The fireworks of colors in the forest begin in autumn. But how was that again with the trees and the leaves?
Many trees have a problem these days: the dark, cold season begins. The sun’s rays increasingly lose their strength, and so photosynthesis is less and less worthwhile. Even more: the trees not only threaten to freeze to death, they also threaten to die of thirst.
A large beech tree, for example, can lose 400 liters of water a day, solely due to evaporation through the leaves. Normally this is not a problem, after all, new water is always refilled with the help of the roots. Not so in winter: when the ground is frozen, the tree can no longer absorb additional water. However, since, provided that the air temperature is appropriate, the evaporation continues through the leaves, the tree literally dries up.
It gets even worse when not only the ground but also the ambient temperature falls below 0 degrees Celsius. Now the leaves are also acutely threatened with freezing. If the water in them solidifies to ice, the expanding volume bursts the cell walls and literally tears the leaf to pieces. For many trees, it is much more attractive to take a break and wait for the sun’s rays in spring.
Since the leaves do more harm than good, they have to be disposed of. Practical: Before being dropped, any pollutants that may still be present can be transported into the leaves and thrown away with them. In this way, detoxification also takes place. However, this is not as simple as it sounds. Rather, the tree has to expend quite a bit of energy to prepare for winter. The leaves are not simply thrown off, because they still contain valuable ingredients such as nitrogen or phosphorus compounds that the tree can use again in the next growth phase in spring – throwing these away would be a waste that nature does not allow itself . Important plant juices are therefore transported into the trunk and also into the roots to be stored there until the next spring.
The bright colors of the autumn leaves resulting from this process are a side effect of the breakdown of chlorophyll. Because this plant pigment is converted into NCC, a colorless substance whose full name is Nonfluorescent Chlorophyll Catabolite. Since the green of the chlorophyll is now missing in the leaves, other dyes that were previously covered by this come into their own. The yellow-orange carotenoids, for example, which are also responsible for the color of the carrots, now dominate the leaf color. Other dyes, on the other hand, are specially produced in autumn, such as the anthocyanins, which also color blackberries and aubergines red-violet.
After the important constituents of the leaves have been removed, the tree could actually shed the leaf. The problem: fungi or bacteria could penetrate the resulting “wound” and damage the tree. The solution is extremely practical: a cork-like separating layer slowly forms at the interface between the leaf stem and branch, which gradually interrupts the supply veins. After the usable substances have been removed, neither water can flow in nor pests can penetrate. The place is now sealed in the truest sense of the word.
All of this can happen very quickly: In just one week, a deciduous tree can break down chlorophyll and thus prepare for winter. A warm, sunny late summer, however, delays this process and gives us a beautiful, long and colorful autumn. In some parts of the world this is the most beautiful time of the year. In Canada it is called “Indian Summer” and in Lapland “Ruska (-Aika)”.
In some places it can be nice and colorful here in autumn, but there is a good reason why Canada and Lapland are famous for their autumn. There are huge forests here with a much greater biodiversity than here. In North America, for example, there are many hundreds of tree species, while in Western Europe there are just a few dozen. It goes without saying that the color fireworks tend to be shot less often in our monocultures than in regions of great diversity. But: a walk in the forest is also worthwhile with us.
