Tree leaves change color as autumn approaches because the days are shorter and the temperatures are cooler. As the length of the days shortens, the leaves stop their production of chlorophyll, a pigment that provides the leaves’ green color. Other pigments in the leaves, mostly yellow, are then able to show through.
The yellow color is mostly seen in aspen, birch, hickory, willow, and yellow poplar trees. Sugars that are trapped in the leaves as the trees prepare for winter form red pigments, also called anthocyanins. Some trees with red leaves are the dogwood, red and silver maple, oak, sumac, and sassafras.