Purpose: To examine and understand mechanisms of hydrological functions and water transport of Larix gmelinii ecosystems, we quantified water consumption (ET), including soil evaporation (E) and vegetation transpiration (T), and the role of environmental factors. Methods: Stable isotope technology was utilized to make high-frequency in situ observations on the oxygen isotope composition (δ18O) of water vapor from six vertical heights (0.1–40 m above ground) in the cold-temperate Larix gmelinii forests in Northeast China. The dynamics of δ18O and its responses to environmental factors were detected during the water consumption process. One-way analysis of variance, independent-samples t test, and path analysis were used in data analysis. Results: The oxygen isotope composition of water vapor decreased with increasing vertical height. Contribution of T to ET varied between 79 and 96%. Values of δ18O in water vapor produced by E (δ18OE) corresponded most closely to water content at 10-cm soil depth (SWC10), while those produced by T (δ18OT) were associated with different environmental factors: air temperature (Ta) for the shrub-based stand, relative humidity (RH) for the herb-based stand, and temperature at 5-cm soil depth (Ts5) for the moss-based stand. Solar radiation (SR) was the dominant factor controlling the δ18O of water vapor produced by ET (δ18OET). Conclusion: Transpiration is the primary means of ecosystem water loss in Larix gmelinii forests. Overall, water loss from T of shrub-based Larix gmelinii stand is greater than those of herb-based and moss-based stands. Meteorological and soil factors mutually affect ecosystem water loss to atmosphere.