For residents in the northern states and the high country of the West and Southwest, one of autumn's pleasant events is the annual color change of the leaves of various vegetation species. As the leaves of deciduous trees slowly change from their deep, summertime green to an array of red, orange, and yellow, the landscape often becomes filled with natural beauty.
In fact, some locales hold events in an effort to welcome tourists who come as "leaf peepers". The U.S. Department of Agriculture maintains a telephone hotline number and U.S. Forest Service Fall Color Hotline Web page containing fall foliage status updates for National Forests across the continental United States. Additional information is also available through the tourism bureaus of the various states.
During summer, leaves on most plants appear green because the dominant pigment in the leaves is chlorophyll, a substance that absorbs most colors of sunlight except for the green color, which is reflected back to our eyes. Green chlorophyll aids in the manufacture of food for the plants. In autumn, leaves on deciduous trees and shrubs (those that lose their leaves and are not "evergreens") turn various shades of yellow, red or orange when the green chlorophyll pigments become less active and decrease in amount, unmasking the reddish carotenoid or xanthophyll pigments that also reside in the leaf. The small amounts of carotenoid pigment present in the leaves are the same orange and yellow chemicals that provide the yellow-orange color to carrots. Differing amounts of red and yellow pigments create the array of colors that depend upon the type of tree:
| Aspen
Birch Hickory Oak |
Yellow
Red to Yellow Golden Brown Red to Brown |
Silver Maple
Sugar Maple Sumac Tamarack |
Red
Red to Yellow Red Yellow |
Shorter daylight hours, coupled with longer, cool nights (where temperatures fall below 45 degrees Fahrenheit), appear to be responsible for initiating the annual color change in healthy trees more than does the occurrence of the first frost. As a result, the food producing process terminates and the chlorophyll breaks down, exposing the underlying orange and yellow colors. Red pigments, such as anthocyanin, may also be formed as sugars are trapped in the leaves during cool nights. Additional factors, such as disease, soil conditions and water stress accumulated over the growing season may also contribute to the onset of early colors, especially in maples.
The intensity of sunlight also affects the brightness of the foliage. Bright sunlight typically accentuates the brilliant colors, while thick clouds or overcast sky conditions favors pastel shades.
Colors decline as leaves fall. Natural leaf fall in autumn is an adaptation that deciduous vegetation perfected to prevent excessive water loss through the broad leaves during winter. Leaf fall can be hastened by heavy rains, snow, strong winds or combination of these factors.
The timing of peak fall coloration depends upon various weather and climate factors. The monitoring of fall colors is an example of phenology (a contraction of the word phenomenology), a scientific treatment of the influence of climate on the periodic annual phenomena of plant and animal life. According to Hopkins Law (named for A. D. Hopkins, an entomologist with the U.S. Dept. of Agriculture, and no known relation to this author), most phenological events tend to progress northward and upward in altitude during spring, while southward and downward in fall. A delay is also noted in the eastward direction. If the peak of autumnal foliage coloration conformed to this empirical relationship, the southwestward progression would be at a rate of 4 days per 1 degree of latitude and downward at 1 day for 100 feet of altitude.
Normally, the peak period for fall colors moves south across the nation, starting in mid September along the Canadian border especially in the Northeast and at those high elevations in the Rocky Mountains where aspen flourish. The wave of color travels southward and downward in altitude, so by late October and early November, the peak has reached the southern Appalachians and the Ozarks. The duration of this peak period depends factors such as the amount of leaf moisture, wind speed, and amount of precipitation.
URL: datastreme/learn/w_sup.html
Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 2001, The American Meteorological Society.