## 4 Discussion  
The economic benefit of the current tree coverage, calculated by the i-Tree Canopy tool, is estimated at about USD 24 million annually (\(\approx\)USD 8 per citizen of the MCN per year). If the assessed potentially plantable area was used for tree plantations, the total benefits would be equal to \(\approx\)USD 37 million/year (\(\approx\)USD 12 per citizen). At the urban level, further economic savings that are not currently included within the perspective of the tool, which, as seen, operated a linear correlation among tree cover and ecological functions provided, would also be found. For instance, tree shadings would affect the local temperature, inducing a reduction in the expenses for electricity that would vary from USD 4 to USD 166 per tree annually due to reduced use of cooling and heating systems [62]. In order to undertake the OBC Project, which originated the idea behind the research presented in this work, the MCN administration committed to funding USD 969 million, within which planting trees is only one of the actions planned. The total annual savings of the present and potential trees land coverage are estimated at \(\approx\)USD 55 million/year purchasing power parity value (PPP) adjusted (considering the value of 0.6708 for Italy in 2019 [63]). It is equivalent to \(\approx\)USD 18 per citizen of the MCN per year and \(\approx\)USD 99,117 per square kilometre of tree coverage. [PERSON] et al. (2017) investigated 10 of the biggest megacities worldwide, with an average of 1.8 \(\times\) 10\({}^{7}\) citizens, including among the tree benefits also the cost of the avoided kWh of electricity and the avoided Mbtu of energy. The median value of the additional tree coverage in the investigated megacities was estimated at 17.8%, providing an average USD 967,000/km\({}^{2}\) of tree coverage and USD 32/capita, PPP adjusted. [PERSON] et al. (2015) assessed the tree population for the Greater London Area, calculating an average benefit of about USD 891,000/km\({}^{2}\) and USD 23/capita (PPP adjusted) for a canopy cover equal to 21% of the territory [64]. Scaling up the economic benefits calculated in this work to the projected number of three million trees planted, it would be reasonable to expect a payback time reduced to 24 years from the entire tree coverage within the investigated area, considering the \begin{table}
\begin{tabular}{l l c} \hline \hline
**Impact Category** & **Unit** & **Growing + Planting** \\ \hline Global warming & kg CO2 eq & 3.36 \(\times\) 10\({}^{6}\) \\ Stratospheric ozone depletion & kg CFC11 eq & 1.98 \(\times\) 10\({}^{0}\) \\ Ionizing radiation & kBq Co-60 eq & 1.40 \(\times\) 10\({}^{5}\) \\ Ozone formation, human health & kg NOxeq & 2.23 \(\times\) 10\({}^{4}\) \\ Fine particulate matter formation & kg PM25 eq & 8.29 \(\times\) 10\({}^{3}\) \\ Ozone formation, terrestrial ecosystems & kg NOxeq & 2.28 \(\times\) 10\({}^{4}\) \\ Terrestrial acidification & kg SO2 eq & 1.57 \(\times\) 10\({}^{4}\) \\ Freshwater eutrophication & kg Peq & 9.03 \(\times\) 10\({}^{2}\) \\ Marine eutrophication & kg Neq & 9.27 \(\times\) 10\({}^{1}\) \\ Terrestrial cootoxicity & kg 1,4-DCB & 1.73 \(\times\) 10\({}^{7}\) \\ Freshwater ecotoxicity & kg 1,4-DCB & 2.65 \(\times\) 10\({}^{5}\) \\ Marine ecotoxicity & kg 1,4-DCB & 3.43 \(\times\) 10\({}^{5}\) \\ Human carcinogenic toxicity & kg 1,4-DCB & 2.02 \(\times\) 10\({}^{5}\) \\ Human non-carcinogenic toxicity & kg 1,4-DCB & 1.22 \(\times\) 10\({}^{7}\) \\ Land use & m\({}^{2}\) a cropeq & 7.68 \(\times\) 10\({}^{6}\) \\ Mineral resource scarcity & kg Cu eq & 5.50 \(\times\) 10\({}^{4}\) \\ Fossil resource scarcity & kg oil eq & 9.44 \(\times\) 10\({}^{5}\) \\ Water consumption & m\({}^{3}\) & 1.47 \(\times\) 10\({}^{4}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Characterized ReCiPe midpoint impacts for growing and planting 2.34 million tree seedlings. total investment by the MCN administration and disregarding other benefits provided from the presence of urban forests.