Water Use Efficiency of Five Tree Species and Its Relationships with Leaf Nutrients in a Subtropical Broad-Leaf Evergreen Forest of Southern China

Water use efficiency (WUE) is key to linking the water, carbon, and nutrient cycles in terrestrial ecosystems. However, the coupling between WUE and leaf nutrients is still poorly understood in subtropical forests. Here, the stable carbon isotope technique was employed to estimate the leaf-scale WUE of five common tree species (Castanopsis eyrei, Symplocos laurina, Machilus grijsii, Ternstroemia gymnanthera, and Rhododendron ovatum) in different habitat types (i.e., hillside, near the top of the peak, and peak) in a subtropical broad-leaf evergreen forest on the western slope of Wuyi Mountain, southern China. In addition, leaf carbon (C), nitrogen (N), and phosphorus (P) contents were also measured to assess plant nutrient utilization and its relationship with WUE. From the hillside to the peak, soil water content showed a decreasing trend, whereas the soil total C, N, and P contents showed an increasing trend. Regardless of species, the leaf delta 13Cp value and WUE showed an increasing trend from the hillside to the peak, mainly due to an increase in soil water deficit and light. The leaf N and P contents showed an increasing trend from hillside to peak due to an increase in soil nutrients, while the leaf C: N ratio, C:P ratio, and N:P ratio showed a decreasing trend. The regression analysis showed that leaf-scale WUE was positively correlated with the leaf N and P contents but negatively correlated with the leaf N:P ratio, especially for the three species (C. eyrei, S. laurina, and T. gymnanthera). These results indicated that the differences in soil water availability, light, and soil development resulting from different habitats have a significant impact on leaf-scale WUE and nutrient status on Wuyi Mountain. Therefore, there may be a close relationship between WUE and leaf nutrients, which would help us to better understand the water-, carbon-, and nutrient-coupled relationships for the evergreen broad-leaved tree species in this region.