Why Leaves Turn Colors in Fall

by Josh Pulito

Most of the bright colors that see we see in the fall are present in the leaves all year long. We just can’t see them because chlorophyll (the green color) is so strong and dominant that it covers up the xanthophyll (yellow) and carotene (orange) pigments which are always present in the leaves. As the darkest color, green is the most visible. Once fall sets in with its shorter days and colder nights the chlorophyll no longer has the conditions it needs to produce sugar (energy) for the tree. The chlorophyll then begins to break down and reveals the yellow and orange pigments beneath. The colors were there the whole time but we just couldn’t see them! The red color is a little more complicated. The red that we see in leaves is caused by anthocyanin which is not always present. Anthocyanin is made by the tree in response to shorter days and the cold. Anthocyanin protects the leaf from sunlight allowing the tree to pull final energy (sugar) from the leaf to store through the winter before it falls to the ground. Since anthocyanin needs sugar and cold to be produced expect the best red and purple colors from maples, oaks and sumacs on dry years with lots of sunlight and cold nights. These are the conditions that are needed to see the best fall foliage colors whether it be red, orange or yellow. Fall foliage really stands out against the blue sky at this time of year. It is not just to the bright leaves that we owe this striking contrast. The sky is especially blue in the fall because the sun is lower in the sky and there are less clouds and humidity making it clearer. I wish you a sunny, clear, cool and dry fall with lots of great colors to appreciate!