Why Leaves Turn Colors in Autumn
In many places, the green of summer gives way to brilliant yellows, oranges, and reds each autumn. The change is not a sudden painting of the leaves. It is the uncovering of colors that were there all along, combined with some new ones.
Leaves look green because they contain chlorophyll, the pigment that captures sunlight for photosynthesis. During the growing season, chlorophyll is constantly broken down and rebuilt. Leaves also hold yellow and orange pigments called carotenoids, but the abundant green hides them.
As autumn arrives, the days grow shorter and the nights grow cooler. The tree responds by slowing and then stopping its production of chlorophyll. The green fades, and the carotenoids show through. This is why birches and aspens often turn a clear yellow.
Reds and purples have a different origin. In many kinds of trees, the leaves make new pigments called anthocyanins in autumn, when sugars build up in the leaf. Bright, sunny days and cool nights tend to produce the strongest reds, so the display varies from one year to the next.
Not every tree takes part in this show. Evergreens such as pines and spruces keep most of their needles through the year and replace them gradually instead of all at once. Brilliant autumn color is best known in regions with cool autumns and cold winters, such as parts of North America, Europe, and eastern Asia. Dropping leaves also helps a tree through winter, because the ground may freeze and the roots cannot take up much water, and bare branches lose far less of it to the air.
Eventually, a layer of cells forms where each leaf joins the twig, and the leaf falls. The tree then rests through the winter and saves its energy for spring.