Each fall, a spectacular performance takes place in forests, parks and cities. The leaves of each deciduous tree lose their green color and turn brown, orange, red, yellow and even purple. In the Eastern United States especially the landscapes seem to catch fire. There are many factors that cause this colorful display.
The direct catalyst for a deciduous tree's color change and eventual leaf fall is the waning light of autumn. As days grow shorter, the amount of light available to fuel photosynthesis diminishes. When the daylight hours drop below that tree's particular threshold, this triggers the autumnal biochemical process whose visible effect is the changing color of the leaves.
Loss of Chlorophyll
Leaves get their green color from chlorophyll, the pigment necessary for photosynthesis. Chlorophyll absorbs sunlight, sunlight powers photosynthesis and photosynthesis produces the sugars that feed the tree.
Some of the sugars produced via photosynthesis remain stored in the tree. This stored supply will feed the tree during the winter months when photosynthesis does not occur.
In response to the diminishing amount of sunlight available each day, the tree begins reducing the rate at which it produces sugar. Eventually chlorophyll production stops entirely. What green pigment is left breaks down. When this happens, the leaf loses its green color.
When leaves lose their chlorophyll, the loss reveals other pigments. Carotenoids make the yellow, orange and brown colors you see in bananas, buttercups, corn and carrots. Anthocyanins make reds, blues and purples, as in cranberries, blueberries, Concord grapes and plums.
Carotenoids have been available since initial development of the leaf. Anthocyanins, however, are produced as the leaf prepares to fall. In response to the shorter days of autumn, the abscission layer, which is a layer of cells at the base of each leaf, begins to swell into a cork-like material that constricts the veins that carry fluids and nutrients to the leaf and carry wastes away. When this happens, trapped sugars in the leaf trigger the production of anthocyanins.
Tannin is a waste product that gets trapped along with the sugars in the closed-off autumn leaf. This gives fall leaves their brown color.
Different species of tree display different characteristic colors depending on their own particular pigment blends. Different species also have different timing triggers for when their leaves begin changing color. Oaks, for example, continue producing chlorophyll after other species have begun shutting down that process.
Colors will also vary between seasons depending on temperature. If the days are warm and sunny and the nights are crisp but not freezing, the combination bright light with an especially high sugar production and thus concentration in the leaves will lead to high production of anthocyanins, resulting in remarkably brilliant colors.