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Red leaves are a bit more fickle than their yellow and orange counterparts. See more tree pictures.

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Why do leaves change color and turn red?

­Enthusiastic snowball fights in winter or lazy summer days at the beach notwithstanding, autumn might be the most beloved season of them all. Every year, people watch in awe as the trees begin their magical transformation from green powerhouses to veritable smorgasbords of color. In the northeastern United States, millions of "leaf peepers" come from all over the world to observe nature's fine art gallery [source: Haberman].

New England doesn't have a monopoly on the color-changing business, and those leaf peepers are far from alone in their wonderment. From Europe to Alaska, people revel in nature's annual opportunity to mix up the palette. Pity those who live in areas that get cheated out of these glorious displays of color; the lucky ones are left debating which hue they most favor -- brilliant yellows, fiery oranges or vibrant reds?

If you're partial to crimson, you might wonder why nature seems to go overboard with the red some years and forget it entirely during others. Native American lore explained the appearance­ of the magnificent colors as coming from above: After hunters killed the Great Bear in the sky, the story goes, its blood splashed down and turned the leaves red. And the yellow leaves? They get their tint from the bear's fat splashing out of the pot that it was being cooked in.

If this explanation doesn't quite satisfy your puzzlement of why leaves change color, scientists have one of their own. Instead of hunters, bears and lard, theirs involves photosynthesis, pigments and sunlight. To fully understand the chemistry behind the color show, we'll need to revisit first-grade science class.

As the green chlorophyll recedes, yellows, oranges and reds get a chance to shine through.

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Anthocyanins: Painting the Town Red

For a large part of the year, leaves are a tree's workhorses, constantly converting carbon dioxide, water and sunlight into energy in a process called photosynthesis. The special ingredient for this process, the pigment chlorophyll, is w­hat gives leaves their bright, green color for much of the year. But while chlorophyll is the star of the show, it has some help in the form of the pigments carotene and xanthophyll. Xantho is Greek for "yellow," and carotene is what gives items like carrots and egg yolks their orangish color. These two pigments are always present in leaves and help absorb sunlight, which they transfer to chlorophyll for photosynthesis.

As summer nears its end and days get shorter, the increased amount of darkness incites trees to prepare for a sort of hibernation. Leaves won't be able to continue photosynthesizing during winter due to the dry air and lack of sunlight, so the tree does two things. First, it forms a separation layer made of corklike cells at the base of each leaf to seal it off from the tree. Second, it stops producing chlorophyll since it won't need this pigment until the days start to lengthen once again in the spring. With chlorophyll out of the picture, the yellow and orange pigments get a chance to shine.

The red hues, which come from pigments called anthocyanins, are slightly more complicated. Whereas all trees contain chlorophyll, carotene and xanthophyll, not all of them produce anthocyanins. Even the ones that do have anthocyanins only produce it under certain circumstances.

Remember that layer of cells at the base of the leaf? Its purpose is to protect the tree during the colder winter and prevent it from drying out. When the separation layer is complete, the leaves fall off in the tree's attempt to conserve energy. But before the leaves fall off and the tree closes up shop, it wants to pull in as much sugar and nutrients as possible from its leaves, which is where the anthocyanin comes in.

Although scientists offer several different reasons for why some trees produce anthocyanins and autumn leaves change color, the prevailing theory is that anthocyanins protect the leaves from excess sunlight and enable the trees to recover any last remaining nutrients. The reason you'll see more vibrant reds during some years is that lots of sunlight and dry weather increase the sugar concentration in tree sap, triggering the tree to release more anthocyanins in a last-ditch effort to gather up energy to get through the winter. In addition, near-freezing weather, low nutrient levels and other plant stressors seem to trigger increased levels of anthocyanins.

If it's been especially rainy and overcast, you won't see much red foliage. Without bright sunlight, the trees don't need the added protection that the red pigments provide, so they don't bother producing them.

­So if autumn just isn't the same for you without the occasional splash of red, hopefully the weather will cooperate. If not, you'll just have to make due with the more reliable yellows and oranges. Or you could always join the leaf-peeping caravan in New England on its endless search for the ultimate display of color.

Lots More Information

Sources

  • "Autumn Color is Nature's Sunscreen, Say Researchers." University of Wis­consin-Madison. ScienceDaily. Oct. 1, 2001. (Sept. 8, 2008)http://www.sciencedaily.com/releases/2001/10/011001072513.htm
  • Haberman, Shir. "Leaf peepers storm N.H., Maine." Seacoastonline.com. Oct. 9, 2007. (Sept. 12, 2008)http://www.seacoastonline.com/apps/pbcs.dll/article?AID=/20071009/NEWS/710 090335
  • "New England's Fall Foliage." Discover New England. (Sept. 9, 2008)http://www.discovernewengland.org/foliage/index.shtml
  • Nye, Bill. "Ask Bill Nye." MSN Encarta. 2008. (Sept. 8, 2008)http://encarta.msn.com/encnet/features/columns/?article=bnfallingleaves
  • Shakhashiri, Bassam Z. "The Chemistry of Autumn Colors." University of Wisconsin-Madison. (Sept. 8, 2008)http://scifun.chem.wisc.edu/CHEMWEEK/fallcolr/fallcolr.html
  • "Why Do Autumn Leaves Turn Red? Soil May Dictate Fall Colors." Science Daily. Oct. 29, 2007. (Sept. 8, 2008)http://www.sciencedaily.com/releases/2007/10/071025112042.htm
  • "Why do leaves change color?" Environmental Education for Kids. Wisconsin Department of Natural Resources. September 2008. (Sept. 8, 2008)http://www.dnr.state.wi.us/org/caer/ce/eek/veg/trees/treestruecolor.htm

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