Responses of the urban roadside trees to traffic environment
城市行道树对交通环境的响应

Ficus virens Ait. var. sublanceolata (Miq.) Corner, Lagerstroemia speciosa (L.) Pers., Mangifera indica L., Ficus microcarpa L. Cinnamomum burmanii (Nees) Bl., Ficus benjamina L.Michelia alba DC., and Bauhinia blakeana (Dunn) are common urban roadside trees in Foshan, Guangdong. They are subjected to many pollutants from automobile emissions, such as SO_2 and the heavy metals Pb and Cd. The comparatively pollution-free parks far from the traffic environment were used as a control site to study the responses of urban roadside trees to the traffic environment. The results are as follows: The eight roadside trees differed from park trees in their absorption of the heavy metals Pb and Cd, sulfur content, electric conductivity, pH value in the leaves and bark, and stomatal density. There were three sampling sites: next to the automobile lane (P_1), the pavement (P_2), and in the comparatively pollution-free parks away from traffic (P_0). Pb, Cd, and sulfur content and electric conductivity in the leaves and bark of the urban roadside trees demonstrated significant differences (5% level) following the trend of P_1>P_2>P_0. The pH value was also significant (5% level); however, it showed the reverse results: P_1P_2>P_0. However, the stomata density of the lower epidermis in leaves of Michelia alba DC showed no significant difference at the 5% level. For the other tree varieties (Lagerstroemia speciosa (L.) Pers., Mangifera indica L., and Cinnamomum burmanii (Nees) Bl.), the stomata density of the lower epidermis of leaves decreased closer to the traffic lanes (i.e., they were significantly different (5% level) with the opposite trend: P_1