Why Tree Leaves Change Color in the Fall

 As the days grow shorter and colder towards the end of the year the green leaves that adorned deciduous trees and shrubs throughout the summer begin to change into colors of red, orange, and yellow, and fall off the tree. Most trees with broad flat leaves lose their leaves every fall, while most trees with needles and a few with flat leaves keep them throughout the winter. Leaves that stay on the tree throughout the year have specially designed protective features, such as a sturdy needle shape, or a wax coating.  Most broad leaf plants do not have these features though. They must lose their large delicate leaves or risk having them destroyed by winter weather. When the days grow shorter there is less sunlight available for the tree to use in photosynthesis, which is the process by which most plants use the green pigment chlorophyll to convert water, carbon dioxide, and light into carbohydrates. Oxygen is produced as a byproduct of this chemical reaction. When the lack of sunlight slows down production to a certain point, the tree breaks down the chlorophyll and proteins in the leaves and stores them in its roots. During the spring when new leaves are being produced these nutrients are sent back up to the branches in the form of a sugary sap. As the green chlorophyll is drawn out of the leaves, the pigments whose colors are usually overpowered by the green chlorophyll become visible. The orange and yellow colors are carotenoids. Among other things they help absorb certain wavelengths of light that the chlorophyll cannot. The use of the red anthocyanins that form during the removal of the chlorophyll is not known for sure, but they are thought to help protect the nutrients from the light as they are being broken down in the leaves. When the nutrient removal is complete, special cells in the abscission layer form a thin layer of cork between the twig and the leaf stem that causes the leaf to fall off. The leaves fall to the ground where they decompose and add nutrients back to the soil around the tree's roots.