Where do the colors come from? Sunlight rules most living things with its golden edicts. When the days begin to shorten, soon after the summer solstice (June 21), a tree reconsiders its leaves. All summer it feeds them so they can process sunlight, but in the dog days of summer the tree begins pulling nutrients back into its trunk and roots, pares down, and gradually chokes off its leaves. A corky layer of cells forms at the leaves' slender petioles, then scars over. Undernourished, the leaves stop producing the pigment chlorophyll and photosynthesis ceases. Animals can migrate, hibernate, or store food to prepare for winter. But where can a tree go? It survives by dropping its leaves, and by the end of autumn only a few fragile threads of fluid-carrying xylem hold leaves to their stems.
A turning leaf stays partly green at first, then reveals splotches of yellow and red as the chlorophyll gradually breaks down. Dark green seems to stay longest in the veins, outlining and defining them. During the summer, chlorophyll dissolves in the heat and light, but it is also being steadily replaced. In the fall, on the other hand, no new pigment is produced, and so we notice the other colors that were always there, right in the leaf, although chlorophyll's shocking green hid them from view. With their camouflage gone, we see these colors for the first time all year and marvel, but they were always there, hidden like a vivid secret beneath the hot glowing greens of summer.