Carbon dioxide (CO2) is one of the most impactful greenhouse gases (GHG) leading to global warming. Planting urban forests can help to mitigate climate change effects as trees remove CO2 from the atmosphere thanks to their photosynthetic activity. However, by setting up a new urban forest, GHG emissions occur during cultivation in the nursery, planting, and maintenance operations. A total of 170 urban trees belonging to four genera (Tilia, Acer, Ulmus, and Cupressus) were planted in Florence (Italy), and species-specific leaf-level net photosynthetic CO2 uptake (An_leaf) was modeled considering An_leaf response to different environmental factors (i.e., light, air temperature, relative humidity, and atmospheric CO2 concentration). An_leaf was scaled to canopy level and the total tree CO2 balance was estimated considering the respiration rate of woody biomass too. Moreover, carbon storage by trees during nursery cultivation was assessed through allometric formulas. Regarding CO2 emissions, a Life Cycle Analysis was accomplished to calculate the Carbon Footprint (CF) linked to nursery cultivation, tree planting, and maintenance over time. In addition, seasonal CO2 soil respiration was measured. To run the model, tree growth over time was estimated, hourly meteorological data and soil temperature were recorded in situ for two consecutive years, while three future climatic scenarios were considered for the entire park-life span (50 years). Results showed that the CF was equal to 14.7 t CO2 equivalent with maintenance over time as the most CO2-emitting phase (62 %). The model highlighted that 13 years are needed to reach a positive CO2 balance. This study allowed to determine when a new urban forest becomes a real carbon sink in a Mediterranean climate, thus helping to achieve the European goal of carbon neutrality. © 2024 The Authors

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