Why Do Fall Leaves Change Color?

Why Do Trees Change Color?

Nature is so fascinating! The main reason for the eye-popping color change is not autumn’s chilly weather, but sunlight—or rather, the lack of daylight.  Day and night are roughly equal in length on the autumnal equinox in late September, but afterward, nights are growing longer and days shorter.

As the autumn days shrink, the reduced daylight tells deciduous plants that it’s time to stop gathering energy and get ready for the dormant season—winter.

All leaves have different types of chemicals in them; one of these chemicals, chlorophyll, is responsible for absorbing sunlight and gives leaves their green color. As chemical changes begin to take place inside the plant, it causes a corky wall of cells (called the “abscission zone”) to form between the twig and the leaf stalk. This corky wall eventually causes the leaf to drop off in the breeze.

As the corky cells multiply, they begin to seal off the vessels that supply the leaf with nutrients and water and also block the exit vessels to some extent, trapping simple sugars in the leaves. The combination of reduced light, lack of nutrients, and less water triggers the trees to start the process of breaking down the chlorophyll and the green color fades.

Because the green color was “masking” other color pigments, we start to see the yellows and reds showing through and you see leaves change colors!

Of course, not all leaves turn vivid colors in autumn. Only a few of our many species of deciduous trees—notably maple, aspen, birch, oak, and gum—produce truly stellar performances for our annual autumn spectacular in North America.

Photo: October morning at Lake George. Credit: Pavels.

What Causes the Bright Yellow and Red Colors of Fall Leaves?

Once the waning hours of daylight trigger these changes and the green chlorophyll is gone, other pigments begin to reveal their bright faces!

• Carotenoids give leaves their brilliant yellow and orange colors.
• Anthocyanins are found in deep red and purplish leaves
• The presence of tannins means leaves will turn brownish or tan.

Yellow carotenoids exist in the leaf all summer, but are masked by chlorophyll during the growing season. Red anthocyanins, on the other hand, are freshly produced by plants as fall conditions ramp up. Surprisingly enough, scientists aren’t entirely sure why trees bother to produce a new pigment while otherwise trying to save their precious resources for the winter ahead. Some suggest that the bright red color could deter insect pests from feeding on leaves, or that red attracts birds that feed on (and spread) the trees’ fruit. 

However, another interesting theory is that the red pigment acts as a sort of sunscreen for the leaves, staving off damage from bright autumn sunlight and allowing the leaves to stay on the tree for longer than they would if they were to remain green or even yellow. 

Sugar trapped in autumn leaves by the corky wall is largely responsible for the vividness of the colors. Some additional anthocyanins are also manufactured by sunlight acting on the trapped sugar. This is why fall foliage is so sparkling after several bright fall days and more muted during rainy spells.

Finally, as autumn carries on, leaves begin to turn brown once all their nutrients are re-absorbed by the tree. The brown color is the result of the leftover tannins, a chemical that exists in many leaves, especially oaks.

What Weather Conditions Bring the Best Fall Foliage?

While the daylight is the main factor that brings on fall foliage, several other factors contribute to how bright fall colors are: temperature, precipitation, and soil moisture.

• In general, cooler nights with decreasing temperatures throughout the day lead to more vivid colors. 
• A wet growing season followed by an autumn with lots of sunny days, dry weather, and cold, frostless nights will produce the most vibrant palette of fall colors. This vividness is especially true of red leaves, such as those on sugar maples and red maple trees.
• Drought conditions during late summer and early fall can also trigger an early “shutdown” of trees as they prepare for winter, causing leaves to release early from trees without reaching their full color potential. Check your long range forecast to see whether a dry autumn is in your future.
• Of course, if freezing temperatures and a hard frost hit, it can kill the process within the leaves, leading to poor fall color and early leaf drop. Check the frost dates in your area!

Which Trees Produce Which Colors, and Which Trees Change Color First?

• Aspen: Golden
• Beech: Golden brown
• Birch: bright yellow
• Canada Red Choke Cherry: red to reddish-purple
• Poplar: golden yellow
• Sugar Maple: orange-red
• Black Maple: glowing yellow
• Red Maple: bright scarlet
• Silver Maple: muted green
• Sassafras: orange-yellow
• Tupelo: yellow/orange and then red
• Dogwood: purple-red
• Oaks: brown or russet
• Hickory: golden bronze
• Maple-Leaf Viburnum: pinkish purple

Which trees change first depends on where you live. Tulip Poplar trees start changing colors as early as August. Next are the maple trees, which give us early oranges and yellows, and of course some regions have the brilliant red maples. Oak, Hickories, and Beech trees are the last to change color. Everything else is in between. 

Where Can You Find the Best Fall Foliage?

Does your area experience fall foliage? Some level of autumn foliage changes in most regions of North America, but it’s New England, the upper Midwest, the Rocky Mountains, and parts of the Appalachians that hold the jackpot for leaf peepers. The right climate and light conditions and an abundance of the tree varieties that hoard colorful pigments come together in these places.

While tradition has it that Columbus Day weekend is when the color peaks in New England, the mythical maximum occurs in northern Maine in mid- to late September and “travels” south, reaching the Connecticut shore by late October. 

Wondering when the leaves will change in your area? See our animated foliage map as well as some our our favorite destinations for leaf peeping!