Tramways covered by trees in Milan. The study looked at Milan and Bologna to understand how different tree species behave in heavily polluted cities. Credit:Marco Bottigelli/ Moment/ Getty Images.
Trees can sequester carbon and pollutants from the air, and identifying the most efficient species for this role is important when planning green areas in cities. A new study in Atmosphere, based on the Italian cities of Milan and Bologna, provides some indications.
The study1 was a collaboration between Italy’s National Research Council (CNR) and Council for Agricultural Research and Economics (CREA), and ARIANET, an air quality consulting company. The authors used a model called AIRTREE that was previously developed by CNR and CREA to predict how tree leaves exchange CO2, water, ozone, and fine particles with the atmosphere, and applied it to large urban areas for the first time.
They created vegetation maps of the two cities, integrating 10-metre resolution images from the Sentinel 2 satellites of the European Space Agency with urban green censuses provided by the Milan and Bologna municipalities. “Usually, these censuses only include around 20% of the urban vegetation, so they’re not complete,” says Ilaria Zappitelli, a PhD student at CREA and first author of the study. “Thanks to the integration of these two data sources, we obtained a better understanding of the vegetation cover in these municipalities”. The maps, along with air quality data from the CRESCO database, were then fed into the AIRTREE model.
Initially, the researchers simulated the maximum possible level of pollution in the two cities, assuming a total absence of trees. Then they calculated the absorption capacity of the various tree species with respect to pollutants such as carbon dioxide, particulate matter, nitrogen dioxide, finding that ones with the highest uptake were the European nettle trees (Celtis australis), the London plane (Platanus x acerifolia), the Siberian elm (Ulmus pumila), and the Northern red oak (Quercus rubra).
“The main contribution of this work is to underline the importance of the biometric specificity of each plant species, especially the number of leaves per square metre,” says Silvano Fares, a CNR researcher and corresponding author of the study. “Characteristics such as photosynthesis and the transpiration capacity of the plant were verified in situ for each species in order to obtain more precise data”.
The model may be used to plan urban reforestation by relying on specific data about the climate in each location, and on how different tree species interact with it. “It will be made available to everyone through an open-source tool, as part of the activities of the new Italian National Biodiversity Future Center,” says Fares.
