What triggers the leaf fall
Trees as cannibals
Before the autumn leaves fall, many trees utilize the nutrients from the leafy green
In these days, parks and deciduous forests develop their most intense autumn colors. Almost everything can be seen, from green alders to yellow beeches and already bare horse chestnuts. But why are the leaves so colorful? For the same reason as they later throw off the dried-up leaves: It is about the economic use of the scarce sunlight and nutrients. In autumn, messenger substances in the leaf stimulate the formation of a cork tissue at the base of the leaf. This tissue grows between the branch and the petiole and impairs the supply of water and nutrients to the leaf. At the same time, the green leaf pigment is broken down. The individual breakdown products are transported back into the bark of the tree, because the leaf pigment chlorophyll contains nitrogen and magnesium, two substances that the tree has to manage. And so there is actually a discoloration behind the color of the autumn leaves. After the green leaf pigment has broken down, carotenoids remain that were previously hidden by the green. In summer, these red and yellow leaf pigments had the task of intercepting excess light energy and converting it into fluorescent light. In doing so, they protected the photosynthetic apparatus, which binds the carbon dioxide, against an excessive energy load. Now in autumn they have lost their job. When the leaf dies when oxygen is supplied, some carotenoids are converted into phaeophytin, which is brown. However, some trees do not use chlorophyll recycling because they suffer less from nutrient scarcity. The ash, for example, only grows on well-supplied soils, where there is no shortage of nitrogen and magnesium. Therefore, it can afford to shed its leaves green. Alder and locust trees, which can use the nitrogen from the air with the help of symbionts, forego the expense of recycling the green dye in their households. It differs from species to species which environmental change triggers the shedding of the leaves. Some trees shed their leaves because the days are getting shorter, other trees react more to the lower temperatures. The cherry, for example, sheds its leaves when night temperatures drop below a certain threshold several times. The plane tree is different. It only loses its leaves when there is no longer enough light. In cities, for example, you can see that plane trees are still green on the side facing a street lamp even in November, while the side of the crown, which is not artificially illuminated at night, is already bare. The length of the day also determines the fall of leaves in the beech. The short life of most of the deciduous leaves has decisive advantages over the evergreen needles. A bare tree withstands winter storms better, but more importantly, the lower water loss in winter, which in the second half of winter often clogs conifers so much that they come into spring with brown needles.
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