil profile and used hydrogen and oxygen stable isotopes to identify the contribution of water to transpiration in shallow (0–2 m) and deep (>2 m) soil. We also measured sap flow changes using thermal dissipation probes during the growing season. The apple orchard had a maximum rooting depth of about 16 m, with a deep soil water deficit of about 771 mm. The corresponding figures for the black locust forest were about 25 m and 1926 mm. In previous years, the apple trees consumed 51.4 mm yr–1 of deep soil water and black locust consumed 120.4 mm yr–1. In the 2018 growing season, soil water content varied from 0–200 cm depth, with no significant changes beyond 200 cm in either species. Shallow soil water contributed, on average, 68.1% and 80.9% to transpiration in the apple orchard and black locust forest, respectively, and both they had a similar amount of transpiration during the growing season. These results indicate that rapid deep soil desiccation leads to a lack of sustainable root water uptake in deep soil, and the dominance of transpiration by precipitation in this region. This study increased our understanding on how intensive water extraction from deep soil affects tree transpiration, which is significant for sustainable afforestation in the Loess Plateau and other regions with similar hydrogeology.