Leaves are yellow in autumn, why do they fall to the ground in winter?

As the autumnal equinox approaches, the weather is getting colder and colder. The original green leaves on the tree turned yellow and red, and finally they all fell to the ground. These situations tell us that the weather is cold, autumn has come and gone, and when the light sets, we can already feel the breath of winter.

Why do green leaves turn yellow and fall off? People always think that this is the result of dry weather and water loss of leaves. This makes sense. In fact, the main reason is that trees are going to hibernate. Leaves send useful substances such as chlorophyll, water, nitrogen, phosphorus, protein and carbohydrates back to the trunk and roots, waiting for withering and death. At the same time, a group of special cells in the petiole began to become fragile, and they were easily broken when exposed to wind and rain, and the leaves fell to the ground.

Botanists also pointed out that the color depth of leaves and the time of falling leaves in autumn are related to rainfall or snowfall. If in a dry year, the color of the leaves will not change much, and the leaves will fall early in order to leave more water for the trees. It seems that storing nutrients, retaining more water and falling leaves are a means for trees to "protect their cars" and protect themselves.

No matter the yellow leaves and red leaves, no matter how beautiful the autumn scenery is, under the cold wind in winter, all the leaves of deciduous trees fall off, leaving only bare trunks. Then, in the tropical region where all seasons are summer, don't the leaves need to fall? That's not true. Tropical trees also shed their leaves. However, their leaves are always in dry season. Take China's Guangdong, Yunnan, Hainan and other places as examples, many trees shed their leaves every February-March. In these places, it seems that "a leaf falls to know the autumn on earth" should be changed to "a leaf falls to know the drought"