The Science Behind Autumn Leaf Color Changes

As we enter the fall season with great anticipation and expectations of a brilliant display of fall colors, it is helpful to understand this phenomenon. The living process within a tree which produces the fall color display is called leaf senescence. Senescence is an ordered shutting-down of the summer growth process and the conservation of valuable resources needed by the tree for next springs renewed growth.

In August, the process starts as the days get shorter and the temperatures cool. A pigment called phytochrome is a dormancy timer that reads the seasons of the year and tells the tree to begin shutting down for the winter.

This change slows the production of chlorophyll, the green pigment which gives leaves their green color. As remaining chlorophyll in the leaf deteriorates and is no longer being replaced, other color pigments become visible. Carotenoids and xanthophylls, which are always present in the leaf, produce the yellow and orange colors which we first notice as the green disappears. Two of my favorites are the Sugar maple with the combination of yellows and oranges and the Ginkgo with its bright yellow.

Many of the materials a tree manufactured during the growing season are withdrawn from the soon-to-be dead leaves. The last bit of tree food is stored away in the living cells of the outer growth rings, twigs, branches, and roots, while waste materials are left behind. At about the same time leaf abscission begins. This allows the leaf to slowly detach from the tree. As this corky abscission layer forms, some of the sugars produced during photosynthesis become trapped in the leaf.

Sugars trapped in the leaves of tree species producing pigments called anthocyanins, combine to produce the brilliant reds and purples. The more acidic the cell sap is, the deeper the reds become. The more alkaline the cell sap, the more purple will predominate. True Red maples and White ash come to mind. Some species produce tannins which give their leaves a brown to bronze appearance like the Bur oak and Sycamore.

The most vivid fall colors are produced when bright sunny days combine with cool nights below 45 degrees, but not freezing. Dry soil conditions are also favorable. Freezing temperatures do not contribute to fall colors. Freezing temperatures kill leaf cells and stop the whole process.

Mother Nature has always provided us with beautiful fall colors.
Some years are just more spectacular than others.