Why Leaves Change Color in the Fall

Why Leaves Change Color in Autumn

Maple leaves turning red in autumn
Maple leaves turning red in autumn. Photo courtesy USFWS.

Have you ever wondered why leaves change color in the fall, or why leaves on
different trees (or even the same tree) turn different colors? The simple
answer is the trees have stopped photosynthesizing, or making energy from
light. Trees enter a period of rest in the autumn, because this season marks
the start of cooling temperatures and decreasing sunlight. In the spring and
summer growing season trees make chlorophyll, the compound responsible for
photosynthesis. The chemical properties of chlorophyll are such that it
absorbs red and blue light, causing leaves to appear green. Chlorophyll
decomposes in the presence of sunlight, and so trees must continually make it.
In the fall, when trees stop making chlorophyll, other pigments begin to show
through. There are two primary classes of these pigments, carotenoids and
anthocyanins.

Carotenoids – Yellow, Orange, Brown, and Gold
Carotenoids (carotene) are a class of pigments that are present in leaves
year-round. These chemicals are generally more stable and durable than
chlorophyll. Once chlorophyll is no longer present, carotenoids cause leaves
to appear gold, yellow, orange, or brown. This pigment is also responsible for
the yellow and orange colors of bananas, pumpkins and other squash, corn,
carrots, and yellow flowers like sunflowers, daffodils, or buttercups. The
amount of carotenoids in the leaves of a particular tree species does not vary
much from year to year, nor is it affected much by weather.

Anthocyanins – Red and Purple
Anthocyanins cause leaves to appear red or purple. They also give color to
foods like strawberries, cherries, blueberries, red or purple grapes, red
apples, plums, cranberries, and raspberries. Unlike carotenoids, anthocyanins
are not always present in leaves. They are formed by a chemical reaction that
takes place in the presence of sugar and sunlight. Bright sunlight during fall
days results in excess production of sugar in leaves. More sunlight means more
sugar and therefore more spectacular reds and purples from anthocyanins.
Anthocyanins produce red colors when the internal environment of the leaf is
acidic and purple colors in low-acid conditions.

Why Are Red and Purple Leaves More Spectacular in Some Years?
The weather during any particular autumn has a profound influence on the hue of
red and purple fall colors. Anthocyanins produce the brightest colors when the
spring has been warm and wet, the summer has been mild and not too dry, and
fall days are relatively warm and sunny with cool nights above freezing.
Periods of drought tend to reduce the intensity and duration of fall color
displays, as does wet weather during fall. Freezing temperatures in the autumn
will kill leaves, causing them to fall off before completing the autumn color
cycle.

Why Do Leaves Fall in the Fall?
Leaves on broadleaf (non-evergreen) trees are full of sap and will freeze in
the winter. Too many frozen leaves can kill or injure a tree. Therefore,
trees begin to seal off leaves in response to increasingly colder temperatures.
Trees try to recover as many nutrients as they can before they shed each leaf.
As temperatures decrease, the veins that carry nutrients to and from each leaf
constrict. At the same time, a layer of cells, called the separation layer,
begins to form at the base of each leaf. When this layer of cells is complete,
the leaf falls off, a process known as dehiscence.

Looking for a fun way to enjoy the changing of the leaves with your family,
church, scouting or other group while giving something back to your community?
Take along a picnic and a few trash bags to your favorite local fall color
recreation spot. Spend a few minutes picking up trash at the site or along the
way before enjoying your picnic lunch. This is a great way to help impart a
responsible land ethic to the youngsters in your life and take in some
beautiful scenery at the same time. After all, the more we do to care for our
valued landscapes, the more valuable they become.

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