Evergreen trees—your pines, spruces, firs, etc.— went the protection route. Their leaves, or needles, are covered in a waxy coating to resist freezing, allowing them to live for years or even decades before falling off and being replaced. The leaves of deciduous trees, on the other hand, are cast off with the arrival of winter. The chemical processes that prepare them for their send-off also treat us to the season’s vibrant colors.
Color Coding Green: The green color of leaves throughout spring and summer comes from chlorophyll, a pigment vital to photosynthesis. As we get closer to autumn and some parts of the planet get fewer hours of sunlight, trees respond by stopping the food-making photosynthesis process and slowing the production of chlorophyll until, eventually, they stop producing it altogether and the green color of the leaf fades
Yellow and Orange: Along with chlorophyll, there are yellow and orange pigments, carotene and xanthophyll, inside some trees’ leaves. For most of the year, these pigments are masked by chlorophyll, but as the chlorophyll breaks down and the green color dissipates, the yellow to orange colors become visible.
Red: Another class of pigment that occurs in leaves is the anthocyanins. Anthocyanins, unlike carotene and xanthophyll, are not present in leaves year-round. It isn’t until the chlorophyll begins breaking down that the plant begins to synthesize anthocyanin. Why do trees begin producing a different pigment in leaves they’re getting ready to lose? The prevailing theory is that anthocyanins protect leaves from sun damage, lower their freezing point, allow them to remain on the tree longer, and buy the tree more time to recover nutrients from its leaves. The colors that anthocyanins produce are dependent on the pH of the leaves’ cell sap. Very acidic sap results in a bright red color, while less acidic sap leads to a purplish red.
Brown: The humdrum color is the result of waste products trapped in the leaves.
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