you are lucky, you live in one of those parts of the world where Nature has one
last fling before settling down into winter's sleep. In those lucky places, as
days shorten and temperatures become crisp, the quiet green palette of summer
foliage is transformed into the vivid autumn palette of reds, oranges, golds,
and browns before the leaves fall off the trees. On special years, the colors
are truly breathtaking.
How does autumn color happen?
For years, scientists have
worked to understand the changes that happen to trees and shrubs in the autumn.
Although we don't know all the details, we do know enough to explain the basics
and help you to enjoy more fully Nature's multicolored autumn farewell. Three
factors influence autumn leaf color-leaf pigments, length of night, and
weather, but not quite in the way we think. The timing of color change and leaf
fall are primarily regulated by the calendar, that is, the increasing length of
night. None of the other environmental influences-temperature, rainfall, food
supply, and so on-are as unvarying as the steadily increasing length of night
during autumn. As days grow shorter, and nights grow longer and cooler,
biochemical processes in the leaf begin to paint the landscape with Nature's
Where do autumn colors come from?
A color palette needs pigments, and there are three types that are
involved in autumn color.
- Chlorophyll, which gives leaves their
basic green color. It is necessary for photosynthesis, the chemical reaction
that enables plants to use sunlight to manufacture sugars for their food. Trees
in the temperate zones store these sugars for their winter dormant period.
- Carotenoids, which produce yellow,
orange, and brown colors in such things as corn, carrots, and daffodils, as
well as rutabagas, buttercups, and bananas.
- Anthocyanins, which give color to such
familiar things as cranberries, red apples, concord grapes, blueberries,
cherries, strawberries, and plums. They are water soluble and appear in the
watery liquid of leaf cells.
Both chlorophyll and carotenoids are present in the chloroplasts of
leaf cells throughout the growing season. Most anthocyanins are produced in the
autumn, in response to bright light and excess plant sugars within leaf cells.
During the growing season, chlorophyll is continually being produced
and broken down and leaves appear green. As night length increases in the
autumn, chlorophyll production slows down and then stops and eventually all the
chlorophyll is destroyed. The carotenoids and anthocyanins that are present in
the leaf are then unmasked and show their colors.
Certain colors are characteristic of particular species. Oaks turn
red, brown, or russet; hickories, golden bronze; aspen and yellow-poplar,
golden yellow; dogwood, purplish red; beech, light tan; and sourwood and black
tupelo, crimson. Maples differ species by species-red maple turns brilliant
scarlet; sugar maple, orange-red; and black maple, glowing yellow. Striped
maple becomes almost colorless. Leaves of some species such as the elms simply
shrivel up and fall, exhibiting little color other than drab brown.
The timing of the color change also varies by species. Sourwood in
southern forests can become vividly colorful in late summer while all other
species are still vigorously green. Oaks put on their colors long after other
species have already shed their leaves. These differences in timing among
species seem to be genetically inherited, for a particular species at the same
latitude will show the same coloration in the cool temperatures of high
mountain elevations at about the same time as it does in warmer lowlands.
How does weather affect autumn color?
The amount and brilliance of the
colors that develop in any particular autumn season are related to weather
conditions that occur before and during the time the chlorophyll in the leaves
is dwindling. Temperature and moisture are the main influences.
A succession of warm, sunny days and cool, crisp but not freezing
nights seems to bring about the most spectacular color displays. During these
days, lots of sugars are produced in the leaf but the cool nights and the
gradual closing of veins going into the leaf prevent these sugars from moving
out. These conditions-lots of sugar and lots of light-spur production of the
brilliant anthocyanin pigments, which tint reds, purples, and crimson. Because
carotenoids are always present in leaves, the yellow and gold colors remain
fairly constant from year to year.
The amount of moisture in the soil also affects autumn colors. Like
the weather, soil moisture varies greatly from year to year. The countless
combinations of these two highly variable factors assure that no two autumns
can be exactly alike. A late spring, or a severe summer drought, can delay the
onset of fall color by a few weeks. A warm period during fall will also lower
the intensity of autumn colors. A warm wet spring, favorable summer weather,
and warm sunny fall days with cool nights should produce the most brilliant
What triggers leaf fall?
