When do poplars usually shed their leaves?

There are two kinds of fallen leaves: one is that when drought or cold season comes, all the leaves die and fall off, leaving only branches, such as peaches. Another: After new leaves appear in spring and summer, the old leaves gradually fall off. As far as the whole tree is concerned, it is evergreen all year round and is an evergreen tree, such as Cinnamomum camphora. Causes of defoliation: Internal cause: After a certain period of physiological activities, a large number of metabolites, such as minerals, accumulate in cells, causing physiological function decline and death (chlorophyll destruction). External factors: the cold weather, lower soil temperature, decreased rainfall, greatly weakened the water absorption capacity of roots, but the transpiration intensity did not weaken, leading to water shortage and defoliation. 3.6.2 Why do fallen leaves fall off? Why are the leaves so smooth after falling off? ◆ The reason is that some cells (originally calcium pectin, soluble pectin and pectin acid) at or near the base of petiole are produced due to changes in cell or biochemical properties. ◆ The separation zone includes: separation layer and protective layer: cells divide to produce a group of small thin-walled cells. For the separation layer, the cell wall gels and the cells become free. Due to the weak supporting force, combined with the gravity of the leaves and the shaking of the wind, the leaves fall off from the separation layer. Then several layers of cells below the abscission layer are embolized to form a protective layer on the cross section to avoid water loss and invasion of pests and diseases. The significance of defoliation: it is an adaptation of plants to reduce transpiration, maintain water in the body and spend the cold and dry season. This habit is formed in the long-term evolution of plants. Defoliation mechanism: adapting to bad environment, defoliation can reduce transpiration and make plants spend cold and dry seasons, which is formed by long-term evolution of plants; Short sunshine accelerates delamination formation; The increase of abscisic acid and ethyl acetate can accelerate leaf abscission.