Quantification of the ecosystem services provided by public and private green in a residential area of Bologna, Northern Italy

In a densely populated district of the urban area of Bologna, a city located in the highly polluted Pianura Padana region (Italy) impacted by urban heat island (UHI) and urban pollution island (UPI) phenomena, the provision of ecosystem services and the relative contributions of private and public vegetation were evaluated using two different models: i-Tree and Envi-met. A comprehensive inventory of public and private green within the study area (1km2) was built combining open datasets and data collected through targeted field surveys. Different ecosystem services were evaluated (carbon dioxide storage and sequestration, pollutants removal, volatile organic compounds emission, oxygen production, avoided runoff, energy saving and air temperature reduction). The survey revealed that private green was dominant in the area, with more than 3000 plants between trees and shrubs belonging to 154 different species, compared to around 1800 public trees with only 68 species. The provision of ecosystem services by private green was relevant, with a greater contribution in energy saving (73%), carbon dioxide sequestration (55%) and oxygen release (55%); the contribution of public vegetation, composed mainly of adult trees, was higher in carbon dioxide storage (56%), pollutant removal (58%), mitigation of urban heat island (62% for air temperature reduction, 51% for thermal comfort improvement), and avoided run-off (59%). Overall, private green contributes by 49.7% to the total ecosystem services highlighting that, when it is not considered, the impact of urban forests on adaptation and mitigation to UHI and UPI, as well as air quality, may be severely underestimated.