A healthier water use strategy in primitive forests contributes to stronger water conservation capabilities compared with secondary forests.

Water conservation is an important ecological function of forest ecosystems, plant water use strategy is a key factor in regulating forest ecosystem water balance. However, there are still insufficient studies on the water conservation capacity and water use strategies of different forest types, especially in climate-sensitive areas. In this study, we determined the stable isotope values (δD, δO and d-excess) of plant water, soil water and precipitation from two typical stand types (primary forest and secondary forest) on Changbai Mountain to reveal plant water use and evaluated the water conservation capacity. The results indicated that rainwater infiltrated into the soil combined with piston flow and preferential flow in the primary forest, and preferential flow was the only form of flow in the secondary forest. The main tree species in the primary forest formed a relatively stable water use niche. Among them, the water use pattern of Quercus mongolica Fisch. ex Ledeb (Qm.) was transformed between shallow and deep soil layers with strong ecological plasticity. The dominant species in secondary forest derived water from similar soil layers with intense interspecific competition. By comparing the water use patterns, the secondary forest conformed to the hypothesis of "two water worlds", while the primary forest conformed to the hypothesis of one reservoir. The primary forest ecosystem had stronger water conservation capacity than secondary forest ecosystem due to the regulable water use strategies of plants and the stable water conservation capacity of the soil. These results will provide theoretical support and a reference for plan future forest management strategies in the climate-sensitive areas.