Water stable isotope characteristics and water use strategies of co-occurring plants in ecological and economic forests in subtropical monsoon regions

Identifying the spatio-temporal variation in water use by co-occurring species in mixed forests can determine whether there is hydrological niche partitioning or niche overlapping, i.e., whether plant co-occurrence brings about resource complementation or interspecific competition, and provides a reference for plantation management. Water stable isotopic compositions in precipitation, soil, and four plants (xylem and leaf) in ecological and economic forests and hydrometeorological factors were continuously monitored for a year, and the water uptake by plants was determined using a Bayesian MixSIR model. The isotopic compositions of soil water and plant water in the ecological forest (Camphora camphora and Cunninghamia lanceolate) and economic forest (Castanea mollissima and Camellia pitardii) showed similar seasonal variation, and no significant difference between their mean isotopic values was found (p > 0.05). The isotopic compositions in leaf water of different plants in the two forests were controlled by xylem water isotope and meteorological factors, rather than soil moisture conditions. With decreasing soil water availability, the water sources of C. camphora, C. lanceolate, and C. pitardii changed from shallow to deep, while the water use pattern of C. mollissima was insensitive to changing soil moisture conditions. The water source overlapping between C. camphora and C. lanceolate in the ecological forest might generate water resource competition during the dry period, limiting their ecological functions. On the other hand, the water source partitioning between C. mollissima and C. pitardii in the economic forest facilitated the water resource allocation. Our study suggested that the overlapping and partitioning of hydrological niches among species in mixed forests were species-specific and provided a scientific basis for fully exploiting the ecological and economic benefits of silviculture ecosystems.