When the nights start to get longer in autumn, the trees begin their preparation for the dormant winter period. These carotenoids are in the leaves year round, but in the spring and summer months the amount of chlorophyll in the leaves is so great that the green blocks out the orange and yellow pigments. Some leaves are a little more complex and get a red or purple color. Old stories say the reason for this is Jack Frost moving through the forests during the fall, breathing his icy breath on the leaves and pinching their stems with his icy fingers to make them the shades of red and purple. The scientists have a different explanation for the red and purple colors.
Senescence is the term used to describe the aging and death of a tree or a part of it.
As the leaf dies, the anthocyanin pigments are produced. Reduction of chlorophyll also plays a small part in the coloring of the red leaves, as does intensity of light, temperature and the water supply that the tree has. Because of these factors, trees or branches that are shaded might not get the bright colors that those in the bright sunlight will get. Because of genetic differences, not all trees change at the same time. Some trees start changing their colors in early August, while others will hold on until late October.