In Paris, the heatwave and drought are causing the leaves to fall before autumn.

On Parisian sidewalks, the early falling leaves already give the streets an autumnal feel. This phenomenon is a protective mechanism that allows trees to limit the effects of intense heat and lack of water.

"The leaves on the ground are a consequence of the heat waves of recent months. The tree is going into a resting, protective phase," explains Amaury Pollet, one of the heads of the forestry department of the City of Paris.

On Boulevard des Batignolles, in western Paris, the three tree species present—European hackberry, linden, and elm—are nevertheless "in very good health," he assures us. This leaf drop is said to be a "natural reaction" to the heat waves and droughts, as the tree is "very resilient."

On this sunny August day, the trees on the boulevard remain largely covered with green leaves and continue to offer shade to pedestrians.

A survival strategy in the face of water shortages

“Trees cope relatively well with heat waves. But when a heat wave is combined with a drought, at that point there is no more water, and the tree adopts a survival strategy,” emphasizes Serge Muller, professor emeritus at the National Museum of Natural History.

Transpiration normally helps lower the temperature of a tree's leaves. When water becomes scarce, their early leaf fall helps protect the tree from more serious damage.

“Trees transpire water and this transpiration lowers the temperature of the leaves. The leaves fall before the stress imposed by the drought causes irreparable damage to the tree,” adds Bastien Castagneyrol, a researcher at the National Research Institute for Agriculture, Food and the Environment (Inrae).

According to the two experts, we will have to wait until next spring to measure the real consequences of these climatic events. Bastien Castagneyrol believes that "we are not going to see trees dying en masse because of a drought," but fears "a gradual weakening" if these phenomena are repeated.

However, not all species react in the same way. Some are particularly vulnerable to drought, while others, such as plane trees, are more resistant.

"A plane tree is used to these heat waves, will lose its leaves and will regenerate very easily, like a Montpellier maple," explains Laurent Chadirac, an expert in the tree and wood service of the City of Paris.

Plane trees represent about 35% of the Parisian tree rows, while more than 800 tree species are listed in the capital.

Diversifying tree species to strengthen resilience

To better adapt its tree heritage to climate change, the City of Paris has accelerated the diversification of species, favouring, where possible, regional or native species that are favorable to biodiversity.

In the Place Prosper-Goubaux, in the 17th arrondissement, a common hornbeam shows signs of water stress. Laurent Chadirac points out the progressive browning of its green leaves, one of the signs of their deterioration.

The City decided a few years ago to gradually replace this rather forest-oriented species with hop hornbeam, considered more resilient and better able to withstand water stress and heat.

This new tree, of more southern origin, was fitted with a tensiometer probe installed in the soil. This allows its water needs to be assessed and the results to be extrapolated to other trees of the same species.

Since the heat waves, residents are also more concerned about the health of trees and in particular about watering young plantings during their first three years, notes the City of Paris.

“Generally speaking, municipalities use the diversity of trees as a factor in resistance and resilience to extreme climatic events,” explains Bastien Castagneyrol.

Over the past ten years, the City of Paris reports having planted 170.000 trees. It highlights their ability to absorb CO2, improve air quality, and cool the urban environment.

Mediterranean and exotic plant species for the city

On Rue de Naples, in the 8th arrondissement, two species of small trees native to Asia, a Parrotia persica and a Koelreuteria paniculata , have been planted in vegetated strips. According to Amaury Pollet, the whole area is doing quite well.

Serge Muller points out that temperatures are higher in urban areas than in forests. The researcher therefore advocates for the use of adapted exotic species, particularly those from the Mediterranean, Asia, or America.

He acknowledges that this strategy is already "more or less" adopted by the City of Paris and other municipalities, while believing that they could go further in this direction.