Changes in leaf photosynthetic characteristics and water use efficiency along with tree height of 4 tree species

As main photosynthetic organs, leaves are very sensitive to exterior environments. Water deficiency obviously affects the biological and physiological characteristics of leaves. Xylem pathways increase when trees grow tall, which results in the increase in water gravity as well as pathway resistance. Accordingly, the physiological characteristics of leaves change along with tree height. In this research, the photosynthetic characteristics and carbon isotope ratio (δ13C) in the leaves of 4 tree species, Platanus hispanica, Robinia pseudoacacia, Fraxinus chinensis and Ginkgo biloba, were measured. The results showed that the leaf photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (Cond) and internal CO2 concentration (Ci) reduced along with tree height, while the leaf δ13C increased along with tree height. The One Way ANOVA and LSD tests showed that the leaf photosynthetic characteristics and δ13C varied significantly at different tree heights (P < 0.05). The decrease in leaf photosynthetic capability and the increase in δ13C along with tree height indicate that the leaves at the tree tops suffer from water stress. These results support the hydraulic limitation hypothesis.