In summer, leaves capture the sun’s energy and turn it into food through a process called photosynthesis, made possible by a pigment called chlorophyll, which keeps leaves green in summer. Chlorophyll absorbs sunlight, turning it into the sugars trees need. 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 be damaged when water within the leaves freezes and expands.
To prepare for winter, trees grow a corky layer which prevents water from entering leaves. Without water, the tree stops making chlorophyll, and the old chlorophyll fades away. Leaves then show their yellow and orange pigments, which were always there! For red leaves, the explanation differs. 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 will not turn red.
