Question: How do Parisian trees react to drought?
Answer: This summer we are seeing an early autumn on the plane trees and the chestnut trees, with leaf fall, which is a quite usual and harmless survival mechanism since they will probably reproduce some by autumn.
The air is increasingly dry and increasingly hungry for water, which accelerates evaporation for the plants, which sweat more to avoid burning, while continuing with difficulty their work of photosynthesis. In this very difficult context, some species are therefore temporarily getting rid of their leaves.
On the other hand, the beech, like the ash, is experiencing a climatic decline, due to water stress. These very old and very seasoned trees no longer die from classic causes but from the lack of adaptation to this new climate.
Q: Should these less suitable trees be replaced?
A: We are torn between the need to preserve the historical heritage, these species chosen with great care more than a century ago (during the Alphand plan, Haussmann's great gardener, editor's note) and the need to change them when we see that they are withering away from the climate.
For example, we now avoid when we can, so unless there is a heritage necessity like on the Champs-Elysées, planting rows of plane trees because this species no longer has a future.
We are introducing more and more non-indigenous species, mainly Mediterranean, such as hackberry from Provence, service tree or even pine and they behave well. We even have a few palm trees, but we're not making rows of olive trees!
We talk a lot about the heat, but every winter we cross our fingers. If it's -10°C, it's over.
Do not abandon these trees or start replacing everything. Plants must be stimulated so that they adapt and mutate. You have to nurture their power of resilience.
Q: How can urban trees be helped to become more resilient?
A: Paris soils are very hostile. The water tables are deep (more than 16m), the ground is made of gravel and the source rock is limestone which is not nutritious. The trees therefore mainly drink from rain and watering, in particular cleaning the soil, which helps them a lot.
For essences, it will be necessary to favor plants that are super dynamic at the root level, to compensate for water stress, those with a network of roots that go down well and are able to merge with fungi that will help them to supply themselves with water.
For their planting pits, we have good dimensions, between 9 and 12m3 according to the Alphand plan, we must not go below. It is necessary to better protect the trees from trampling, to sensitize the Parisians to move away from the trunks and to be wary of the grass, appreciated by the users but which makes the war of the roots in the basement.