Ever wonder why leaves change color? Did you know that fall’s vivid colors are actually hidden underneath summer’s green? The main reason for color change is not autumn’s chilly weather, but light—or rather, the lack of it.
Why Do Leaves Change Color?
First of all, not all leaves turn vivid colors in the fall. 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.
Several factors contribute to fall color (temperature, precipitation, soil moisture), but the main agent is light, or actually the lack of it. The amount of daylight relates to the timing of the autumnal equinox, when day and night are roughly equal, but nights are growing longer.
As the autumn days grow shorter, the reduced light starts chemical changes in deciduous plants, causing 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 pigment chlorophyll, which contains valuable nutrients and is the source of the “green” in leaves. As chlorophyll is broken down and the green color fades, yellows and reds are revealed.
Where Do the Bright Yellow and Red Colors Come From?
Once the green chlorophyll is gone, other pigments begin to reveal their bright faces. These pigments, carotenoids (yellow) and anthocyanins (red), are responsible for the lovely colors of fall.
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. Suprisingly enough, scientists aren’t enitirely 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?
In general, 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.
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 leaf and lead to poor fall color. Check the frost dates in your area!
Also, drought conditions during late summer and early fall can trigger an early “shutdown” of trees as they prepare for winter, causing leaves to fall early from trees without reaching their full color potential.
Where Can You Find the Most Beautiful 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.
When to Look for Fall Leaves
Wondering when the leaves will change in your area? Below is an animation of the changing leaves from past years, based on the foliage reports from our readers. Although every year is different, this map should give you an idea of when leaves typically start to change across the country.
How are the fall leaves looking in your area? Let us know below! We hope you have a beautiful autumn.