Street trees in italian cities: story, biodiversity and integration within the urban environment

Since the nineteenth century, street trees have become a significant component of urban landscapes, with multiple ecological roles and social-economic functions. However, data about their abundance and diversity, and information about the criteria used for the species selection, are still limited. Besides providing a historical overview, we analyze and compare data on abundance, diversity and criteria of species selection of street trees in 15 Italian cities chosen considering their representativeness along a bioclimatic transect across Italy. The biodiversity of urban street trees resulted quite rich with 277 species (about 60% are exotic); among these, Platanus x hispanica Mill. ex Munchh., Celtis australis L. and Quercus ilex L. resulted as the most common species. Besides the general suitability for the local climatic conditions, several factors seem to concur in the species selection, such as their aesthetic appeal and dimension. Other selection criteria based on the species' ecology or functionality were not evident.