Why do Leaves Turn Color?
by Dave Erler, Sr., Naturalist

When I was a young boy, I believed
a lot more than I understood. There
was a special time of year between the spring visit by the Easter Bunny and Santa’
s grand arrival. It was the time of year that Jack Frost did his thing, painting the
leaves of autumn their brilliant colors.

Well, somewhere around the second grade a neighbor kid took the wind from
my sail about the spring hare and the big guy from the North Pole. I think I
figured out for myself that the Jack Frost stuff was in the same boat with them.
(Fortunately, I had enough sense to continue buying into the tooth fairy until I
shed my last molar.)

Although many autumns came and went past my eighth year, it wasn’t until my
college dendrology (study of woody plants) course that I began to really ponder
why the oaks turned scarlet, the maples blaze orange and the aspens a golden

The Science Behind the Beautiful Leaves

Essentially, the process of color change in leaves comes down to chemical and
environmental factors. The green color we all see in leaves most of the growing
season is chlorophyll. Chlorophyll is the key ingredient in the food-making
process of photosynthesis. Hidden behind the green pigment are two yellow
pigments called carotene and xanthophylls. Chlorophyll is unstable and is
regularly being broken down and resynthesized. Carotene and xanthophylls are
produced as the leaf unfolds and are stable, remaining in the leaves until they
turn brown. The red color we see is caused by pigments called anthocyanins
which are produced late in the growing season. These red pigments or
anthocyanins are water soluble and are located in the leaf’s upper layer. The
anthocyanins depend upon high sugar and tannin concentrations which are
produced best by bright sunny days and cool nights. Because the anthocyanins
are located in the upper layer of leaf cells, it can cover the other pigments either
partially or completely.

Early Frost: Better Color? Not Necessarily

Although I came to realize Jack Frost didn’t paint the leaves their brilliant colors,
I long held the common belief that early frosts were important in triggering the
color change. In actuality, frost can be damaging to the process of color change
by destroying the cell tissues. The whole process is prompted by the shortening
of day length and the resulting decrease in the amount of sunlight absorbed by
the leaf. As days shorten, photosynthesis decreases. At the same time a thin layer
of cells form at the point where the leaf stem attached to the branch, effectively
reducing the flow of water necessary for sugar and chlorophyll production.

So what’s this got to do with clear days, cool nights and leaf color? As the
chlorophyll production comes to a halt, the leaves will appear yellow as the
carotene and xanthophylls pigments are revealed. Clear days and cool nights
(below 45 and above 32 degrees F) allow for the red anthocyanin pigments to be
produced and trapped in the upper leaf cells. If present in high concentrations
the reds can mask the yellows, or in lesser concentrations blend to give us
various hues of orange.

Cloudy Weather: Redder Trees

An autumn with a lot of cloudy weather reduces the production of the reds and
the usually accompanying warmer nights allows for the sugars to be transported
out of the leaves, further reducing the amount of red pigment. Early frosts can
kill the process before peak color occurs. Other factors can affect a particular
autumn or an individual tree’s color. Stress caused by insects, disease, root
damage and even road salt can result in premature color change. Elevation, tree
orientation to afternoon sunlight and genetics can, in combination with rainfall
and temperature, account for the varying brilliance of color year to year.

If all this sounds complicated, take heart in knowing the trees will continue to
change color whether we human leaf peepers understand the process or not.
However, we shouldn’t take this annual color show for granted. As color sighted
animals, humans can witness this amazing display. Dogs, cats, foxes, raccoons
and most other mammals witness only subtle changes in shades of grey. But
whether you accept the scientific explanation for the process or still believe Jack
Frost is responsible for nature’s colorful farewell to summer, the important thing
is to make sure you get out and experience it!

For exceptional leaf peeping, visit the
Squam Lakes Natural Science Center and
take a short hike up the nature trail to Mt. Fayal. See spectacular panoramic
views of the entire Squam Region, with colors so bright and bountiful it’s
breathtaking. Or consider a view from the water with Science Center Lakes

Squam Lake on a pontoon and see the colors from a slightly different angle,
with brilliant reflections beaming from the water’s edges. Some of the smaller
islands seem to be on fire with an array of deep and bright oranges, reds, and
yellows. Call the Science Center at 603-968-7194 or visit their website at
nhnature.org for more information.

About the author:

Dave Erler is a Senior Naturalist at the Squam Lake Natural Science Center.
Previously, Dave was a naturalist with the National Park Service and University
of Minnesota Extension Service before coming to the Center in 1979.  Dave was
recognized as the New England Environmental Education Alliance’s Educator of
the Year in 2002.
Leaf.  Photo courtesy of Squam Lakes Natural Science Center
Dave Erler & friend
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