Out In the Woods

Fallen Leaves Along a Sanford Footpath

Photo Credit: Kevin McKeon

Leaf Colors

By Kevin McKeon, Maine Master Naturalist

In Sanford, and many other places worldwide, the landscape’s leaf colors slowly change from summer’s green to the vibrant reds, oranges, yellows and purples of autumn. Even the brown leaves that linger add to this colorful mural. So, what causes nature to offer us this wonderful creation?

Leaves make food in a chemical process called photosynthesis. This involves using energy from the sun, absorbing carbon dioxide from the air and sucking up water from the ground to make sugar. All this happens within chloroplasts—tiny green structures that live within the leaves’ cells. The tree’s roots then add nutrients from the soil to this sugar to build its woody structure. The waste product of photosynthesis is oxygen. The chloroplasts appear green to us because the other colors get absorbed during the food-making process, so only the green gets reflected back to our eyes. Some leaves are red and thought to protect some sun-sensitive plants from excess sunlight. The green, food-making chloroplasts are there, but the plant has evolved to use other colors, so reflect back the red.

“Photoperiod” describes the length of time sunlight is available. As the days get shorter and the photoperiod decreases, the leaves receive reduced levels of light energy thus producing less food for the tree. The tree recognizes that its leaves are becoming less efficient, so it begins to release them from their duties. But there’s still some food stored in these leaves, so the tree moves this food—the sugars and starches—into the twigs. When the leaf is no longer needed, the tree closes the leaf’s veins, weakening the attachment point to the twig, called the abscission layer. (Apple trees do the same thing to the apples, causing the fruit to fall.) The leaf stem then splits and falls to the soil, providing homes and protection for the various little creatures that live under the leaves and food for the tiny soil microbes. Thus, the soil becomes nourished with nature’s fertilizers, ready to feed everything else.

As the green chlorophyl fades away, the previously hidden colors of the leaves’ pigments come into view. As the leaf dies, various chemical reactions add additional colors to the leaf. The tree then knows to stop wasting energy in an attempt to grow. It conserves food and energy until the days once again get longer, its leaves begin to grow again from new buds and the increasing sunshine allows for more efficient food-making.

Evergreens like pine, fir and spruce hold their leaves (needles) for 3 years or longer, able to protect themselves with a thick, waxy coating and small surface area, allowing both continued photosynthesis and conservation of water.

What about those brown leaves that persist throughout the winter into spring? This is called marcescence. Scientists currently offer various theories for this. Maybe the leaf remains to protect next year’s leaf bud or the bitter-tasting leaf discourages deer and rabbits from eating the bud. Or maybe, since beech and oak originated in the tropics, they simply haven’t yet learned to drop their leaves!