What Makes Leaves Change Color in the Fall?

Posted: September 2, 2015 

By Rex Bastian

A beautiful fall landscape is a sign your trees have been hard at work

Nature does its best every autumn to put on a spectacular show of color in many towns across America. Over just a few weeks, bright green leaves transform into dark browns, buttery yellows, deep reds and vibrant orange. The changing leaves delight and amaze onlookers from September through November, marking the fall season.

And while this awe-inspiring phenomenon receives a warm reception every year, the science behind it is often misunderstood. Many believe that cool weather alone causes leaves to change color. Though temperature can dictate intensity, it is just one of the factors that play a part in painting the striking fall landscape. Cool air, changes in rainfall, and most of all, shortening day length, all play a role in the changing color of tree leaves.

Here’s what’s really going on behind the scenes as green leaves turn to shades of autumn.

Leaves are busy creating necessary foods for growth during the spring and summer. 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 breakdown 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.

Those red, pink and purple pigments come from anthocyanins, which are manufactured from sugars the tree produces. Shorter days and cooler nights signal to leaves that it’s time to prepare for winter, and leaf veins leading into and out of the leaves begin to close, trapping excess sugars. Combined with bright light, the sugars are used in the production of anthocyanin pigments. These hues are the most variable 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. 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.

The fallen leaves decompose, providing the soil with much-needed nutrients and a food source for many organisms.

How weather impacts leaf color:

Have you noticed that 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 brown.

Weather is responsible for the onset of fall and the intensity of autumn color. Combine a warm, wet spring with a mild summer and bright autumn days with cool nights for vivid colors. On the other hand, a late spring or severe drought delays the onset of fall color for weeks.

Dull autumn colors are the result of a warm fall. Wind and rain during the fall color season often knocks leaves off trees before they can produce their best color, too.

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, well-cared for trees showcase the most glorious leaf colors. To promote your trees’ ability to offer their best fall color, talk to a certified arborist at Davey Tree.  Find a trusted tree care specialist at www.davey.com.

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