Nine species of landscape trees cultivated on the campus of Jilin Agricultural University were selected to investigate reasonable sampling positions for evaluating the trees ecological effects in terms of increasing humidity and reducing temperature through leaf area transpiration. The crown of each variety of tree was divided into four layers; surface layer (part A), outer layer (part B), middle layer (part C) and inner layer (part D). The increased humidity and reduced temperature values of unit leaf area caused by transpiration in each layer were determined and compared with different layers within and among species. The results showed that except for the species Juglans mandshurica and Cornus alba, nine tested species all showed the same feature in terms of the capacity to increase humidity and reduce temperature per unit leaf area in four layers: the surface layer (part A) > the outer layer (part B) > the middle layer(part C) > the inner layer (part D). The average values for increase in humidity of the surface layer (A), outer layer (B), middle layer (C) and inner layer (D) in the nine species were 2.87, 2.57, 2.12 and 1.73 kg m^-2 d^-1 respectively, and the average value was 2.32 kg m^-2 d^-1. Compared with the average values, the values of the surface layer (A) and outer layer (B) increased by 23.71% and 10.77% respectively. The values of the middle layer (C) and inner layer (D) decreased by 8.62% and 26.46% respectively. The average values for decrease in temperature in parts A, B, C and D were 0.55, 0.50, 0.41 and 0.33℃ respectively, and the average value was 0.45℃. Compared with the average values, the values of the surface layer (A) and outer layer (B) increased by 22.22% and 11.11% respectively. The values of the middle layer (C) and inner layer (D) decreased by 8.89% and 26.67% respectively. Among the nine species, the capacity to increase humidity and reduce temperature in arbor life forms was better than that in shrub life forms. For example, the average values for increase in humidity and decrease in temperature in five arbor life forms-Juglans mandshurica, Catalpa ovata, Euonymus bungeanus, Armeniaca mandshurica and Crataegus pinnatifida-were 2.50 kg m^-2 d^-1and 0.49℃ respectively. The average values for increase in humidity and decrease in temperature in the shrub life forms Prunus triloba, Cornus alba, Lonicera ruprechtiana and Hydrangea paniculata were 2.10 kg m^-2 d^-1and 0.41℃. The variations in standard deviation (SD) and variable coefficient (VC) among species were larger than those within species. The values of SD within species were between 0.91 (Euonymus bungeanus) and -0.23 (Lonicera ruprechtiana), and the average value was 0.54. The values of VC within species were between 0.47 (Euonymus bungeanus) and -0.13 (Lonicera ruprechtiana), and the average value was 0.26. The values of SD among species were between 1.39 (surface layer) and -0.88 (inner layer); the average value was 1.07. The values of VC among species were between 0.48 (outer layer) and -0.51 (inner layer); the average value was 0.47. The variations in the capacity to increase humidity and reduce temperature in different layers of the arbor life forms were larger than those in the shrub life forms. The average SD values of the arbor life forms were 0.64 and 0.12; the average VC value of the arbor life forms was 0.28. The average SD values of the shrub life forms were 0.49 and 0.08; the average VC value of the shrub life forms was 0.23. The result of the study suggests that it is reasonable to select part B or part C near the top of part B as the optimal sampling positions for evaluating the ecological function of landscape trees.