Fall foliage colors

how they do it… and some ideas about why

In early October, I made a quick visit to southern Wisconsin. My timing was just about perfect for enjoying the splendor of colorful autumn foliage. The rolling hills were swathed in reds and yellows, with just enough touches of green and brown to set off the brighter colors. I walked through a maple forest: golden canopy, golden carpet of fallen leaves, and a golden rain of more leaves coming down. Oaks, which usually turn brown and russet, were producing bright reds and lovely soft yellows—a color I didn’t know they could make. Shrubby sumacs at the forest edges presented blaze orange, all possible shades of red, and even almost purple. Hickories, ashes, and grape vines added a range of yellows. Virginia creeper vines sprawled up tree trunks and over bushes, providing splashes of red between the ground and the canopy. Individual maples often carried crowns of red, orange, and yellow, all on one tree. Altogether, a splendid show! It’s easy to see why this annual event has become a tourist attraction, so much so that states (especially in the Midwest and Northeast) provide maps and dates of where and when visitors can enjoy the best color spectacular.

Around here, we are surrounded by somber spruce and hemlock greens and dull alder brown. Bits of color are seen here and there: cottonwoods and willows give us some golds and yellows; dwarf dogwood leaves turn crimson, a nice framework for the scarlet berries; and highbush cranberry bushes offer pinks and reds. In the alpine zones, we can find gold and russet from the deer cabbage and dull reds from the low-growing blueberries. But nowhere do we get the broad brush-strokes of brilliant color that are seen back east.

Several folks have asked me about the causes of fall foliage colors. So here is a short version of the physiological explanation, as it applies to trees, at least. In fall, as days get shorter and nights get longer, deciduous trees gradually shut off the circulation to and from leaves; nutrients such as nitrogen can be withdrawn from the leaves and stored in the roots. A corky layer develops at the base of the leaf stem and gradually pinches off the flow of water and nutrients. So less water rises from the roots to the leaves, and less carbohydrate (sugars) passes from the leaves to the rest of the plant. Chlorophyll, the green pigment essential for photosynthesis of carbohydrate, begins to disintegrate. As it breaks down, yellow and orange pigments already in the leaf become exposed to our view.

Red and purple colors are, in most species, caused by anthocyanins, which are synthesized in fall. A series of warm, sunny days and cool nights allows photosynthesis to continue until the chlorophyll is gone, but the sugars produced by this process can no longer get completely out of the leaf. These excess sugars are used to produce anthocyanins, particularly when the leaf is exposed to bright light. So we may see that the top of a tree, or the outer part of the crown, produces redder leaves than lower or inner branches. Sometimes a single leaf is yellow on one side of the midvein, where it was shaded by another leaf, and bright red on the unshaded side.

The leaf is still alive, so its respiration continues to produce carbon dioxide, which in summer is used to make sugars. But when photosynthesis is reduced in fall, excess carbon dioxide may accumulate in the leaves, making the sap more acidic. At least with some anthocyanins, the level of acidity can affect the development of color: high levels favor red hues, but lower acidity in the sap may allow blue tones to develop.

Eventually the leaf dies. Some leaves, such as those of alder, just turn brown, which is the color of dead cells. More colorful leaves may retain their color for a few days even after they fall to the ground.

That’s a brief account of some leaf physiology and the mechanisms of color change. In short, it begins to answer the question of how leaf colors develop—the physiological mechanism. But there is a broader question that begs recognition: Why do certain kinds of trees produce red colors at all? Is there any adaptive value to the tree? And here, there are many suggestions but many fewer answers.

Some researchers suggest that red pigments help protect an autumn leaf from too-intense sunlight. But the evidence is equivocal, and in any case it is not clear why a dying leaf of a deciduous tree would need such protection. This suggestion may well work, however, for species such as dwarf dogwood, whose persistent leaves turn crimson in the open but stay green in the forest. Other researchers think that red color may deter certain autumn-active herbivorous insects that might lay eggs on the tree—eggs that would hatch into leaf-munching caterpillars the next spring. Again, the evidence is equivocal. Another idea is that, for species that produce edible fruits, bright leaf colors might serve as ‘flags’ to attract fruit-eating vertebrates that would disperse the seeds. But this notion has, to date, no support. And perhaps there are no advantages to the trees at all; the lovely colors could be by-products of other features that do have adaptive value. It is likely that no single explanation fits all species. So, for now, the fundamental question of possible adaptive values of fall foliage colors remains unanswered.

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