The aim of this study was to evaluate the traffic-related effects on morphological and anatomical traits of Fraxinus rotundifolia and Morus alba along the Malayer-Hamedan road. In the study area, populations of two species of F. rotundifolia (30 individual trees) and M. alba (30 individual trees) were selected for sampling. The results showed that the leaf and stomata dimensions at distances near to the roadside were significantly lower compared to longer distances from the road. For F. rotundifolia, the number of veins showed the lowest plasticity, while in case of M. alba, the stomata length (P = 0.52) and the number of veins (P = 0.54) showed the lowest plasticity related to the environmental conditions. Results of discriminant analysis for population grouping for the two species of F. rotundifolia and M. alba confirmed the accuracy of grouping 74.8% and 79.5%, respectively. In case of trees that were farther away from the road, guard and epidermal cells were located at the same level. We found that the leaf stomata in M. alba were closed more than in F. rotundifolia at the same distance. Totally, the results of this research show that the air pollution stress impacted the tree morphological traits. From the two species, F. rotundifolia was more resistant in terms of pollution stress.
