Plant 170,000 trees in the capital by 2026. This is the goal set by the Paris City Hall for the second term of office run by Anne Hidalgo (2020-2026). An ambitious bet, which would amount to planting nearly ten times more trees in six years than during the previous term, when the target – which the city claims to have reached – was set at 20,000. which could reach those of Canberra (in Australia) within thirty yearsParis is looking for answers in revegetation, and relies in particular on trees, CO2 absorbers, to improve air quality and limit “urban heat islands”, these localized increases in temperature due to the nature of the soil.
And for good reason: trees can reduce air conditioning needs by up to 30%, according to the UN, and guarantee temperatures under their foliage 10 to 20 degrees lower than those on the bitumen in full sun, within the same perimeter. Hence the will of the mayor of Paris. “To date, the capital has about 200,000 trees in total, and just over 38,000 trees out of the 170,000 targeted for 2026 have been planted since the end of 2020″, argues to L’Express the deputy in charge of revegetation at the town hall of Paris Christophe Najdovski. If the approach is welcomed in principle by the associations, they however deplore “a blow of com”, giving “the pride of quantity rather than quality”.
Can Anne Hidalgo’s team keep their promise? For environmental associations, this objective is untenable. “This figure seems quite unrealistic”, confides for example the co-president of the National Tree Monitoring Group (GNSA), Alexis Boniface. And his colleague Thomas Brail, activist and founder of the association, added: “I find it difficult to understand how we are going to succeed in putting all the trees if we continue to make such significant progress in terms of concreting”. According to the calculations of the architect and scientific adviser of France Nature Environnement (FNE) Paris Tanguy Le Dantec, “reaching 170,000 trees would mean freeing up a total area of 5.7 square kilometers, i.e. the area of the 17th arrondissement, this which is materially impossible”.
Controversial Miyawaki method
To meet its objectives, the mayor of Paris intends to use an innovative planting method straight from Japan and consisting of planting trees very close to each other. On the ground, it would allow the Town Hall to massively vegetate the embankments of the ring road, where the town hall is aiming for 70,000 new trees by 2026. This method called “Miyawaki” is however far from unanimous, and heightens the tensions between elected municipal officials and associations. On the one hand, the municipality maintains that “each tree planted today is intended to live at least until 2100” and boasts of choice of tree species “diverse” and “resistant” to high temperatures (hackberry, holm oak…).
On the other hand, the associations are less optimistic and point to the poor quality of the trees planted: “it amounts to planting three per square meter, but the plants are so tight that 99% will not survive in 70 years. “, warns Tanguy Le Dantec, who doubts that the final balance of trees in Paris can really be increased with this method. And to add: “In a normal temperate forest, it’s 300 trees per hectare. The technique used by the city consists of putting a hundred times more! Knowing that an adult tree needs a minimum of 20 square meters of surface, one sprout in Miyawaki will eventually eliminate almost all the others. These are gimmicky plantations, which will not survive, and which simply allow the city to increase its balance sheet”. In Europe, one of the few scientific studies on the effectiveness of the Miyawaki method reports tree mortality of between 61% and 84% after twelve years, due to the high density, which exacerbates the phenomenon of natural competition between young trees for access to light. For the GNSA, “it is impossible to know how many will remain in place by 2100, but for sure, many of them will have withered”.
Replantings not equivalent
Beyond the viability of the new plantations, the associations also insist on the need not to sacrifice the trees already present for the benefit of real estate expansion. For them, if the plantations carried out in areas that are currently not vegetated are beneficial, the replantings following felling are much more questionable. “It must be understood that a freshly planted young tree cannot replace an existing tree, which has a much higher ecological value”, explains the co-president of the GNSA. “Young trees cannot draw water from deep and do not play the role of a natural refrigerator as much.” In Paris, “3,000 trees are felled each year, 90% of which for phytosanitary reasons and therefore only 10% for development projects”, however tempers the deputy in charge of revegetation at the Hôtel de Ville.
But the associations have made this 10% their hobbyhorse, and have one objective: to prevent their slaughter. “The concreting projects implemented at the gates of Paris (Porte Maillot, Porte de Vincennes, Porte des Lilas, Porte d’Italie, etc.) are problematic. At the Porte de Montreuil, 250 trees will be felled, and 76 old forty years have already been replanted last April, as part of a real estate project. Even if 216 trees are going to be replanted, a tree lost in the ground is obviously not compensated for by a young tree replanted on slab”, denounces the president of France Nature Environnement (FNE) Paris Christine Nedelec. And Tanguy Le Dantec adds: “To replace a hundred-year-old tree and find an equivalent leaf area, and therefore a comparable ecological benefit, it’s not one tree that needs to be replanted, it’s 125! We can’t mix tea towels and napkins”.
Contested expert opinions
As for the “90% of trees felled for phytosanitary reasons”, the associations display a certain skepticism. For the trees felled as part of the development of the Porte de Montreuil, an impact study published on the website of the town hall of the 20th arrondissement reported a “good or average sanitary condition” of these, which did not prevent the Town Hall from brandishing the argument of a “state of irreversible decline” of 110 of them to legitimize the slaughter. Some expert reports from the Town Hall attesting to the phytosanitary risks of certain trees are also contested by the associations, which attribute to them a lack of transparency, as these are rarely made public.
It now remains to be seen whether the policies put in place by the municipality will be sufficient to enable it to achieve another objective that it has set itself, by 2030 this time: increase its canopy index to 23%. This figure, which corresponds to the volume – projected on the ground – of the leaves of all the trees in a city, is now around 8-9% in the capital, according to the scientific adviser of FNE Paris, making of the metropolis one of the worst students nationally and worldwide, while this index exceeds 20% in Lyon and Strasbourg, and reaches up to 28% in Toronto, a world benchmark in this area.