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Fall Colors - What Causes Them, Where to Find Them

This article was initially posted in September 2008, but we thought it was well worth a revisit. 

Last autumn we found this informative article about why leaves change colors pasted into the Recreation Report  published by the Wenatchee National Forest. We got permission to use that article and it is pasted in below. We also discovered the Forest Service had a very nice gallery of autumn pictures taken in 2007 around Central Washington. These pictures are well worth viewing and will give you ideas on where to go to looking for fall colors this year.

Why Leaves Change Color...

...visitors to the Wenatchee National Forest will see vibrant fall colors in most areas of the forest. Leaves have already reached their peak colors in the higher elevation areas of the forest but are still incredibly beautiful in the lower elevations. Every year at this time visitors revel in the beauty of the trees, knowing well that it is only a fleeting pleasure. Before long though, the leaves will flutter away and become a part of the rich carpet that covers the forest floor.

Recent Photos of Fall Colors on the Forest

The timing of color change and leaf fall are primarily regulated by the increasing length of night. None of the other environmental influences -- temperature, rainfall, food supply, and so on--are as unvarying as the steadily increasing length of night during autumn. As days grow shorter, and nights grow longer and cooler, biochemical processes in the leaf begin to paint the landscape with Nature's autumn palette.

A color palette needs pigments, and there are three types that are involved in autumn color.

  • Chlorophyll, which gives leaves their basic green color. It is necessary for photosynthesis, the chemical reaction that enables plants to use sunlight to manufacture sugars for their food. Trees in the temperate zones store these sugars for their winter dormant period.
  • Carotenoids, which produce yellow, orange, and brown colors in such things as corn, carrots, and daffodils, as well as rutabagas, buttercups, and bananas.
  • Anthocyanins, which give color to such familiar things as cranberries, red apples, concord grapes, blueberries, cherries, strawberries, and plums. They are water soluble and appear in the watery liquid of leaf cells. Most anthocyanins are produced in the autumn, in response to bright light and excess plant sugars within leaf cells.

During the growing season, chlorophyll is continually being produced and broken down and leaves appear green. As night length increases in the autumn, chlorophyll production slows down and then stops and eventually all the chlorophyll is destroyed. The carotenoids and anthocyanins that are present in the leaf are then unmasked and show their colors.

The amount and brilliance of the colors that develop in any particular autumn season are related to weather conditions that occur before and during the time the chlorophyll in the leaves is dwindling. Temperature and moisture are the main influences.

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. 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.

In early autumn, in response to the shortening days and declining intensity of sunlight, leaves begin the processes leading up to their fall. The veins that carry fluids into and out of the leaf gradually close off as a layer of cells forms at the base of each leaf. These clogged veins trap sugars in the leaf and promote production of anthocyanins. Once this separation layer is complete and the connecting tissues are sealed off, the leaf is ready to fall.

The evergreens-pines, spruces, cedars, firs, and so on are able to survive winter because their needle-like or scale-like foliage is covered with a heavy wax coating and the fluid inside their cells contains substances that resist freezing. Thus the foliage of evergreens can safely withstand all but the severest winter conditions, such as those in the Arctic. Evergreen needles survive for some years but eventually fall because of old age.

Needles and leaves that fall are not wasted. They decompose and restock the soil with nutrients and make up part of the spongy humus layer of the forest floor that absorbs and holds rainfall. Fallen leaves also become food for numerous soil organisms vital to the forest ecosystem.

Although it has been explained above why leaves change color, it is just nice to go out and enjoy the spectacle and beauty of all that color that is so fleeting. Take the time to bask in the fall colors before they are all gone...

 (This information was provided by the US Forest Service).