Results
Air quality and climate benefits
The city scale is relevant for climate policy, although cities cover less than 2% of the Earth surface, urban areas produce more than 70% of GHG emissions. Science-informed nature-based solutions are recommended as a proper tool for adapting to climate change. Trees help to regulate climate by taking up CO2 from the air and storing carbon. Urban forests represent one of the recognized nature-based solutions for increasing the sustainability and resilience of cities to climate change. In addition to the reduction of air temperature, the concrete actions focus on the reduction of air pollution, including CO2 but also O3 (and its precursors) classified as “Short-Lived Climate Forcer” with a shorter lifetime in atmosphere than CO2 (that is hundreds of years) and their reduction can reduce air temperature faster than acting only on reducing CO2 levels. Acting locally on the reduction of CO2, O3 and NO2 is a “win- win strategy”.
Within the test area (with 400 efficient mixed tree species), we observed air quality improvement of 16.3% PM2.5, 7.6% PM10, 2.6% O3, 1.7% NO2 and 0.9% CO2 compared to surrounding area. Each reforested area eliminates annually: 1.5 tons O3, 170 kg NO2, 150 kg PM10, 21 tons CO2 and increase carbon stocks (2.6 tons). At city scale, compared to the current tree cover, each 5%-point increase in tree canopy cover could reduce by 3.1% and 1.4% the annual PM2.5 and NO2 mean concentrations, and by 1.3% the summertime O3 concentrations in each city. In 2030, each reforested area will have eliminated at least: 12 tons O3, 1.4 tons NO2, 1.2 tons PM10, 168 tons CO2 and increased carbon stocks (20.8 tons).
Below small groups of trees, midday air temperature at 1.5 m above ground is 1.5°C cooler than in open area, and 2.0°C cooler by 2030. Reduced air temperature can improve air quality because the emission of many pollutants and/or ozone-forming chemicals are temperature dependent. Decreased air temperature will reduce O3 formation.
Ecological benefits and environmental footprint
Greenness - In 2022, 2.2-ha of new green areas with 800 trees newly planted in the 2 front- runners cities. Compared to 2021, the number of public trees, i.e. managed by the municipality, increased by 1.3% and 0.5% in Aix-en-Provence and Florence, respectively. By 2030, we expect a 5% increase in tree cover in both cities. Public green areas increased by 0.5% and 0.4% in Aix-en-Provence and Florence, respectively.
Biodiversity - The new green spaces are important for biodiversity enhancement and conservation. The new green spaces contribute to enhance biodiversity by providing habitat and food for pollinators and birds and supporting native pollinator communities within the city. By selecting attractive plant species for pollinators, the new green areas act as an extensive network of pollinator-friendly habitats. The Ecosystem Service Provision Index (ESPI) increased by 19% between 2021 and 2024. With proper planning and management of green spaces, cities play a major role for biodiversity.
Soil biodiversity - Initial and upgraded status of ecosystems in terms of biodiversity of a range of micro- (bacteria, fungi) and macro-organisms (e.g., invertebrates, plants) and important ecosystem functions and services have been quantified and included in the ESPI.
Environmental footprint - A Life Cycle Analysis was accomplished to calculate the Carbon Footprint linked to nursery cultivation, tree planting, and maintenance over time. The Carbon Footprint was equal to 14.7t 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.
Socio-Economic benefits
Direct economic impact of the newly planted areas
LIFE AIRFRESH has provided an impulse to start the greening actions by re-naturing the first 2.2-ha area in two cities. The direct economic impact is mainly constituted by the implementation of green spaces (e.g., 146,650€ in Aix-en-Provence and 19,000€ in Florence) and maintenance (ab. 9,600€ a year for irrigation and pruning).
Economic valuation of air pollutant removal
Each reforested area has eliminated in 2023: 1.5 tons O3, 170 kg NO2, 150 kg PM10, increase carbon stocks (2.6 tons). In Italy and France, each reforested area could prevent damages valued at €19,640 per year i.e., reaching a total benefit of ab. €78,560 in 2024 and ab. €314,240 by 2030 for both test areas.
Quantification and economic valuation of health benefits
We have estimated the health benefits for each 5%-point in tree canopy in each city as we expect a 5% increase of tree cover by 2030 in both cities. In Aix-en-Provence and Florence, each 5%-point in tree canopy could prevent 2 and 10 premature deaths annually, respectively as well as 6 and 40 hospital admissions will be “prevented” annually. By increasing tree cover by 5% in each city by 2030, we expect a health benefit of about €8.6 and €43.1 million each year in Aix-en-Provence and Florence, respectively.
Raising public / citizens’ awareness
At least 470,840 people (incl. 760 decision-makers, policymakers, and city planners) were informed about the benefits of re-naturing cities for combatting air pollution and mitigating climate change and for improving biodiversity, health, and well-being of citizens. Assuming that 5% take into account the advice, the estimated impact is 23,500 people, in particular people who are most vulnerable, with less polluting and healthier habits. In 2022, we organized a tree planting ceremony in each city to draw the citizen's attention to the importance of nature, trees, and biodiversity in the city. In 2021, only 7% of people heard about the project, while 64% have been informed about the LIFE AIRFRESH project and activities at the end of the project.
New socializing open areas and Socio-cultural inclusiveness
The new green spaces can also markedly improve the quality of life of citizens by providing an area to meet others, to play sports and part of an active healthy lifestyle with opportunities for activities such as biking and jogging. The new area also provides a neighbourhood-park effect. A cross-sectional observational study of 702 people aged 18+ years has been conducted in both cities to collect data on level of satisfaction with green spaces.
From the results (Annex 23), we observe a clear improvement and better understanding related to the air pollution issues and urban trees. Focusing on the AIRFRESH test area, the results indicated that people spent on average 77 minutes in nature in the past week in 2021, while they spent 304 minutes in the past week in 2024 with an increase of the number of visits (on average, 0.6 in 2021 vs 2.0 times per week in 2024) in particular for walking and playing with children. Based on literature, from 120 minutes onwards in nature weekly, people have reported better levels of health or well-being than those who did not spend any time in a green space (White et al., 2019). People have attributed a good satisfaction score (6.9 out of 10). The new green space is used by all ages and social categories, regardless of income, and contributes to an attractive green townscape and thus communicates the image of a nature-oriented city.