Scientific American Instant Egghead
You've probably heard different explanations
for why leaves change color:
lower temperatures,
less daylight,
less rain.
But, the correct answer is
less daylight.
Temperature can affect the intensity of leaf color,
which I will explain in a minute,
but less daylight actually causes color change.
Most leaves contain chlorophyll, a green pigment,
that starts the process of photosynthesis.
Chlorophyll helps convert sunlight, carbon dioxide and water
into sugars that travel to the branches and the roots,
feeding the tree.
Chlorophyll breaks down and is replenished
all summer long.
As daylight hours shorten, however,
veins that bring water and nutrients to the leaf
gradually close off.
The old chlorophyll disappears,
and no new chlorophyll forms.
The green color fades.
Yellow and orange pigments, which were there all along,
become visible.
But, what about the red colors?
They actually form during the fall.
As veins close, sugars get trapped in the leaf.
They react with other chemicals to form the red pigments.
You'll remember, I said intensity is connected to temperature.
If days are bright, and nights are cold,
more sugars get trapped,
and the reds intensify.
Eventually, the drier autumn weather
triggers a hormone telling the tree to drop its leaves.
Otherwise, the leaves would use up water
that the tree needs to keep its roots alive during the winter.
Okay, what about evergreens?
The needles have a waxy coating, and less surface area,
which reduces water loss.
They also contain a chemical like anti-freeze,
so the roots and the leaves can survive the winter.
Now that you know the science behind colorful autumn leaves,
let's identify some of them:
oaks turn red and brown;
hickories turn bronze;
aspen and poplar turn gold and yellow;
sugar maples turn orange;
black maples turn yellow;
and red maples turn -- what else?-- brilliant red.
For Scientific American's Instant Egghead,
I'm Mark Fischetti.