Context and Background
Urban challenges: air pollution and climate change
Mass urbanization is one of the most urgent challenges of the 21st century, i.e. 82% of European population will live in cities in 2030. Air pollution and urban heat island are two major problems currently affecting urban areas. These environmental problems are a major public health issue, with half a million premature deaths in Europe Union and considerable economic costs, estimated at €644 billion in Europe in 2015 (OECD, 2016). By 2050, the contribution of outdoor air pollution to premature mortality is estimated to double (Lelieveld et al., 2015).
Mediterranean cities are expected to be more strongly affected by climate change than most of the other regions of the world. The air temperature could significantly increase at regional scale by 2-5°C, and up to + 7°C in cities by 2100. To date, tropospheric ozone (O3), particulate matter (PM2.5, PM10) and nitrogen dioxide (NO2) are the most threatening air pollutants in terms of harmful effects on people (Pascal et al. 2014).
The summer 2019 showed two heat waves and air pollution peaks across Europe, which raised a considerable awareness in public authorities about the imperative urgency to both mitigate the effects of air pollution and climate change (e.g. heat waves, effects on health) and adapt to them.
Greening cities: win-win strategies co-benefitting air quality and climate
Cities have to cope with rising poor air quality impacting human health, quality of life and citizens’ well-being. A logical way to improve air quality is reducing air pollutants emissions, but it was suggested that green urban infrastructure, especially trees, could be also used to clean polluted urban air. The vegetation facilitates deposition of particulate matter and gases on plant surfaces and absorbs gaseous air pollutants (NO2 and O3) through leaf stomata (Fig. 2).
Greening and re‐naturing cities, keywords of the 2030 EU biodiversity strategy, is a solution addressing multiple benefits important for human well-being: reduction of air pollution and greenhouse gases, carbon sequestration, air temperature regulation, runoff mitigation, noise reduction as well as recreational, social (e.g. improvement of health) and aesthetic benefits (WHO, 2016).
Environmental hazards in summer 2019 have also opened the door to testing new ways of planning and managing urban forests e.g. in Milan (3 million trees planted by 2030) and the region “Sud” in France (1 million trees by 2022). However, a few municipalities have hurriedly planted any tree species anywhere, and these tree planting strategies have degraded air quality and increased asthma exacerbation.
Suitable selection of plant species for quality of life in cities
Disservices of urban forests can be 1) environmental, such as the release of biogenic volatile organic compounds (BVOC) leading to ozone formation; 2) social (e.g. allergenic pollen) and 3) financial such as pruning. To efficiently reduce air pollution levels in cities and maximize the health benefits for citizens, both municipalities and city planners urgently need a suitable selection of tree species as well as guidelines for tree planting and maintenance strategy to maximize the urban forest benefits and minimize disservices.