From now on, the Metropolis will no longer plant paths of this type, very airy with a single species of large trees. The plantations will be much denser. (Picture HJ)
The temperatures abnormally high in October upset the flora in the Lyon region. Fruit trees have blossomed again, deciduous trees have not lost their leaves… Pierre Athanaze, Vice-President of Lyon Metropolis in charge of the Environment, Animal Protection and Risk Prevention, tells us how he works with its teams to better adapt the urban area’s green spaces to global warming. A flowery interview.
What was the impact of the October heat wave on the vegetation of the Metropolis of Lyon?
People were impressed by the temperatures we had in the fall. But for the trees, what was very problematic was the drought which started very early this year, in February. Then there was the heat wave which was added. We had already experienced this overlapping of a drought and a heat wave during the summers of 2018, 2019, 2020. This year what was special was the duration of this episode. We had more days with a heat wave than without a heat wave for three months this summer. And we are still in a drought situation. In 2021, a rainier summer, there are a lot of plants that have rested, but they have not been able to completely replenish their water reserves.
With this prolonged drought, is it possible to continue the revegetation of Grand Lyon that you advocate?
On what we have planted in terms of trees this year, we have barely more mortality than a normal year. We had indeed the prefectural authorization to water our young trees until noon this summer. On the other hand, trees between 8 and 12 years old suffered a lot. They didn’t have the necessary water reserves and you couldn’t water them. It is also normal, we cannot water the 100,000 trees (excluding parks and natural spaces) which are planted in the metropolis. A tree is watered for up to two or even three years.
This summer was very spectacular, but we had anticipated it by discussing with the departmental directorate of Territories (DDT). We were able to get permission to water the recently planted trees. It is our joker to fight against global warming. It would have been crazy not to be able to plant new trees this year.
Are you planting new tree species to deal with global warming?
What you need to know is that there is not one species that will withstand everything in terms of climate. We always plant according to the place and its specificity. If we had planted lime trees on the Cours Charlemagne, which is an oven in the summer, it would have been a failure. We therefore planted Mediterranean or exotic species. Conversely, when we plant urban forests, we will plant local species. These trees will protect each other. So there is no question of replacing all our native trees with exotic species, as we are sometimes criticized for doing. When you plant an exotic species, there is little moss or insects that come to pass on it, because it is an unknown species.
Where things need to change is in the planting conditions. You need to plant more densely. Before, the Metropolis planted trees separated by 12 to 15 meters to put parking spaces in the middle. Now, we prefer to plant benches of trees that allow you to immediately feel the freshness. Taller trees are planted on the extremities of this line of trees and in the middle are planted shrubs which will be protected by the shade cover of the taller trees. We also avoid surrounding the trees with borders that prevent water from arriving. It is practically more important to plant the trees in benches with different strata than to plant this or that species.
Will we still see plane trees in a few years on the avenues of Lyon?
Yes, we will still see plane trees. But we have the colored canker which prompts us to cut down the affected plane trees immediately. The prefecture obliges us to cut down within two months when there is canker and we cannot replant a plane tree where there has been canker. In the 1980s, 75% of plane trees were planted. There were a lot of reasons to do it, because the plane tree is pruning tolerant for example. but we were in mono-culture and the first parasites arrived.
Is global warming making trees more vulnerable to pests?
A tree that has had water stress will attract insects and fungi. If you go to the Jura, the spruces are stressed by the heat wave and the bark beetles come on the stressed trees. We have the same thing in town with the plane tree tiger. We had very few storms this summer and the plane tree tiger has multiplied. In the evening in the alleys lined with plane trees, the tiger came in through the windows of the inhabitants.