Why Do Leaves Change Color in the Fall?

Natascha Batchelor
Updated September 10, 2015
orange fall leaves
Chlorophyll, the chemical that gives leaves their bright green color, begins to break down and disappear as the weather cools. (Photo courtesy of The Davey Tree Expert Company)

Soon enough, summer trips to the beach and refreshing lemonade will be replaced by autumn's cool breeze, football games and pumpkin lattes.

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But the real sign fall is here is the brilliant foliage — the most beloved sign of autumn. Bright green leaves transform into browns, buttery yellows, deep reds and vibrant shades of orange. 

The science behind fall foliage

Many believe that cool weather alone causes leaves to change color. While temperature can dictate intensity, it’s just one of the factors that play a part in painting a striking fall landscape. Changes in rainfall and shorter days also affect fall leaf colors.

During the spring and summer, leaves are busy creating the foods necessary for the tree’s growth. But in fall, leaves reduce their food-making process because of changing temperatures and the amount of sunlight. Chlorophyll, the chemical that gives leaves their bright green color, begins to break down and disappear.

Carotenes, the substances that gives leaves their yellow and orange hues, are present in leaves throughout the growing season. The larger amount of green chlorophyll masks the colors of carotenoids until fall’s arrival when chlorophyll production wanes and other pigments get their chance to shine.

The pigments that produce the most exciting colors — reds, pinks and purples — come from anthocyanins, which are manufactured from sugars the tree produces. As days shorten and nights cool, excess sugars are trapped in the leaf as veins leading into and out of the leaves close. Combined with bright light, the sugars are used in the production of anthocyanin pigments. These colors vary the most from year to year, because their formation is the most dependent on favorable fall environmental conditions and sunlight.

By late autumn, the yellow and red pigments, following the green, disintegrate in the leaves. This final step exposes the brown tannins.

When leaves fall, they decompose and provide the soil with nutrients. These leaves also become a food source for many organisms.

Sugar maples, birches and other broad-leafed trees shed their leaves in the fall. But before they do, they salvage as many nutrients as possible to store through the cold months and reuse the following spring. 

bridge and fall leaves
Autumn leaf colors vary from year to year depending on the weather. (Photo courtesy of The Davey Tree Expert Company)

Varying leaf colors

Autumn leaf colors vary from year to year. Some years, those bright and beautiful reds, purples and yellows last for weeks, while other years, autumn leaves quickly turn to brown.

The factor responsible for the onset of fall and the intensity of autumn color is the weather. A warm, wet spring combined with a mild summer and bright autumn days with cool nights results in more vivid colors. On the other hand, a late spring or severe drought will delay the onset of fall colors for weeks. A warm fall will also dull autumn colors. Wind and rain during the fall color season will often knock the leaves off the tree before they can produce their best color.

Trees under stress may discolor or drop leaves early. Extreme heat, drought, insects, disease and damaged roots can all be great stressors for trees. The best time of year to examine your trees for signs of stress is early September.

Healthy trees showcase the most glorious leaf colors. To promote your trees’ ability to offer their best fall color, talk to a certified arborist.

As of Sept. 10, 2015, this service provider was highly rated on Angie's List. Ratings are subject to change based on consumer feedback, so check Angie's List for the most up-to-date reviews. The views expressed by this author do not necessarily reflect those of Angie's List.

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