Why do the leaves fall off the trees in the fall?

Many trees shed their leaves at certain times of the year such as survival strategy in unfavorable environmental conditions. Trees that lose all their leaves seasonally are known as deciduous or deciduous trees, while trees that do not are known as evergreen or evergreen trees.

Both deciduous and evergreen are distributed in all climates, but evergreen trees tend to predominate in tropical and subtropical climates, while deciduous trees tend to predominate in temperate and cold climates. Some examples of deciduous trees in temperate and cold climates are ash, poplar, beech, birch, cherry, walnut or willow. In tropical and subtropical climates, examples of deciduous with acacia, baobad, ceiba or guanacaste.

leaf abscission

The separation of any small part, such as the leaves, from the rest of the plant is known in botany by the generic name of abscission. The seasonal abscission of leaves in deciduous trees is produced by a active mechanism in which various phytohormones that produce metabolic changes in the leaves to cause their fall.

The production of these hormones is activated when the days become shorter and the incidence of solar radiation decreases announcing the arrival of winter. Light changes are detected by the phytochromeswhich detect light in the red spectrum, and cryptochromeswhich detect light in the blue spectrum.

In addition to solar incidence, other factors such as soil moisture, environmental temperature and, most likely, some genes may influence, as some species begin to shed their leaves as soon as the end of summer, while others do so at mid to late fall.

Phytohormones that promote leaf abscission produce two main effects. On the one hand, the plant absorbs a large amount of nutrients from its leaves before they break off and stop producing chlorophyll; carotenoids and xanthophylls begin to predominate, orange, yellow, red and brown pigments, which is why the leaves lose their green color before falling.

On the other hand, hormones stimulate the growth of a layer of special cells at the base of the petiole (the axis that joins the leaf to the stem), the abscission layer, which acts as a "scissor" for the sheet to come off. As this cell layer grows, the leaf becomes less attached to the plant until the wind and other movements end up ripping the leaf off the tree. In some cases it is possible to see this cell layer without the need of a microscope.

Leaves are the food factory of plants. They absorb carbon dioxide from the air and energy from solar radiation to combine it with water and synthesize glucose. With this glucose the plant can manufacture cellulose, lignin and other substances that allow it to grow, develop and reproduce.

Metabolic activity increases during the spring, summer, and early fall. But in cold seasons, metabolic activity slows down. If during the arrival time of winter one day it is warmer, the metabolic activity can be reactivated, but if there is a pronounced drop in temperatures afterwards, the leaf will be with water in your pipes that can freeze with the consequent death of the leaf.

With dead leaves, the tree can die when spring comes if it does not have energy reserves to form new leaves. To prevent this, the deciduous tree absorbs all the nutrients it can from the leaf and then gets rid of them when the cold begins to arrive.

This is just one of the mechanisms that plants have developed to overcome unfavorable seasons. Most deciduous trees have broad leaves with ample surface area to absorb solar radiation, but this greater surface area also makes them more susceptible to damage at low temperatures.

Evergreen trees adapted to low temperatures usually have pointed leaves, such as pines or cypresses, covered with waxes and resins that protect them from the coldand also produce various substances with antifreeze action that allow them to withstand low temperatures without dying.

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