Tree canopy cover is a very important indicator of the ecological quality of urban centres. It is the surface area of tree canopies or, in other words, the area of leaves, branches and plant stems that covers the ground when observed from above. "It doesn't just indicate the quantity of trees we have in our cities, but also their quality. Every tree has a different leaf surface area, and a healthy tree usually has a large leaf surface area," explained Maria Chiara Pastore, scientific director of the Forestami project, during an international conference on tree canopy cover held at the Poldi Pezzoli museum in Milan.

The event, organised together with Gruppo Prada as part of the Forestami Academy, took place close to the International Day of Forests on 21 March. This year's theme, "Forests and Economies", fits perfectly with tree canopy cover. As Gherardo Chirici, professor of urban ecology and remote sensing at the University of Florence and scientific director of the Observatory for Urban Forests of the Fondazione per il futuro delle città, explained, measuring canopy surface area is the first step towards determining the ecosystem services provided by trees, including the amount of carbon dioxide sequestered, the amount of particulate matter removed from the atmosphere, and their contribution to temperature mitigation.

Tree canopy cover is the surface area of tree canopies or, in other words, the area of leaves, branches and plant stems that covers the ground when observed from above.

What role does tree canopy cover play in the European Nature Restoration Law

The Nature Restoration Law (Nature restoration law), the first European regulation to directly address biodiversity, "establishes that protecting nature is not enough, because ecosystems also need to be regenerated," as Benjamin Rolles, policy officer at the European Commission, pointed out. And above all, "it recognises the role of cities in this process, with specific targets that include tree canopy cover." Until 2030, cities will be required to commit to avoiding any reduction in tree cover; after that, they will need to work towards increasing it.

"Small municipalities have limited resources to carry out urban green monitoring. The Nature Restoration Law has finally included them in national monitoring plans, recognising the role of trees in cities," added Ana Macias, lecturer in landscape architecture at the School of Engineering in Fuenlabrada. "Tree canopy cover is an indicator that goes beyond the number of trees and allows complex geographical analyses to be carried out at the level of individual neighbourhoods, making it possible to measure climate vulnerability and monitor environmental justice." When assessing national implementation plans, according to Dr Macias, the EU will need to take into account different urban morphologies: tree sizes can vary considerably between the northern and southern regions of Europe, not least because streets in Mediterranean cities are often very narrow.

How tree canopy cover is measured

Tree canopy cover can be estimated by analysing aerial or satellite imagery and integrating it with other territorial data, such as impermeable surfaces and infrastructure, in order to identify both existing coverage and areas suitable for new planting. The "sentinels" of Copernicus — the satellites used in the European Union's Earth observation programme — play an essential role in this regard. Satellite monitoring is based on the fact that each plant species reflects light differently, and that healthy trees display different reflectance properties compared to those under stress, as highlighted by Phillip Harwood, project manager of the Stakeholder Engagement Facility at the European Space Agency.

What the 3-30-300 rule is

"Tree canopy cover is also a useful indicator for measuring the psychophysical wellbeing of citizens," added Professor Chirici. The 3-30-300 rule, devised by forest ecologist Cecil Konijnendijk, establishes that cities should have at least three trees visible from every home and 30 per cent tree canopy cover in every neighbourhood, and that every citizen should be no more than 300 metres from the nearest green space: these are some of the factors that affect people's quality of life.

As Chirici explained, the spatial distribution of urban green areas should respect criteria of accessibility, equitable allocation, and ecological connectivity. It is on assessments of this kind that the interventions set out in Milan's Green Plan are based — a voluntary strategic instrument that complements general urban planning and aims to improve the quality of urban landscapes, air quality, climate adaptation, the protection of waterways from evapotranspiration, and the consolidation of riverbanks. Among the measures envisaged are the Milano Green Circle 90/91 project, which aims to transform the trolleybus ring of lines 90 and 91 into an ecological corridor, and the remediation of the "Goccia" area, a former industrial site between the Bovisa and Villapizzone neighbourhoods.

What is digital twin modeling

"The starting point was a complete census of the municipal green heritage," stressed Paola Viganò, director of the Green Areas department at the City of Milan. "We also used digital twin modeling to create digital twins of 30,000 trees out of the 250,000 catalogued, selected on account of their positioning along tree-lined avenues and roads." Digital twin modeling is a technological tool based on the use of photogrammetry, laser scanners and artificial intelligence to acquire virtual copies of physical objects, such as trees, in order to monitor their condition, simulate their evolution over time and support more efficient and sustainable decisions in urban green space management.

Monitoring is undoubtedly an essential aspect: all trees require proper care, especially during the growth phase. And the role of citizens is fundamental in ensuring that trees grow healthily: "Citizens who wish to contribute to the Forestami project can take part both in planting activities and in monitoring programmes, by adopting a tree or helping to check the health of adult plants and of younger, more fragile ones," explained architect Stefano Boeri, president of the Fondazione Forestami's steering committee.

What is the goal of Forestami Academy's new educational project with Gruppo Prada

Gruppo Prada believed from the very beginning in the importance of engaging and educating citizens, and is now ready to invest in a new chapter of Forestami Academy: this year, thanks to a new educational project, one hundred fourth- and fifth-grade primary school classes in the metropolitan city of Milan will be able to access a dedicated programme on tree canopy cover. "We are convinced that children will start a constructive dialogue with their parents," concluded Maria Chiara Pastore. The initiative also aims to contribute to Goal 4.7 of the 2030 Agenda: "Ensure that all learners acquire the knowledge and skills needed to promote sustainable development." Because only in this way will it be possible to cultivate a widespread awareness, one capable of taking root in the present and growing into the future.