Background Water migration and use are important processes in trees. However, it is possible to overestimate transpiration by equating the water absorbed through the plant roots to that diffused back to the atmosphere through stomatal transpiration. Therefore, it is necessary to quantify the water transpired and stored in plants. Method The δ2H/δ18O technique and heat ratio method were used to explore the water usage of coniferous and broad-leaved tree species, including the proportions of water used for transpiration and water storage. Results Platycladus orientalis and Quercus variabilis had strong plasticity in their water usage from different sources. Platycladus orientalis primarily used groundwater (30.5%) and the 60–100-cm soil layer (21.6%) throughout the experimental period and was sensitive to precipitation, absorbing water from the 0–20-cm layer (26.6%) during the rainy season. Quercus variabilis absorbed water from all sources (15.7%–36.5%) except from the 40–60-cm soil layer during the dry season. In addition, it did not change its water source but increased its groundwater uptake during the rainy season. The annual mean water fluxes of P. orientalis and Q. variabilis were 374.69 and 469.50 mm·year− 1, with 93.49% and 93.91% of the water used for transpiration, respectively. However, nocturnal sap flow in P. orientalis and Q. variabilis was mainly used for water storage in the trunk rather than transpiration, which effectively alleviated drought stress and facilitated the transport of nutrients. Conclusions The water stored in both species comprised 6%–7% of the total water fluxes and, therefore, should be considered in water balance models.

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Water migration and use are important processes in trees. However, it is possible to overestimate transpiration by equating the water absorbed by plant roots with that diffused back to the atmosphere via the stomata. The δ2H/δ18O technique and heat ratio method were used to explore the patterns of water use of coniferous and broad-leaved tree species to determine the proportions of water used for transpiration and water storage. Our results indicate that both species showed strong plasticity in their use of water sources. The species Platycladus orientalis (Oriental arbor-vitae) and Quercus variabilis (Chinese cork oak) primarily absorbed water from groundwater and the 60–100 cm soil layer, and Q. variabilis also absorbed water from the 0–20 cm and 20–40 cm soil layers during the dry season. Only P. orientalis was sensitive to precipitation and used water from the 0–20 cm layers. Q. variabilis did not change its water source but increased its uptake of groundwater during the rainy season. We observed reverse flow and hydraulic redistribution in P. orientalis, which alleviated the spatial heterogeneity of soil water and provided water for neighboring trees. Nocturnal sap flow in P. orientalis and Q. variabilis facilitated the storage of water in the trunk. The water used for storage in both species comprised 6–7% of the total quantity and therefore, should be considered in water balance models.