During the growing months, leaves produce chlorophyll, a green-colored pigment. Through chlorophyll the tree uses the sunlight’s energy to combine carbon dioxide and water to make glucose, which serves as food for the tree. This process is called photosynthesis.
As summer ends, shorter days and colder air alerts the tree that winter is coming. The tree severs the leaves’ connection to water and minerals from the roots. The leaves begin to die and chlorophyll production ends. The sunlight breaks down chlorophyll which reveals carotenoid pigments previously masked by the deep green. The remaining chlorophyll produces lots of sugar, which is not used up by the tree. Red pigments called anthocyanins are created through excess sugar and more sunlight. Sunny days and cool nights provide the best weather for good anthocyanin production. You may notice that the leaves may be more red where the tree receives direct sunlight. Shaded leaves don’t produce as many anthocyanins and appear less brilliant.