Why do tree leaves turn yellow in the fall?

A striking characteristic of autumn, the yellowing of leaves is linked to the reduction in the quantity of light received by vegetation.

It’s autumn, since Sunday, September 22, and the vegetation is starting to change color in places, to slowly but surely take us towards these pleasant shades of yellow, red and orange. In autumn, the days get shorter. Since the beginning of September, we have been losing a few minutes of sunshine every day. We lost another four minutes between Sunday and Monday, September 23. This decrease in the amount of light received by the vegetation pushes the trees to perform less photosynthesis. They slow down their metabolism and the chlorophyll, which is green in color, is destroyed, to make way for tannins and pigments, which color the leaves red, orange-yellow, until the leaf dies, which then takes on a brown color and ends up falling off.

Temperature or water reserves also play a role. When this yellowing occurs earlier, sometimes in August, it is due to high temperatures or water stress. This was not the case in 2024. The trees are still quite green, some are even continuing to flower due to the mild weather and abundant rain. Some 70% of water tables are above their usual level, indicates the latest report from the Bureau of Geological and Mining Research (BRGM), with the black spots being in Roussillon, Languedoc and certain areas of Corsica. However, everywhere in France, with shorter days, the foliage will end up changing color.

This mechanism allows vegetation to cope with the drop in temperatures, knowing that in any case the cold destroys most of the leaves and makes photosynthesis impossible. Furthermore, biologists at the Natural History Museum explain that by renewing their foliage every year, trees also get rid of certain parasitic fungi or unwanted insects. This is called autumn cleaning. There are also evergreen trees. These have leaves that are more resistant to the cold. If you look at pine needles, olive or camellia leaves, they are thicker and covered with a sort of varnish that protects them from the drop in temperatures. As a result, this vegetation has another strategy to save its energy. It keeps its leaves so that it does not have to grow them back in the spring.