Urban plant assemblages by land use type in Milan: floristic, ecological and functional diversities and refugium role of railway areas.
The investigation of plant assemblages growing in urban ecosystems offers the chance to observe new interactions among biodiversity, urban planning and public health, in the context of a changing world with increasing city surfaces and populations. In this context, the aim of this work is the study of floristic, ecological and functional diversity of land use types of Milan, such as (a) Built-up areas (urbanized and productive zones), (b) Roads (borders and flowerbed of main boulevards and avenues), (c) Railways (tracks borders, stations and adjacencies) and (d) Green areas (parks, gardens, and orchards). A floristic survey was performed through stratified sampling and a floristic database was built. The following bioecological information was added to each sampling location: species richness, %alien species, %families, %life forms, functional diversity, %Grime's CSR strategies, Ellenberg's indicator values (EIVs), urbanity index and functional diversity estimators. Differences among land use types were detected with linear mixed models. In total, about 300 taxa were recorded, and 34% of the species surveyed were aliens. The Railways were prominent with regard to several traits such as floristic richness, number of insect-pollinated species, mean EIV for light and soil reaction and functional diversity estimators. On the other hand, the urbanity index was the lowest in Railways. In general, Urban areas and Roads had lower trait values, while Green areas had intermediate performances. Our results showed that despite the expected high presence of ruderal and alien species, typical of disturbed environments, the urban landscapes of Milan are diversified in their ecological functions and included areas rich in uncommon species limited to peculiar habitats. The railway landscape of Milan represents a reservoir and refuge for many native species, incorporating microhabitats not present in other city landscapes. Therefore, railway areas should be included in the planning of green networks to improve connectivity and support the nature in urban landscapes.