Autumn Jewels: The Science Behind the Scenery
by Cheryl M. English
As the descendant of a New England family, I have always considered as one of the perks of
living here in Michigan the magnificent display of fall color, cascading from Sault Ste. Marie
down to just north of Toledo, 
the gorgeous, glowing garnets, topazes and citrines of deciduous trees and shrubs (complemented,
of course, by the emerald of evergreens) as they put on their final - and perhaps most glorious -
show. Beginning in August and September with poison ivy and sumac and ending in November with the
larches and weeping willows, the show is a non-stop, two month kaleidoscope of autumn jewels.
We've known for some time how they do it - the science of the transition from summer cool to fall warm. The mystery was not how, but why, a mystery that may finally (unlike the origin of the cat's purr!) be solved.
How They Do It
Although the leaves on most trees appear to be green from spring through summer, all the time they are producing chlorophyll as a product of photosynthesis (which is also incidentally a green pigment), carotene and xanthophyll (both yellow pigments) are also present. As the days become shorter and cooler in the autumn, the chlorophyll in the leaves begins to break down, revealing the yellow pigments it had masked through the warm months. These two pigments - carotene and xanthophylls - are the source of the clear yellow of the gingko, as well as the yellow and gold hues of redbud, larch, hickory, birch and witch hazel.
But what about the orange, red and purple hues found in some maples, dogwood and oaks? Those
are produced by anthocyanins, which may mask the yellow pigments. Anthocyanin production increases
with the presence of 
increased sugars in the
leaves. As the fall season - with its sunny days and cool nights - increases the sugars in leaves, the oranges, reds and purples intensify. This also
explains the two-tone effect on certain species, such as green ash and amelanchier, with leaves of
a redder or more purple hue occurring nearer the top or on the outside of the tree.
The tans and browns of oaks are caused by tannins that accumulate as the chlorophyll dissipates. Frost and freezing temperatures will halt the coloration process, blackening the leaves and drawing the show to a close.
Why They Do It
Although the chemistry of fall color has long been understood, the why of it has long eluded biologists. Until recently, it was assumed to simply be a consequence of the change of seasons as photosynthesis wound down, a by-product of dying leaves. Trees need energy to produce these other pigments which are only revealed in autumn. This is energy the trees cannot reclaim as it stays in the leaf when it dies. In other words, if the pigments do not in some way help trees to survive, they seem to be wasted - not typical of Mother' Nature's work ethic.
Evolutionary biologists propose that bright autumn leaves warn insects to leave them alone. In autumn, insects such as aphids choose the trees in which they will lay their eggs. When the eggs hatch the next spring, the larvae will feed on that tree, often with serious effects. A tree, however, can ward off such pests with toxins. The hypothesis is that trees with strong chemical defenses might protect themselves even further by letting egg-laying insects know what would be in store for their young. As insects evolved to avoid trees with the brightest leaves, natural selection favored those trees that could become even brighter. In other words, the trees advertise their lethality through their fall colors.
On the other hand, bright leaves occur on trees that have no insects to warn. In far northern climes, by the time the leaves change color, all the insects that feed on foliage are already gone. The work of a number of plant physiologists suggests that those fall colors may actually serve as a sunscreen. The interior of an autumn leaf is a veritable frenzy of activity. The materials necessary for photosynthesis Ð including chlorophyll - are breaking down, while the nutrients are being sent to the tree's tissue, to help it grow and reproduce come spring. The leaves need energy in order to perform these tasks, energy generally available only through photosynthesis. But photosynthesis no longer functions effectively. Autumn leaves cannot capture all of the sunlight striking them and the excess cause damage to leaf tissue. Under the circumstances, the less intense light of October can do more harm to a leaf than the blazing hot sun of July.
Anthocyanins, those pigments producing orange, red and purple hues, may protect autumn leaves by blocking some of the sunlight. Experiments tend to confirm this hypothesis. The link between bright fall leaves and lack of spring damage may indicate the trees were doing a good job of protecting their leaves in the fall and maximizing the process of approaching dormancy. This, however, does not explain the success of trees - such as birches - that produce no anthocyanins. It is unclear how they protect themselves from autumn sunlight.
Of course, who's to say that perhaps both functions are not at work? Mother Nature is known for
her thoroughness and multi-tasking.
Maybe those bright leaves serve to protect trees from both the sun and insects. Nevertheless,
the phenomenon is, regardless of its scientific basis, a most glorious transition to the rigors
of winter, a blazing last hurrah.
Until recently, it was assumed to be simply a consequence of the change of seasons as photosynthesis wound down, a by-product of dying leaves. Trees need energy to produce those other pigments which are only revealed in autumn.