Fall: Deciduous trees color their leaves until they finally shed them. Why do leaves actually change color in the fall? Here is the answer to these questions
We are currently noticing the change of season in the increasingly shorter days and falling temperatures. But even a visit to the forest leaves no doubt that autumn is here. Why the discoloration of the leaves is essential for the survival of the trees and what hormones, among other things, have to do with it, you will learn in this article.
When do trees start to change color?
Just like us, trees notice that with the onset of autumn, the days become shorter. This means there is less light available to the tree. At the same time, temperatures begin to drop. The tree responds by shutting down its plant metabolism. At the same time, an aging process is set in motion in the leaves. By the way, this all happens through plant hormones.
The trees lose an immense amount of ballast during leaf shedding. The birch, for example, sheds up to 28 kg / 56 lbs of leaves on average.
What exactly happens during this process?
As already mentioned, the shedding of leaves is not a coincidence, but a process directly controlled by hormones. Let’s take a closer look at this process. The shedding of leaves takes place in two steps:
In the first step, the tree begins to decompose important components within the leaf, or to conduct them back to the shoot. There they are stored in the twigs, trunk or roots until the next spring. These components include chlorophyll, proteins and minerals such as nitrogen. Chlorophyll is responsible for the green coloration of leaves and covers all other color pigments in summer. Due to the deprivation of chlorophyll, leaves lose their green color and other hues such as yellow or red become visible. Responsible for this are other chemical ingredients such as carotenoids, xantophylls and anthocyanins. Carotenoids are found in carrots, for example. While the first two ingredients were present in the leaf all along, the anthocyanins are formed as a by-product during the process of leaf discoloration and shedding.
But how does leaf shedding occur? For this, it is necessary to take a closer look at the second step. After the nutrients have been removed, the tree forms a separating tissue at the base of the leaf stalk. A slight breeze is now enough for the leaves to fall. After the tree drops the leaf, increased cell divisions occur at the wound site. As a result, the wound quickly closes again and a new, waterproof tissue is formed. In this way, the wound protects itself from the entry of fungi or bacteria.
In plant physiology, leaf shedding is referred to as “abscission.”
Why do trees do that with leaves anyway?
On the low flame through the winter. The discoloration and shedding of leaves is essential for the survival of trees, because it is the only way they can survive the cold winter season. As you already know, the tree shuts down its plant metabolism in the fall. It would be a great effort for it to continue supplying its leaves with nutrients throughout the winter. It therefore draws these back into the tree from the leaves for “hibernation”.
Protection from snow and ice. The tree protects itself from storms and large amounts of snow in winter by shedding its leaves. The weight of snow can otherwise cause branches to break. The lack of foliage also means that the tree offers less of an attack surface to a powerful autumn storm.
Protection from thirst. The leaves release water via the leaf surface. This is known as transpiration. If the tree were still wearing its leaves in winter, it would probably die of thirst. When the sun shines, the leaves transpire, but the Buam cannot draw water from the frozen ground.
Detox for the tree. Leaf shedding means a kind of detox for the trees, because toxins can also be released with the leaves. These are produced in the course of a growing season by toxic metabolic end products or are absorbed from the environment.
Great opportunity for the very young. The lack of a leaf canopy leads to the growth of early bloomers in the spring. These require sufficient light for their growth and take advantage of the short period in which the trees do not yet have leaves.
Not all trees shed their leaves at the same time. The oak, for example, keeps its leaves on the tree for a very long time. This allows it to photosynthesize longer than other deciduous trees, but makes it more susceptible to autumn storms. Here, the wind has more surface to attack and causes the branches and trunks to break more frequently.
Leaf drop is affected by certain environmental factors such as extreme drought, pollutants in the soil or air, extreme infestations of predators such as insects or fungi. These stressors can cause premature leaf drop, even as early as summer. Here’s how you can tell if a tree may be diseased.
But how do the conifers do it?
Coniferous trees do not lose their “leaves”. They are called evergreen trees. But how do they survive the cold season anyway? Even though they don’t shed their leaves, conifers also lower their metabolism. Their needles have a much smaller surface area and are surrounded by a wax layer, which inhibits transpiration. In addition, the stomata, through which water evaporates, are significantly narrowed in winter and during drought.
There are some coniferous species that, unlike usual, also shed their needles. These include, among others, the European larch, the sequoia and the swamp cypress.
What happens to the old leaves?
The shedding of leaves is not only essential for the survival of the trees, but is also part of the natural cycle of the forest. Microorganisms decompose the old leaves and make them available for the soil. Mechanical shredding is carried out by beetles, woodlice and worms. Fungi and bacteria break down components that are difficult to digest, such as lignin. In the end, new humus is created from the old leaves, as well as from branches or roots. In other words, new food for trees and plants.
Ash, alder and elder do not change color and shed their green leaves.
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