We all enjoy the beautiful colors of autumn, but why the leaves change color is a mystery to most leaf-watchers. In summer, leaves capture the sun's energy and turn it into food through a process called photosynthesis. Photosynthesis is made possible through a green pigment called chlorophyll, which keeps leaves green in summer. Chlorophyll absorbs sunlight, turning it into the sugars trees live on. Yellow and orange pigments are present, but they are overpowered by green chlorophyll.
In winter, trees shed their leaves to slow the evaporation of water, which is difficult to replace when the ground is frozen. Also, trees may become damaged when water within their leaves freezes and expands. To prepare for winter, trees grow a corky layer which prevents water from entering their leaves. Without water, the tree stops making chlorophyll, and the old chlorophyll eventually fades away.
The leaves then show their yellow and orange pigments, which were there all along! For red leaves, there is a different explanation. The corky layer not only stops water from entering the leaves, it also prevents sugars from exiting the leaves. When the sun shines on the leaves, the sugars react with a chemical in the leaf to form a red pigment. A leaf that is in complete shade, with no sunlight, will not turn red.