The variety of colors seen in trees, from summer green to the vibrant hues of autumn, results from chemical compounds called pigments within the leaves. These pigments interact with sunlight, absorbing some wavelengths of light while reflecting others, which determines the color we perceive. Throughout the growing season, a complex mixture of these chemicals is at work, but the change in seasons shifts which pigments are visible.
The Reason Trees Are Green
The pervasive green color of most leaves during the spring and summer is due to a pigment called chlorophyll. Chlorophyll is the most abundant chemical in the leaf structures, and its primary function is to capture light energy to power photosynthesis. This process converts water and carbon dioxide into sugars, which serve as the tree’s food source.
We see the color green because of how chlorophyll absorbs light from the sun’s visible spectrum. The pigment efficiently absorbs light from the red and blue regions, using that energy for photosynthesis. Green wavelengths of light are not absorbed; instead, they are reflected away from the leaf surface, making the leaves appear green.
The Yellow and Orange Pigments
While chlorophyll is the dominant pigment, other colors are present in the leaf tissue throughout the entire year. These are the carotenoids, which include both carotenes, responsible for orange hues, and xanthophylls, which produce yellow tones. These pigments are always in the leaf but are completely overshadowed by the high concentration of green chlorophyll during the active growing season.
Carotenoids play an active role in the leaf’s daily function. They assist in photosynthesis by absorbing certain light wavelengths that chlorophyll cannot capture effectively. These yellow and orange pigments also serve a protective role, shielding the chlorophyll from excessive light energy. When the green pigment eventually fades, the stable carotenoids become visible, revealing the underlying yellow and orange shades.
How Trees Initiate Color Change
The shift from green to the autumn palette begins when the tree senses the environmental changes that signal the end of the growing season. The primary trigger is the decreasing photoperiod, or the shortening of daylight hours, which is a more reliable signal than temperature alone. Cooler temperatures also play a role, but the length of the day dictates the start of the seasonal process.
In response to these signals, the tree initiates a process to conserve resources before the leaves drop. It stops producing new chlorophyll, and the existing green pigment begins to break down into colorless compounds. Simultaneously, a specialized layer of cells, known as the abscission layer, forms at the base of the leaf stem. This sealing process allows the tree to recycle valuable nutrients, such as nitrogen and phosphorus, back into its branches and roots for storage over winter.
The gradual disappearance of the dominant green chlorophyll unmasks the stable yellow and orange carotenoids that have been present all along. This breakdown of the green pigment ensures the tree salvages maximum energy and nutrients before shedding the leaf.
Why Some Trees Turn Red and Purple
The red and purple colors seen in certain species, such as maples and oaks, are created by a distinct group of pigments called anthocyanins. Unlike the carotenoids, anthocyanins are not present in the leaf throughout the summer but are newly manufactured late in the season. Their production requires the tree to use some of the sugars that were created during the final days of photosynthesis.
Anthocyanin synthesis is intensified by environmental conditions that include bright, sunny days and cool, but not freezing, nights. The sunny days allow the leaf to produce high levels of sugar, while the cool nights slow the movement of these sugars out of the leaf. This sugar accumulation then drives the chemical reaction that creates the red and purple pigments within the leaf’s cell sap.
The biological purpose of this late-season pigment production is thought to be a form of sun protection. The red layer acts as a sunscreen, shielding the leaf from intense light and ultraviolet radiation. This temporary protection allows the tree more time to efficiently complete the process of extracting and reabsorbing the remaining nutrients before the leaf detaches.