As fall season is upon us, and the trees and shrubs begin to change to their fall foliage, many people may ask; “What causes leaf color to change from green to yellow or red in the fall?”
Many suppose that Jack Frost causes this color change, but he doesn’t. Some of the leaves begin to turn before we have any frosts. Native Americans from the Midwest, thought it was because the celestial hunters had slain the Great Bear – his blood dripping on the forests changing the leaves of many trees to red. Other trees were turned yellow by the fat that splattered out of the kettle as the hunters cooked meat for supper.
The real explanation is a combination of light, temperature and available water. With less light on short days, trees and shrubs return to an energy-saving mode to survive the winter. Short days and falling temperatures signal trees and shrubs to quit producing chlorophyll, which converts sunlight into sugars through a process called photosynthesis.
As chlorophyll breaks down, leaves reflect the remaining leaf pigments that were there all along. Yellow is produced by carotenoids, the same pigment that gives carrots their color. The red color is produced by anthrocyanins.
These pigments, normally masked by chlorophyll, help protect leaves from excessive sunlight. Anthocyanins are actually toxic, and contribute to a plant’s natural defense against insects.
Many of our native trees, and trees introduced to Nevada, turn yellow in the fall. Aspen in the high country can turn entire mountainsides golden-yellow. Huge areas are actually clones of the same tree that have spread via underground root suckers.
Moisture during the fall keeps leaves bright and colorful for a longer period of time. When conditions are dry, leaf color fades to brown, and leaves drop quickly. Cooler temperatures also support a longer period of color, which explains why fall color lasts longer in the mountains than along the Front Range.
The normal season for autumn color is September and October. On average, the best color occurs in late September and early October. Of course, with Nevada’s temperamental weather, fall color can occur earlier or later. So get out and enjoy the fall season and its many splendid colors.
