Le Télégramme: Why are trees losing their leaves this summer?
Aurélie Gousset: Two reasons explain this phenomenon: successive heat waves and the lack of water in the soil. Like humans, trees need to transpire: 75 liters of water per day for a birch, 300 liters for a poplar and 1,000 liters for an oak. To compensate for this loss, the tree must recover water at ground level. With the drought and the lack of rain, the tree will therefore adopt strategies to save itself, such as closing its stomata (pores on the surface of the leaves which allow gas exchange in the tree) in order to sweat less. The leaf will therefore dry up and fall from the tree.
Will some trees be more affected?
Some have preferences. Ash trees, willows and poplars will be very sensitive to lack of water. In the middle mountains, beeches will be affected by the heat because they prefer cool temperatures. There can also be differences within the same species. In our laboratory, we work on moderate water stress. We observe in particular, in the long term, a disappearance of beeches in the middle mountains, replaced by more Mediterranean species.
Is this a phenomenon that is accelerating?
Trees are quite resilient if they lose their leaves once. A one-time event will therefore not affect them. The problem, however, is that there are more and more heat waves. In the event of a lack of water, the trees can make air bubbles, which prevent the circulation of water, but which are not reversible. If the heat waves and the water deficit are repeated, this is a phenomenon that is likely to increase.
Does it matter if the tree is in town or in the countryside?
The trees will be more affected in the city because of the bitumen that surrounds them. If the rain falls, then the water will slide on the road and not soak into the ground. In parks or in the countryside, water will more easily seep into the ground. The “ideal” situation for a tree is to be in the forest.
Can this loss of leaves have an impact on the fruit?
Fruits are an important carbon sink and do not photosynthesise. They therefore require a large investment for the tree. To protect its reserves, the tree can therefore decide to get rid of the smallest fruits and preserve the “best”. There are also a number of oak trees that have already lost acorns and hazel trees that have lost hazelnuts.
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