During the spring and summer the leaves are normally green due to the presence of a pigment called chlorophyll. Chlorophyll serves a vital function: it captures the solar rays and uses the heat energy to produce the plants food, by transforming carbon dioxide and water to carbohydrates, such as sugar and starch.
In the autumn with less daylight and cooler temperatures, the leaves stop their food making process. The chlorophyll breaks down, the green color disappears, and the yellow and orange become visible, giving the leaves their autumn splendor. At the same time, other chemical changes may occur, which form additional colors by the development of red anthocynin pigments, which produce the bright red leaves. An early frost may decrease the presence of bright colors, whereas rain tends to intensify the colors.
The change of leaf colors in the fall has also been suggested as an adaptation to attract birds and herbivores. Many plants with berries and leaf color attract birds, particularly those that are bright red. The birds get a meal and the undigested seeds are carried off and deposited with the birds droppings. Poison Ivy is especially notable for having bright red autumn foliage to attract birds to the white berries.
In autumn, other changes also take place. For example, at the point where the leaf is attached to the tree, a special layer of cells develops and gradually severs the tissue that supports the leaf. At that time the tree seals the cut. As a result, a leaf scar is left behind when the leaf is blown off by the wind or falls to the ground.
Most broad-leaved trees shed their leaves in autumn. However, the leaves from some oak trees may stay on the tree and stay green until the growth starts again in the spring. This is true for both the White and Emery oaks seen at the Highlands Center.
Contributed by Herdis Maclellan, Highlands Center Naturalist