Why Leaves Turn Color in the Fall
Marianne C. Ophardt
Washington State University Cooperative Extension
Area Extension Agent
Tree leaves contain different types of pigments. The predominant one is usually green and it comes from the chlorophyll needed for photosynthesis... the process by which the leaf captures sunlight and uses that energy to make sugars out of water and carbon dioxide. In the fall as the leaf begins the process of senescence or dying and falling off the tree, photosynthesis stops and chlorophyll breaks down, revealing the underlying yellow and orange pigments in the leaf...these are carotene (orange-yellow pigment), and xanthophyll (yellow). Red and purple colors come from anthocyanin pigments. Anthocyanins are not masked but actually start to build up in the leaves of certain trees as the chlorophyll breaks down.
The weather that leads to the best fall colors are those which promote the highest levels of sugars in the leaves. Bright, sunny warm days and cool nights will lead to the most brilliant hues of oranges through reds and purples. Heavy frosts and overcast days can diminish fall color, while a mild drought can favor anthocyanin production and fall red color.&
Its important to point out that some trees such as red maple, dogwood, sweetgum, and dogwood are capable of exhibiting fall color under the right conditions and other plants, such as sycamore, black locust, black walnut, linden, catalpa, and elm will never provide an attractive autumnal display.
Conifers are cone bearing trees. Most conifers are needled evergreens. While most conifers don’t lose their leaves in the fall, they do lose some of their oldest needles each year. This is usually a gradual, unnoticed process, but in some years needles may turn bright yellow in the fall and drop over a short period of time. There are a few deciduous conifers, such as larch and dawn redwood, which lose all their needles each fall.