Abstract:As main photosynthetic organs leaves are very sensitive to exterior environments. Water deficiency obviously affects the biological characteristics of leaves, and leads to the changes in their photosynthetic characteristics and water use efficiencies. Xylem pathways increase when trees grow, which results in the increase in water gravity as well as pathway resistance. Accordingly the biological characteristics of leaves changes along tree height. In this research the photosynthetic characteristics and carbon isotope rate (δ13Ｃ) in the leaves of 4 tree species, Platanus hispanica, Robinia pseudoacacia, Fraxinus chinensis, and Ginkgo biloba are measured. The results show that the leaf photosynthetic rate (Pn), transpiration ratio (Tr), stomata conductance (Cond) and internal CO2 concentration (Ci) reduce along tree heights, while the leaf δ13Ｃ increases along tree height. The δ13C values at the tree tops of Platanus hispanica, Robinia pseudoacacia, Fraxinus chinensis and Ginkgo biloba are respectively 1.126‰, 2.310‰, 2.290‰ and 2.276‰ larger than that at the lower crowns. The One Way ANOVA and LSD tests show that the leaf photosynthetic characteristics and δ13C vary significantly at different heights (P<0.05). The decrease in leaf photosynthetic capability and the increase in δ13C along tree height indicate that the leaves at the tree tops suffer from water stress. These results support the hydraulic limitation hypothesis.