Abstract
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 (δ 13C) 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 CO 2 concentration ( Ci) reduced along with tree height, while the leaf δ 13C increased along with tree height. The One Way ANOVA and LSD tests showed that the leaf photosynthetic characteristics and δ 13C varied significantly at different tree heights ( P < 0.05). The decrease in leaf photosynthetic capability and the increase in δ 13C along with tree height indicate that the leaves at the tree tops suffer from water stress. These results support the hydraulic limitation hypothesis.
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