When is the maple leaf red?

The specific reason for turning red

All leaves contain green chlorophyll. Trees use chlorophyll to capture light energy and store it in the form of chemicals such as sugar with the help of other substances in leaves. In addition to chlorophyll, many leaves also contain other pigments such as carotene, yellow pigment and red pigment. Although these pigments can't carry out photosynthesis like chlorophyll, some of them can transfer the captured light energy to chlorophyll. In spring and summer, the content of chlorophyll in leaves is much richer than other pigments, so the leaves show the green color of chlorophyll, while the colors of other pigments are invisible. When autumn comes, the days get shorter and the nights get longer, which makes the trees begin to shed their leaves.

Trees don't produce a lot of chlorophyll as they did in spring and summer before falling leaves, and the existing pigments, such as chlorophyll, will gradually decompose. In this way, with the gradual reduction of chlorophyll content, the colors of other pigments will gradually appear on the leaves, so the leaves will show yellow, red and other colors.