Urban sustainability: CO2 uptake by green areas in the historic centre of Siena

Nature provides free assets and ecological services essential for human health and economic activity. For this reason, our ecosystems need to be protected and managed without affecting their integrity in the long run. The absorption of carbon dioxide (CO2) by vegetation is one of the most important services provided by the ecosystem, which needs to be preserved over time, because it regulates the planetary energy and entropic balance. In the cities, population growth, together with progressive urbanization, often leads towards a reduction of green areas and related ecological systems. Therefore, urbanization processes should be planned, also keeping in mind maintenance of a right equilibrium between built and green areas. In this study, the green areas in the historic centre of the city of Siena (Tuscany, central Italy) were identified and investigated. It was found that the total surface area of the historic centre was 169.64 ha, of which 71.54 ha was occupied by valleys and other green areas. The real contribution of this natural heritage to the CO2 absorption capacity of the ecosystem, was 330.50 t CO2 yr-1, depending on the vegetation types present in the green areas (e.g. trees, olive groves, vineyards, bamboo, grass and vegetables). Data showed remarkable carbon-storage efficiency untypical of a highly populated urban area (1.95 t CO2 haoverall-1 yr-1). In an urban system, the presence of large green areas and their proper management are necessary to ensure its sustainability.