In autumn the chestnuts fall, which the schoolchildren pick up with joy. The days become shorter and the trees are adorned with delicious colors, whose ephemeral character reinforces their fragile beauty. Gradually, the majority of the deciduous trees in our regions will lose their foliage. It is said that trees put their activity on hold. They are preparing to go into a form of hibernation during which they are not completely inactive. Here's how it happens.
In order to protect themselves from the rigors of the cold that they sense coming, trees form a layer of insulating cells at the base of the petiole, thus blocking the vessels that supply the leaf with water. The petiole, which means "little foot" in Latin, refers to the part of the leaf that connects the blade (the leaf itself) to the stem. The petiole has the same internal structure as a stem and is composed of a bundle of conductive vessels, which are blocked in autumn by the formation of a layer of cork allowing the abscission (detachment of the leaf).
The leaves, no longer hydrated, dry out and die. Other complex biochemical phenomena take place in the leaf, explaining its progressive color change. The green color is indeed due to the presence of chlorophyll in the leaf. Chlorophyll is the main assimilating pigment in plants. It is involved in the process of photosynthesis to intercept the light energy that will allow the tree to capture and transform nutrients from the soil. When autumn comes, the chlorophyll is gradually destroyed, the green color disappears and other colors appear, which were masked by the chlorophyll. Depending on the species, the leaves become yellow, orange, brown or red.
🌳 Trees adapt to the changing seasons
In the fall, trees enter a phase known as paradorming. They prepare themselves to face winter, which takes them a little more than a month. If they lose their leaves, it is to limit the evaporation of water, which however will continue in a very limited way throughout the winter, and not to give a grip to the frost in the woody tissues. As soon as the frost arrives, the water supply to the leaves is stopped. Thus, deciduous trees and shrubs no longer absorb water when the first frosts arrive and winter can surprisingly be considered a dry season for them.
Coniferous trees, with scales or needles, have foliage covered with an insulating cuticle that limits the effects of evapotranspiration. As long as the temperature is above 0°C, photosynthesis continues, so they do not lose their foliage, which is called persistent.
Some species of deciduous trees will keep their dried leaves on the branches until the following spring. This is particularly true of beech and oak trees. Their leaves are said to be marcescent.
🍂 Why do the leaves change color ?
Because they die!
Leaves are green in spring and summer. This color is due to the presence of chlorophyll. In autumn, the leaves start to die. The chlorophyll is destroyed, little by little. The green color disappears and other colors appear. They were masked by the chlorophyll. The leaves become yellow, red or orange. It depends on the trees you are looking at. What a show!
☀️ The solar collector
Through chlorophyll, the leaf absorbs the light of the Sun. It is transformed into energy. The tree uses this energy to capture and transform nutrients from the soil. When the leaves fall, the tree slows down its life rhythm... until the next spring!
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