The Role of Chlorophyll Breakdown
Leaves appear green during spring and summer due to the abundance of chlorophyll, the pigment essential for photosynthesis. As autumn approaches, shorter days and cooler temperatures signal trees to prepare for winter dormancy. The production of new chlorophyll slows down, and the existing chlorophyll begins to break down, revealing other pigments that were present in the leaves all along.
Unmasking Yellows and Oranges: Carotenoids
With the degradation of chlorophyll, yellow and orange pigments called carotenoids become visible. These pigments, like beta-carotene (found in carrots), are always present in the leaves but are masked by the dominant green of chlorophyll. Carotenoids also play a role in photosynthesis by absorbing light energy and protecting chlorophyll from damage.
Creating Reds and Purples: Anthocyanins
Reds, purples, and crimson hues are produced by pigments called anthocyanins, which are not present in the leaves during the growing season. These pigments are synthesized in the fall in the leaf's cell sap from sugars trapped in the leaves once veins begin to close. Bright, sunny autumn days and cool (but not freezing) nights tend to produce the most vibrant anthocyanin displays.
Environmental Factors and Their Impact
The timing and intensity of fall colors are primarily influenced by climate factors. A succession of warm, sunny days followed by cool, crisp, frost-free nights is ideal for brilliant color. Sunny days boost sugar production, while cool nights slow the transport of these sugars out of the leaves, promoting anthocyanin formation. Drought or early frost can diminish the intensity and duration of fall foliage.