The Leaves of Autumn

Weeks before the maples and oaks turn color, I have already been satiated by the brilliant hues of the understory trees, shrubs, and vines. From the time I spot the first scarlet and purple leaves on black gum trees in late August until the understory leaves fall in mid-October, I am surrounded by gold, purple, orange, pink, and every shade in between during my daily walks.

Scientists have known for years how plants turn color by flooding their leaves with an enzyme that breaks down the green of chlorophyll and carries certain chemicals, such as nitrogen and magnesium, out of the leaves before they drop. This process unveils colorful pigments that have been in the leaves throughout the growing season. Carotene pigments make leaves orange and yellow while the pigment anthocyanin is responsible for red and purple leaves. Whether leaves turn purple or red often depends on the acidity or alkalinity of the tree. For instance, the more acidic red maples turn red and the more alkaline ash trees turn purple.

ALthough the signal to turn color is related to longer nights and cooler weather which eventually stop the trees’ growth system, each species has a different response time. Here on our mountain, black gum, striped maples, witch hazel, and black birch turn first and all the oak species and quaking aspen last. In between is the grand pageantry of red and sugar maples.

For a long time, scientists hadn’t a clue about why leaves turn color. One researcher, James Poling, back in 1977, wrote in LEAVES: THEIR AMAZING LIVES AND STRANGE BEHAVIOR that …”as far as botanists can determine, the chemical energy that goes into the painting of a leaf is of no benefit at all to the plant…”

Since then, several scientists have been looking at the question and coming up with a variety of answers. Botanist Edmund Stiles has been particularly interested in many of the trees, shrubs, and vines that turn color early such as black gum, flowering dogwood, Virginia creeper, spicebush, poison ivy, sumacs, wild grape, and sassafras. He believes they would have to have a good reason to give up a month of photosynthesis and thinks that their leaves, which he calls “foliar fruit flags,” are signaling to migrating fruit-eating birds that their fruits are ripe.

Many of these plants, such as spicebush, dogwood, black gum, and sassafras, have fruits high in fats that rot quickly if they are not eaten. And since birds are the dispersers of the seeds of these plants, it is important to them that the birds eat their fruits and defecate their seeds while they are still viable.

Furthermore, migrant birds would not know, as resident birds would, where the fruits are, so the bright leaves, seen against a mostly green landscape, would serve as signals to migrating fruit-eating birds including robins, cedar waxwings, eastern bluebirds, veeries, brown thrashers, gray catbirds, and hermit, Swainson’s, gray-cheeked, and wood thrushes.

Most of Stiles’s “foliar fruit flag” species also have inconspicuous fruits hidden by leaves. Without the colored leaves, the birds might not look for the fruit. The green vines are particularly difficult to spot as they twine along the ground or up tree trunks, but when they turn color, they are strikingly beautiful and easy to see. Poison ivy has clusters of gray or whitish fruits and compound red leaves. Virginia creeper hides its flat-topped clusters of bluish-black fruits with scarlet leaves while wild grapevines produce compact clumps of purple grapes beneath golden leaves. Spicebush too has gold leaves and contrasting red clusters of fruit that, when crushed, smell like allspice.

Black gum trees sometimes produce small clusters of half-inch long, bluish black fruits beneath their flamboyantly colored leaves. The fruits of sassafras are dark blue and shiny under a veil of yellow and red leaves while the purplish red leaves of flowering dogwood hide showy clusters of red fruit. Staghorn umacs leaves also turn red and have showy, upright clusters of fuzzy, red fruit.

Stiles studied staghorn sumac in detail to prove his hypothesis because it is a dioecious species (one that has male and female flowers on separate plants) which forms clones of plants. He hypothesized that nonfruiting male clones would not have many foliar fruit flags because they have no fruit.

