Updated: Dec 14, 2021
— New York, NY
As summer ends and fall begins, the leaves are a constant reminder of the changing seasons. Magical as they may seem, the changing colors can be attributed to various environmental shifts that affect deciduous trees. Rain, the cold weather, and most importantly, sunlight all play their own role in the biochemical process that is admired around the world!
The deciduous trees grow during the spring and summer before fall begins. During that time, chlorophyll builds up within the leaves, producing the familiar green color. The trees’ chlorophyll produces carbohydrates, which act as fuel and are stored for the changing process. As the days become shorter, chlorophyll stops producing and what is left breaks down, transforming the leaves from green to the colors of fall.
Chlorophyll’s chemical process also produces the chemicals necessary to result in the reds, oranges, and yellows of the fall trees. The four main chemicals are carotenoids, xanthophylls, anthocyanins, and tannins, each of which serve a different purpose. Carotenoids and xanthophylls become visible as the chlorophyll breaks down, having been hidden by chlorophyll’s green during the warmer seasons. Anthocyanins, which are produced as the veins of the leaves close, create the red colors of the trees. Finally, tannins, which are present in almost all deciduous tree leaves, are left behind once all of the other colors dissipate and produce the final brown color. The chemical reactions are specific to types of trees, for example, oak trees often turn red and brown or to a rusty color.
As for water and weather, they play their own role in determining the appearance of the new colors. Drought in the summer and freezing temperatures early in the fall both damage the foliage. If there is drought, the leaves are scorched, which can lead to early changing of colors or minimized changing. The rain knocks leaves to the ground, causing the leaves to fall earlier than they should.
The changing and falling of the leaves are beautiful processes that can feel mysterious, but they can be explained through science. There are still aspects of the chemical reactions occurring in deciduous trees that scientists do not fully understand. Thankfully, at the most fundamental level, we know why we are greeted with such beautiful sights each fall.