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The brilliant colors deciduous trees and shrubs display in autumn are actually the grand finale of the complicated process of photosynthesis. That’s how plants produce food. It takes place in the leaf cells that contain chlorophyll, the pigment that makes leaves green. Leaves make chlorophyll all through the growing season. But as temperatures get cooler and the days grow shorter, chlorophyll production slows down and eventually stops. As the chlorophyll declines, the green starts to fade away, revealing pigments that had been masked by green. This is when the show begins.
Carotenoids are the yellow pigments that turn aspens and ginkgos stunning shades of yellow and gold. Tannins are responsible for the dull browns of some oaks. And since all leaves contain at least some tannin, that’s why most of them eventually turn to brown, especially after they fall from the tree.
Anthocyanin turns sumacs, below, and maples red and dogwoods purple. It’s a bit more temperamental than the other two pigments, and needs lots of sunlight to reach its colorful peak. If the leaf doesn’t get enough sun, the color stops at yellow, orange or even brown.
Of course temperatures come into play as well. Early freezes reduce colorful foliage because the leaves are killed before they can go through this elaborate process. Long, warm fall days and bright sun will create the most colorful fall show.