Fall Foliage: Why Do Leaves Change Color?

New England is known for its exquisite show of color during the autumn season. “Leaf peepers” come from all over the globe just to witness the phenomenon, and it’s no wonder as there are only five regions in the world to experience the changing of the leaves. We are still about 3-4 weeks out from forecasted peak foliage, but the extremities of trees are already beginning to turn shades of red, orange and gold. But what causes this beautiful display?

During the growing season, leaves are the largest contributors to a tree’s growth. It is in the leaf where chlorophyll absorbs energy from the sun, transforming carbon dioxide and water into carbohydrates which fuel growth in a process known as photosynthesis. Chlorophyll is also the chemical which gives leaves their green color! But there are secondary pigments known as carotenoids and flavonoids in leaves which are not visible through the chlorophyll—until…

As the temperature begins to drop and the length of daylight shortens, photosynthesis slows down and then stops altogether. This causes a breakdown in the chlorophyll, and the green pigment fades away, revealing a myriad of brilliant autumn colors! Some trees undergo a second chemical process which creates pigments known as anthocyanins; these tend to be red and purple in color. Anthocyanins are thought to be produced as a form of protection, helping the tree to recover nutrients in the leaves before the next growing season.

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Weather conditions can also impact the brilliance of fall foliage. A warm, wet spring, an average summer, or a fall with warm sunny days and cool nights present the best conditions for a spectacular autumn. Severe frosts will kill the leaves early causing them to turn brown and drop, and a warm, wet period during the fall will lower the intensity of colors. Droughts can delay foliage by weeks. So, as many things in life and nature, the conditions must be “just right”!

Most trees shed their leaves to conserve energy for the winter months. A layer of cells called the separation layer develops at the base of the leaf stem which severs the connection to the tree. The tree closes off the cut, and leaves will either fall of their own accord or be blown off by the wind. This process contributes to the breakdown in chlorophyll as the flow of nutrients is cut off.

Isn’t science awesome? Happy Fall!