© Janet Davis
I love autumn when sugar maples take on those Technicolor hues of red and orange, birches turn clear yellow and burning bush goes into spontaneous combustion. Then again, I’ve always been a sucker for fireworks.
If you listen to folks around you, theories on what causes leaves to change seem to vary almost as much as the colors on the trees. Lots of sun? Drought? Indian summer? Early frost?
The fact is, when rainfall has been adequate and there is an extended period of warm, dry, sunny autumn days combined with cool, but not freezing nights (below about 7C or 45F), those trees genetically capable of changing color do so in spectacular fashion. But the chain of events leading to those gorgeous autumn leaves is more complicated than a change in the weather.
Pigments and Photosynthesis
During the growing season, a
tree conducts photosynthesis. Simply
put, a leaf surface absorbs radiant energy in the form of sunlight. When sunlight hits the leaves, the pigment
chlorophyll contained in the chloroplasts -- microscopic bodies within leaf
cells -- absorb the blue and red waves
but not the green waves.
Another group of pigments contained in leaves (and in higher concentrations in trees like birch, ginkgo, linden and aspen) are the carotenoids, which absorb light waves in the blue-green and blue spectrum. Together, chlorophyll and the carotenoids absorb red, blue and blue-green light waves; thus the light that is reflected back from leaves – what we see as “leaf color” – is green or yellow-green.
A third pigment group present in mature leaves of late-summer and fall are the anthocyanins, which absorb blue, blue-green and green light waves, reflecting back red. Anthocyanins are dissolved in the cell sap of plants and are sensitive to the plant’s pH. The more acidic the sap, the more red color the leaf exhibits in fall; lower acidity dictates a duller purple color. The level of anthocyanin in the sap is a function of the sugar content in leaves and fruit (grape skins, raspberries, blueberries and ripe apple skin get their red, blue and purple colors from anthocyanins). In other words, anthocyanin is not synthesized until the concentration of sugar in the leaf sap has reached a certain level in late summer and autumn. Also, leaves that are exposed to the sun will have higher sugar concentrations, therefore more anthocyanin in the sap. Thus we have leaves with red fall color in areas exposed to the sun, but green where they were in shade. Similarly, many apples are red on the “sunny” side and green on the other. Not all plants have anthocyanin-rich leaves; those that do include red maple, red oak, Virginia creeper and sumac, all of which prefer acidic soil. Some scientists have speculated that anthocyanins act as a kind of sunscreen for leaves in fall, protecting them from damaging UV radiation while they finish the job of translocating sugars into the stem and roots. Others speculate that red leaves signal insects to stay away. We also know that red leaves often indicate stress factors like drought.
As days become shorter and
temperatures cool down in late summer, the normal biochemistry in a leaf begins
to change. Chlorophyll, which is very
unstable, breaks down quickly, leaving the two accessory pigments, the
carotenoids and anthocyanins. The
carotenoids continue absorbing light and cause the leaves to visibly reflect
the rich yellows, golds and apricots of redbud, birch, ginkgo, honey locust,
aspen and katsura. And where anthocyanin
levels are high, we now see reflected the rich reds, pinks and scarlets of the
maples, oaks, liquidambar and burning bush.
Together, these two pigment groups produce the glorious tapestry of the
deciduous forest in autumn.
Carbon Dioxide, Water and Photosynthesis
Fall color, of course, is a pleasant side-effect of the most important physical process on the planet. As the leaf pigments described above absorb solar light waves, carbon dioxide is also being absorbed into the leaves through tiny, valve-like pores (stomata) on the leaf surface. And as CO˛ enters the leaf through these pores, water is being drawn up from the soil by the tree roots, transported up the trunk and through the branches into the leaves. The sun’s energy splits this water, H˛0, into hydrogen atoms and molecular oxygen, producing chemical energy that is used immediately to manufacture sugars and other compounds vital to a tree’s growth.
The oxygen we need to support life on our
planet is a waste product of photosynthesis; its release into the atmosphere as
water vapor through the leaves (again through the stomata) is 
why trees are called the
“lungs of the earth”.
A square yard of leaf surface working throughout the daylight hours in June, July and August, can produce 1-1/2 pounds of sugars, and a full-grown maple tree with its massive leaf canopy produces almost 2 tons of these leaf sugars in a summer.
Leaf Abscission
As the growing season ends and color changes are taking place, the tree begins sealing off the leaf stalks with a corky, impervious layer of cells known as the “abscission layer”. Abscission is the term botanists use to describe the separation of the leaf from the tree (what we call falling leaves). It is such a complex process involving hormones, enzymes and proteins that a California scientist published a 400-page book devoted entirely to the topic.
That takes us back to the weather. When autumn days are sunny, sugar production remains plentiful. But cool nights and the timely creation of the abscission layer result in a high level of sugars remaining behind in the leaves to trigger production of brilliant red anthocyanins. If days are rainy and overcast, sugar production falls off, so autumn seems characterized more by mellow gold and buff that spectacular scarlet and red. An early hard-freeze, of course, puts an end to the color parade.
And that, more or less, is how leaves produce their spectacular color show each year.