Abstract: This study aims to examine leaf functional traits and tolerance to air pollution of greening tree species in heterogeneous urban habitats. We measured leaf morphological and physiological traits of Sophora japonica, Aesculus chinensis, Hibiscus syriacus and Buxus sinica in both road and residential habitats in Lanzhou. The scanning electron microscope was used to observe leaf epidermis structure and stomatal characteristics, and then the air pollution tolerance index, i.e. APTI, was calculated. The correlations between leaf functional traits and species’ tolerance to air pollution were explored. The results showed that: (1) In road habitat, the specific leaf area of the four tree species was lower (P<0.01), the duration of green foliage was shorter, and leaf thickness was higher than that in residential habitat. Stomatal width, stomatal opening level and opening ratio of the four tree species grown in road habitat were lower, but stomatal density was higher. All species allocated more resources to structure their leaf tissues and maintain their ability to conduct air/water exchange, in order to adapt to the road habitat. In road habitat, leaf ascorbic acid content in the tested species increased except for Aesculus chinensis. Hibiscus syriacus and Sophora japonica showed a higher value of leaf ascorbic acid content under the road habitat than that under residential habitat. The total chlorophyll content of the four tree species decreased in road habitat. Hibiscus syriacus showed a signi ficant difference of leaf pH between both habitats (P<0.01); Buxus sinica showed a significant difference of leaf relative water content (P<0.05), while other tree species showed no differences. (2) The extents of correlations differed among various leaf functional traits, with close relationships among stomatal traits, leaf morphological and physiological traits. (3) Sophora japonica showed strong tolerance to air pollution in both habitats (APTI>17), Hibiscus syriacus was intermediate (12<APTI<17), while Buxus sinica and Aesculus chinensis were sensitive (APTI<12). Sophora japonica and Hibiscus syriacus have epidermis trichomes, with coarse epidermis texture and deeper wrinkles and grains on the surface. Results of path analysis between APTI, leaf morphological and physiological traits showed that the duration of green foliage had a significant negative effect on APTI value (P<0.05), while both stomatal density and stomatal opening ratio had a significant effect on APTI (P<0.01). Our results provide a reference for selection and configuration of urban greening tree species.

Key words: urban habitat, leaf functional trait, stomatal trait, air pollution tolerance index, urban tree species.