Tree Leaves in the Fall

Leaf Color

You've noticed that, at the end of summer, the days get shorter and cooler, and that the sun is lower in the sky. Trees also know this is happening. The chlorophyll in the leaves needs lots of sunlight to do its job correctly. As the days grow shorter, the chlorophyll disappears. In the summer, there is so much chlorophyll in the leaves that it hides the other bright pigments, such as yellows, reds, and oranges. The intensity of color in the leaves varies according to tree species, the health of the tree, and soil acidity.

The leaves of some trees--such as the beech--don't show a dramatic change in color in the fall; they simply turn brown. The leaves of the lilac and some other trees stay green until they drop. The most spectacular, however, are the trees whose leaves turn flashy shades of orangish red, such as the sugar maple and the red maple, or golden yellow, such as the elm and birch.

When the Leaves Fall

The main role of leaves is to carry out photosynthesis. When the chlorophyll disappears from leaves in the fall, they can no longer photosynthesize, so the tree drops them. The tree does this by growing a thin layer of a kind of cork between the leaf and where its stem attaches to the branch. The leaf then falls off. Even after they drop, however, leaves still have a role to play. The dead leaves form a blanket on the ground around the foot of the tree that protects its roots from the cold and freezing, while holding in moisture. Insects and fungi break the leaves down into humus, which nourishes the roots of the tree.