Fall colors
I have always wondered why leaves change color during the fall season. Some have told me it is a killer frost, which causes the tree sap to flow into
the root system for the winter. Then others have told me that it has to do with amount of sunlight (photosynthesis) which the trees receive. Can you
set me straight?
 Fall
leaves. Photo courtesy of the USDA Forest Service.
You've got the right idea.
"Every autumn across the Northern Hemisphere, diminishing daylight hours and falling temperatures induce trees to
prepare for winter," says
Bassam Shakhashiri, chemistry professor at the University of Wisconsin-Madison.
Broadleaf, green trees turn to brilliant shades of yellow, orange, and red. Their color changes because the tree's
growth factory shuts down for the winter. Winter is a bad time for growing: too little water, too little sun, and too
much cold. So the tree stops producing chlorophyll, stores sugars for antifreeze protection, and sheds tender leaves.
The Leaf Factory
"During summer, the leaves of trees are factories producing sugar from carbon dioxide and water by the action of light on chlorophyll [photosynthesis]," says
Shakhashiri.
To get energy, the large chlorophyll molecules absorb red and blue light from sunlight and reflect green light. That's why trees look green during summer.
Chlorophyll molecules use the light energy to change carbon dioxide and water into oxygen, sugars, and starches. The leaves exhale the waste oxygen and keep
the food.
Water and nutrients flow up from roots through branches and into the leaves. Sugars manufactured during photosynthesis flow out from the leaves to the rest of
the tree and the chemical energy of the sugar fuels tree growth.
The tree stores any excess energy.
 How
a leaf functions. Copyright ©2007 Conservation Commission of Missouri. All
Rights Reserved.
Chlorophyll, however, breaks down easily in bright sunlight. So
leaves constantly churn out chlorophyll all summer long and the
leaves stay green. The leaves of many trees (for example, birches)
also contain the yellow pigment, carotene, which traps blue-green
and blue light.
Fall Comes
As the nights get shorter and cooler, the tree must protect itself for
winter. Its stems, twigs, and buds are tough enough to withstand
winter, but not so its fragile broad, thin leaves. The watery sap in
leaf cells freezes easily. So the tree seals off its leaves and sheds
them.
To begin the sealing-off process, the tree grows a corky membrane
between each branch and leaf stem. The membrane hinders the
flow of nutrients into the leaf which stops the leaf from making new
chlorophyll. The old chlorophyll quickly decomposes and the leaf's
green color fades.
If the leaf contains carotene, birch trees for example, the fading leaf
changes from green to yellow. Carotene, a more stable compound
than chlorophyll, persists in leaves even after all the chlorophyll is gone. The leaves of those trees now look yellow. The birches, aspens, and cottonwoods shimmer gold
in a breeze.
The sealing membrane not only stops the inward nutrient flow but also blocks the outward flow of sugar, trapping it in the leaf. The blocked leaf sugar in some
trees, like maples, reacts to form a red pigment, called anthocyanins. The leaves now take on a reddish tinge. The brighter the daylight during this time, the more
brilliant the leaf color we see. Indeed, the maples, oaks, and sumacs can inflame the forest with scarlet.
Further Reading
The
structure of a leaf, Missouri Department of Conservation
Hot spots for
fall color in the nation, USDA Forest Service
The chemistry of fall colors, University of Wisconcin
(Answered Oct. 11, 2000; updated Nov. 27, 2007)
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