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Rocky Mountain Region |
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Every fall across the Northern Hemisphere, diminishing daylight
hours and falling temperatures induce trees to prepare for winter
and they shed billions of tons of leaves. In the Rocky Mountain
Region, a spectacular color show precedes the shedding of leaves.
Formerly green leaves turn to brilliant shades of yellow, orange,
and red as a result of transformations in leaf pigments. The green pigment in leaves is chlorophyll, which
absorbs red and blue light from sunlight.
Therefore, the light the leaves reflect is diminished in red
and blue and appears green. The molecules of chlorophyll are large
(C55H70MgN4O6). They
are not soluble in the aqueous solution that fills plant cells.
Instead, they are attached to the membranes of disc-like structures,
called chloroplasts, inside the cells. Chloroplasts are the site
of photosynthesis, the process in which light energy is converted
to chemical energy. In chloroplasts, the light absorbed by chlorophyll
supplies the energy used by plants to transform carbon dioxide and
water into oxygen and carbohydrates, which have a general formula
of
In this endothermic transformation, the energy of the light absorbed by chlorophyll is converted into chemical energy stored in carbohydrates (sugars and starches). This chemical energy drives the biochemical reactions that cause plants to grow, flower, and produce seed.
Chlorophyll is not a very stable compound; bright sunlight causes it to decompose. To maintain the amount of chlorophyll in their leaves, plants continuously synthesize it. The synthesis of chlorophyll in plants requires sunlight and warm temperatures. Therefore, during summer chlorophyll is continuously broken down and regenerated in the leaves of trees.
Another pigment found in the leaves of many plants is carotene. Carotene absorbs blue-green and blue light. The light reflected from carotene appears yellow. Carotene is also a large molecule (C40H36) contained in the chloroplasts of many plants. When carotene and chlorophyll occur in the same leaf, together they remove red, blue-green, and blue light from sunlight that falls on the leaf. The light reflected by the leaf appears green. Carotene functions as an accessory absorber. The energy of the light absorbed by carotene is transferred to chlorophyll, which uses the energy in photosynthesis. Carotene is a much more stable compound than chlorophyll. Carotene persists in leaves even when chlorophyll has disappeared. When chlorophyll disappears from a leaf, the remaining carotene causes the leaf to appear yellow.
The prime example is the early season color change showing the unique characteristic of aspen growth. Aspen propagate primarily by sprouting from an expanding root system, creating groups of trees, or clones, ranging in size from several trees to many acres. These clones are genetically identical. One factor in how and when a tree changes color is the balance of various chemicals in the plant. This balance is, to some degree, genetically determined and varies between clones. Because of these differences, it is possible to see hillsides with one small group of trees that have already changed to vibrant gold colors standing among otherwise green aspen. This color change allows viewers to readily see individual aspen clones.
During summer, the tree leaves are factories producing sugar from carbon dioxide and water using by the action of light on chlorophyll. Chlorophyll causes the leaves to appear green. (The leaves of some trees, such as birches and cottonwoods, also contain carotene; these leaves appear brighter green, because carotene absorbs blue-green light.) Water and nutrients flow from the roots, through the branches, and into the leaves. Photosynthesis produces sugars that flow from the leaves to other tree parts where some of the chemical energy is used for growth and some is stored. The shortening days and cool nights of fall trigger changes in the tree. One of these changes is the growth of a corky membrane between the branch and the leaf stem. This membrane interferes with the flow of nutrients into the leaf. Because the nutrient flow is interrupted, the chlorophyll production in the leaf declines, and the green leaf color fades. If the leaf contains carotene, as do the leaves of birch and hickory, it will change from green to bright yellow as the chlorophyll disappears. In some trees, as the sugar concentration in the leaf increases, the sugar reacts to form anthocyanins. These pigments cause the yellowing leaves to turn red. Red maples, red oaks, and sumac produce anthocyanins in abundance and display the brightest reds and purples in the fall landscape.
In recent years, fall colors have been attracting more travelers to prime color regions: New England, Michigan, Wisconsin, and Colorado. The right combination of tree species and likely weather conditions produce the most spectacular displays in these regions. States in these regions maintain a fall foliage "hotline," keeping color watchers apprised of the peak viewing locations and times. The Forest Service also operates a Fall Foliage Hotline at 800-354-4595. A detailed report can be found on the Web at www.fs.fed.us/news/fallcolors/. .
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U.S. Forest Service, Rocky Mountain Region |