Autumn Leaves Change Colors as Trees Prepare for Winter: The Foliage Forecast

As summer ends and autumn begins, residents of the northeast United States are looking forward to some spectacular changes in foliage. Each year, leaves change from green to vibrant hues of red, yellow and orange. But what causes these changes in color? And why does it occur at around the same time each year?

In order to understand why leaves change color, you need to learn a bit more about the leaves themselves.  Tree leaves possess: chlorophyll, which gives the leaves their green color; carotenoids, which produce yellow, orange and brown colors in things such as corn, carrots and daffodils; and anthocyanins, which give the color to things such as cranberries, red applies and blueberries.

During the growing season in spring and summer, chlorophyll is constantly being produced and broken down. This causes leaves to appear green as trees store up nutrients, and take advantage of this period of growth. But when autumn comes around, trees begin to shut down in preparation for the cold months ahead, and chlorophyll production slows down in response to the cooler temperatures. Eventually, chlorophyll production stops entirely and all chlorophyll in the leaf is destroyed.

Once the chlorophyll is no longer present in the leaf, only the carotenoids and anthocyanins are left in the sugars and wastes in the leaf. This means that leaves begin to show their "colors" depending on what carotenoids and anthocyanins are present.

While the lack of chlorophyll is why trees change color, the specificities of the coloration vary between tree species. For example, in some trees, like maples, glucose is trapped in leaves; this glucose, loaded with anthocyanins, changes to a red color during the cool nights of autumn. In oaks, though, the brown color of the leaves is made from the wastes left in the leaves.

It's not just the color change that takes place, though. At the point where the stem of the leaf attaches to the tree, a special layer of cells develops. These cells eventually sever the tissues that support the leaf. At the same time, the tree seals the cut, so when the leaf is finally blown off or falls from its own weight, it leaves behind a leaf scar.

You may have noticed that some years produce more spectacular colors than others in the autumn. This is because weather has a major influence on color intensity. For example, low temperatures above freezing will favor anthocyanin formation, producing the bright reds in maples. Early frost, though, will weaken the red color.

The best colors are usually seen during a succession of warm, sunny days and cool, but not freezing, nights. A few bouts of occasional rainfall, in particular, will help things along. During these warm, sunny days, a lot of sugars are produced in the leaf; however, the cool nights cause leaf veins to close and prevent the sugars from moving out. This "backload" of sugars in the leaf means that there are more spectacular colors to be seen.

When can you see these changes? You can start seeing leaves turn in late September in the northeast, though obviously weather patterns can cause these changes to vary widely.

This year has seen an abnormally warm season so far. If nights start cooling off, this could set the stage for a spectacular scene for fall foliage in the northeast. However, it will still be a couple more weeks before this prediction is certain.

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