Early November color at Fort Mountain State Park’s lake in Murray County. Why some leaves turn red and others turn yellow or orange in fall depends on chemicals and pigments in the leaves. CHARLES SEABROOK

Red, orange or yellow — how autumn leaves get their color

Over the next two or three weeks, fall leaf color should be at its prime in Georgia’s mountains. Leaf watchers by the thousands will be hitting mountain highways, trails and overlooks for a peep at the dazzling display.

While you’re admiring the colorful foliage, you might ponder this: Why do some trees like maples have fiery red leaves while others like tulip poplars have mostly yellow or orange leaves?

The answer has to do with a few key chemicals and pigments in leaves. First and foremost is chlorophyll, which makes leaves appear green. During summer, chlorophyll is the dominant leaf chemical, converting sunlight into food for the tree through photosynthesis.

So dominant is chlorophyll that it hides any other colors present in a leaf. The vivid yellow and orange pigments — carotenoids — are in a leaf all summer, too, but hidden.

Come fall, cooler temperatures, shorter days and other factors signal that chlorophyll’s job is over for the year and that the tree is preparing to shed its leaves and go into winter dormancy. The chlorophyll breaks up. Now, no longer hidden, the yellow and orange carotenoids can show off their colors for a short time before the leaves drop off.

Red, however, is another matter. It’s made on purpose. As chlorophyll diminishes in fall, a red pigment, anthocyanin, which was not previously present, is produced in the leaves of certain tree species, such as maples, oaks and dogwoods. The anthocyanin makes their fall leaves predominantly red.

Scientists don’t fully understand why trees produce anthocyanin. One hypothesis is that the red leaves play a protective role as the tree prepares for winter senescence.

Regardless of color, most autumn leaves will have fallen to the ground, their nutrients recycled, by late November.

IN THE SKY: From David Dundee, Tellus Science Museum astronomer: The moon will be new on Sunday. Both Mercury and Venus are low in the west just after dark. Venus sets shortly thereafter. The two planets will appear near the crescent moon on Monday evening. Jupiter is low in the southwest around dusk and sets a few hours later. It will appear near the moon on Thursday night. Saturn is low in the south just after dark and sets in the west around midnight.

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