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The pigment chlorophyll is an organic compound
that has the ability to absorb energy from sunlight,
which a plant can then use to produce sugar from
carbon dioxide and water through photosynthesis.
Chlorophyll primarily absorbs from the blue and red wavelengths of the spectrum
while reflecting green — hence the greenish color of deciduous leaves. Once the
leaves emerge in spring, they begin to produce more and more chlorophyll, and
they turn from light green to a darker, richer hue. As fall sets in, less chlorophyll is
produced, and the leaves begin to lose their green color.
There are other plant chemicals in leaves that have distinctive colors. Ca-
rotenoids, which produce yellow, orange, and brown hues, are found in leaves
throughout the growing season, but are masked by the chlorophyll green. Anthocy-
anins, which are associated with reds and purples, are produced mainly in fall, and
aren’t present in all trees. When chlorophyll production ceases in a leaf, the green
color fades, and the other
two pigments become
dominant. The actual fall
leaf color depends on the
species of tree.
Fortunately, nature
provides us with a full
color palette in fall. As-
pen and birch are known
for their brilliant yellow
leaves, which can stand
in stark contrast to the
surrounding evergreens of
more northern latitudes.
Oak leaves are typically
brown, red, or russet. Sourwood and black tupelo, found mainly in the Southeast,
are known for their brilliant crimson leaves. The flowering dogwoods of the East-
ern states have purplish-red leaves. Also in the Eastern states, yellow leaves befit
the yellow poplar. Hickories go golden bronze; beech become light tan. Some trees,
such as elms, don’t take on bright colors; their leaves just turn brown and fall off.
When most people think of fall colors, they think of maple trees, which
produce some of the most spectacular color displays. The black maples of the
Midwest turn bright yellow. The red maples of the Eastern United States become a
brilliant scarlet. From the Midwest to the Northeast, the sugar maple is a fall color
highlight, with leaves ranging from gold to orange to scarlet.
This happens every fall, because trees are genetically programmed to behave
in this fashion. They respond to day length, or, more precisely, the length of night.
Longer nights herald the arrival of colder temperatures. As this is a constant every
year, it’s a great mechanism for plant life cycle events. Although all deciduous
trees will lose their leaves in fall, they do it at different times, varying by species.
Sourwood trees in the South can turn in late summer. Oaks are among the last to
turn in fall, and may even keep some leaves until the next spring.
Cooling temperatures are also a cue for the approach of the first freeze, but
temperature patterns aren’t as reliable as changes in day length, so this is a second-
ary cue. Abnormally warm weather can delay the onset of leaf changes, but they’ll
still occur due to the lengthening nights. Conversely, colder-than-normal tempera-
tures in early fall can hasten the process.
With the significance of night length and temperature on fall color, it’s easy to
see why latitude plays a major role in when colors develop. The farther north youWith the significance of night
length and temperature on fall
color, it’s easy to see why latitude
plays a major role in when colors
develop. The farther north you
travel, the sooner fall days
become shorter.