Have you ever wondered what makes the leaves change color in the fall? Or why some years are more vibrant than others? It is quite a fascinating phenomenon, and it all starts with the seasonal temperature and day light hour change. In the fall, the days become shorter and the evenings become cooler. This is what triggers deciduous trees (trees that lose their leaves in the winter) to begin their process of preparation for the winter months to come. This is also why the foliage changes color around the same time every year.

It starts with the expansion of the abscission layer between the stem and leaf, which slowly blocks the movement of water and sugar back and forth between the leaf and stem. This causes the leaf to lose the ability or resources to replenish chlorophyll, which gives leaves their green coloration. The chlorophyll rapidly breaks down and we are left with xanthophylls and carotenoids, which are responsible for the yellow and orange pigments you see on trees like aspen and birch. These pigments are usually present in the leaf throughout the growing season but are masked by the green pigment of the chlorophyll.  Anthocyanins are responsible for red and purple coloration and are created by the buildup of sugars trapped in the leaf. As fall progresses into winter all of the other pigments break down, as the chlorophyll did, and the only pigment that remains are the tannins, which are responsible for the brown color of the fallen leaves – though some trees retain their brown leaves throughout the winter, such as oak and beech.

Sassafras leaves changing color in Ganondagan State Historic Site. Xanthophylls and carotenoids are what make up the yellow and orange colors respectively and anthocyanins are responsible for the pink color seen in these leaves. Photo by Tom Hughes.

Oak leaves turning brown on the forest floor in Thacher State Park. Tannins are responsible for the brown color observed on the oak leaves under this Jefferson’s Salamander. Photo by Lilly Schelling.

Factors that affect the color and duration of fall foliage are temperature, sunlight and moisture. Ideal conditions for colorful fall foliage are a good growing season followed by dry, warm, sunny days and cool nights. If there was stress in the growing season, such as a drought or flood, the abscission layer may form early and the leaves will fall off before changing color. Additionally, too low of temperatures (freezing) in the beginning of fall will rapidly break down the products responsible for bright colors and the only pigment left will be brown. Other factors that can affect colorful fall foliage are heavy rain and windy storms, as these conditions will cause the leaves to fall.

When you are out admiring the fall colors this year, try to identify which pigment products are responsible for the colors you are seeing on the trees. A great way to view the fall foliage is from a canoe or kayak, but remember to wear your personal floatation device as the water will be chilly! The New York Fall Foliage Report is a great tool for tracking the color changes across the state: http://www.iloveny.com/seasons/fall/foliage-report/#.VgQGwstVhBd.

References:

The United States National Arboretum

Post by Lilly Schelling, OPRHP.