Why Do Leaves Change Their Color?
Excerpted from: SUNY College of Environmental Science
Every autumn we revel in the beauty of the fall colors. The mixture of red, purple, orange and
yellow is the result of chemical processes that take place in the tree as the seasons change
from summer to winter.
During the spring and summer the leaves have served as factories where most of
the foods necessary for the tree's growth are manufactured. This food-making process takes
place in the leaf in numerous cells containing chlorophyll, which gives the leaf its green color.
This extraordinary chemical absorbs from sunlight the energy that is used in transforming carbon
dioxide and water to carbohydrates, such as sugars and starch.
Along with the green pigment are yellow to orange pigments, carotenes and xanthophyll pigments
which, for example, give the orange color to a carrot. Most of the year autumn colors are masked by
great amounts of green coloring.
But in the fall, because of changes in the length of daylight and changes in temperature, the leaves
stop their food-making process. The chlorophyll breaks down, the green color disappears, and the
yellow to orange colors become visible and give the leaves part of their fall splendor.
At the same time other chemical changes may occur, which form additional colors through the
development of red anthocyanin pigments. Some mixtures give rise to the reddish and purplish
fall colors of trees such as dogwoods and sumacs, while others give the sugar maple its brilliant
The autumn foliage of some trees shows only yellow colors. Others, like many oaks, display mostly
browns. All these colors are due to the mixing of varying amounts of the chlorophyll residue and
other pigments in the leaf during the fall season.
What Is Chlorophyll?
Green plants have the ability to make their own food. They do this through a process called
photosynthesis, which uses a green pigment called chlorophyll. A pigment is a
molecule that has a particular color and can absorb light at different wavelengths, depending
on the color. There are many different types of pigments in nature, but chlorophyll is unique
in its ability to enable plants to absorb the energy they need to build tissues.
Chlorophyll’s job in a plant is to absorb light — usually sunlight. The energy absorbed
from light is transferred to energy-storing molecules. Through photosynthesis, the plant uses
the stored energy to convert carbon dioxide from the air and water into glucose.
As the fall colors appear, other changes are taking place. At the point where the stem of the leaf is
attached to the tree, a special layer of cells develops and gradually severs the tissues that support
the leaf. At the same time, the tree seals the cut, so that when the leaf is finally blown off by the wind
or falls from its own weight, it leaves behind a leaf scar.
Most of the broad-leaved trees in the north shed their leaves in the fall. However, the dead brown
leaves of the oaks and a few other species may stay on the tree until growth starts again in the spring.
In the South, where the winters are mild, some of the broad-leaved trees are evergreen —
that is, the leaves stay on the trees during winter and keep their green color.
Most of the conifers, like pines, spruces, firs, hemlocks and cedars, are evergreen in
both the North and South. The needle- or scale-like leaves remain green or greenish the year round,
and individual leaves may stay on for 2 to 4 or more years.
Temperature, light, and water supply have an influence on the degree and the duration of fall color.
Low temperatures above freezing will favor anthocyanin formation producing bright reds in maples.
However, early frost will weaken the brilliant red color. Rainy and/or overcast days tend to increase
the intensity of fall colors. The best time to enjoy the autumn color would be on a clear, dry, and cool
(not freezing) day.
Wisconsin provides us with some brutal winters, sometimes non-existent springs, and varying
degree summers. The one season we know we can count on as Wisconsinites is fall. The fall season
brings apple picking, game-days, moderately chilled weather, pumpkin carving, trick-or-treating, and
of course, changing tree colors. These varying colors entice us to go outside and view the beauty
we are surrounded by here in the Midwest. The real question is, what trees will provide us with
these beautiful fall colors? Here are some suggestions.
Northern Red Oak
Beavers — Nature's Hydrologist, Part 2
Garter Snakes — The Gardener's Friend
Wisconsin Native Salamanders
Goundhog or Woochuck: All The Facts
Voles, Both The Good and The Bad