Leaves may turn red to screen the Sun
![[Bill Hoch, U of Wisconsin-Madison] Korean Maple](red-leaves.jpg) Q: I heard on the radio that autumn color is Nature's sunscreen. What do they mean? -Shirley E., New York,
New York
A: It's the latest theory. According to researchers Bill Hoch, Eric Zeldin, and Brent McCown at the University of
Wisconsin-Madison, red pigments in the leaves act as a sunscreen.
[Bill Hoch, U of Wisconsin-Madison] Korean Maple
"The pigments shade sensitive photosynthetic tissue in fall while trees reabsorb nutrients from their leaves," says
horticulturist Bill Hoch,. "Trees need to store as many of those nutrients as they can before the leaves drop."
During summer, the leaves of trees are factories producing sugar from carbon dioxide and water by the action of light
on a green pigment (chlorophyll) in the leaves. This process is called photosynthesis. 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.
But wait, before the tree sheds those leaves-it must extract as much food from the leaf factories as it can. Moreover, it needs the energy produced by
photosynthesis to do this last-minute work. The tree, consequently, has a big problem: It needs chlorophyll to produce energy and it's quit making
chlorophyll. Worse--chlorophyll breaks down easily in bright sunlight, i.e., sunlight destroys it. That may be why the leaves turn red: to shade the
chlorophyll so it doesn't break down and thereby conserve the supply needed to produce energy for the last-minute work.
The researcher's theory explains why:
- The outer leaves of maple trees, for example, are redder than the shaded leaves inside the leaf canopy
- Leaves on the south-facing side of the tree are redder than shaded leaves on the north-facing side
- Autumn colors are their brightest and most colorful when the fall is dry with bright sunny days, and cold nights.
(Answered by April Holladay, science correspondent, October 17, 2001)
Further Surfing:
WonderQuest USATODAY.com: Why leaves change color
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