Our Maine Street's Aroostook Issue 14 : Fall 2012 | Page 29

are responsible for red autumn leaves. Anthocyanins
appear red, purple, or blue; they are the main pigments
responsible for the color of several Maine treats including
cranberries, blueberries, and apples. Unlike chlorophyll
and carotenoids, which are present throughout the life of
all leaves, anthocyanins are only formed by certain species
of trees. As the woody membrane covers the base of the leaf
stem and reduces nutrient exchange, the sugar content of
the leaf sap increases. Anthocyanins are then formed when
the excess sugar reacts with cell proteins in the presence of
sunlight. Anthocyanins, which are abundant in red maples
and many types of oaks, change their pigment depending
on the pH of the surrounding fluid; more acidic cell sap
results in red leaves, while less acidic cell sap results in
purple leaves.

The woody membrane that forms at the base of each
leaf is also the culprit which causes the leaves to fall at the
end of the season. In order to protect the tree from freezing
temperatures, the membrane thickens as winter approaches.
Eventually, it thickens to the point where nutrients are
blocked completely, and the leaf separates from the tree.
After that, it’s just a matter of gravity, wind, and rain that
causes the leaf to fall.

Because sunlight is one of the key factors in
the degradation of chlorophyll and the formation of
anthocyanins, weather greatly influences the intensity of
autumn colors. Low temperatures and bright sunlight
contribute to the degradation of chlorophyll, which
speeds up the yellowing process. Bright sunlight, dry
weather, and temperatures that stay above freezing enhance
the production of anthocyanins, creating vibrant reds.

Therefore, the most spectacular autumn colors are created
by sunny days followed by cool (but not freezing), dry
nights, especially when preceded by a warm, wet spring
and favorable summer weather. Wet or warm autumns
tend to result in muted colors, and hard frosts or freezing
temperatures tend to cause leaves to fall prematurely.

So as you’re enjoying the autumn splendor of
Aroostook County this year, stop for a moment and
consider the chemical processes at work; there’s always more
than meets the eye. Although color varies quite a bit, even
within each species, the Maine Forest Service offers these
general guidelines regarding what colors you’ll see in the
foliage and which trees do they belong to:

Yellow leaves can be seen on green ash, black ash,
basswood, beech, birch, butternut, elm, boxelder, mountain
maple, silver maple, striped maple, sugar maple, mountainash, poplar, serviceberry, willow, and witch-hazel. Red or
scarlet leaves can be seen on hornbeam, red maple, mountain
maple, sugar maple, black oak, red oak, scarlet oak, white
oak, sumac, and tupelo. Purple leaves can be seen on white
ash and witch-hazel.
*The Maine School of Science and Mathematics (MSSM) is
Maine’s premier immersive, residential high school offering
an advanced curriculum in science, technology, engineering,
and math (STEM), as well as the humanities. MSSM students
and faculty from throughout our beautiful state are excited

to bring STEM content to Our Maine Street’s Aroostook.

FALL 2012

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