Autumn is a beautiful time of year, and one of the things that makes it so special is the changing of the leaves. As the days get shorter and the nights get cooler, trees begin to shed their leaves in preparation for winter. The colors of autumn leaves vary depending on the type of tree. Some trees, such as maples, turn bright reds and oranges, while others, such as oaks, turn yellows and browns. The colors are caused by a chemical reaction in the leaves as they lose their chlorophyll, the green pigment that helps them photosynthesize. An Autumn Leaves Sticker.