Why do leaves fall in autumn?

The main reasons for falling leaves in autumn are as follows:

Self-protection mechanism of trees. As the temperature drops, trees will lose their leaves to reduce the waste of nutrients and avoid wasting too much nutrients.

The disappearance of yellow pigment. In autumn, the green pigment on the tree will gradually decrease, which will lead to the leaves gradually turning yellow. Yellow will reduce the water on the tree and make the leaves fall from the tree.

Transfer of nutrients. In autumn, trees need to do a good job of wintering and store nutrients in the trunk and roots, so trees will leave unused leaves and leave nutrients to the roots.

Wind action. In autumn, the wind is strong, so it is easy to blow off the leaves. The tree will peel off the leaves before they fall, so as not to hinder the growth of the tree.

Autumn:

Autumn is one of the four seasons. Traditionally, China takes "beginning of autumn" in the 24 solar terms as the starting point of autumn. Entering autumn means that rainfall and humidity tend to decrease or decrease, and everything in nature begins to mature from lush.

At present, the four seasons are usually divided according to the average temperature of modern scholar Zhang Baokun. The index is that the average temperature is below 10℃ in winter, above 22℃ in summer, and 10~22℃ in spring or autumn.

Autumn is a season of alternating hot and cold. Early autumn is still in the heat, but it hasn't come out yet. This is the hottest, wettest and sultry period of the year. After late autumn, it is sunny during the day, cool and dry at night, with large temperature difference between morning and evening, less rain and insufficient water. Therefore, autumn dryness has become the main feature of late autumn.

Astronomy:

In autumn, the direct point of the sun gradually moves south from the northern hemisphere, passes through the equator after the autumnal equinox, and the sun directly shines in the southern hemisphere. Seen from the northern hemisphere, the angle of the sun gradually becomes lower, and the gap between day and night becomes smaller. At the autumnal equinox, the length of day and night is equal.

In autumn, the distance between the earth and the sun is from far to near. Seen from the ecliptic plane, the sun is on the background of Leo, Virgo and Libra.

Phenology:

It is autumn in beginning of autumn so far, which means that precipitation and humidity are at the turning point of the year, and there is a tendency to decrease or decrease. In nature, everything begins to grow from lush to rich and mature. Autumn is the harvest season, and many plants mature in autumn.

In autumn, in nature, the qi of yin and yang begins to change, the yang is gradually declining, the yin is gradually flourishing, the climate turns from hot to cold, and everything is born with cold and gradually falls. This is the season of alternating hot and cold.

As the temperature gradually drops, the leaves of many deciduous perennial plants will gradually change color, wither and fall off, leaving only branches for winter. Panicum miliaceum will come to the end of his life, wither and die.

Seasonal variation covers many aspects of climate characteristics, and it is not accurate to judge autumn only by temperature. The most obvious change in autumn is that the leaves of vegetation change from lush green to yellow, and the leaves begin to fall and the crops begin to mature.