what is the real impact of urban forests?

Decryption
The third urban forest in Paris should see the light of day in June, on the square in front of the Hôtel de Ville. Acting as islands of freshness, these groves are supposed to respond to the challenges of global warming in cities.

In the bitter cold of mid-January, a few workers are busy in front of the Paris City Hall. In a few months, the appearance of the emblematic square has changed significantly: it is now disfigured on its left by a gaping hole two meters deep, consolidated by wooden scaffolding. On the other side, in perfect symmetry, the ditch has already been filled with earth and trees with still wrapped roots have appeared.

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Eight months of work will still be necessary to deliver the third urban forest in the capital – after Place de la Catalunya in the 14^e district, and the woods of Charonne in the 20^e. With the aim of cooling this mineral heat island in the heart of 4^e district of Paris, around a hundred trees should be planted by spring. Rows of Burgundy oaks and common hornbeams, species common in the Ile-de-France forests, but also of American honeylocusts and Julian hackberries, supposed to better resist climate change, will soon bud.

In total, it’s almost 1,000 m²or between 25 and 30% of the space, which will be planted in the ground. To increase “the green surface”the town hall has chosen to cover the two fountains with vegetation, which will be dried up. “This new landscape will guarantee historic views, particularly of Notre-Dame de Paris” on the other side of the Seine, enthused the socialist mayor Anne Hidalgo during the visit to the site. All for a total construction cost of 6 million euros.

“Putting trees always has an effect »

It’s a fact, the mini urban forest is trendy. “Nature regains its rights over the city”affirmed with great fanfare the Paris town hall in 2019, when announcing the creation of the city’s first green groves, Place de la Catalogne. “It’s going to be beautiful but also much more pleasant and refreshing”welcomed Friday January 17 Anne Hidalgo, dressed in an orange construction vest. The city councilor can rely on the results obtained by the two other Parisian islands of greenery. According to Amélie Astruc, head of division within the green spaces department of the City of Paris, the 470 trees that make up the urban forest of Place de la Catalogne would have made it possible to lower the temperature in its direct surroundings. “up to 4°C”.

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If the socialist mayor’s detractors denounce greenwashing, Marc Saudreau, bioclimatologist at the National Institute of Agronomic Research (INRAE) and member of the Cooltrees project on cooling cities using trees, brushes aside the criticism: “Putting trees always has an effect. » Under the tree cover, “the temperature felt can be between 6 and 10°C lower than what one might feel in direct sunlight”explains the researcher. However, announcing that we are going to plant 150 trees “doesn’t mean much”he explains:

“If the trees are stunted, there can be very many of them without it having any impact. The important thing is that they are well leafed, to provide shade and sweat a lot. »

By encouraging the establishment of insects and birds, the mini-urban forest aims to become a shelter for biodiversity. Planting also contributes to the restoration of degraded soils and better CO capture.₂ – “several tens of kilos per year” – and the filtration of rainwater. “Not to mention the psychological benefits: greenery feels good”concludes Marc Saudreau.

Space, water and minerals

The cooling power of plants also has a size limit. Without a significant supply of water and minerals, trees have little chance of expanding their root systems. To develop, they also need a layer of humus of 30 to 40 centimeters and to form their vertical anchorage in the first ten years. “Two meters deep doesn’t seem like much for planting”concedes Marc Saudreau. Finally, their age is a risk factor. “Planting so many large trees in such a small space is a first”recognizes Amélie Astruc. “The chances of recovery are better when trees are planted younger”admits the specialist, who justifies the choice of having selected older trees by the need for “made immediate and majestic”.

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However, the urban forest alone does not solve anything: Marc Saudreau points out the imperative to demineralize cities. “Between bitumen, impermeable concrete surfaces, the metal of buildings… Cities accumulate solar radiation all day long and release heat at nighthe points out. Planting is good. But we must also stop artificializing. » Behind the controversy over the Place des Arbres, it is the densification of Paris which is at stake, while, lacking large intramural parks, the city displays a ratio of nature per inhabitant less than half at 10 m² prescribed by the World Health Organization. In the meantime, the town hall promises to continue to green the streets of the capital.

Work on the fourth urban forest, place du Colonel-Fabien (10^e et 19^e arrondissements) will begin this week – they will last “un the”indicates Thomas Chevandier, deputy in charge of public construction and construction sites. While 50 trees are being planted on the Quai aux Fleurs, a citizen vote will allow, in March, Parisians to decide on the “garden streets “. Before the end of Anne Hidalgo’s mandate in 2026, the municipal team will have “debitumé 60 hectares”promises the councilor, who wishes to increase this figure to « 300 hectares » in 2040.