It's one of those questions -- like why is the sky blue? -- that can stump a parent on an otherwise enjoyable fall walk. So let's answer the question.
Most simply, to survive the winter, deciduous trees need to store nutrients in their roots, which means they must absorb the nutrients in their leaves. Changes in color are triggered as the trees absorb essential nutrients.
Throughout the warm sunny months, trees are lush and green because they're working hard. Tree leaves are green because of the abundance of the pigment chlorophyll, which is essential to converting sunlight, water, and carbon dioxide into energy-rich sugars.
Yellow colors that were always present in leaves become unmasked as the chlorophyll is broken down and absorbed. Called carotenoids, these are the yellow pigments that give trees like birch, beech, and tulip their bright fall colors.
Red and orange colors, like those that characterize the famous red maples of New England, are made by different pigments, called anthocyanins. Unlike the ever-present yellows that simply become unmasked when chlorophyll recedes, red pigments are actually created as a tree is going dormant to protect leaves from the sun and give leaves extra time to unload nutrients, acccording to Bill Hoch, an assistant professor at Montana University's College of Agriculture.
As summer wanes, changes in tree leaves are triggered by the cooler temperatures, changes in rainfall and weather, and most of all, the shortening of daylight hours.
While leaves will always change color as the amount of sunlight wanes, several weather conditions can affect how brilliant they become.
According to the U.S. Forest Service: A succession of warm, sunny days and cool, crisp but not freezing nights seems to bring about the most spectacular color displays. The amount of moisture in the soil also affects autumn colors. Like the weather, soil moisture varies greatly from year to year. The countless combinations of these two highly variable factors assure that no two autumns can be exactly alike.