In early autumn, in response to the shortening days and declining
intensity of sunlight, leaves begin the processes leading up to their fall. The
veins that carry fluids into and out of the leaf gradually close off as a layer
of cells forms at the base of each leaf. These clogged veins trap sugars in the
leaf and promote production of anthocyanins. Once this separation layer is
complete and the connecting tissues are sealed off, the leaf is ready to fall.
What does all this do for the tree?
Winter is a certainty that all vegetation in the
temperate zones must face each year. Perennial plants, including trees, must
have some sort of protection to survive freezing temperatures and other harsh
wintertime influences. Stems, twigs, and buds are equipped to survive extreme
cold so that they can reawaken when spring heralds the start of another growing
season. Tender leaf tissues, however, would freeze in winter, so plants must
either toughen up and protect their leaves or dispose of them.
The evergreens-pines, spruces, cedars, firs, and so on-are able to
survive winter because they have toughened up. Their needle-like or scale-like
foliage is covered with a heavy wax coating and the fluid inside their cells
contains substances that resist freezing. Thus the foliage of evergreens can
safely withstand all but the severest winter conditions, such as those in the
Arctic. Evergreen needles survive for some years but eventually fall because of
The leaves of broadleaved plants, on the other hand, are tender and
vulnerable to damage. These leaves are typically broad and thin and are not
protected by any thick coverings. The fluid in cells of these leaves is usually
a thin, watery sap that freezes readily. This means that the cells could not
survive winter where temperatures fall below freezing. Tissues unable to
overwinter must be sealed off and shed to ensure the plant's continued
survival. Thus leaf fall precedes each winter in the temperate zones.
||What happens to all those fallen
Needles and leaves that fall are not wasted. They decompose and
restock the soil with nutrients and make up part of the spongy humus layer of
the forest floor that absorbs and holds rainfall. Fallen leaves also become
food for numerous soil organisms vital to the forest ecosystem.
It is quite easy to see the benefit to the tree of its annual
leaf fall, but the advantage to the entire forest is more subtle. It could well
be that the forest could no more survive without its annual replenishment from
leaves than the individual tree could survive without shedding these leaves.
The many beautiful interrelationships in the forest community leave us with
myriad fascinating puzzles still to solve.
Where can I see autumn color in the United
You can find autumn color in parks and
woodlands, in the cities, countryside, and mountains - anywhere you find
deciduous broadleaved trees, the ones that drop their leaves in the autumn.
Nature's autumn palette is painted on oaks, maples, beeches, sweetgums,
yellow-poplars, dogwoods, hickories, and others. Your own neighborhood may be
planted with special trees that were selected for their autumn color.
New England is rightly famous for the spectacular autumn colors
painted on the trees of its mountains and countryside, but the Adirondack,
Appalachian, Smoky, and Rocky Mountains are also clad with colorful displays.
In the East, we can see the reds, oranges, golds, and bronzes of the mixed
deciduous woodlands; in the West, we see the bright yellows of aspen stands and
larches contrasting with the dark greens of the evergreen conifers.
Many of the Forest Service's 100 plus scenic byways were planned with
autumn color in mind. In 31 States you can drive on over 3,000 miles of scenic
byways, and almost everyone of them offers a beautiful, colorful drive sometime
in the autumn.
When is the best time to see autumn
Unfortunately, autumn color is not very predictable, especially in
the long term. Half the fun is trying to outguess Nature! But it generally
starts in late September in New England and moves southward, reaching the Smoky
Mountains by early November. It also appears about this time in the
high-elevation mountains of the West. Remember that cooler high elevations will
color up before the valleys. The Forest Service's Fall Color Hotline
(1-800-354-4595) can provide you with details as the autumn color display
Persons of any race, color, national origin, sex,
age, or religion, or with any handicapping condition are welcome to use and
enjoy all the facilities, programs, and services of the U.S. Department of
Agriculture. Discrimination in any form is strictly against agency policy and
should reported to the Secretary of Agriculture, Washington, DC 20250.