Why do leaves turn color in the fall?

Depending on where you live in the world, the final months of the year may bring cooler temperatures, shorter days, and bright autumn leaves. It is precisely these cooler temperatures and the decrease in daylight which results in changes in the leaves of some trees and bushes. 

Throughout the growing seasons of spring and summer, the tree is hard at work transforming sunlight and carbon dioxide into sugars- some of which are stored for the...

Depending on where you live in the world, the final months of the year may bring cooler temperatures, shorter days, and bright autumn leaves. It is precisely these cooler temperatures and the decrease in daylight which results in changes in the leaves of some trees and bushes. 

Throughout the growing seasons of spring and summer, the tree is hard at work transforming sunlight and carbon dioxide into sugars- some of which are stored for the dormant seasons of fall and winter. The cells of leaves contain pigment called chlorophyll, which is responsible for the absorption of sunlight and giving leaves their bright, green color. Leaves contain other pigments which are not as noticeable for much of the year. This includes carotenoids (a bright yellow-orange pigment) and anthocyanins (which create a reddish-purple hue.)

Have you ever seen a plant which has been watered too much, but has received too little sunlight? The leaves tend to turn yellow. Similarly, towards the end of summer and beginning of fall, leaves on trees will take on a yellow color. This is because the tree is responding to a decrease in sunlight, and is not producing more chlorophyll to replace that which has already broken down. In the height of autumn, we can observe these yellow leaves turning orange, red, and even plum or brown. The carotenoid pigments are still present, and a buildup of sugars which have not been stored in the tree gives way to high levels of anthocyanins. 

Differing amounts of sunlight and temperature fluctuations may make this process a slow or fast one, with relatively dull or brilliant color displays. The United States Forest Service says that the best conditions for bright, colorful leaves is several days of bright light and crisp- but not too chilly- nighttime. This ensures that the production of sugars is high in the leaf, but becomes "trapped" as the veins of the leaf close off in the cool night.