170,000 trees to fight against global warming
It was one of Anne Hidalgo’s great campaign promises in Paris: to make the capital green. The tree plan of the town hall is presented today to the Council of Paris. The goal is to plant 170,000 trees in 6 years.
It is on average 2.1 degrees cooler under the canopy of trees than outside, in summer.
The capital already has 500,000 trees, 300,000 in the Bois de Boulogne and Vincennes and 200,000 in intramural Paris. According to Christophe Najdovski, deputy mayor of Paris, in charge of green spaces and biodiversity, “It is about enriching this heritage and ensuring that we can plant wherever possible: in the streets, in squares, in new green spaces. We must move from a city with gardens to a garden city ”. We must therefore plant wherever possible and create mini urban forests. The first will be Place de Catalogne, next to Montparnasse station.
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The consultation will begin and other plantings are planned, for example, on the forecourt of the Town Hall. The tree is the ally of cities in the fight against climate change and in particular against the heat waves to come. Christophe Najdovski affirms that there is a need for massive greening in the cities, “We have to make sure that we are within 300 meters of an island of freshness, especially during a heat wave. The tree captures pollutants, it shelters biodiversity and also provides psychological well-being ”. Paris also wants to preserve the existing. The tree felling tax will be raised by 32% and a charter will be signed with public and private operators to avoid felling or to compensate during construction sites. CNRS researchers have installed temperature sensors in the heart of the forests and compared them with the temperatures around. In Europe, the average result shows that it is 2.1 degrees cooler under the canopy of trees than outside, in summer.
“Under a canopy cover of less than 40%, the insulating effect will not be sufficient”
The trees play the role of parasols and above all they refresh the atmosphere thanks to their perspiration. According to Jonathan Lenoir, CNRS researcher and main author of this study, “We have an effect of transpiration of the trees which is done by the leaves with their stomata which will open to let water vapor out. It is this effect of pumping cold water from the ground to the atmosphere which, through perspiration, will cause the temperature of the ambient air to drop ”.
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Can mini urban forests play this same insulating role? This will obviously depend on their size but also on the tree cover according to the researcher, “If we have more than 50% canopy cover, the effect will be positive because the temperatures will drop during the summer periods. On the other hand, under cover of less than 40%, the insulating effect will not be sufficient ”. This insulating effect can disappear if the trees are subjected to heat waves and severe droughts. They protect themselves, sweat less and waste away. We must therefore choose the species of trees that will be planted. The mayor of Paris has been working on the subject since 2018 and should soon publish a guide to the most authorized species.
Baptiste Gaborit