Why do leaves change colors, fall? Editor explains

Every fall leaves change colors, people travel far and wide to see the bright and interesting colors, then the leaves fall off and become yardwork for homeowners. But why do leaves change colors? And why are many of the colors so bright? George Stanley, editor of the Milwaukee Journal Sentinel, takes a closer look at how leaves become colorful and transform into one of rural America's most beautiful natural delights. (Sentinel photo: Holy Hill National Shrine of Mary in Hubertus, Wis.)

"Some poor souls believe money makes the world go round; actually, it is the color green," Stanley writes. "Tiny pools of chlorophyll float in leaf cells moistened by an intricate network of veins. Columns of water molecules, bonded by surface tension, rise through those veins from the tree's roots. As water molecules evaporate from leaf surface into the air, the entire column below is pulled upward, drawing a continuous stream of moisture through the plant."

"Each morning, as light reaches out to touch the leaves, their pores open to admit carbon dioxide from the air," he writes. "Meanwhile, the pools of chlorophyll absorb the light's energy, which splits H2 from O in water molecules. Liberated hydrogen molecules then hook up with carbon dioxide to make sugar. Sugar is whisked through the veins from leaves to other cell factories in the plant. Some of the tree cells fuel growth by burning sugar. Others store energy by converting the sugar into fats and starches. Still others combine sugar with minerals that have been absorbed along with water through the roots, to manufacture proteins, vitamins and other nutrients. Unable to generate energy out of sunlight, animals take what they need from plants—or by eating other animals that have tapped into the greenery."

"Green is not the only pigment present in the leaves," he writes. "Pools of yellow and orange act as a sun screen of sorts for the chlorophyll, absorbing overflows of energized oxygen—a byproduct of photosynthesis that would otherwise pollute cell chemistry. Our eyes can't see the yellows and oranges through the overwhelming greens of the growing season. The background pigments finally get their time in the sun when day lengths shorten, letting the tree know its leaves have finished their work."

"Broad leaves, in winter, would do more harm than good to a tree," he writes. "Snow caught by the leaves would be a heavy burden for branches to bear. Even worse, cold weather shuts down sap flow, making it impossible to move water to the leaves, or sugar from them. Water evaporating from winter leaf surfaces could not be replenished through roots locked in frozen soil. Leaves continue absorbing sunlight and producing sugar on sunny days well into September. But cool nights will freeze up sap lines, shutting down the sugar flow out of the leaves. In maple leaves, among others, this oversupply of sugar can yield pinks, reds and purples, the colors affected by the pH of the sap."

"At the same time, decreasing sunlight triggers a ring of corky cells to develop at the base of each leaf, shutting off its water supply," he writes. "Chlorophyll quickly breaks down and the green fades, while yellow, orange, red and purple remain. The result is an impressionist artist's dream, as tiny dots of pigment paint the landscape with brilliant, scattered light." (Read more)