The fall of leaves from the trees characterizes the approach or onset of autumn, which, of course, is repeated every year. Probably, many of you understand that if the trees did not shed their leaves in autumn, they would simply die. And how they can die because of this, we will talk a little later.
Why do leaves fall from trees in autumn?
I think some of you have heard the second name of the leaves - the lungs of plants. In summer, in the leaves, by exposure to light, the process of photosynthesis proceeds, when, thanks to the green pigment (chlorophyll), mineral salts and carbon dioxide begin to turn into organic substances and oxygen.
And in this regard, by the autumn period, many by-products of photosynthesis and unnecessary substances begin to accumulate in the leaves. But besides this, at low temperatures, chlorophyll will not be produced, and therefore the leaves will simply simply cease to perform their functions, and then, having become no longer needed, they will begin to fall off.
Let's also try to figure out whether the fall of the leaves is a biological phenomenon, which is due to the vital activity of the plant, or whether the cause is still a drop in temperature and the onset of autumn bad weather.
If in the summer, and even better in the spring, we plant some kind of young tree, for example, an oak or a maple, in a pot with earth, for example, an oak or a maple, and put it in a room or in a greenhouse, then in the fall the leaves from these plants will still fall off, even without looking that you took good care of him.
As you know, autumn bad weather simply cannot penetrate into a room or into the same greenhouse, and therefore there are no frosts here, but the leaves will still fall off at the right time.
It follows that leaf fall is not a consequence of unfavorable conditions.
But we can also go the other way to understand that leaf fall is a biological process.
At the end of the summer period in some tree, through the base of the leaf petiole, we make a cut, namely in the place where the petiole is attached to the stem, in connection with which the so-called “leaf pad” is formed there.
But if you look through a microscope, you can see that a special separating (cork) layer has formed on the cut. The cells of this layer have smooth walls, but in addition they can easily separate from each other.
By the period of leaf fall, the connection between them may be broken, and the leaf itself on the tree can be kept thanks to vascular bundles in the form of the smallest “water pipes” connecting the leaf with the rest of the plant.
Vascular bundles can also be seen with the naked eye on leaf scars, located in the form of 3-5 (in some cases more) large dots.
They perform the function of carrying water and mineral salts from the roots to the leaves and nutrients (carbohydrates) that are produced by the leaves during assimilation.
But there comes a period when the petiole of the leaf and the mother plant lose their last connection.
Even a small gust of wind is enough for this, but in some cases the leaves fall even in calm weather, which of course is associated with a sharp fluctuation in temperature (during freezing or thawing).
But it can also happen under the influence of the gravity of the leaf blade, which can become heavier due to the settled dew.
Why is it necessary for the leaves to fall from the trees in autumn?
First reason . Water evaporates over the entire area of the leaves of trees. In the summer, by extracting water from the soil, the tree can replenish the amount of moisture lost.
But with a cold snap, it becomes more and more difficult to extract water and soil, and in the winter period it is completely difficult to extract water from frozen ground. Trees that would have had leaf cover even in winter would simply dry out due to lack of moisture.
The second reason . Probably, many people paid attention to how, after a heavy snowfall, namely, under the weight of snow, a strong inclination of tree branches to the ground occurs. In some cases, they may break.
Now imagine if there were still leaves on the trees in the winter, how many times the amount of snow on the branches could increase, since the leaf surface is larger compared to the surface of the branches.
Thus, due to the fact that the leaves fall from the trees in autumn, there is additional protection against mechanical damage, which can be caused by snow pressure.
Third reason . During autumn leaf fall, trees can get rid of excess mineral salts that accumulate throughout the summer period. As we said above, increased evaporation of water occurs in the leaves, which in summer can be replenished by sucking it from the soil by the roots.
But in the water obtained by the roots from the soil, a large number of different salts. That is, we can say that the roots are obtained along with water and saline solutions.
Some of these salts are used by trees as food, and the rest will be deposited in leaf cells.
The more the leaves evaporate moisture, the more they will mineralize by autumn. That is, due to the large amount of salts accumulated by the leaves, they are mineralized in the fall.
With an excess of mineral salts, the normal operation of the leaves will be disrupted. And in this regard, we can say with confidence that the shedding of old leaves is a necessary condition for the normal functioning of trees and shrubs.