On October 3, 1981 he drove three transects–from Stroudsburg, Pennsylvania to Palmerton along route 209, Palmerton to Easton on routes 248, 512, and U.S. route 22, and from Easton to Oldbridge, New Jersey along route 178, noting both the fruiting and nonfruiting clones of staghorn sumac. Along all three transects the fruiting female clones had many more colored leaves than the male clones, strengthening his theory.

Stiles offered his explanation for early leaf color in some plants in THE AMERICAN NATURALIST in 1982. More recently other scientists have been tackling why leaves color in general. Two British scientists think that bright colors signal sap-eating aphids to let the trees alone because they will get a mouthful of thickening, unpalatable leaves to chew on and encounter heightened chemical defenses. The leaves are producing “‘pick on someone else’ signals to specialist autumn-flying insects,” researcher Sam Brown claims.

Scientists at the University of Wisconsin have a another theory about why leaves turn scarlet and, in addition, why some colors are more vibrant one year than another. They believe that the red pigments (anthocyanins) act as sunscreens by shading sensitive photosynthesis tissue in the autumn while trees reabsorb nutrients from their leaves.

In an anonymously-authored paper entitled “Fall Color Acts as Sunscreen,” which they published in December 2001, they wrote that “the pigments protect leaves dwindling ability to generate energy. Besides high light levels, plant stressors such as near-freezing temperatures, drought, and low nutrient levels trigger increased pigment levels.”

This theory is plausible because autumn color is brightest when the weather is dry and sunny and the nights cold but not freezing. Also, the scientists pointed out that “the outer leaves of trees such as maples are more colorful than leaves shaded inside the canopy or those with a northern exposure,” which further strengthens their hypothesis.

Clearly scientists are still working on this theory and others to understand why leaves turn color. To those of us afield, what really matters is that the leaves do turn color and we have a front seat for the almost two-month-long, light and color extravaganza. The leaves brighten the forest even when it is raining, and after they fall we have color beneath our feet as well for a few days. In early October, I feel as if I am trodding on fields of gold created by black birch, witch hazel, and the other yellow leaves that fall then. The red and gold maples leaves form a similar carpet in mid-October. At the end of the month, the wine-red, burnt orange, brown, and beige leaves of the oaks and bright gold leaves of the aspens are finally blown to the ground.

The color quickly fades, though, because even before leaves fall, they have been colonized by saprophytic fungi. These fungi can start the leaves’ rotting by breaking down their cellulose and lignin, creating a cloying, sour smell that I detect after the first late autumn rains. Then their decomposition is taken over by litter fungi primarily of the Basidiomycetes class of most gilled mushrooms.

“Mushrooms are a symbol of renewal and a symbol of the season. THEY are the shining fruits of the great autumn leaf harvest,” writes Peter Marchand in his excellent book AUTUMN: A SEASON OF CHANGE. Perhaps not so coincidentally, the variety and colors of fall mushrooms are almost as amazing as those of autumn leaves.

The fungi are joined by earthworms, snails, small arthropods, nematodes, slime molds, and other decomposer organisms which can amount to five tons an acre in productive forest soil. For instance, earthworms in rich leaf compost can weigh 900 pounds an acre and slugs and snails 400 pounds. In most northern, mixed deciduous forests, it takes approximately ten years for the total decomposition of leaves to occur, but every year the leaves that fall return a little more than 100 pounds per acre of nitrogen, phosphorus, potassium, calcium, and magnesium to the soil, supplying from 75 to 85% of the trees’ needs for these elements every year.

No matter what scientists may eventually discover about why leaves turn color, and I suspect there will be many reasons, once leaves fall they are immensely important to the continuing life of the forest. They are also important to streams and rivers, feeding aquatic insects and crustaceans called “shredders” which, in turn, feed fish.

“Consider,” Henry David Thoreau wrote in his journal in 1853, “what a vast crop is thus annually shed upon the earth. This, more than any mere grain or seed, is the great harvest of the year.” A great harvest that not only brightens our October days but reminds us that from death comes a renewal of life.