Why do leaves change color?
It's one of those questions like, why is the sky blue? that can stump a parent on an otherwise enjoyable fall walk. So let's answer the question.
Most simply, to survive the winter, deciduous trees need to store nutrients in their roots, which means they must absorb the nutrients in their leaves. Changes in color are triggered as the trees absorb essential nutrients. Here's how it works:
Throughout the warm sunny months, trees are lush and green because they're working hard. Tree leaves are green because the abundance of the pigment chlorophyll, which is essential to converting sunlight, water and carbon dioxide into energy-rich sugars. If plants hadn't figured out the trick of photosynthesis, we'd all be out of luck, since the energy humans need to live comes from plants, or the animals that eat plants. Tree leaves are also busy using other essential nutrients, like nitrogen and phosphorus (the same main ingredients in most store-bought fertilizers, or in compost), so these nutrients are abundant in summer foliage.
As summer wanes, changes in tree leaves are triggered by the cooler temperatures, changes in rainfall and weather, and most of all, the shortening of daylight hours. Much of the change happens without our knowing it, as trees begin to absorb essential nutrients and store them in their roots so they are available for the following spring. As the trees absorb the last of the chlorophyll, however, the brilliant colors we associate with autumn begin to appear. (To find out where to find peak foliage in the U.S., and other tourism resources, visit the U.S. Forest Service's Fall Colors website fs.fed.us/fallcolors, or call 1-800-354-4595.)
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Or they should, anyway. While leaves will always change color as the amount of sunlight wanes, several weather conditions can affect how brilliant they become. High heat or drought can rob leaves of their brilliance, or make leaves drop early. Insufficient rain in September can also hurt peak color.
But why do leaves change from green to yellow, orange and red under the best conditions?
Yellow colors that were always present in leaves become unmasked as the chlorophyll is broken down and absorbed. Called carotenoids, these are the yellow pigments that give trees like birch, beech and tulip their bright fall colors. Why are these yellow pigments there in the first place? To protect leaves from the byproducts of photosynthesis, which would otherwise cause damage. (Carotenoids play a similar role in the human body, as it turns out. Beta carotene and lycopene are among the best-known carotenoids, because they are healthy antioxidants in our food.)
Photo: Carl Derrick
Red and orange colors, like those that characterize the famous red maples of New England, are made by different pigments, called anthocyanins. For years, scientists were stumped by them: Unlike the ever-present yellows that simply become unmasked when chlorophyll recedes, red pigments are actually created as a tree is going dormant. But why would a tree expend energy to produce a new pigment just as it's hunkering down for the winter? And why do some trees make red pigments, when others don't? Further, the reds of New England are so famous in part because they are unique to the new world. Why are European autumns so predominantly yellow?
In 2001, Bill Hoch, then a grad student at the University of Wisconsin-Madison, found an explanation that has stuck. (He followed up his research with a supporting study in 2003, and he is now an assistant professor at Montana State University's College of Agriculture.)
The new red pigments protect leaves from the sun, giving some species extra time to absorb those essential leaf nutrients, according to Hoch. If chlorophyll is the beach umbrella, anthocyanins are the sunscreen. As chlorophyll exits leaves, anthocyanins are created to buy leaves time to unload nutrients. If you've ever noticed maples turn a deep burgundy before they achieve that crimson red, it's because the burgundy is a mix of outgoing green chlorophyll and incoming red anthocyanins, and the crimson is pure anthocyanin.
"When the leaves are senescing (transferring nutrients from leaves to roots), they're more susceptible to light damage because they're taking themselves apart," Hoch said. "It's like taking parts off your car while you're still trying to drive it."
Anthocyanins, incidentally, are found in many other plants, including foods like eggplant, red cabbage, oranges and many berries. There's some research suggesting that they, too, are important for human health.
But why don't all trees' leaves turn red? It turns out, Hoch said, that yellow trees are those that colonize open land first so-called pioneer species that are tolerant of direct sunlight. Those that turn red are species that follow in the succession of species that come to dominate a landscape, and they tend to benefit from more protection from the sun. It's not that the red leaves lack the yellow pigment; the red pigment is an addition, and in fall it is so intense that it masks the yellow, just as green does in summer.
"Those pioneer species are less susceptible to light damage," Hoch suggests.
But why are European trees more yellow? It could be that falls there tend to be warmer and cloudier, so there was never any selective advantage for trees to evolve red pigments that would be protective of the sun.
Photo: Sherry Ziolkowski
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Because red pigments are created in fall, specific weather conditions can have a big effect on their brilliance. According to the U.S. Forest Service:
A succession of warm, sunny days and cool, crisp but not freezing nights seems to bring about the most spectacular color displays. During these days, lots of sugars are produced in the leaf but the cool nights and the gradual closing of veins going into the leaf prevent these sugars from moving out. These conditions-lots of sugar and lots of light-spur production of the brilliant anthocyanin pigments, which tint reds, purples, and crimson. Because carotenoids are always present in leaves, the yellow and gold colors remain fairly constant from year to year.
The amount of moisture in the soil also affects autumn colors. Like the weather, soil moisture varies greatly from year to year. The countless combinations of these two highly variable factors assure that no two autumns can be exactly alike. A late spring, or a severe summer drought, can delay the onset of fall color by a few weeks. A warm period during fall will also lower the intensity of autumn colors. A warm wet spring, favorable summer weather, and warm sunny fall days with cool nights should produce the most brilliant autumn colors.
Climate change, then, could have a lasting effect on fall foliage, as the cycle of drought and deluge changes, and temperatures warm overall. A longer growing season can keep trees more green later into the fall, when a shock of frost can make them drop leaves before they complete the color-change process. Data from the Mohonk Preserve in New York's Shawangunk Mountains, where naturalists have tracked the weather and the foliage for more than a century, suggest that the peak of fall color is indeed coming later days or even weeks later, on average.
It's only one consequence of global warming for northern forests. Maples, which fill the forests with crimson, prefer colder climes, and require cold winters to produce adequate sap for syrup. Over time, those trees may grow only farther north, where the climate is more suitable. Insect pests, too, are spreading north as winter freezes grow more mild, and those pests might affect not only fall color, but the value of timber harvests and the health of habitat for wildlife.
Photo: Kimber Stannard
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