Leaves
While temperature can
dictate intensity, it is just
one of the factors that play
a part in painting a striking
fall landscape. Cooler
temperatures, changes in
rainfall, and most of all,
shortening day length, play a
role in the changing color of
tree leaves.
This is what’s really going on
behind the scenes as green
leaves turn to shades of
autumn. During the spring
and summer, leaves are busy
creating the foods necessary
for the tree’s growth.
But in fall, leaves reduce their
food-making process because
of changing temperatures
and the amount of sunlight.
Chlorophyll, the chemical
that gives leaves their
bright green color, begins to
breakdown and disappear.
Carotenes, the substances
that gives leaves their yellow
and orange hues, are present
in leaves throughout the
growing season.
The larger amount of green
chlorophyll masks the colors
of carotenoids until fall’s
arrival when chlorophyll
production wanes and other
pigments get their chance to
shine.
The pigments that produce
the most exciting colors
—reds, pinks and purples
— come from anthocyanins,
which are manufactured from
sugars the tree produces.
As days shorten and nights
cool, excess sugars are
trapped in the leaf as veins
leading into and out of the
leaves close.
Combined with bright light,
the sugars are used in the
production of anthocyanin
pigments. These colors
are the most variable from
year to year, because their
formation is the most
dependent on favorable fall
environmental conditions
and sunlight.
By late autumn, the yellow
and red pigments, following
the green, disintegrate in
the leaves. This final step
exposes the brown tannins.
When leaves fall, they
decompose and provide the
soil with nutrients. These
leaves also become a food
source for many organisms.
Sugar maples, birches and
other broad-leafed trees
shed their leaves in the fall.
But before they do, they
salvage as many nutrients as
possible to store through the
cold months and reuse the
following spring.
Autumn leaf colors vary from
year to year. Some years,
those bright and beautiful
reds, purples and yellows last
for weeks, while other years,
autumn leaves quickly turn
to brown.
The factor responsible for
the onset of fall and the
intensity of autumn color is
the weather.
A warm, wet spring combined
with a mild summer and
bright autumn days with cool
nights results in more vivid
colors. On the other hand, a
late spring or severe drought
will delay the onset of fall
color for weeks. A warm fall
will also dull autumn colors.
Wind and rain during the fall
color season will often knock
the leaves off the tree before
they can produce their best
color.
Trees under stress may
discolor or drop leaves
early. Extreme heat, drought,
insects, disease and
damaged roots can all be
great stressors for trees. The
best time of year to examine
your trees for signs of stress
is early September.
Healthy, well-cared for trees
showcase the most glorious
leaf colors. To promote
your trees’ ability to offer
their best fall color, talk to
a certified arborist at Davey
Tree. Find a trusted tree care
specialist at www.davey.com.
Landscape & Urban Design Issue 21
21