Why Do Tree Leaves Change Color? Which Leads to The Change?
|Colorful Leaves. Photo: Mixi|
What makes a tree’s leaves change from green to yellow, orange, and red is actually always present in the leaf. It just happens to be the shorter days and cooler nights that allow the brilliant colors of autumn to shine through.
What Kind of Trees Make the Colorful Autumn Leaves?
You already know that trees are classified as either evergreen or deciduous, depending on their genus and species. A deciduous tree puts out new leaves each spring, and those leaves live for just one growing season. It is these annual, deciduous leaves that give us showy fall colors.
Why are Leaves Green in Spring and Summer?
During spring and summer, trees make chlorophyll and food for the tree. Chlorophyll is created by the reaction of specialized leaf cells to sunlight and it’s what makes leaves look green.
But fall’s cooler weather and shorter days signal to the tree that dormancy is coming and it’s time to close up shop for the winter. This end-of-season signal also means that the process that makes chlorophyll (and, therefore, makes leaves green) will stop, too.
Why Do Leaves Change Color In The Fall?
|Photo: The Cornell Daily Sun|
There are several reasons why leaves change color in the fall, but the most significant contributing factors are shorter daylight hours and longer nighttime hours, and how those factors affect the chemical process inside each leaf.
It all comes down to biological pigments (also known as "biochromes"), which are molecular substances that manifest in living things as specific colors by absorbing or reflecting wavelengths of light.
You might already know a little something about chlorophyll — it's the green pigment produced by plants during the photosynthesis process. Some of the other pigments found in plants are carotenoids, which are responsible for oranges, and anthocyanins, which yield red and purple leaves. While chlorophyll and carotenoids are present throughout the growing season, most anthocyanins are produced exclusively in late summer and early autumn.
As the days become shorter and the nights become longer, the amount of light required for photosynthesis wanes, and chlorophyll production gradually comes to a halt. Without any new chlorophyll being produced, the leaves' characteristic green color begins to break down and vanish. This mechanism essentially "unmasks" the colors of the carotenoids and anthocyanins that were lurking beneath.
While the waning hours of sunlight are the most significant contributing factor to affect changing foliage colors, temperature, and humidity can also play a role in the intensity of these seasonal displays. For example, warm, sunny days coupled with cool, mild nights are a particularly potent recipe for brilliance.
As the National Forest Service explains: "During these days, lots of sugars are produced in the leaf but the cool nights and the gradual closing of veins going into the leaf prevent these sugars from moving out. These conditions — lots of sugar and lots of light — spur production of the brilliant anthocyanin pigments, which tint reds, purples and crimson."
FUN FACT: If you examine a twig or branch, you’ll often find the remnants, or scars, of previous leaf abscission on the bark’s surface. In many species, these leaf scars are an important way to identify a tree, particularly during dormancy when its branches are bare.
What Makes Autumn Leave Colors Appear?
Chlorophyll is a strong green pigment, strong enough to mask other colors that are present in leaves during the growing season. But once the green chlorophyll in a leaf is gone, other colors in the leaf are exposed.
These are the most common of the previously camouflaged colors in leaves that are displayed when chlorophyll is removed:
* Xanthophylls, cells that we see as yellow
* Carotenoids, cells that we see as yellow-orange and orange
* Anthocyanins, cells that we see as pink and red
What Determines Which Color the Leaves will be in Fall?
The exact color of orange, yellow, or red that a tree displays will depend on its species, as well as a tree’s overall health. Trees stressed from summer heat or drought often have disappointing fall colors and leaves can turn a dull brown instead of a blazing red or orange. Some trees, such as many oaks, turn brown naturally.
Some tree species have carotenoids, giving leaves a yellow or orange hue. Carotenoids are so abundant in certain tree leaves that they can sometimes compete with chlorophyll, which is why you might see yellowish-green leaves during the growing season, and why yellow leaves are often the first to show in the fall. Hardwood species such as ash, alder, birch, cottonwood, and maple will have leaves with a yellow or orange tint. This pigment is also what gives carrots, corn, bananas, egg yolks and many other things in nature their vibrant color.
Anthocyanin pigments are responsible for a leaf’s red color, and are developed by sunlight. Sometimes you can spot it in the Spring on buds and the edges of new leaves, but it really stands out in Autumn on the broad leaves of maple, oak, and dogwood trees. They can combine with carotenoids, making the deep orange and bronze colors we often see in hardwood species. You can also see anthocyanin pigments in red apples, strawberries, plums and cranberries.
Does The Weather Make A Difference?
The color and brightness of a tree’s leaves can actually depend on the weather. According to the U.S. National Arboretum, weather conditions can affect fall foliage during each season and stage of the growing process.
A wet Spring is ideal (no problem here). Drought conditions early on in the growing season can cause a tree to seal off its leaves too soon, and they will drop before they’ve had a chance to develop their fall coloring.
In the Summer and early Fall, too much moisture can actually mute colors. It’s the kind of fall seasons we love— sunny days and cool nights— that will bring about the brightest leaves. Lots of sun will cause chlorophyll to decompose more rapidly, allowing yellows and oranges to be revealed sooner and boosting the formation of more anthocyanins. The fact that red pigments depend on sunshine is why you might see more red leaves on trees (or parts of trees) that get a lot of sun exposure.
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