Changes in tree composition and diversity of streetscapes and their impact on allergenic risk of pollen during urban expansion: a case study in Chengdu, China

Street trees are an important component of urban landscapes and provide diverse ecological and social services to city dwellers. However, allergenic pollen released by street trees also harm human health. During China’s rapid urban expansion, little has been documented about the changes of the composition, diversity, and allergenic risk of street trees. Therefore, the streetscape of the old and new districts in Chengdu City, the first designated “Park City” in China, was selected as the study site of this study. All 502 streets, including 56,125 trees, in the old district and 413 streets, including 70,134 trees, in the new district, were placed into five street categories according to their width for cross-analysis. The two districts exhibited remarkably similar landscape compositions, with 38 shared tree species contributing to a uniform visual interface, though more tree species were recorded in the new district (48 species) compared to the old district (43 species). The per capita number of street trees in the new district was 0.11, exceeding 0.04 in the old district. However, there were no significant differences among the five street categories of the two districts in mean diameter at breast height (DBH), tree height, and α-diversity. T An excess of big trees (DBH ≥ 15.2 cm) was found in the new district. There were significantly fewer Platanus acerifolia and Erythrina variegate in the new district; however, there was a much higher percentage of flowering ornamental trees and colored-leaf trees, especially in precinct and community streets. This reflected the contemporary demands from city dwellers for more colorful and manageable streetscapes. As the street width decreased, the values of the urban green zone allergenicity index (I_UGZA) in the new district increased, while I_UGZA values were bimodal in the old district. The I_UGZA value in the precinct and community streets of the old district greatly exceeded the upper limit of 1.0; the I_UGZA value of community streets in the new district also exceeded 1.0. These street categories exhibited a very high allergenic risk of pollen from their tree landscapes. Overall, the new district’s streetscapes exhibited a lower allergenic risk of pollen than the old district due to a reduced proportion of high-VPA (potential allergenicity) tree species, demonstrating clearer health benefits. In addition, Pearson’s correlation analysis identified that tree species richness was the key factor significantly affecting the I_UGZA value. Based on these findings, this study suggests that optimizing streetscapes management—such as selecting tree species with low VPA, intensifying crown pruning before flowering seasons, and applying targeted pollen-suppression agents—could further mitigate pollen allergenic risks and enhance urban livability in Chengdu.