You already know about photosynthesis. That's when a plant takes sunlight, water, and carbon dioxide and turns it into the sugars, called glucose, which it needs for growth. Another beneficial byproduct happens to be oxygen. The chemical in plants that makes it all happen is chlorophyll. As you may know, chlorophyll is green. And that's why trees, and grass, and millions of other plants are green.
During the cold days of winter, there isn't enough sunlight and water for photosynthesis so plants take a rest from making their own food supply and shut down the whole process. After the autumnal equinox the days begin getting shorter and trees recognize the changing conditions. It's their clue to start getting ready for winter. The chlorophyll fades away. With the chlorophyll gone, leaves return to their natural color.
|Aspens this week in Colorado|
The technical names of the color chemicals are "carotenoids" for yellows and oranges and browns, and "anthocyacins" for the reds and purples. They're the same chemicals that make corn yellow, carrots orange, apples red, or grapes purple.
The trees continue their seasonal preparation by completely shutting down the veins to the leaves so the nutritional sucrose remains in the body of the tree and roots. In some trees some of the sugar is trapped during this process, hence the formation of red-producing anthocyacins. After new cells complete a barrier at the base of the leaf, it's ready to depart from the tree. A strong breeze, steady rain, or steady snow will cause the leaves to fall.
Decreasing sunlight is the primary factor in triggering the change in trees. Moisture levels in the soil and specific periods of warm days and cool nights are also big factors in how much color change you'll see; this can be quite variable. That's why some years produce longer and more brilliant displays of color while some years produce unexciting blandness.
Some parts of the United States make an industry of predicting when the colors will be at their peak and millions of people travel hundreds of miles to see the natural spectacle. Here in Colorado, prime leaf-peeping is happening now in the high country. Take advantage of this yearly event, travel to a color area near you, and when a child asks, "Why do the leaves change color?", you'll be able to give the correct answer.