If 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 autumn palette.
Where do autumn colors come from?
A color palette needs pigments, and there are three types that are involved in
• 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
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 autumn colors.
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 old age.
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 leaves?
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 States?
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
When is the best time to see autumn color?
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 western Maryland and moves eastward, reaching the Eastern Shore by
mid-October. Remember that cooler high elevations will color up before the